Shahin Internship Report on Non-Destructive Testing in Saj Engineering and Trading Company

Internship report on Non-Destructive Testing

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Practicum Report

On

Non-Destructive Testing (NDT)

Submitted To

Registrar

IUBAT—International University of Business Agriculture and Technology

Submitted By

1. Md. Shahin Manjurul Alam ID# 07207013

Program: BSME

December 14, 2010

IUBAT-International University of Business Agriculture & Technology


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Non-Destructive Testing

(NDT)


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Request for the Report

December 14, 2010

Engr. Abdul Wadud

Faculty and Course Coordinator

Department of Mechanical Engineering

CEAT- College of Engineering and Technology

IUBAT- International University of Business Agriculture and Technology

4, Embankment Drive Road, Uttara Model Town, Sector 10, Dhaka 1230, Bangladesh.

Subject: Request for the report.

Dear Sir

With due respect, I would like to submit this report as partial fulfillment of the BSME program,

the topic of ―Non-Destructive Testing (NDT)‖. It was superlative opportunity for me to work

on this topic to actualize my theoretical knowledge in the practical area and to have an enormous

experience on that system. Now I am looking forward for your kind assessment regarding this

report.

I would be very kind of you, if you please take the trouble of going through the report and

evaluate my performance regarding this report.

Sincerely Yours,

….…………………

1. Md. Shahin Manjurul Alam

ID # 07207014

BSME


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Letter of Transmittal

December 14, 2010

Engr. Abdul Wadud

Faculty and Course Coordinator

Department of Mechanical Engineering

CEAT- College of Engineering and Technology

IUBAT- International University of Business Agriculture and Technology

4, Embankment Drive Road, Uttara Model Town, Sector 10, Dhaka 1230, Bangladesh.

Subject: Letter of Transmittal of the Practicum Report.

Dear Sir

I have pleasure in submitting the practicum report on “Non-Destructive Testing (NDT)”.

According to your requirement I had worked in Saj Engineering & Trading Company. It was a

challenging work because, in our country Saj Engineering & Trading Company is the only one

company which has the latest equipments for NDT services and expert NDT practitioners. It

was certainly a great opportunity for me to work on this paper to actualize my theoretical

knowledge in the practical arena.

Though there were many hindrances arose during I was conducting data and information for this

project, I tried my level best to achieve our goal to make a realistic and informative research

paper.

Thank you, Sir

Sincerely Yours,

….……………………..


ID # 07207014

BSME


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SAJ ENGINEERING & TRADING COMPANY

205/5 Elephant Road (1st floor) Dhaka-1205, Bangladesh

Phone: +88 02 9677628, +88 02 8616859 Fax: +88 02 9677625 www.sajetc.com

To Whom It May Concern

This is to certify that Md. Shahin Manjurul Alam, student of IUBAT has continuing his

Internship program with us from 1st October, 2010 till today. The subject matter of the internship

program was Non-Destructive Testing (NDT).

During his internship he followed instructions according to the satisfaction of the management.

He was very keen to learn the lessons and enthusiastic in completing any assignment that was

given to him time to time.

We wish all the best for his future endeavors.

-----------------------------

Jahangir Kabir

CEO

Saj Engineering & Trading Company


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Student's Declaration

This is to inform that the Practicum Report on “Non-Destructive Testing (NDT)” has only been

prepared as a partial fulfillment of the Bachelor of Science in Mechanical Engineering (BSME)

Program. I hereby declare that the project embodied in this report in the result of my own

handwork and has not been submitted for another degree to another university.

Authors,

….………………………


ID # 07207014

BSME


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Acknowledgement

This Practicum Report which is entitled as “Non-Destructive Testing (NDT)” is the concrete

effort of a number of people.

In the process of conducting this research project, I would like to express my gratitude and

respect to some generous persons for their immense help and enormous cooperation.

First of all I would like to pay my gratitude to Honorable Vice Chancellor Prof. Dr. M.

Alimullah Miyan for giving me chance to prepare my research about this splendid topic.

I am very much grateful to some of my faculties specially Engr. Abdul Wadud, Respected

Course Coordinator of ME department of IUBAT, for his helping hand. I also say my warmest

thanks to Engr. Sarwar Iqbal, respected Faculty of ME department of IUBAT who had taken

many courses. I would like to thanks Engr. Amirul Islam for his painstaking guidance and

constant inspiration to do this report.

After that I would like to express my special gratitude to Md. Jahangir Kabir, CEO of Saj

Engineering & Trading Company, Engr. Amit Hasan, Service Engineer and Ferdous Ahmed

Marketing Executive of Saj Engineering & Trading Company for their keen interest and valuable

suggestions regarding preparing this report. I will never forget Engr. Md. Rashedul Alam,

Marico Bangladesh Ltd, who recommended us to do our internship on Non-Destructive Testing

in Saj Engineering Trading Company.

Finally I also feel it is important to acknowledge and thanks to my classmates especially to those

who participated in the data collection and who helped a lot to provide a valuable forum for the

exchange of ideas and information.


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Company Profile

Saj Engineering & trading company was established on 1998 and trying to develop specializing

on the supply full range of Non-Destructive Testing (NDT) equipments and its consumables to

Aviation, fertilizer, power generation, shipbuilding, Defense, training institute and NDT service

provider etc.

Company Goal

It is our goal to provide NDT practitioners with quality on the latest technology at reasonable prices and

ensure that these are readily available at our customer's convenience and satisfaction. Through the years,

we have endeavored to represent only the well-known manufacturers of NDT equipment in the world.

Our Customer

BIMAN BANGLADESH AIR LINES.

BANGLADESH AIR FORCE.

BANGLADESH ARMY.

BANGLADESH NAVY.

BANGLADESH SHIPPING CORPORATION.

BANGLADESH INLAND WATER TRANSPORT AUTHORITY.

BANGLADESH INLAND WATER TRANSPORT CORPORATION

DOCKYARD & ENGINEERING WORKS LTD.

KHULNA SHIPYARD LTD.

BANGLADESH POWER DEVELOPMENT BOARD.

CHITTAGONG PORT AUTHORITY.

ATOMIC ENERGY CENTER

ENGINEERING UNIVERSITY

ALL INSPECTION COMPANY IN BANGLADESH


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Our Sole Agent

MITSUBISHI HEAVY INDUSTRIES MARINE LTD. JAPAN

DAIKAI ENGINEERING PTE LTD. SINGAPORE (Daihatsu Diesel co ltd.)

IHI MARINE CO LTD. JAPAN

ISS MACHINERY CO LTD. JAPAN

MacGREGOR (SGP) PTE LTD. SINGAPORE

SKL MOTOREN-UND SYSTEMTECHNIK,GERMANY

ROSTOCK DIESEL GmbH, GERMANY

RS ―UNISCHIFF GmbH‖ GERMANY

VRM SERVICES, SINGAPORE

OLYMPUS SINGAPORE PTE LTD.

OLYMPUS NDT , CANADA

SIMPLEX MARINE PTE LTD(BLOM +VOSS,GERMANY)

BRANDNER ENGINEERING –AIRCON SAVER, GERMANY

PT SELAMAT SEMPURNA TBK(SAKURA), INDONESIA

ITW, INDIA(ITW, USA- FORTUNE 200 COMPANY)

ICM, BELGIUM

Emerson, USA


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Table of Contents

Topic Name Page

1. Preparatory Part:

A. Title Fly…………………………………………………………………………..…… 10

B. Topic Name…………………………………..…………………………….………… 10

C. Request for Report ………………………………………………………...………… 10

D. Letter of Transmittal…………………………............................................................. 10

E. To whom it May we concern …………………………................................................ 10

F. Student Declaration …………………………...............................................................10

G. Acknowledgement …………………………………………………………...…….…10

H. Table of Content ………………………………………………………………… 10- 10

J. Executive Summary …………………………………………………………………...10

2. Text of the Report

Introduction 2.1 Origin of the report

2.2 Objective

2.2.1 Broad Objective

2.2.2 Specific Objectives

2.3 Background

2.4 Methodology

2.5 Limitations

3. Company Overview

3.1 Company Profile…………………………………………………………….1

3.1 Company Goal………………………………………………………..1

3.2 Our Customer……………………………………………………….…………..1

3.3 Our Sole Agent…………………………………………………………………2

4. Introduction of Non-destructive Testing (NDT)………………………….3

4.1 History of NDT……………………………………………………….…3

4.3 Importance of NDT……………………………………………………...4

4.5 NDT Methods…………………………………………………………...5


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4.6 Six Most Common NDT Methods……………………………………....5

4.6.1 Visual and Optic Testing (VT)…………………………………………..6

4.6.2 Dye Penetrant Testing (DPT)……………………………………………6

4.6.3 Magnetic Particle Testing (MPT)………………………….……………..6

4.6.4 Eddy Current Testing (ECT)…………………………………….………6

4.6.5 Radiography Testing (RT)……………………………………….………7

4.6.6 Ultrasonic Testing (UT)…………………………….………………7

5. Visual Testing (VT)

5.1 Introduction………………………………………………………………7

5.1 Physical Principle………………………………………………………..8

5.3 Inspection Requirements………………………………………………..9

5.4 Practical Considerations………………………………………………..10

6. Dye Penetrant Testing (DPT)

6.1 Introduction Dye Penetrant Testing…………………………………………...10

6.2 History Dye Penetrant Testing………………………………………..………..11

6.3 DPT for Detectability of Flaws…………………………………..…………….11

6.4 Process for Dye Pretrant Testing………………………………………………..12

6.5 Common Uses of Dye Penetrant Testing………………………………………..14

6.6 Effectiveness of Dye Penetrant Testing…………………………………………15

6.7 Advantages of Dye Penetrant Testing………………………………………….16

6.8 Disadvantages of Dye Penetrant Testing ……………………………………….17

7. Magnetic Particle Testing (MPT)

7.1 Introduction of Magnetic Particle Testing………………………………………17

7.2 History of Magnetic Particle Testing……………………………………………18


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7.3 Basic Principles…………………………………………………………………19

7.4 Magnetic Field Orientation and Flaw Detectability…………………………….20

7.5 Portable Equipments for Magnetic Particle Testing……………………….……22

7.6 Permanent magnets……………………………………………………………..22

7.7 Electromagnets…………………………………………………………………..23

7.8 Prods……………………………………………………………………………..24

7.9 Portable Coils and Conductive Cables………………………………………….25

7.10 Portable Power Supplies………………………………………………………...25

7.11 Lights for Magnetic Particle Inspection…………….………………………….26

7.12 Dry Particle Inspection…………………………………………………………27

7.13 Examples of Dry Magnetic Particle Inspection…………………………………27

7.14 Advantages of Magnetic Particle Testing………………………………………28

7.15 Disadvantages of Magnetic Particle Testing……………………………………28

8. Eddy Current Testing (ECT)

8.1 Introduction of Eddy Current Testing…………………………………………29

8.2 History of Eddy Current Testing……………………………………………….30

8.3 Present State of Eddy Current Inspection………………………………………30

8.4 Research to Improve Eddy current measurements………………………………31

8.4.1 Photoinductive Imaging (PI)……………………………………………………32

8.4.1 Pulse Eddy Current……………………………………………………………..32

8.5 Eddy Current Instruments………………………………………………………32

8.6 Probes - Mode of Operation…………………………………………………….33

8.6.1 Absolute Probes…………………………………………………………34

8.6.2 Differential Probes……………………………………………………...34


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8.6.3 Reflection Probes………………………………………………………..35

8.6.4 Hybrid Probes…………………………………………………………..35

8.8.5 Probes – Configurations………………………………………………..36

8.8.6 Surface Probes………………………………………………………….36

8.8.7 Bolt Hole Probes……………………………………….……………….37

8.8.8 ID or Bobbin Probes……………………………………………………37

8.8.9 OD or Encircling Coils………………………………………………….38

8.9 Surface Breaking Cracks…………………………………………………………38

8.10 Surface Crack Detection Using Sliding Probes………………………………….40

8.11 Probe Types……………………………………………………………………...40

8.11.1 Fixed Sliding Probes…………………………………………………….40

8.11.2 Adjustable Sliding Probes……………………………………………….40

8.12 Reference Standards………………………………………………………….41

8.13 Inspection Variables……………………………………………………………42

8.13.1 Liftoff signal Adjustment……………………………………………...42

8.13.2 Scan Patterns…………………………………………………………..42

8.13.3 Signal Interpretation…………………………………………...………42

8.13.4 Probe Scan Deviation…………………………………………………..43

8.13.5 Crack Angle Deviation…………………………………………………43

8.13.6 Electrical Contact……………………………………………………....44

8.14 Tube Inspection by Eddy Current…………………………….………………..44

8.15 Thickness Measurements of Thin Material…………………….……………….45

8.16 Corrosion Thinning of Aircraft Skins………………………….……………….45


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8.17 Thickness Measurement of Thin Conductive Sheet, Strip and Foil…….……...46

8.18 Thickness Measurement of Thin Conductive Layers…………………………..47

8.19 Pulsed Eddy Current Inspection……………………………………………….47

8.20 EC Standards and Methods…………………………………………………….48

9. Radiography Testing (RT)

9.1 History of Radiography…………………………………………………………50

9.2 A Second Source of Radiation…………………………………………………51

9.3 Health Concerns………………………………………………………………..52

9.4 Present State of Radiography……………………………………………….….53

9.5 Future Direction of Radiographic Education…………………………………..54

9.6 Properties of X-Rays and Gamma Rays………………………………………..55

9.6.1 X-Radiation……………………………………………………………..55

9.6.1 Bremsstrahlung Radiation………………………………………………56

9.6.1 Gamma Radiation……………………………………………………….57

9.7 Types Radiation Produced by Radioactive Decay………………………………57

9.7.1 Alpha Particles…………………………………………………………58

9.7.2 Beta Particles…………………………………………………………..58

9.7.3 Gamma-rays…………………………………………………………..58

9.8 Filters in Radiography………………………………………………………….58

9.9 Radiation Safety………………………………………………………………..59

9.10 Radiographic Film………………………………………………………………60

9.10.1 Film Selection………………………………………………………….61


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9.10.2 Film Packaging…………………………………………………………62

9.10.3 Film Handling………………………………………………………….63

9.10.4 Film Processing………………………………………………………..63

9.10.4.1 Manual Processing & Darkrooms…………………………..64

9.10.4.2 Automatic Processor Evaluation……………………………65

9.11 Radiograph Interpretation – Welds……………………………………………..65

9.12 Discontinuities…………………………………………………………………..66

9.13 Welding Discontinuities…………………………………………………………66

9.13.1 Cold Lap……………………………………………………………….66

9.13.2 Porosity………………………………………………………………...67

9.13.3 Cluster porosity…………………………………………………………67

9.13.4 Slag inclusions…………………………………………………………68

9.13.5 IP and LOP……………………………………………………………..69

9.13.6 Incomplete fusion………………………………………………………69

9.13.7 Internal concavity or suck back………………………………………..70

9.13.8 Internal or root undercut……………………………………………….70

9.13.9 External or crown undercut…………………………………………….71

9.13.10 Offset or mismatch…………………………………………………….72

9.13.11 Inadequate weld reinforcement………………………………………72

9.13.12 Excess weld reinforcement……………………………………………73

9.13.13 Cracks…………………………………………………………………73

9.13.14 Discontinuities in TIG welds………………………………………...74


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9.13.15 Tungsten inclusions…………………………………………………..74

9.13.16 Oxide inclusions……………………………………………………...74

9.13.17 Discontinuities in Gas Metal Arc Welds (GMAW)…………………..75

9.13.18 Burn-Through…………………………………………………………75

9.14 Real-time Radiography…………………………………………………………76

9.15 Advantages of Radiography……………………………………………………76

9.16 Disadvantages of Radiography…………………………………………………76

10. Ultrasonic Testing (UT) 10.1 Introduction of Ultrasonic Testing………………………………………………77

10.2 Basic Principles of Ultrasonic Testing…………………………………………..77

10.3 History of Ultrasonics…………………………………………………………..79

10.4 Present State of Ultrasonics…………………………………………………….80

10.5 Future Direction of Ultrasonic Inspection………………………………………82

10.6 Wavelength and Defect Detection………………………………………………83

10.7 Sound Propagation in Elastic Materials…………………………………………85

10.8 Speed of Sound…………………………………………………………………85

10.9 Applications of Non-Destructive Testing………………………………………..86

10.10 Piezoelectric Transducers……………………………………………………….88

10.11 Characteristics of Piezoelectric Transducers……………………………………90

10.12 Radiated Fields of Ultrasonic Transducers…………………………………….91

10.13 Transducer Types………………………………………………………………93

10.13.1 Contact Transducers…………………………………………………94

10.13.2 Immersion transducers……………………………………………….94


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10.13.3 More on Contact Transducers………………………………………..94

10.13.4 Dual element transducer…………………………………………….95

10.13.5 Delay line transducers……………………………………………….95

10.13.6 Angle beam transducers……………………………………………..96

10.13.7 Normal incidence shear wave transducers……………….………….96

10.13.8 Paint brush transducers……………………………………………...96

10.14 Couplant………………………………………………………………………..97

10.15 Pulser-Receivers………………………………………………………………..97

10.16 Angle Beams I………………………………………………………………….99

10.17 Angle Beams II………………………………………………………………….99

10.18 Calibration Methods……………………………………………………………100

10.19 Weldments (Welded Joints)……………………………………………………101

10.20 Advantages of Ultrasonic Flaw Detection…………………………………….103

10.21 Disadvantages of Ultrasonic Flaw Detection…………………………………104

11. Applications of Non-Destructive Testing………………………………104

11.1 Aerospace Industry……………………………………….………………….104

11.2 Aircraft Overhaul………………………………………………….…………104

11.3 Automotive Industry……………………………………………….………...104

11.4 Petrochemical & Gas Industries………………………………………….…..104

11.5 Railway Industry…………………………………………………….…..…...104

11.6 Mining Industry………………………………………………………….…...104

11.7 Agricultural Engineering. ………………………………………….………....104

11.8 Power Generation…………………………………………………………….105

11.9 Iron Foundry………………………………………………………………….105


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11.10 Shipbuilding Industry…………………………………………………………105

11.11 Steel Industry…………………………………………………………………105

11.12 Pipe & Tube Manufacturing Industry………………………………………...105

12. Recommendation…………………………………………………………………..105

13. Conclusion…………………………………………………………………………..106

4 References…………………………………………………………………………...106


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Executive Summary

Non-destructive testing in its present form has been carried out, by specialised service companies

and manufacturers, for decades. Röntgen Technische Dienst bv in The Netherlands was

established more than 60 years ago, in 1937, and that year marked the beginning of radiographic

inspection of welds in The Netherlands. Similar situations exist in other countries. In fact,

welding industry would not have experienced the growth and the wide range of applications it

has today if there were no such thing as NDT.

NDT has a very important formal status. Requirements for performance of NDT, acceptance

criteria and requirements for personnel qualification are implemented in codes and standards.

The NDT procedure is part of the contract. During the many years that NDT methods have been

used in industry a well-established situation has evolved, enabling the use of NDT for the

evaluation of welds against Good Workmanship Criteria on a routine basis, thus maintaining

workmanship standards and minimising the risks of component failure.

In addition, NDT plays an important part in industrial maintenance. During plant shutdowns for

instance, many thousands of ultrasonic wall thickness measurements are taken on piping, vessels,

furnace tubes etc. All these thickness readings have to go into extensive data bases, and this

process is, thanks to modern computers and data loggers, ever more automated.

The ultimate aim was, to find a way to accept and reject weld defects on the basis of their

significance for weld integrity. For let us be honest: in conventional NDT we are doing

something completely different. We base our judgement on density differences on a film, or on

echo amplitudes on a screen. Parameters that have very little to do indeed with significance of

defects for weld integrity.

In maintenance practice, we base our decisions on NDT that is performed during shutdowns. A

significant amount of money could be saved if we would have NDT methods that minimize the

time required for that shutdown, or, a step further.


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4. Introduction of Non-destructive Testing (NDT)

Non-destructive Testing is one part of the function of Quality Control and is complementary to

other long established methods. By definition non-destructive testing the use of noninvasive

techniques to determine the integrity of a material, component or structure or quantitatively

measure some characteristic of an object. It is the testing of materials, for surface or internal

flaws or metallurgical condition, without interfering in any way with the integrity of the material

or its suitability for service. The technique can be applied on a sampling basis for individual

investigation or may be used for 100% checking of material in a production quality control

system. Whilst being a high technology concept, evolution of the equipment has made it robust

enough for application in any industrial environment at any stage of manufacture - from steel

making to site inspection of components already in service. A certain degree of skill is required

to apply the techniques properly in order to obtain the maximum amount of information

concerning the product, with consequent feed back to the production facility. Non-destructive

Testing is not just a method for rejecting substandard material; it is also an assurance that the

supposedly good is good. The technique uses a variety of principles; there is no single method

around which a black box may be built to satisfy all requirements in all circumstances.

4.1 History of NDT

Nondestructive testing has been practiced for many decades, with initial rapid developments in

instrumentation spurred by the technological advances that occurred during World War II and

the subsequent defense effort. During the earlier days, the primary purpose was the detection of

defects. As a part of "safe life" design, it was intended that a structure should not develop

macroscopic defects during its life, with the detection of such defects being a cause for removal

of the component from service. In the early 1970's, two events occurred which caused a major

change in the NDT field. First, improvements in the technology led to the ability to detect small

flaws, which caused more parts to be rejected even though the probability of component failure

had not changed. However, the discipline of fracture mechanics emerged, which enabled one to

predict whether a crack of a given size will fail under a particular load when a material's fracture

toughness properties are known. Other laws were developed to predict the growth rate of cracks

under cyclic loading (fatigue). With the advent of these tools, it became possible to accept

structures containing defects if the sizes of those defects were known. This formed the basis for

the new philosophy of "damage tolerant" design. Components having known defects could


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continue in service as long as it could be established that those defects would not grow to a

critical, failure producing size.

A new challenge was thus presented to the nondestructive testing community. Detection was not

enough. One needed to also obtain quantitative information about flaw size to serve as an input

to fracture mechanics based predictions of remaining life. The need for quantitative information

was particularly strongly in the defense and nuclear power industries and led to the emergence of

quantitative nondestructive evaluation (QNDE) as a new engineering/research discipline. A

number of research programs around the world were started, such as the Center for

Nondestructive Evaluation at Iowa State University (growing out of a major research effort at the

Rockwell International Science Center); the Electric Power Research Institute in Charlotte, North

Carolina; the Fraunhofer Institute for Nondestructive Testing in Saarbrucken, Germany; and the

Nondestructive Testing Centre in Harwell, England.

4.3 Importance of NDT

NDT plays an important role in the quality control of a product. It is used during all the stages of

manufacturing of a product. It is used to monitor the quality of the:

1. Raw materials which are used in the construction of the product.

2. Fabrication processes which are used to manufacture the product.

3. Finished product before it is put into service.

Use of NDT during all stages of manufacturing results in the following benefits:

1. It increases the safety and reliability of the product during operation.

2. It decreases the cost of the product by reducing scrap and conserving materials, labor and

energy.

3. It enhances the reputation of the manufacturer as producer of quality goods. All of the

above factors boost the sales of the product which bring more economical benefits to the

manufacturer. NDT is also used widely for routine or periodic determination of quality of

the plants and structures during service. This not only increases the safety of operation

but also eliminates any forced shut down of the plants.


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4.5 NDT Methods

The number of NDT methods that can be used to inspect components and make measurements is

large and continues to grow. Researchers continue to find new ways of applying physics and

other scientific disciplines to develop better NDT methods.

The methods covered are:

Visual Testing

Microwave

Thermography

Magnetic Particle Testing

Tap Testing

Radiography Testing

Acoustic Microscopy

Acoustic Emission

Magnetic Measurements

Ultrasonic Testing

Flux Leakage

Laser Interferometry

Eddy Current

Dye Penetrant Testing

4.6 Six Most Common NDT Methods

There are six NDT methods that are used most often. They are

1. Visual and Optical Testing (VT)


3. Magnetic Particle Testing(MPT)

4. Eddy Current Testing(ECT)


6. Ultrasonic Testing (UT)

http://www.ndt-ed.org/EducationResources/CommunityCollege/PenetrantTest/cc_pt_index.htm

http://www.ndt-ed.org/EducationResources/CommunityCollege/Radiography/cc_rad_index.htm

http://www.ndt-ed.org/EducationResources/CommunityCollege/Ultrasonics/cc_ut_index.htm


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4.6.1 Visual and Optic Testing (VT)

Visual inspection involves using an inspector's eyes to look for defects. The inspector may also

use special tools such as magnifying glasses, mirrors, or borescopes to gain access and more

closely inspect the subject area. Visual examiners follow procedures that range from simple to

very complex.

4.6.2 Dye Penetrant Testing (DPT)

Test objects are coated with visible or fluorescent dye solution. Excess dye is then removed from

the surface, and a developer is applied. The developer acts as blotter, drawing trapped penetrant

out of imperfections open to the surface. With visible dyes, vivid color contrasts between the

penetrant and developer make "bleedout" easy to see. With fluorescent dyes, ultraviolet light is

used to make the bleedout fluoresce brightly, thus allowing imperfections to be readily seen.

4.6.3 Magnetic Particle Testing (MPT)

This NDT method is accomplished by inducing a magnetic field in a ferromagnetic material and

then dusting the surface with iron particles (either dry or suspended in liquid). Surface and near-

surface imperfections distort the magnetic field and concentrate iron particles near imperfections,

previewing a visual indication of the flaw.

4.6.4 Eddy Current Testing (ECT)

Electrical currents are generated in a conductive material by an induced alternating magnetic

field. The electrical currents are called eddy currents because they flow in circles at and just

below the surface of the material. Interruptions in the flow of eddy currents, caused by

imperfections, dimensional changes, or changes in the materials conductive and permeability

properties, can be detected with the proper equipment.


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4.6.5 Radiography Testing (RT)

Radiography involves the use of penetrating gamma or X-radiation to examine parts and

products for imperfections. An X-ray generator or radioactive isotope is used as a source of

radiation. Radiation is directed through a part and onto film or other imaging media. The

resulting shadowgraph shows the dimensional features of the part. Possible imperfections are

indicated as density changes on the film in the same manner as medical X-ray shows broken

bones.

4.6.6 Ultrasonic Testing (UT)

Ultrasonic use transmission of high-frequency sound waves into a material to detect

imperfections or to locate changes in material properties. The most commonly used ultrasonic

testing technique is pulse echo, wherein sound is introduced into a test object and reflections

(echoes) are returned to a receiver from internal imperfections or from the part's geometrical

surfaces.

5. Visual Testing (VT)

5.1 Introduction of Visual Testing

Visual inspection is by far the most common nondestructive testing (NDT) technique. When

attempting to determine the soundness of any part or specimen for its intended application, visual

inspection is normally the first step in the examination process. Generally, almost any specimen

can be visually examined to determine the accuracy of its fabrication. For example, visual

inspection can be used to determine whether the part was fabricated to the correct size, whether

the part is complete, or whether all of the parts have been appropriately incorporated into the

device. While direct visual inspection is the most common nondestructive testing technique,

many other NDT methods require visual intervention to interpret images obtained while carrying

out the examination. For instance, penetrant inspection using visible red or fluorescent dye relies

on the inspector‘s ability to visually identify surface indications. In arriving at a definition of

visual inspection, it has been noted in the literature that experience in visual inspection and

discussion with experienced visual inspectors revealed that this NDT method includes more than


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use of the eye, but also includes other sensory and cognitive processes used by inspectors. Thus,

there is now an expanded definition of visual inspection in the literature:

―Visual inspection is the process of examination and evaluation of systems and components by

use of human sensory systems aided only by mechanical enhancements to sensory input as

magnifiers, dental picks, stethoscopes, and the like. The inspection process may be done using

such behaviors as looking, listening, feeling, smelling, shaking, and twisting. It included a

cognitive component wherein observations are correlated with knowledge of structure and with

descriptions and diagrams from service literature.‖ The human eye is one of mankind‘s most

fascinating tools and is capable of assessing many visual characteristics and identifying various

types of discontinuities.

5.2 Physical Principle

The human eye is one of mankind‘s most fascinating tools. It has greater precision and accuracy

than many of the most sophisticated cameras. It has unique focusing capabilities and has the

ability to work in conjunction with the human brain so that it can be trained to find specific

details or characteristics in a part or test piece. It has the ability to differentiate and distinguish

between colors and hues as well. The human eye is capable of assessing many visual

characteristics and identifying various types of discontinuities. The eye can perform accurate

inspections to detect size, shape, color, depth, brightness, contrast, and texture. Visual testing is

essentially used to detect any visible discontinuities, and in many cases, visual testing may locate

portions of a specimen that should be inspected further by other NDT techniques. Many

inspection factors have been standardized so that categorizing them as major and minor

characteristics has become common. Surface finish verification of machined parts has even been

developed, and classification can be performed by visual comparison to manufactured finish

standards. In the fabrication industry, weld size, contour, length, and inspection for surface

discontinuities are routinely specified many companies have mandated the need for qualified and

certified visual weld inspection. This is the case particularly in the power industry, which

requires documentation of training and qualification of the inspector. Forgings and castings are

normally inspected for surface indications such as laps, seams, and other various surface

conditions.

5.3 Inspection Requirements


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Requirements for visual inspection typically pertain to the vision of the inspector; the amount of

light falling on the specimen, which can be measured with a light meter; and whether the area

being inspected is in anyway obstructed from view.In many cases, each of these requirements is

detailed in regulatory code or other inspection criteria.

Mechanical and/or optical aids may be necessary to perform visual testing. Because visual

inspection is so frequently used, several companies now manufacture gages to assist visual

inspection examinations. Mechanical aids include: measuring rules and tapes, calipers and

micrometers, squares and angle measuring devices, thread, pitch and thickness gages, level

gages, and plumb lines. Welding fabrication uses fillet gages to determine the width of the weld

fillet, undercut gages, angle gages, skew fillet weld gages, pit gages, contour gages, and a host of

other specialty items to ensure product quality.

At times direct observation is impossible and remote viewing is necessary which requires the use

of optical aids. Optical aids for visual testing range from simple mirrors or magnifying glasses to

sophisticated devices, such as closed circuit television and coupled fiber optic scopes. The

following list includes most optical aids currently in use :

Mirrors (especially small, angled mirrors).

Magnifying glasses, eye loupes, multilens magnifiers, measuring magnifiers.

Microscopes (optical and electron).

Optical flats (for surface flatness measurement).

Borescopes and fiber optic borescopes.

Optical comparators.

Photographic records

Closed circuit television (CCTV) systems (alone and coupled to

borescopes/microscopes).

Machine vision systems.

Positioning and transport systems (often used with CCTV systems).

Image enhancement (computer analysis and enhancement).

5.4 Practical Considerations


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Visual inspection is applicable to most surfaces, but is most effective where the surfaces have

been cleaned prior to examination, for example, any scale or loose paint should be removed by

wire brushing, etc. Vision testing of an inspector often requires eye examinations with standard

vision acuity cards such as Jaeger, Snellen, and color charts. Vision testing of inspectors has

been in use for about 40 years. Although many changes in NDT methods have taken place over

the years and new technologies have been developed, vision testing has changed little over time.

Also little has been done to standardize vision tests used in the industrial sector. For those

seeking certification in the area of visual testing, (Visual and Optical Testing) provides a useful

reference.


6.1 Introduction Dye Penetrant Testing

This method is frequently used for the detection of surface breaking flaws in non ferromagnetic

materials. The subject to be examined is first of all chemically cleaned, usually by vapors phase,

to remove all traces of foreign material, grease, dirt, etc.

from the surface generally, and also from within the

cracks. Next the penetrant (which is a very fine thin oil

usually dyed bright red or ultra-violet fluorescent) is

applied and allowed to remain in contact with the

surface for approximately fifteen minutes. Capillary

action draws the penetrant into the crack during this

period. The surplus penetrant on the surface is then

removed completely and thin coating of powdered chalk

is applied. After a further period (development time) the chalk

draws the dye out of the crack, rather like blotting paper, to form a

visual, magnified in width, indication in good contrast to the background. The process is purely a

mechanical/chemical one and the various substances used may be applied in a large variety of

ways, from aerosol spray cans at the most simple end to dipping in large tanks on an automatic

basis at the other end. The latter system requires sophisticated tanks, spraying and drying

Fig: Dye Penetrant Testing


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equipment but the principle remains the same. Dye Penetrant Testing is a method that is used to

reveal surface breaking flaws by bleedout of a colored or fluorescent dye from the flaw. The

technique is based on the ability of a liquid to be drawn into a "clean" surface breaking flaw by

capillary action. After a period of time called the "dwell," excess surface penetrant is removed

and a developer applied. This acts as a blotter. It draws the penetrant from the flaw to reveal its

presence. Colored (contrast) penetrants require good white light while fluorescent penetrants

need to be used in darkened conditions with an ultraviolet "black light".

6.2 History of Dye Penetrant Testing

A very early surface inspection technique involved the rubbing of carbon black on glazed

pottery, whereby the carbon black would settle in surface cracks rendering them visible. Later, it

became the practice in railway workshops to examine iron and steel components by the "oil and

whiting" method. In this method, a heavy oil commonly available in railway workshops was

diluted with kerosene in large tanks so that locomotive parts such as wheels could be submerged.

After removal and careful cleaning, the surface was then coated with a fine suspension of chalk

in alcohol so that a white surface layer was formed once the alcohol had evaporated. The object

was then vibrated by being struck with a hammer, causing the residual oil in any surface cracks

to seep out and stain the white coating. This method was in use from the latter part of the 19th

century to approximately 1940, when the magnetic particle method was introduced and found to

be more sensitive for ferromagnetic iron and steels. A different (though related) method was

introduced in the 1940's. The surface under examination was coated with a lacquer, and after

drying, the sample was caused to vibrate by the tap of a hammer. The vibration causes the brittle

lacquer layer to crack generally around surface defects.

6.3 DPT for Detectability of Flaws

The advantage that a Dye Penetrant Testing (DPT) offers over an unaided visual inspection is

that it makes defects easier to see for the inspector. There are basically two ways that a penetrant

inspection process makes flaws more easily seen. First, DPT produces a flaw indication that is

much larger and easier for the eye to detect than the flaw itself. Many flaws are so small or

narrow that they are undetectable by the unaided eye. Due to the physical features of the eye,

http://www.ndt-ed.org/EducationResources/CommunityCollege/PenetrantTest/Introduction/Keywords/pt1.htm


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there is a threshold below which objects cannot be resolved. This threshold of visual acuity is

around 0.003 inch for a person with 20/20 vision.The second way that DPT improves the

detectability of a flaw is that it produces a flaw indication with a high level of contrast between

the indication and the background also heDPTng to make the indication more easily seen. When

a visible dye penetrant inspection is performed, the penetrant materials are formulated using a

bright red dye that provides for a high level of contrast between the white developer. In other

words, the developer serves as a high contrast background as well as a blotter to pull the trapped

penetrant from the flaw. When a fluorescent penetrant inspection is performed, the penetrant

materials are formulated to glow brightly and to give off light at a wavelength that the eye is

most sensitive to under dim lighting conditions.

6.4 Process for Dye Pretrant Testing

1. Surface Preparation: One of the most critical steps of a Dye Penetrant Testing is the

surface preparation. The surface must be free of oil, grease, water, or other contaminants

that may prevent penetrant from entering flaws. The sample may also require etching if

mechanical operations such as machining, sanding, or grit blasting have been performed.

These and other mechanical operations can smear metal over the flaw opening and

prevent the penetrant from entering.

2. Penetrant Application: Once the surface has been thoroughly cleaned and dried, the

penetrant material is applied by spraying, brushing, or immersing the part in a penetrant

bath.

3. Penetrant Dwell: The penetrant is left on the surface for a sufficient time to allow as

much penetrant as possible to be drawn from or to seep into a defect. Penetrant dwell

time is the total time that the penetrant is in contact with the part surface. Dwell times are

usually recommended by the penetrant producers or required by the specification being

followed. The times vary depending on the application, penetrant materials used, the

material, the form of the material being inspected, and the type of defect being inspected

for. Minimum dwell times typically range from five to 60 minutes. Generally, there is no

harm in using a longer penetrant dwell time as long as the penetrant is not allowed to dry.


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4. Excess Penetrant Removal: This is the most delicate part of the inspection procedure

because the excess penetrant must be removed from the surface of the sample while

removing as little penetrant as possible from defects. Depending on the penetrant system

used, this step may involve cleaning with a solvent, direct rinsing with water, or first

treating the part with an emulsifier and then rinsing with water.

5. Developer Application: A thin layer of developer is then applied to the sample to draw

penetrant trapped in flaws back to the surface where it will be visible. Developers come

in a variety of forms that may be applied by dusting (dry powdered), dipping, or spraying

(wet developers).

6. Indication Development: The developer is allowed to stand on the part surface for a

period of time sufficient to permit the extraction of the trapped penetrant out of any

surface flaws. This development time is usually a minimum of 10 minutes. Significantly

longer times may be necessary for tight cracks.

7. Inspection: Inspection is then performed under appropriate lighting to detect indications

from any flaws which may be present.

8. Clean Surface: The final step in the process is to thoroughly clean the part surface to

remove the developer from the parts that were found to be acceptable.

6.5 Common Uses of Dye Penetrant Testing

Dye penetrant Testing (DPT) is one of the most widely used

nondestructive Testing (NDT) methods. Its popularity can be

attributed to two main factors: its relative ease of use and its

flexibility. DPT can be used to inspect almost any material

provided that its surface is not extremely rough or porous.

Materials that are commonly inspected using DPT include the

following:

Metals (aluminum, copper, steel, titanium, etc.)

Glass


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Many ceramic materials

Rubber

Plastics

DPT offers flexibility in performing inspections because it can be applied in a large variety of

applications ranging from automotive spark plugs to critical aircraft components. Penetrant

materials can be applied with a spray can or a cotton swab to inspect for flaws known to occur in

a specific area or it can be applied by dipping or spraying to quickly inspect large areas. In the

image above, visible dye penetrant is being locally applied to a highly loaded connecting point to

check for fatigue cracking.

Dye Penetrant Testing can only be used to inspect for flaws that break the surface of the sample.

Some of these flaws are listed below:

Fatigue cracks

Quench cracks

Grinding cracks

Overload and impact fractures

Porosity

Laps

Seams

Pin holes in welds

Lack of fusion or braising along the edge of the bond line

As mentioned above, one of the major limitations of a penetrant inspection is that flaws must be

open to the surface. To learn more about the advantages and disadvantages of DPT, proceed to

the next page.

6.6 Effectiveness of Dye Penetrant Testing

small round defects than small linear defects. Small round defects are generally easier

to detect for several reasons. First, they are typically volumetric defects that can trap

significant amounts of penetrant. Second, round defects fill with penetrant faster than


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linear defects. One research effort found that elliptical flaw with length to width ratio of

100, will take the penetrant nearly 10 times longer to fill than a cylindrical flaw with the

same volume.

deeper flaws than shallow flaws. Deeper flaws will trap more penetrant than shallow

flaws, and they are less prone to over washing.

flaws with a narrow opening at the surface than wide open flaws. Flaws with narrow

surface openings are less prone to over washing.

flaws on smooth surfaces than on rough surfaces. The surface roughness of the part

primarily affects the removability of a penetrant. Rough surfaces tend to trap more

penetrant in the various tool marks, scratches, and pits that make up the surface.

flaws with rough fracture surfaces than smooth fracture surfaces. The surface

roughness that the fracture faces is a factor in the speed at which a penetrant enters a

defect. In general, the penetrant spreads faster over a surface as the surface roughness

increases. It should be noted that a particular penetrant may spread slower than others on

a smooth surface but faster than the rest on a rougher surface.

flaws under tensile or no loading than flaws under compression loading. In a 1987

study at the University College London, the effect of crack closure on detectability was

evaluated. Researchers used a four-point bend fixture to place tension and compression

loads on specimens that were fabricated to contain fatigue cracks. All cracks were

detected with no load and with tensile loads placed on the parts. However, as

compressive loads were placed on the parts, the crack length steadily decreased as load

increased until a load was reached when the crack was no longer detectable.

6.7 Advantages of Dye Penetrant Testing

The method has high sensitivity to small surface discontinuities.

The method has few material limitations, i.e. metallic and nonmetallic, magnetic and

nonmagnetic, and conductive and nonconductive materials may be inspected.

Large areas and large volumes of parts/materials can be inspected rapidly and at low cost.

Parts with complex geometric shapes are routinely inspected.


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Indications are produced directly on the surface of the part and constitute a visual

representation of the flaw.

Aerosol spray cans make penetrant materials very portable.

Penetrant materials and associated equipment are relatively inexpensive.

6.8 Disadvantages of Dye Penetrant Testing

Only surface breaking defects can be detected.

Only materials with a relatively nonporous surface can be inspected.

Precleaning is critical since contaminants can mask defects.

Metal smearing from machining, grinding, and grit or vapor blasting must be removed

prior to LPI.

The inspector must have direct access to the surface being inspected.

Surface finish and roughness can affect inspection sensitivity.

Multiple process operations must be performed and controlled.

Post cleaning of acceptable parts or materials is required.

Chemical handling and proper disposal is required.

7. Magnetic Particle Testing

7.1 Introduction of Magnetic Particle Testing

This method is suitable for the detection of surface and near surface discontinuities in magnetic

material, mainly ferrite steel and iron. Magnetic particle Testing (MPI) is a nondestructive

testing method used for defect detection. MPI is fast and relatively easy to apply, and part

surface preparation is not as critical as it is for some other NDT methods. These characteristics

make MPI one of the most widely utilized nondestructive testing methods. MPI uses magnetic

fields and small magnetic particles (i.e.iron filings) to detect flaws in components. The only

requirement from an inspectability standpoint is that the component being inspected must be

made of a ferromagnetic material such as iron, nickel, cobalt, or some of their alloys.

Ferromagnetic materials are materials that can be magnetized to a level that will allow the

inspection to be effective. The method is used to inspect a variety of product forms including


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castings, forgings, and weldments. Many different industries use magnetic particle inspection for

determining a component's fitness-for-use. Some examples of industries that use magnetic

particle inspection are the structural steel, automotive, petrochemical, power generation, and

aerospace industries. Underwater inspection is another area where magnetic particle inspection

may be used to test items such as offshore structures and underwater pipelines.

7.2 History of Magnetic Particle Testing

Magnetism is the ability of matter to attract other matter to itself. The ancient Greeks were the

first to discover this phenomenon in a mineral they named magnetite. Later on Bergmann,

Becquerel, and Faraday discovered that all matter including liquids and gasses were affected by

magnetism, but only a few responded to a noticeable extent.

The earliest known use of magnetism to inspect an object took place as early as 1868. Cannon

barrels were checked for defects by magnetizing the barrel then sliding a magnetic compass

along the barrel's length. These early inspectors were able to locate flaws in the barrels by

monitoring the needle of the compass. This was a form of nondestructive testing but the term

was not commonly used until sometime after World War I.

In the early 1920‘s, William Hoke realized that magnetic particles (colored metal shavings)

could be used with magnetism as a means of locating defects. Hoke discovered that a surface or

subsurface flaw in a magnetized material caused the magnetic field to distort and extend beyond

the part. This discovery was brought to his attention in the machine shop. He noticed that the

metallic grindings from hard steel parts (held by a magnetic chuck while being ground) formed

patterns on the face of the parts which corresponded to the cracks in the surface. Applying a fine

ferromagnetic powder to the parts caused a build up of powder over flaws and formed a visible

indication. The image shows a 1928 Electyro-Magnetic Steel Testing Device (MPI) made by the

Equipment and Engineering Company Ltd. (ECO) of Strand, England.

In the early 1930‘s, magnetic particle inspection was quickly replacing the oil-and-whiting

method (an early form of the liquid penetrant inspection) as the method of choice by the railroad

industry to inspect steam engine boilers, wheels, axles, and tracks. Today, the MPI inspection


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method is used extensively to check for flaws in a large variety of manufactured materials and

components. MPI is used to check materials such as steel bar stock for seams and other flaws

prior to investing machining time during the manufacturing of a component. Critical automotive

components are inspected for flaws after fabrication to ensure that defective parts are not placed

into service. MPI is used to inspect some highly loaded components that have been in-service for

a period of time. For example, many components of high performance racecars are inspected

whenever the engine, drive train or another system undergoes an overhaul. MPI is also used to

evaluate the integrity of structural welds on bridges, storage tanks, and other safety critical

structures.

7.3 Basic Principles

In theory, magnetic particle inspection (MPI) is a relatively simple concept. It can be considered

as a combination of two nondestructive testing methods: magnetic flux leakage testing and visual

testing. Consider the case of a bar magnet. It has a

magnetic field in and around the magnet. Any place that

a magnetic line of force exits or enters the magnet is

called a pole. A pole where a magnetic line of force exits

the magnet is called a north pole and a pole where a line of force enters the magnet is called a

south pole.

When a bar magnet is broken in the center of its length, two complete bar magnets with magnetic

poles on each end of each piece will result. If the magnet is just cracked but not broken

completely in two, a north and south pole will form at

each edge of the crack. The magnetic field exits the

north pole and reenters at the south pole. The magnetic

field spreads out when it encounters the small air gap

created by the crack because the air cannot support as

much magnetic field per unit volume as the magnet can.

When the field spreads out, it appears to leak out of the

material and, thus is called a flux leakage field.


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If iron particles are sprinkled on a cracked magnet, the particles will be attracted to and cluster

not only at the poles at the ends of the magnet, but also at the poles at the edges of the crack.

This cluster of particles is much easier to see than the actual crack and this is the basis for

magnetic particle inspection.

The first step in a magnetic particle inspection is to magnetize the component that is to be

inspected. If any defects on or near the surface are present, the defects will create a leakage field.

After the component has been magnetized, iron particles, either in a dry or wet suspended form,

are applied to the surface of the magnetized part. The particles will be attracted and cluster at the

flux leakage fields, thus forming a visible indication that the inspector can detect.

7.4 Magnetic Field Orientation and Flaw Detectability

To properly inspect a component for cracks or other defects, it is important to understand that the

orientation between the magnetic lines of force and the flaw is very important. There are two

general types of magnetic fields that can be established within a component.

A longitudinal magnetic field has magnetic lines of force that run

parallel to the long axis of the part. Longitudinal magnetization of a

component can be accomplished using the longitudinal field set up

by a coil or solenoid. It can also be accomplished using permanent

magnets or electromagnets.


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A circular magnetic field has magnetic lines of force that run

circumferentially around the perimeter of a part. A circular magnetic

field is induced in an article by either passing current through the

component or by passing current through a conductor surrounded by

the component.

The type of magnetic field established is determined by the method used to magnetize the

specimen. Being able to magnetize the part in two directions is important because the best

detection of defects occurs when the lines of magnetic force are established at right angles to the

longest dimension of the defect. This orientation creates the largest disruption of the magnetic

field within the part and the greatest flux leakage at the surface of the part. As can be seen in the

image below, if the magnetic field is parallel to the defect, the field will see little disruption and

no flux leakage field will be produced.

An orientation of 45 to 90 degrees between the magnetic field and the defect is necessary to form

an indication. Since defects may occur in various and unknown directions, each part is normally

magnetized in two directions at right angles to each other. If the component below is considered,

it is known that passing current through the part from end to end will establish a circular

magnetic field that will be 90 degrees to the direction of the current. Therefore, defects that have

a significant dimension in the direction of the current (longitudinal defects) should be detectable.

Alternately, transverse-type defects will not be detectable with circular magnetization.


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7.5 Portable Equipments for Magnetic Particle Testing

To properly inspect a part for cracks or other defects, it is important to become familiar with the

different types of magnetic fields and the equipment used to generate them. As discussed

previously, one of the primary requirements for detecting a defect in a ferromagnetic material is

that the magnetic field induced in the part must intercept the defect at a 45 to 90 degree angle.

Flaws that are normal (90 degrees) to the magnetic field will produce the strongest indications

because they disrupt more of the magnet flux.

Therefore, for proper inspection of a component, it is important to be able to establish a magnetic

field in at least two directions. A variety of equipment

exists to establish the magnetic field for MPI. One way

to classify equipment is based on its portability. Some

equipment is designed to be portable so that inspections

can be made in the field and some is designed to be

stationary for ease of inspection in the laboratory or

manufacturing facility.

7.6 Permanent magnets

Permanent magnets are sometimes used for magnetic

particle inspection as the source of magnetism. The two

primary types of permanent magnets are bar magnets and

horseshoe (yoke) magnets. These industrial magnets are


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usually very strong and may require significant strength to remove them from a piece of metal.

Some permanent magnets require over 50 pounds of force to remove them from the surface.

Because it is difficult to remove the magnets from the component being inspected, and

sometimes difficult and dangerous to place the magnets, their use is not particularly popular.

However, permanent magnets are sometimes used by divers for inspection in underwater

environments or other areas, such as explosive environments, where electromagnets cannot be

used. Permanent magnets can also be made small enough to fit into tight areas where

electromagnets might not fit.

7.7 Electromagnets

Today, most of the equipment used to create

the magnetic field used in MPI is based on

electromagnetism. That is, using an

electrical current to produce the magnetic

field. An electromagnetic yoke is a very

common piece of equipment that is used to

establish a magnetic field. It is basically

made by wrapping an electrical coil around

a piece of soft ferromagnetic steel. A switch

is included in the electrical circuit so that the

current and, therefore, the magnetic field can be turned on and off. They can be powered with

alternating current from a wall socket or by direct current from a battery pack. This type of

magnet generates a very strong magnetic field in a local area where the poles of the magnet

touch the part being inspected. Some yokes can lift weights in excess of 40 pounds.


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P

ortable yoke with battery pack

P

ortable magnetic particle kit

7.8 Prods

Prods are handheld electrodes that are pressed

against the surface of the component being

inspected to make contact for passing electrical

current through the metal. The current passing

between the prods creates a circular magnetic field

around the prods that can be used in magnetic

particle inspection. Prods are typically made from

copper and have an insulated handle to help protect

the operator. One of the prods has a trigger switch

so that the current can be quickly and easily turned

on and off. Sometimes the two prods are connected by any insulator (as shown in the image) to

facilitate one hand operation. This is referred to as a dual prod and is commonly used for weld

inspections.

If proper contact is not maintained between the prods and the component surface, electrical

arcing can occur and cause damage to the component. For this reason, the use of prods are not

allowed when inspecting aerospace and other critical components. To help prevent arcing, the

prod tips should be inspected frequently to ensure that they are not oxidized, covered with scale

or other contaminant, or damaged.


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The following applet shows two prods used to create a current through a conducting part. The

resultant magnetic field roughly depicts the patterns expected from an magnetic particle

inspection of an unflawed surface. The user is encouraged to manipulate the prods to orient the

magnetic field to "cut across" suspected defects.

7.9 Portable Coils and Conductive Cables

Coils and conductive cables are used to establish a longitudinal magnetic field within a

component. When a preformed coil is used, the component is placed against the inside surface on

the coil. Coils typically have three or five turns of a copper cable within the molded frame. A

foot switch is often used to energize the coil. Conductive cables are wrapped around the

component. The cable used is typically 00 extra flexible or 0000 extra flexible. The number of

wraps is determined by the magnetizing force needed and of course, the length of the cable.

Normally, the wraps are kept as close together as possible. When using a coil or cable wrapped

into a coil, amperage is usually expressed in ampere-turns. Ampere-turns is the amperage shown

on the amp meter times the number of turns in the coil.

Portable Coil

Conductive Cable

7.10 Portable Power Supplies


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Portable power supplies are used to provide the necessary electricity to the prods, coils or cables.

Power supplies are commercially available in a variety of sizes. Small power supplies generally

provide up to 1,500A of half-wave direct current or alternating current when used with a 4.5

meter 0000 cable. They are small and light enough to be carried and operate on either 120V or

240V electrical service. When more power is necessary, mobile power supplies can be used.

These units come with wheels so that they can be rolled where needed. These units also operate

on 120V or 240V electrical service and can provide up to 6,000A of AC or half-wave DC.

7.11 Lights for Magnetic Particle Inspection

Magnetic particle inspection can be performed using

particles that are highly visible under white light

conditions or particles that are highly visible under

ultraviolet light conditions. When an inspection is

being performed using the visible color contrast

particles, no special lighting is required as long as

the area of inspection is well lit. A light intensity of

at least 1000 lux (100 fc) is recommended when

visible particles are used, but a variety of light

sources can be used. When fluorescent particles are

used, special ultraviolet light must be used.

Fluorescenc e is defined as the property of emitting

radiation as a result of and during exposure to radiation. Particles used in fluorescent magnetic

particle inspections are coated with a material that produces light in the visible spectrum when

exposed to near-ultraviolet light. This "particle glow" provides high contrast indications on the

component anywhere particles collect. Particles that fluoresce yellow-green are most common

because this color matches the peak sensitivity of the human eye under dark conditions.

However, particles that fluoresce red, blue, yellow, and green colors are available.


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7.12 Dry Particle Inspection

In this magnetic particle testing technique, dry particles are

dusted onto the surface of the test object as the item is

magnetized. Dry particle inspection is well suited for the

inspections conducted on rough surfaces. When an

electromagnetic yoke is used, the AC or half wave DC

current creates a pulsating magnetic field that provides

mobility to the powder. The primary applications for dry

powders are unground welds and rough as-cast surfaces.

Dry particle inspection is also used to detect shallow

subsurface cracks. Dry particles with half wave DC is the

best approach when inspecting for lack of root penetration

in welds of thin materials. Half wave DC with prods and

dry particles is commonly used when inspecting large castings for hot tears and cracks.

7.13 Examples of Dry Magnetic Particle Inspection

One of the advantages that a magnetic particle inspection has over some of the other

nondestructive evaluation methods is that flaw indications generally resemble the actual flaw.

This is not the case with NDT methods such as ultrasonic and eddy current inspection, where an

electronic signal must be interpreted. When magnetic particle inspection is used, cracks on the

surface of the part appear as sharp lines that follow the path of the crack. Flaws that exist below

the surface of the part are less defined and more difficult to detect. Below are some examples of

magnetic particle indications produced using dry particles.


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Fig: Before and after inspection pictures of cracks emanating from a hole

7.14 Advantages of Magnetic Particle Testing

(1) It does not need very stringent pre-cleaning operation.

(2) Best method for the detection of fine, shallow surface cracks in ferromagnetic material.

(3) Fast and relatively simple NDT method.

(4) Generally inexpensive.

(5) Will work through thin coating.

(6) Few limitations regarding the size/shape of test specimens.

(7) Highly portable NDT method.

(8) It is quicker.

(9) Simplicity of operation and application.

7.15 Disadvantages of Magnetic Particle Testing

(1) Material must be ferromagnetic.

(2) Orientation and strength of magnetic field is critical.

(3) Detects surface and near-to-surface discontinuities only.

(4) Large currents sometimes required.

(5) ―Burning‖ of test parts a possibility.

(6) Parts must often be demagnetized, which may be difficult.


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8. Eddy Current Testing (ECT)

8.1 Introduction of Eddy Current Testing

This method is widely used to detect surface flaws, to sort materials, to measure thin walls from

one surface only, to measure thin coatings and in some applications to measure case depth. This

method is applicable to electrically conductive materials only. In the method eddy currents are

produced in the product by bringing it close to an alternating current carrying coil. The main

applications of the eddy current technique are for the detection of surface or subsurface flaws,

conductivity measurement and coating thickness measurement. The technique is sensitive to the

material conductivity, permeability and dimensions of a product. Eddy currents can be produced

in any electrically conducting material that is subjected to an alternating magnetic field (typically

10Hz to 10MHz). The alternating magnetic field is normally generated by passing an alternating

current through a coil. The coil can have many shapes and can between 10 and 500 turns of wire.

The magnitude of the eddy currents generated in the product is dependent on conductivity,

permeability and the set up geometry. Any change in the material or geometry can be detected by

the excitation coil as a change in the coil impedance. The most simple coil comprises a ferrite

rod with several turns of wire wound at one end and which is positioned close to the surface of

the product to be tested. When a crack, for example, occurs in the product surface the eddy

currents must travel farther around the crack and this is detected by the impedance change.


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8.2 History of Eddy Current Testing

Eddy Current testing has its origins with Michael

Faraday's discovery of electromagnetic induction in

1831. Faraday was a chemist in England during the

early 1800's and is credited with the discovery of

electromagnetic induction, electromagnetic rotations,

the magneto-optical effect, diamagnetism, and other

phenomena. In 1879, another scientist named

Hughes recorded changes in the properties of a coil

when placed in contact with metals of different

conductivity and permeability. However, it was not

until the Second World War that these effects were

put to practical use for testing materials. Much work

was done in the 1950's and 60's, particularly in the

aircraft and nuclear industries. Eddy current testing

is now a widely used and well-understood inspection technique.

8.3 Present State of Eddy Current Inspection

Eddy current inspection is used in a variety of

industries to find defects and make

measurements. One of the primary uses of eddy

current testing is for defect detection when the

nature of the defect is well understood. In

general, the technique is used to inspect a

relatively small area and the probe design and

test parameters must be established with a good

understanding of the flaw that is to be detected.

Since eddy currents tend to concentrate at the

surface of Eddy current inspection is used in a


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variety of industries to find defects and make measurements. One of the primary uses of eddy

current testing is for defect detection when the nature of the defect is well understood. In general,

the technique is used to inspect a relatively small area and the probe design and test parameters

must be established with a good understanding of the flaw that is to be detected. Since eddy

currents tend to concentrate at the surface of a material, they can only be used to detect surface

and near surface defects. In thin materials such as tubing and sheet stock, eddy currents can be

used to measure the thickness of the material. This makes eddy current a useful tool for detecting

corrosion damage and other damage that causes a thinning of the material. The technique is used

to make corrosion thinning measurements on aircraft skins and in the walls of tubing used in

assemblies such as heat exchangers. Eddy current testing is also used to measure the thickness of

paints and other coatings.

Eddy currents are also affected by the electrical conductivity and magnetic permeability of

materials. Therefore, eddy current measurements can be used to sort materials and to tell if a

material has seen high temperatures or been heat treated, which changes the conductivity of

some materials.

Eddy current equipment and probes can be purchased in a wide variety of configurations.

Eddyscopes and a conductivity tester come packaged in very small and battery operated units for

easy portability. Computer based systems are also available that provide easy data manipulation

features for the laboratory. Signal processing software has also been developed for trend

removal, background subtraction, and noise reduction. Impedance analyzers are also sometimes

used to allow improved quantitative eddy-current measurements. Some laboratories have

multidimensional scanning capabilities that are used to produce images of the scan regions. A

few portable scanning systems also exist for special applications, such as scanning regions of

aircraft fuselages.

8.4 Research to Improve Eddy current measurements

A great deal of research continues to be done to improve eddy current measurement techniques.

A few of these activities, which are being conducted at Iowa State University, are described

below.


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8.4.1 Photoinductive Imaging (PI)

A technique known as photoinductive imaging (PI) was pioneered at CNDE and provides a

powerful, high-resolution scanning and imaging tool. Microscopic resolution is available using

standard-sized eddy-current sensors. Development of probes and instrumentation for

photoinductive (PI) imaging is based on the use of a medium-power (5 W nominal power) argon

ion laser. This probe provides high resolution images and has been used to study cracks, welds,

and diffusion bonds in metallic specimens. The PI technique is being studied as a way to image

local stress variations in steel.

8.4.2 Pulse Eddy Current

Research is currently being conducted on the use of a technique called pulsed eddy current

(PEC) testing. This technique can be used for the detection and quantification of corrosion and

cracking in multi-layer aluminum aircraft structures. Pulsed eddy-current signals consist of a

spectrum of frequencies meaning that, because of the skin effect, each pulse signal contains

information from a range of depths within a given test specimen. In addition, the pulse signals

are very low-frequency rich which provides excellent depth penetration. Unlike multi-frequency

approaches, the pulse-signals lend themselves to convenient analysis. .

Measurements have been carried out both in the laboratory and in the field. Corrosion trials have

demonstrated how material loss can be detected and quantified in multi-layer aluminum

structures. More recently, studies carried out on three and four layer structures show the ability

to locate cracks emerging from fasteners. Pulsed eddy-current measurements have also been

applied to ferromagnetic materials. Recent work has been involved with measuring the case

depth in hardened steel samples.

8.5 Eddy Current Instruments

Eddy current instruments can be purchased in a

large variety of configurations. Both analog and

digital instruments are available. Instruments are


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commonly classified by the type of display used to present the data. The common display types

are analog meter, digital readout, impedance plane and time versus signal amplitude. Some

instruments are capable of presenting data in several display formats.

The most basic eddy current testing instrument consists of an alternating current source, a coil of

wire connected to this source, and a voltmeter to measure the voltage change across the coil. An

ammeter could also be used to measure the current change in the circuit instead of using the

voltmeter. While it might actually be possible to detect some types of defects with this type of

equipment, most eddy current instruments are a bit more sophisticated. In the following pages, a

few of the more important aspects of eddy current instrumentation will be discussed.

8.6 Probes - Mode of Operation

Eddy current probes are available in a large

variety of shapes and sizes. In fact, one of the

major advantages of eddy current inspection is

that probes can be custom designed for a wide

variety of applications. Eddy current probes are

classified by the configuration and mode of

operation of the test coils. The configuration of

the probe generally refers to the way the coil or

coils are packaged to best "couple" to the test

area of interest. An example of different configurations of probes would be bobbin probes, which

are inserted into a piece of pipe to inspect from the inside out, versus encircling probes, in which

the coil or coils encircle the pipe to inspect from the outside in. The mode of operation refers to

the way the coil or coils are wired and interface with the test equipment. The mode of operation

of a probe generally falls into one of four categories: absolute, differential, reflection and hybrid.

Each of these classifications will be discussed in more detail below.


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8.6.1 Absolute Probes

Absolute probes generally have a single test coil that is

used to generate the eddy currents and sense changes in

the eddy current field. As discussed in the physics

section, AC is passed through the coil and this sets up

an expanding and collapsing magnetic field in and

around the coil. When the probe is positioned next to a

conductive material, the changing magnetic field

generates eddy currents within the material. The

generation of the eddy currents take energy from the

coil and this appears as an increase in the electrical

resistance of the coil. The eddy currents generate their

own magnetic field that opposes the magnetic field of the coil and this changes the inductive

reactance of the coil. By measuring the absolute change in impedance of the test coil, much

information can be gained about the test material.

Absolute coils can be used for flaw detection, conductivity measurements, liftoff measurements

and thickness measurements. They are widely used due to their versatility. Since absolute probes

are sensitive to things such as conductivity, permeability liftoff and temperature, steps must be

taken to minimize these variables when they are not important to the inspection being performed.

It is very common for commercially available absolute probes to have a fixed "air loaded"

reference coil that compensates for ambient temperature variations.

8.6.2 Differential Probes

Differential probes have two active coils usually wound in opposition,

although they could be wound in addition with similar results. When the

two coils are over a flaw -free area of test sample, there is no differential

signal developed between the coils since they are both inspecting

identical material. However, when one coil is over a defect and the other

is over good material, a differential signal is produced. They have the


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advantage of being very sensitive to defects yet relatively insensitive to slowly varying

properties such as gradual dimensional or temperature variations. Probe wobble signals are also

reduced with this probe type. There are also disadvantages to using differential probes. Most

notably, the signals may be difficult to interpret. For example, if a flaw is longer than the spacing

between the two coils, only the leading and trailing edges will be detected due to signal

cancellation when both coils sense the flaw equally.

8.6.3 Reflection Probes

Reflection probes have two coils similar to a differential probe, but one coil is used to excite the

eddy currents and the other is used to sense changes in the test material. Probes of this

arrangement are often referred to as driver/pickup probes. The advantage of reflection probes is

that the driver and pickup coils can be separately optimized for their intended purpose. The

driver coil can be made so as to produce a strong and uniform flux field in the vicinity of the

pickup coil, while the pickup coil can be made very small so that it will be sensitive to very small

defects.

8.6.4 Hybrid Probes

An example of a hybrid probe is the split D, differential

probe shown to the right. This probe has a driver coil that

surrounds two D shaped sensing coils. It operates in the

reflection mode but additionally, its sensing coils operate in

the differential mode. This type of probe is very sensitive to

surface cracks. Another example of a hybrid probe is one

that uses a conventional coil to generate eddy currents in the

material but then uses a different type of sensor to detect

changes on the surface and within the test material. An example of a hybrid probe is one that

uses a Hall effect sensor to detect changes in the magnetic flux leaking from the test surface.

Hybrid probes are usually specially designed for a specific inspection application.


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8.7 Probes - Configurations

As mentioned on the previous page, eddy current probes are classified by the configuration and

mode of operation of the test coils. The configuration of the probe generally refers to the way the

coil or coils are packaged to best "couple" to the test area of interest. Some of the common

classifications of probes based on their configuration include surface probes, bolt hole probes,

inside diameter (ID) probes, and outside diameter (OD) probes.

8.7.1 Surface Probes

Surface probes are usually designed to be handheld

and are intended to be used in contact with the test

surface. Surface probes generally consist of a coil

of very fine wire encased in a protective housing.

The size of the coil and shape of the housing are

determined by the intended use of the probe. Most

of the coils are wound so that the axis of the coil is

perpendicular to the test surface. This coil

configuration is sometimes referred to as a

pancake coil and is good for detecting surface

discontinuities that are oriented perpendicular to

the test surface. Discontinuities, such as

delaminations, that are in a parallel plane to the

test surface will likely go undetected with this coil

configuration.

Wide surface coils are used when scanning large areas for relatively large defects. They sample a

relatively large area and allow for deeper penetration. Since they do sample a large area, they are

often used for conductivity tests to get more of a bulk material measurement. However, their

large sampling area limits their ability to detect small discontinuities.


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Pencil probes have a small surface coil that is encased in a long slender housing to permit

inspection in restricted spaces. They are available with a straight shaft or with a bent shaft, which

facilitates easier handling and use in applications such as the inspection of small diameter bores.

Pencil probes are prone to wobble due to their small base and sleeves are sometimes used to

provide a wider base.

8.7.2 Bolt Hole Probes

Bolt hole probes are a special type of surface probe that is designed to be used with a bolt hole

scanner. They have a surface coil that is mounted inside a housing that matches the diameter of

the hole being inspected. The probe is inserted in the hole and the scanner rotates the probe

within the hole.

8.7.3 ID or Bobbin Probes

ID probes, which are also referred to as Bobbin probes

or feed-through probes, are inserted into hollow

products, such as pipes, to inspect from the inside out.

The ID probes have a housing that keep the probe

centered in the product and the coil(s) orientation

somewhat constant relative to the test surface. The

coils are most commonly wound around the

circumference of the probe so that the probe inspects

an area around the entire circumference of the test

object at one time.


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8.7.3 OD or Encircling Coils

OD probes are often called encircling coils. They are similar to ID probes except that the coil(s)

encircle the material to inspect from the outside in. OD probes are commonly used to inspect

solid products, such as bars.

8.8 Surface Breaking Cracks

Eddy current equipment can be used for a variety of

applications such as the detection of cracks (discontinuities),

measurement of metal thickness, detection of metal thinning

due to corrosion and erosion, determination of coating

thickness, and the measurement of electrical conductivity and

magnetic permeability. Eddy current inspection is an excellent

method for detecting surface and near surface defects when

the probable defect location and orientation is well known.

Defects such as cracks are detected when they disrupt the path

of eddy currents and weaken their strength. The images to the

right show an eddy current surface probe on the surface of a

conductive component. The strength of the eddy currents

under the coil of the probe ins indicated by color. In the lower image, there is a flaw under the

right side of the coil and it can be see that the eddy currents are weaker in this area.

Of course, factors such as the type of material, surface finish and condition of the material, the

design of the probe, and many other factors can affect the sensitivity of the inspection.

Successful detection of surface breaking and near surface cracks requires:

1. A knowledge of probable defect type, position, and orientation.

2. Selection of the proper probe. The probe should fit the geometry of the part and the coil

must produce eddy currents that will be disrupted by the flaw.

3. Selection of a reasonable probe drive frequency. For surface flaws, the frequency should

be as high as possible for maximum resolution and high sensitivity. For subsurface flaws,


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lower frequencies are necessary to get the required depth of penetration and this results in

less sensitivity. Ferromagnetic or highly conductive materials require the use of an even

lower frequency to arrive at some

level of penetration.

4. Setup or reference specimens of

similar material to the component

being inspected and with features

that are representative of the defect

or condition being inspected for.

The basic steps in performing an inspection

with a surface probe are the following:

1. Select and setup the instrument and probe.

2. Select a frequency to produce the desired depth of penetration.

3. Adjust the instrument to obtain an easily recognizable defect response using a calibration

standard or setup specimen.

4. Place the inspection probe (coil) on the component surface and null the instrument.

5. Scan the probe over part of the surface in a pattern that will provide complete coverage of

the area being inspected. Care must be taken to maintain the same probe-to-surface

orientation as probe wobble can affect interpretation of the signal. In some cases, fixtures

to help maintain orientation or automated scanners may be required.

6. Monitor the signal for a local change in impedance that will occur as the probe moves

over a discontinuity.

Move the probe over the surface of the specimen and compare the signal responses from a

surface breaking crack with the signals from the calibration notches. The inspection can be made

at a couple of different frequencies to get a feel for the effect that frequency has on sensitivity in

this application.

8.9 Surface Crack Detection Using Sliding Probes


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Many commercial aircraft applications involve the use of multiple fasteners to connect the multi-

layer skins. Because of the fatigue stress that is caused by the typical application of any

commercial aircraft, fatigue cracks can be induced in the vicinity of the fastener holes. In order

to inspect the fastener holes in an adequate amount of time, sliding probes are an efficient

method of inspection. Sliding probes have been named so because they move over fasteners in a

sliding motion. There are two types of sliding probes, fixed and adjustable, which are usually

operated in the reflection mode. This means that the eddy currents are induced by the driver coil

and detected by a separate receiving coil.

Sliding probes are one of the fastest methods to inspect large numbers of fastener holes. They are

capable of detecting surface and subsurface discontinuities, but they can only detect defects in

one direction. The probes are marked with a detection line to indicate the direction of inspection.

In order to make a complete inspection there must be two scans that are orthogonal (90 degrees)

to each other.

8.10 Probe Types

8.10.1 Fixed Sliding Probes

These probes are generally used for thinner material compared to the

adjustable probes. Maximum penetration is about 1/8 inch. Fixed

sliding probes are particularly well suited for finding longitudinal

surface or subsurface cracks such as those found in lap joints. Typical

frequency range is from 100 Hz to 100 kHz.

8.10.2 Adjustable Sliding Probes

These probes are well suited for finding subsurface cracks

in thick multi-layer structures, like wing skins. Maximum

penetration is about 3/4 inch. The frequency range for

adjustable sliding probes is from 100 Hz to 40 kHz.


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Adjustable probes, as the name implies, are adjustable with the use of spacers, which will change

the penetration capabilities. The spacer thickness between the coils is normally adjusted for the

best detection. For tangential scans or 90 degree scanning with an offset from the center, a

thinner spacer is often used.

The spacer thickness range can vary from 0 (no spacer) for inspections close to the surface and

small fastener heads to a maximum of about 0.3 inch for deep penetration with large heads in the

bigger probe types. A wider spacer will give more tolerance to probe deviation as the sensitive

area becomes wider but the instrument will require more gain. Sliding probes usually penetrate

thicker materials compared to the donut probes.

8.11 Reference Standards

Reference/calibration standards for setup of sliding probes typically consist of three or four

aluminum plates that are fastened together within a lap joint type configuration. EDM notches or

naturally/artificially- induced cracks are

located in the second or third layer of the

standard. Reference standards used should

be manufactured from the same material

type, alloy, material thickness, and

chemical composition that will be found on

the aircraft component to be inspected.

Sizes and tolerances of flaws introduced in

the standards are usually regulated by

inspection specifications.

8.12 Inspection Variables


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8.12.1 Liftoff signal Adjustment

Liftoff is normally adjusted to be relatively horizontal. The term "relatively horizontal" is used

here because the liftoff signal often appears a curved line rather than a straight line. Sometimes

liftoff can be a sharp curve and may need to be adjusted to run slightly upwards before moving

downwards.

8.12.2 Scan Patterns

A typical scan is centralized over the fastener head and moves along the axis of the fastener

holes. This scan is generally used to detect cracks positioned along the axis of the fastener holes.

For detecting cracks located transverse or 90 degrees from the axis of the fastener holes, a scan

that is 90 degrees from the axis of the fastener holes is recommended.

8.12.3 Signal Interpretation

When the probe moves over a fastener hole with a crack, the indication changes and typically

will create a larger vertical movement. The vertical amplitude of the loop depends on the crack

length, with longer cracks giving higher indications.

If the crack is in the far side of the fastener, as the probe moves over it, the dot will follow the

fastener line first but will move upwards (clockwise) as it goes over the crack. If the crack is in

the near side, it will be found first and the dot will move along the crack level before coming

down to the fastener level.

If two cracks on opposite sides of the fastener hole are present, the dot will move upwards to the

height by the first crack length and then come back to the fastener line and balance point.

8.12.4 Probe Scan Deviation


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Most probes are designed to give a narrow indication for a good fastener hole so that the loops

from the cracks are more noticeable. Some probes and structures can give wider indications and

a similar result can be obtained if the probe is not straight when it approaches the fastener. It is

important to keep the probe centralized over the fastener heads. Doing this will give you a

maximum indication for the fastener and a crack.

If the probe deviates from the center line, the crack indication will move along the loop that we

saw in Figure A and is now present in Figure B. The crack indication is at "a" when the probe is

centralized and moves toward "b" as it deviates in one direction, or "c" as it deviates in the

opposite direction. Point "b" gives an important indication even if it loses a small amount of

amplitude it has gained in phase, giving a better separation angle. This is because we deviated to

the side where the crack is located.

8.12.5 Crack Angle Deviation

A reduction in the crack indication occurs when the crack is at an angle to the probe scan direction. This happens if

the crack is not completely at 90 degrees to the normal probe scan or changes direction as it grows. Both the fixed

and adjustable sliding probes are capable of detecting cracks up to about 30 degrees off angle.

8.12.6 Electrical Contact

When inspecting fasteners that have just been installed or reference standards that have intimate

contact with the aluminum skin plate, it is not unusual to obtain a smaller than normal indication.

b a c

Fig. A

a b c

Fig. B


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In some extreme cases, the fastener indication may disappear almost completely. This is due to

the good electrical contact between the fastener and the skin. This condition allows the eddy

currents to circulate without encountering a boundary, and therefore, no obstacle or barrier.

Because of this effect, it is recommended to paint the holes before fastener installation.

8.13 Tube Inspection By Eddy Current

Eddy current inspection is often used to detect corrosion, erosion,

cracking and other changes in tubing. Heat exchangers and steam

generators, which are used in power plants, have thousands of tubes

that must be prevented from leaking. This is especially important in

nuclear power plants where reused, contaminated water must be

prevented from mixing with fresh water that will be returned to the

environment. The contaminated water flows on one side of the tube

(inside or outside) and the fresh water flows on the other side. The

heat is transferred from the contaminated water to the fresh water and

the fresh water is then returned back to is source, which is usually a

lake or river. It is very important to keep the two water sources from

mixing, so power plants are periodically shutdown so the tubes and other equipment can be

inspected and repaired. The eddy current test method and the related remote field testing method

provide high-speed inspection techniques for

these applications.

A technique that is often used involves feeding

a differential bobbin probe into the individual

tube of the heat exchanger. With the

differential probe, no signal will be seen on the

eddy current instrument as long as no metal

thinning is present. When metal thinning is

present, a loop will be seen on the impedance

plane as one coil of the differential probe

passes over the flawed area and a second loop will be produced when the second coil passes over


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the damage. When the corrosion is on the outside surface of the tube, the depth of corrosion is

indicated by a shift in the phase lag. The size of the indication provides an indication of the total

extent of the corrosion damage.

A tube inspection using a bobbin probe is simulated below. Click the "null" button and then drag

either the absolute or the differential probe through the tube. Note the different signal responses

provided by the two probes.

8.14 Thickness Measurements of Thin

Material

Eddy current techniques can be used to perform a

number of dimensional measurements. The ability to

make rapid measurements without the need for couplant

or, in some cases even surface contact, makes eddy

current techniques very useful. The type of

measurements that can be made include:

thickness of thin metal sheet and foil, and of metallic coatings on metallic and

nonmetallic substrate

cross-sectional dimensions of cylindrical tubes and rods

thickness of nonmetallic coatings on metallic substrates

8.15 Corrosion Thinning of Aircraft Skins

One application where the eddy current technique is commonly

used to measure material thickness is in the detection and

characterization of corrosion damage on the skins of aircraft.

Eddy current techniques can be used to do spot checks or

scanners can be used to inspect small areas. Eddy current


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inspection has an advantage over ultrasound in this application because no mechanical coupling

is required to get the energy into the structure. Therefore, in multi-layered areas of the structure

like lap splices, eddy current can often determine if corrosion thinning is present in buried layers.

Eddy current inspection has an advantage over radiography for this application because only

single sided access is required to perform the inspection. To get a piece of film on the back side

of the aircraft skin might require removing interior furnishings, panels, and insulation which

could be very costly. Advanced eddy current techniques are being developed that can determine

thickness changes down to about three percent of the skin thickness.

8.16 Thickness Measurement of Thin Conductive Sheet, Strip and Foil

Eddy current techniques are used to measure the thickness of hot sheet, strip and foil in rolling

mills, and to measure the amount of metal thinning that has occurred over time due to corrosion

on fuselage skins of aircraft. On the impedance plane, thickness variations exhibit the same type

of eddy current signal response as a subsurface defect, except that the signal represents a void of

infinite size and depth. The phase rotation pattern is the same, but the signal amplitude is greater.

In the applet, the lift-off curves for different areas of the taper wedge can be produced by nulling

the probe in air and touching it to the surface at various locations of the tapered wedge. If a line

is drawn between the end points of the lift-off curves, a comma shaped curve is produced. As

illustrated in the second applet, this comma shaped curve is the path that is traced on the screen

when the probe is scanned down the length of the tapered wedge so that the entire range of

thickness values is measured. When making this measurement, it is important to keep in mind

that the depth of penetration of the eddy currents must cover the entire range of thicknesses being

measured. Typically, a frequency is selected that produces about one standard depth of

penetration at the maximum thickness. Unfortunately, at lower frequencies, which are often

needed to get the necessary penetration, the probe impedance is more sensitive to changes in

electrical conductivity. Thus, the effects of electrical conductivity cannot be phased out and it is

important to verify that any variations of conductivity over the region of interest are at a

sufficiently low level.

8.17 Thickness Measurement of Thin Conductive Layers


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It is also possible to measure the thickness of a thin layer of metal on a metallic substrate,

provided the two metals have widely differing electrical conductivities (i.e. silver on lead where

= 67 and 10 MS/m, respectively). A frequency must be selected such that there is complete

eddy current penetration of the layer, but not of the substrate itself. The method has also been

used successfully for measuring thickness of very thin protective coatings of ferromagnetic

metals (i.e. chromium and nickel) on non-ferromagnetic metal bases.

Depending on the required degree of penetration, measurements can be made using a single-coil

probe or a transformer probe, preferably reflection type. Small-diameter probe coils are usually

preferred since they can provide very high sensitivity and minimize effects related to property or

thickness variations in the underlying base metal when used in combination with suitably high

test frequencies. The goal is to confine the magnetizing field, and the resulting eddy current

distribution, to just beyond the thin coating layer and to minimize the field within the base

metals.

8.18 Pulsed Eddy Current Inspection

Conventional eddy current inspection techniques use sinusoidal alternating electrical current of a

particular frequency to excite the probe. The pulsed eddy current technique uses a step function

voltage to excite the probe. The advantage of using a step function voltage is that it contains a

continuum of frequencies. As a result, the electromagnetic response to several different

frequencies can be measured with just a single step. Since the depth of penetration is dependent

on the frequency of excitation, information from a range of depths can be obtained all at once. If

measurements are made in the time domain (that is by looking at signal strength as a function of


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time), indications produced by flaws or other features near the inspection coil will be seen first

and more distant features will be seen later in time.

To improve the strength and ease interpretation of the signal, a reference signal is usually

collected, to which all other signals are compared (just like nulling the probe in conventional

eddy current inspection). Flaws, conductivity, and dimensional changes produce a change in the

signal and a difference between the reference signal and the measurement signal that is

displayed. The distance of the flaw and other features relative to the probe will cause the signal

to shift in time. Therefore, time gating techniques (like in ultrasonic inspection) can be used to

gain information about the depth of a feature of interest.

8.19 EC Standards and Methods

British Standards (BS) and American Standards (ASTM) relating to magnetic flux leakage and

eddy current methods of testing are given below. National standards are currently being

harmonized across the whole of Europe, and British Standards are no exception. Harmonized

standards will eventually be identified by the initials BS EN; for example, BS 5411 has been

revised and is now known as BS EN 2360. Harmonization is unlikely to be completed before

2001. The year of updating a British Standard is given in brackets. ASTM standards are

published annually and updated when necessary.

FLUX LEAKAGE METHODS (INCLUDING MAGNETIC PARTICLE INSPECTION)

British Standards (BS)

BS 6072:1981 (1986) Magnetic particle flaw detection

BS 4489:1984 Black light measurement

BS 5044:1973 (1987) Contrast aid paints

BS 5138:1974 (1988) Forged and stamped crankshafts

BS 3683 (part 2):1985 Glossary

BS 4069:1982 Inks and powders

American Society for Testing and Materials (ASTM)


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ASTM E 709 Magnetic particle inspection practice

ASTM E 125 Indications in ferrous castings

ASTM E 1316 Definition of terms

ASTM E 570 Flux leakage examination of ferromagnetic steel tubular products

EDDY CURRENT METHODS

British Standards (BS)

BS 3683 (part 5):1965 (1989) Eddy current flaw detection glossary

BS 3889 (part 2A): 1986 (1991) Automatic eddy current testing of wrought steel tubes

BS 3889 (part 213): 1966 (1987) Eddy current testing of nonferrous tubes

BS 5411 (part 3):1984 Eddy current methods for measurement of coating thickness of

nonconductive coatings on nonmagnetic base material. Withdrawn: now known as BS EN 2360

(2007).



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9.1 History of Radiography

X-rays were discovered in 1895 by Wilhelm Conrad Roentgen (1845-

1923) who was a Professor at Wuerzburg University in Germany.

Working with a cathode-ray tube in his laboratory, Roentgen observed

a fluorescent glow of crystals on a table near his tube. The tube that

Roentgen was working with consisted of a glass envelope (bulb) with

positive and negative electrodes encapsulated in it. The air in the tube

was evacuated, and when a high voltage was applied, the tube

produced a fluorescent glow. Roentgen shielded the tube with heavy

black paper, and discovered a green colored fluorescent light generated

by a material located a few feet away from the tube.

Public fancy was caught by this invisible ray with the ability to pass through

solid matter, and, in conjunction with a photographic plate, provide a picture of

bones and interior body parts. Scientific fancy was captured by the

demonstration of a wavelength shorter than light. This generated new possibilities in physics,

and for investigating the structure of matter. Much enthusiasm was generated about potential

applications of rays as an aid in medicine and surgery. Within a month after the announcement of

the discovery, several medical radiographs had been made in Europe and the United States,

which were used by surgeons to guide them in their work. In June 1896, only 6 months after

Roentgen announced his discovery, X-rays were being used

by battlefield physicians to locate bullets in wounded

soldiers.

In 1922, industrial radiography took another step forward

with the advent of the 200,000-volt X-ray tube that allowed

radiographs of thick steel parts to be produced in a

reasonable amount of time. In 1931, General Electric Company developed 1,000,000 volt X-ray

generators, providing an effective tool for industrial radiography. That same year, the American

Society of Mechanical Engineers (ASME) permitted X-ray approval of fusion welded pressure

vessels that further opened the door to industrial acceptance and use.


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9.2 A Second Source of Radiation

Shortly after the discovery of X-rays, another form of penetrating rays was discovered. In 1896,

French scientist Henri Becquerel discovered natural radioactivity. Many scientists of the period

were working with cathode rays, and other scientists were gathering evidence on the theory that

the atom could be subdivided. Some of the new research showed that certain types of atoms

disintegrate by themselves. It was Henri Becquerel who discovered this phenomenon while

investigating the properties of fluorescent minerals. Becquerel was researching the principles of

fluorescence, wherein certain minerals glow (fluoresce) when exposed to sunlight. He utilized

photographic plates to record this fluorescence.

One of the minerals Becquerel worked with was a uranium compound. On a day when it was too

cloudy to expose his samples to direct sunlight, Becquerel stored some of the compound in a

drawer with his photographic plates. Later when he developed these plates, he discovered that

they were fogged (exhibited exposure to light). Becquerel questioned what would have caused

this fogging. He knew he had wrapped the plates tightly before using them, so the fogging was

not due to stray light. In addition, he noticed that only the plates that were in the drawer with the

uranium compound were fogged. Becquerel concluded that the uranium compound gave off a

type of radiation that could penetrate heavy paper and expose photographic film. Becquerel

continued to test samples of uranium compounds and determined that the source of radiation was

the element uranium. Bacquerel's discovery was, unlike that of the X-rays, virtually unnoticed by

laymen and scientists alike. Relatively few scientists were interested in Becquerel's findings. It

was not until the discovery of radium by the Curies two years later that interest in radioactivity

became widespread.

Radium became the initial industrial gamma ray source. The material allowed castings up to 10

to 12 inches thick to be radiographed. During World War II, industrial radiography grew

tremendously as part of the Navy's shipbuilding program. In 1946, man-made gamma ray

sources such as cobalt and iridium became available. These new sources were far stronger than

radium and were much less expensive. The manmade sources rapidly replaced radium, and use

of gamma rays grew quickly in industrial radiography.


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9.3 Health Concerns

The science of radiation protection, or "health physics" as it is more properly called, grew out of

the parallel discoveries of X-rays and radioactivity in the closing years of the 19th century.

Experimenters, physicians, laymen, and physicists alike set up X-ray generating apparatuses and

proceeded about their labors with a lack of concern regarding potential dangers. Such a lack of

concern is quite understandable, for there was nothing in previous experience to suggest that X-

rays would in any way be hazardous. Indeed, the opposite was the case, for who would suspect

that a ray similar to light but unseen, unfelt, or otherwise undetectable by the senses would be

damaging to a person? More likely, or so it seemed to some, X-rays could be beneficial for the

Today, it can be said that radiation ranks among the most thoroughly investigated causes of

disease. Although much still remains to be learned, more is known about the mechanisms of

radiation damage on the molecular, cellular, and organ system than is known for most other

health stressing agents. Indeed, it is precisely this vast accumulation of quantitative dose-

response data that enables health physicists to specify radiation levels so that medical, scientific,

and industrial uses of radiation may continue at levels of risk no greater than, and frequently less

than, the levels of risk associated with any other technology.

X-rays and Gamma rays are electromagnetic radiation of exactly the same nature as light, but of

much shorter wavelength. Wavelength of visible light is on the order of 6000 angstroms while

the wavelength of x-rays is in the range of one angstrom and that of gamma rays is 0.0001

angstrom. This very short wavelength is what gives x-rays and gamma rays their power to

penetrate materials that light cannot. These electromagnetic waves are of a high energy level and

can break chemical bonds in materials they penetrate. If the irradiated matter is living tissue, the

breaking of chemical bonds may result in altered structure or a change in the function of cells.

Early exposures to radiation resulted in the loss of limbs and even lives. Men and women

researchers collected and documented information on the interaction of radiation and the human

body. This early information helped science understand how electromagnetic radiation interacts

with living tissue. Unfortunately, much of this information was collected at great personal

expense.


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9.4 Present State of Radiography

In many ways, radiography has changed little from the early days of its use. We still capture a

shadow image on film using similar procedures and processes technicians were using in the late

1800's. Today, however, we are able to generate images of higher quality and greater sensitivity

through the use of higher quality films with a larger variety of film grain sizes. Film processing

has evolved to an automated state, producing more consistent film quality by removing manual

processing variables. Electronics and computers allow technicians to now capture images

digitally. The use of "filmless radiography" provides a means of capturing an image, digitally

enhancing, sending the image anywhere in the world, and archiving an image that will not

deteriorate with time. Technological advances have provided industry with smaller, lighter, and

very portable equipment that produce high quality X-rays. The use of linear accelerators provide

a means of generating extremely short wavelength, highly penetrating radiation, a concept

dreamed of only a few short years ago.

While the process has changed little, technology has evolved allowing radiography to be widely

used in numerous areas of inspection. Radiography has seen expanded usage in industry to

inspect not only welds and castings, but to radiographically inspect items such as airbags and

canned food products. Radiography has found use in metallurgical material identification and

security systems at airports and other facilities.

Gamma ray inspection has also changed considerably since the Curies' discovery of radium.

Man-made isotopes of today are far stronger and offer the technician a wide range of energy

levels and half-lives. The technician can select Co-60 which will effectively penetrate very thick

materials, or select a lower energy isotope, such as Tm-170, which can be used to inspect plastics

and very thin or low density materials. Today gamma rays find wide application in industries

such as petrochemical, casting, welding, and aerospace.

9.5 Future Direction of Radiographic Education


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Although many of the methods and techniques developed over a century ago remain in use,

computers are slowly becoming a part of radiographic inspection. The future of radiography will

likely see many changes. As noted earlier, companies are performing many inspections without

the aid of film.

Radiographers of the future will capture images in digitized form and e-mail them to the

customer when the inspection has been completed. Film evaluation will likely be left to

computers. Inspectors may capture a digitized image, feed them into a computer and wait for a

printout of the image with an accept/reject report. Systems will be able to scan a part and present

a three-dimensional image to the radiographer, helping him or her to locate the defect within the

part.

Inspectors in the future will be able to peal away layer after layer of a part to evaluate the

material in much greater detail. Color images, much like

computer generated ultrasonic C-scans of today, will make

interpretation of indications much more reliable and less time

consuming.

Educational techniques and materials will need to be revised

and updated to keep pace with technology and meet the

requirements of industry. These needs may well be met with

computers. Computer programs can simulate radiographic

inspections using a computer aided design (CAD) model of a

part to produce physically accurate simulated x-ray radiographic images. Programs allow the

operator to select different parts to inspect, adjust the placement and orientation of the part to

obtain the proper equipment/part relationships, and adjust all the usual x-ray generator settings to

arrive at the desired radiographic film exposure.

9.6 Properties of X-Rays and Gamma Rays


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They are not detected by human senses (cannot be seen, heard, felt, etc.).

They travel in straight lines at the speed of light.

Their paths cannot be changed by electrical or magnetic fields.

They can be diffracted to a small degree at interfaces between two different materials.

They pass through matter until they have a chance encounter with an atomic particle.

Their degree of penetration depends on their energy and the matter they are traveling

through.

They have enough energy to ionize matter and can damage or destroy living cells.

9.6.1 X-Radiation

X-rays are just like any other kind of electromagnetic radiation. They can be produced in

parcels of energy called photons, just like light. There are two different atomic processes

that can produce X-ray photons. One is called Bremsstrahlung and is a German term

meaning "braking radiation." The other is called K-shell emission. They can both occur

in the heavy atoms of tungsten. Tungsten is often the material chosen for the target or

anode of the x-ray tube.

Both ways of making X-rays involve a change in the state of electrons. However,

Bremsstrahlung is easier to understand using the classical idea that radiation is emitted

when the velocity of the electron shot at the tungsten changes. The negatively charged

electron slows down after swinging around the nucleus of a positively charged tungsten

atom. This energy loss produces X-radiation. Electrons are scattered elastically and

inelastically by the positively charged nucleus. The inelastically scattered electron loses

energy, which appears as Bremsstrahlung. Elastically scattered electrons (which include

backscattered electrons) are generally scattered through larger angles. In the interaction,

many photons of different wavelengths are produced, but none of the photons have more

energy than the electron had to begin with. After emitting the spectrum of X-ray

radiation, the original electron is slowed down or stopped.

9.6.2 Bremsstrahlung Radiation


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X-ray tubes produce x-ray photons by

accelerating a stream of electrons to energies

of several hundred kilovolts with velocities

of several hundred kilometers per hour and

colliding them into a heavy target material.

The abrupt acceleration of the charged

particles (electrons) produces

Bremsstrahlung photons. X-ray radiation

with a continuous spectrum of energies is

produced with a range from a few keV to a

maximum of the energy of the electron beam. Target materials for industrial tubes are

typically tungsten, which means that the wave functions of the bound tungsten electrons

are required. The inherent filtration of an X-ray tube must be computed, which is

controlled by the amount that the electron penetrates into the surface of the target and by

the type of vacuum window present.

The bremsstrahlung photons generated within the target material are attenuated as they

pass through typically 50 microns of target material.

The beam is further attenuated by the aluminum or

beryllium vacuum window. The results are an

elimination of the low energy photons, 1 keV through

l5 keV, and a significant reduction in the portion of

the spectrum from 15 keV through 50 keV. The

spectrum from an x-ray tube is further modified by

the filtration caused by the selection of filters used in

the setup.

The applet below allows the user to visualize an electron accelerating and interacting

with a heavy target material. The graph keeps a record of the bremsstrahlung photons

numbers as a function of energy. After a few events, the "building up" of the graph may

be accomplished by pressing the "automate" button.

9.6.3 Gamma Radiation


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Gamma radiation is one of the three types of natural radioactivity. Gamma rays are

electromagnetic radiation, like X-rays. The other two types of natural radioactivity are

alpha and beta radiation, which are in the form of particles. Gamma rays are the most

energetic form of electromagnetic radiation, with a very short wavelength of less than

one-tenth of a nanometer.

Gamma radiation is the product of radioactive atoms. Depending upon the ratio of

neutrons to protons within its nucleus, an isotope of a particular element may be stable or

unstable. When the binding energy is not strong enough to hold the nucleus of an atom

together, the atom is said to be unstable. Atoms with unstable nuclei are constantly

changing as a result of the imbalance of energy within the nucleus. Over time, the nuclei

of unstable isotopes spontaneously disintegrate, or transform, in a process known as

radioactive decay. Various types of penetrating radiation may be emitted from the

nucleus and/or its surrounding electrons. Nuclides which undergo radioactive decay are

called radionuclides. Any material which contains measurable amounts of one or more

radionuclides is a radioactive material.

9.7 Types Radiation Produced by Radioactive Decay

When an atom undergoes radioactive decay, it emits one or more forms of radiation with

sufficient energy to ionize the atoms with which it interacts. Ionizing radiation can consist of

high speed subatomic particles ejected from the nucleus or electromagnetic radiation (gamma-

rays) emitted by either the nucleus or orbital electrons.

9.7.1 Alpha Particles


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Certain radionuclides of high atomic

mass (Ra226, U238, Pu239) decay by

the emission of alpha particles. These

alpha particles are tightly bound units

of two neutrons and two protons each

(He4 nucleus) and have a positive

charge. Emission of an alpha particle

from the nucleus results in a decrease of

two units of atomic number (Z) and

four units of mass number (A). Alpha particles are emitted with discrete energies

characteristic of the particular transformation from which they originate. All alpha

particles from a particular radionuclide transformation will have identical energies.

9.7.2 Beta Particles

A nucleus with an unstable ratio of neutrons to protons may decay through the emission

of a high speed electron called a beta particle. This results in a net change of one unit of

atomic number (Z). Beta particles have a negative charge and the beta particles emitted

by a specific radionuclide will range in energy from near zero up to a maximum value,

which is characteristic of the particular transformation.

9.7.3 Gamma-rays

A nucleus which is in an excited state may emit one or more photons (packets of

electromagnetic radiation) of discrete energies. The emission of gamma rays does not

alter the number of protons or neutrons in the nucleus but instead has the effect of

moving the nucleus from a higher to a lower energy state (unstable to stable). Gamma ray

emission frequently follows beta decay, alpha decay, and other nuclear decay processes.

9.8 Filters in Radiography

At x-ray energies, filters consist of material placed in the useful beam to absorb, preferentially,

radiation based on energy level or to modify the spatial distribution of the beam. Filtration is

required to absorb the lower-energy x-ray photons emitted by the tube before they reach the


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target. The use of filters produce a cleaner image by absorbing the lower energy x-ray photons

that tend to scatter more.

The total filtration of the beam includes the inherent filtration (composed of part of the x-ray

tube and tube housing) and the added filtration (thin sheets of a metal inserted in the x-ray

beam). Filters are typically placed at or near the x-ray port in the direct path of the x-ray beam.

Placing a thin sheet of copper between the part and the film cassette has also proven an effective

method of filtration.

For industrial radiography, the filters added to the x-ray beam are most often constructed of high

atomic number materials such as lead, copper, or brass. Filters for medical radiography are

usually made of aluminum (Al). The amount of both the inherent and the added filtration are

stated in mm of Al or mm of Al equivalent. The amount of filtration of the x-ray beam is

specified by and based on the voltage potential (keV) used to produce the beam. The thickness of

filter materials is dependent on atomic numbers, kilovoltage settings, and the desired filtration

factor.

9.9 Radiation Safety

Ionizing radiation is an extremely important NDT tool but it can pose a

hazard to human health. For this reason, special precautions must be

observed when using and working around ionizing radiation. The possession

of radioactive materials and use of radiation producing devices in the United

States is governed by strict regulatory controls. The primary regulatory

authority for most types and uses of radioactive materials is the federal

Nuclear Regulatory Commission (NRC). However, more than half of the states in the US have

entered into "agreement" with the NRC to assume regulatory control of radioactive material use

within their borders. As part of the agreement process, the states must adopt and enforce

regulations comparable to those found in Title 10 of the Code of Federal Regulations.

Regulations for control of radioactive material used in Iowa are found in Chapter 136C of the

Iowa Code.


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For most situations, the types and maximum quantities of radioactive materials possessed, the

manner in which they may be used, and the individuals authorized to use radioactive materials

are stipulated in the form of a "specific" license from the appropriate regulatory authority. In

Iowa, this authority is the Iowa Department of Public Health. However, for certain institutions

which routinely use large quantities of numerous types of radioactive materials, the exact

quantities of materials and details of use may not be specified in the license. Instead, the license

grants the institution the authority and responsibility for setting the specific requirements for

radioactive material use within its facilities. These licensees are termed "broadscope" and require

a Radiation Safety Committee and usually a full-time Radiation Safety Officer.

Complicating matters further is the fact that Gamma and X-ray radiation are not detectable by

the human body. However, the risks can be minimized when the radiation is handled and

managed properly. The law requires that individuals receive training in the safe handling and use

of radioactive materials and radiation producing devices. Some of the topics this training should

cover include:

Health concerns associated with exposure to radioactive materials or radiation.

Precautions or procedures to minimize exposure to radiation.

Purposes and functions of protective devices employed.

The permit conditions and the applicable portions of the Radiation Safety Manual.

Worker‘s responsibility to promptly report any condition that may lead to or cause a

violation of the regulations or cause an unnecessary exposure.

Actions to take in the event of an emergency.

Radiation exposure reports that workers have a right to

receive.

9.10 Radiographic Film

X-ray films for general radiography consist of an emulsion-gelatin

containing radiation sensitive silver halide crystal, such as silver


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bromide or silver chloride, and a flexible, transparent, blue-tinted base. The emulsion is different

from those used in other types of photography films to account for the distinct characteristics of

gamma rays and x-rays, but X-ray films are sensitive to light. Usually, the emulsion is coated on

both sides of the base in layers about 0.0005 inch thick. Putting emulsion on both sides of the

base doubles the amount of radiation-sensitive silver halide, and thus increases the film speed.

The emulsion layers are thin enough so developing, fixing, and drying can be accomplished in a

reasonable time. A few of the films used for radiography only have emulsion on one side which

produces the greatest detail in the image.

When x-rays, gamma rays, or light strike the grains of the sensitive silver halide in the emulsion,

some of the Br- ions are liberated and captured by the Ag

+ ions. This change is of such a small

nature that it cannot be detected by ordinary physical methods and is called a "latent (hidden)

image." However, the exposed grains are now more sensitive to the reduction process when

exposed to a chemical solution (developer), and the reaction results in the formation of black,

metallic silver. It is this silver, suspended in the gelatin on both sides of the base, that creates an

image. See the page on film processing for additional information.

9.10.1 Film Selection

The selection of a film when radiographing any

particular component depends on a number of different

factors. Listed below are some of the factors that must

be considered when selecting a film and developing a

radiographic technique.

1. Composition, shape, and size of the part being

examined and, in some cases, its weight and

location.

2. Type of radiation used, whether x-rays from an

x-ray generator or gamma rays from a radioactive source.

3. Kilovoltages available with the x-ray equipment or the intensity of the gamma radiation.

4. Relative importance of high radiographic detail or quick and economical results.


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Selecting the proper film and developing the optimal radiographic technique usually involves

arriving at a balance between a number of opposing factors. For example, if high resolution and

contrast sensitivity is of overall importance, a slower and finer grained film should be used in

place of a faster film.

9.10.2 Film Packaging

Radiographic film can be purchased in a number of different

packaging options. The most basic form is as individual

sheets in a box. In preparation for use, each sheet must be

loaded into a cassette or film holder in the darkroom to

protect it from exposure to light. The sheets are available in a

variety of sizes and can be purchased with or without interleaving paper. Interleaved packages

have a layer of paper that separates each piece of film. The interleaving paper is removed before

the film is loaded into the film holder. Many users find the interleaving paper useful in

separating the sheets of film and offer some protection against scratches and dirt during

handling.

Industrial x-ray films are also available in a form in which each sheet is enclosed in a light-tight

envelope. The film can be exposed from either side without removing it from the protective

packaging. A rip strip makes it easy to remove the film in the darkroom for processing. This

form of packaging has the advantage of eliminating the process of loading the film holders in the

darkroom. The film is completely protected from finger marks and dirt until the time the film is

removed from the envelope for processing.

Packaged film is also available in rolls, which allows the radiographer to cut the film to any

length. The ends of the packaging are sealed with electrical tape in the darkroom. In applications

such as the radiography of circumferential welds and the examination of long joints on an

aircraft fuselage, long lengths of film offer great economic advantage. The film is wrapped


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around the outside of a structure and the radiation source is positioned on axis inside, allowing

for examination of a large area with a single exposure.

Envelope packaged film can be purchased with the film sandwiched between two lead oxide

screens. The screens function to reduce scatter radiation at energy levels below 150keV and as

intensification screens above 150 keV.

9.10.3 Film Handling

X-ray film should always be handled carefully to avoid physical strains, such as pressure,

creasing, buckling, friction, etc. Whenever films are loaded in semi-flexible holders and external

clamping devices are used, care should be taken to be sure pressure is uniform. If a film holder

bears against a few high spots, such as on an un-ground weld, the pressure may be great enough

to produce desensitized areas in the radiograph. This precaution is particularly important when

using envelope-packed films.

Another important precaution is to avoid drawing film rapidly from

cartons, exposure holders, or cassettes. Such care will help to eliminate

circular or treelike black markings in the radiograph that sometimes

result due to static electric discharges.

9.10.4 Film Processing

As mentioned previously, radiographic film consists of a transparent,

blue-tinted base coated on both sides with an emulsion. The emulsion

consists of gelatin containing microscopic, radiation sensitive silver

halide crystals, such as silver bromide and silver chloride. When x-rays,

gamma rays or light rays strike the the crystals or grains, some of the Br-

ions are liberated and captured by the Ag+

ions. In this condition, the

radiograph is said to contain a latent (hidden) image because the change


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in the grains is virtually undetectable, but the exposed grains are now more sensitive to reaction

with the developer.

When the film is processed, it is exposed to several different chemicals solutions for controlled

periods of time. Processing film basically involves the following five steps.

Development - The developing agent gives up electrons to convert the silver halide grains

to metallic silver. Grains that have been exposed to the radiation develop more rapidly,

but given enough time the developer will convert all the silver ions into silver metal.

Proper temperature control is needed to convert exposed grains to pure silver while

keeping unexposed grains as silver halide crystals.

Stopping the development - The stop bath simply stops the development process by

diluting and washing the developer away with water.

Fixing - Unexposed silver halide crystals are removed by the fixing bath. The fixer

dissolves only silver halide crystals, leaving the silver metal behind.

Washing - The film is washed with water to remove all the processing chemicals.

Drying - The film is dried for viewing.

Processing film is a strict science governed by rigid rules of chemical concentration,

temperature, time, and physical movement. Whether processing is done by hand or automatically

by machine, excellent radiographs require a high degree of consistency and quality control.

9.10.4.1 Manual Processing & Darkrooms

Manual processing begins with the darkroom. The darkroom should be located in a central

location, adjacent to the reading room and a reasonable distance from the exposure area. For

portability, darkrooms are often mounted on pickups or trailers.

Film should be located in a light, tight compartment, which is most often a metal bin that is used

to store and protect the film. An area next to the film bin that is dry and free of dust and dirt

should be used to load and unload the film. Another area, the wet side, should be used to process

the film. This method protects the film from any water or chemicals that may be located on the

surface of the wet side.


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Each of step in the film processing must be excited properly to develop the image, wash out

residual processing chemicals, and to provide adequate shelf life of the radiograph. The objective

of processing is two fold: first, to produce a radiograph adequate for viewing, and second, to

prepare the radiograph for archival storage. Radiographs are often stored for 20 years or more as

a record of the inspection.

9.10.4.2 Automatic Processor Evaluation

The automatic processor is the essential piece of equipment in every x-ray department. The

automatic processor will reduce film processing time when compared to manual development by

a factor of four. To monitor the performance of a processor, apart from optimum temperature and

mechanical checks, chemical and sensitometric checks should be performed for developer and

fixer. Chemical checks involve measuring the pH values of the developer and fixer as well as

both replenishers. Also, the specific gravity and fixer silver levels must be measured. Ideally, pH

should be measured daily and it is important to record these measurements, as regular logging

provides very useful information. The daily measurements of pH values for the developer and

fixer can then be plotted to observe the trend of variations in these values compared to the

normal pH operating levels to identify problems.

Sensitometric checks may be carried out to evaluate if the performance of films in the automatic

processors is being maximized. These checks involve measurement of basic fog level, speed and

average gradient made at 1° C intervals of temperature. The range of temperature measurement

depends on the type of chemistry in use, whether cold or hot developer. These three

measurements: fog level, speed, and average gradient, should then be plotted against temperature

and compared with the manufacturer's supplied figures.

9.11 Radiograph Interpretation - Welds

In addition to producing high quality radiographs, the radiographer must also be skilled in

radiographic interpretation. Interpretation of radiographs takes place in three basic steps: (1)


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detection, (2) interpretation, and (3) evaluation. All of these steps make use of the radiographer's

visual acuity. Visual acuity is the ability to resolve a spatial pattern in an image. The ability of an

individual to detect discontinuities in radiography is also affected by the lighting condition in the

place of viewing, and the experience level for recognizing various features in the image. The

following material was developed to help students develop an understanding of the types of

defects found in weldments and how they appear in a radiograph.

9.12 Discontinuities

Discontinuities are interruptions in the typical structure of a material. These interruptions may

occur in the base metal, weld material or "heat affected" zones. Discontinuities, which do not

meet the requirements of the codes or specifications used to invoke and control an inspection, are

referred to as defects.

9.13 Welding Discontinuities

The following discontinuities are typical of all types of welding.

9.13.1 Cold Lap

Cold lap is a condition where the weld filler metal does not properly fuse with the base metal or

the previous weld pass material (interpass cold lap). The arc does not melt the base metal

sufficiently and causes the slightly molten puddle to flow into the base material without bonding.

Fig. : Cold Lap in Welding


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9.13.2 Porosity

It is the result of gas entrapment in the solidifying metal. Porosity can take many shapes on a

radiograph but often appears as dark round or irregular spots or specks appearing singularly, in

clusters, or in rows. Sometimes, porosity is elongated and may appear to have a tail. This is the

result of gas attempting to escape while the metal is still in a liquid state and is called wormhole

porosity. All porosity is a void in the material and it will have a higher radiographic density than

the surrounding area.

.

Fig. : Porosity in Welding

9.13.3 Cluster porosity

Cluster porosity is caused when flux coated electrodes are contaminated with moisture. The

moisture turns into a gas when heated and becomes trapped in the weld during the welding

process. Cluster porosity appear just like regular porosity in the radiograph but the indications

will be grouped close together.


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Fig. : Cluster porosity in Welding

9.13.4 Slag inclusions

Slag inclusions are nonmetallic solid material entrapped in weld metal or between weld and base

metal. In a radiograph, dark, jagged asymmetrical shapes within the weld or along the weld joint

areas are indicative of slag inclusions.

Fig. : Slag Inclusions


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9.13.5 IP and LOP

Incomplete penetration (IP) or lack of penetration (LOP) occurs when the weld metal fails to

penetrate the joint. It is one of the most objectionable weld discontinuities. Lack of penetration

allows a natural stress riser from which a crack may propagate. The appearance on a radiograph

is a dark area with well-defined, straight edges that follows the land or root face down the center

of the weldment.

Fig. : Incomplete penetration (IP) or lack of penetration (LOP) in Welding

9.13.6 Incomplete fusion

It is a condition where the weld filler metal does not properly fuse with the base metal.

Appearance on radiograph: usually appears as a dark line or lines oriented in the direction of the

weld seam along the weld preparation or joining area.

Fig. : Incomplete fusion in Welding


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9.13.7 Internal concavity or suck back

Internal concavity or suck back is a condition where the weld metal has contracted as it cools and

has been drawn up into the root of the weld. On a radiograph it looks similar to a lack of

penetration but the line has irregular edges and it is often quite wide in the center of the weld

image.

Fig. : Internal concavity or suck back in Welding

9.13.8 Internal or root undercut

It is an erosion of the base metal next to the root of the weld. In the radiographic image it

appears as a dark irregular line offset from the centerline of the weldment. Undercutting is not as

straight edged as LOP because it does not follow a ground edge.


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Fig. : Internal or root undercut in Welding

9.13.9 External or crown undercut

External or crown undercut is an erosion of the base metal next to the crown of the weld. In the

radiograph, it appears as a dark irregular line along the outside edge of the weld area.

Fig. : External or crown undercut in Welding


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9.13.10 Offset or mismatch

Offset or mismatch is terms associated with a condition where two pieces being welded together

are not properly aligned. The radiographic image shows a noticeable difference in density

between the two pieces. The difference in density is caused by the difference in material

thickness. The dark, straight line is caused by the failure of the weld metal to fuse with the land

area.

Fig. : Offset or mismatch in Welding

9.13.11 Inadequate weld reinforcement

Inadequate weld reinforcement is an area of a weld where the thickness of weld metal deposited

is less than the thickness of the base material. It is very easy to determine by radiograph if the

weld has inadequate reinforcement, because the image density in the area of suspected

inadequacy will be higher (darker) than the image density of the surrounding base material.

Fig. : Inadequate weld reinforcement In Welding


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9.13.12 Excess weld reinforcement

It is an area of a weld that has weld metal added in excess of that specified by engineering

drawings and codes. The appearance on a radiograph is a localized, lighter area in the weld. A

visual inspection will easily determine if the weld reinforcement is in excess of that specified by

the engineering requirements.

Fig. : Excess weld reinforcement

9.13.13 Cracks

Cracks can be detected in a radiograph only when they are propagating in a direction that

produces a change in thickness that is parallel to the x-ray beam. Cracks will appear as jagged

and often very faint irregular lines. Cracks can sometimes appear as "tails" on inclusions or

porosity.

Fig. : Cracks in Welding


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9.13.14 Discontinuities in TIG welds

The following discontinuities are unique to the TIG welding process. These discontinuities occur

in most metals welded by the process, including aluminum and stainless steels. The TIG method

of welding produces a clean homogeneous weld which when radiographed is easily interpreted.

9.13.15 Tungsten inclusions

Tungsten is a brittle and inherently dense material used in the electrode in tungsten inert gas

welding. If improper welding procedures are used, tungsten may be entrapped in the weld.

Radiographically, tungsten is more dense than aluminum or steel, therefore it shows up as a

lighter area with a distinct outline on the radiograph.

Fig. : Tungsten inclusions

9.13.16 Oxide inclusions

Oxide inclusions are usually visible on the surface of material being welded (especially

aluminum). Oxide inclusions are less dense than the surrounding material and, therefore, appear

as dark irregularly shaped discontinuities in the radiograph.


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Fig. : Oxide inclusions in Welding

9.13.17 Discontinuities in Gas Metal Arc Welds (GMAW)

The following discontinuities are most commonly found in GMAW welds. Whiskers are short

lengths of weld electrode wire, visible on the top or bottom surface of the weld or contained

within the weld. On a radiograph they appear as light, "wire like" indications.

9.13.18 Burn-Through

Burn-Through results when too much heat causes excessive weld metal to penetrate the weld

zone. Often lumps of metal sag through the weld, creating a thick globular condition on the back

of the weld. These globs of metal are referred to as icicles. On a radiograph, burn-through

appears as dark spots, which are often surrounded by light globular areas (icicles).

Fig. : Burn-Through


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9.14 Real-time Radiography

Real-time radiography (RTR), or real-time

radioscopy, is a nondestructive test (NDT) method

whereby an image is produced electronically, rather

than on film, so that very little lag time occurs

between the item being exposed to radiation and the

resulting image. In most instances, the electronic

image that is viewed results from the radiation

passing through the object being inspected and

interacting with a screen of material that fluoresces or gives off light when the interaction occurs.

The fluorescent elements of the screen form the image much as the grains of silver form the

image in film radiography. The image formed is a "positive image" since brighter areas on the

image indicate where higher levels of transmitted radiation reached the screen. This image is the

opposite of the negative image produced in film radiography

9.15 Advantages of Radiography

Information is presented pictorially.

A permanent record is provided which may be viewed at a time and place distant from

the test.

Useful for thin sections.

Sensitivity declared on each film suitable for any material.

Suitable for any material.

9.16 Disadvantages of Radiography

Generally an inability to cope with thick sections.

Possible health hazard.

Need to direct the beam accurately for two-dimensional defects.

Film processing and viewing facilities are necessary, as is an exposure compound.

Not suitable for automation, unless the system incorporates fluoroscopy with an image

intensifier or other electronic aids

Not suitable for surface defects.


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10. Ultrasonic Testing

10.1 Introduction of Ultrasonic Testing

This technique is used for the detection of internal and surface (particularly distant surface)

defects in sound conducting materials. The principle is in some respects similar to echo

sounding. A short pulse of ultrasound is generated by means of an electric charge applied to a

piezo electric crystal, which vibrates for a very short period at a frequency related to the

thickness of the crystal. In flaw detection this frequency is usually in the range of one million to

six million times per second (1 MHz to 6 MHz). Vibrations or sound waves at this frequency

have the ability to travel a considerable distance in homogeneous elastic material, such as many

metals with little attenuation. The velocity at which these waves propagate is related to the

Young‘s Modulus for the material and is characteristic of that material. For example the velocity

in steel is 5900 metres per second, and in water 1400 metres per second. Ultrasonic energy is

considerably attenuated in air, and a beam propagated through solid will, on reaching an

interface (e.g. a defect, or intended hole, or the backwall) between that material and air reflect a

considerable amount of energy in the direction equal to the angle of incidence. For contact

testing the oscillating crystal is incorporated in a hand held probe, which is applied to the surface

of the material to be tested. To facilitate the transfer of energy across the small air gap between

the crystal and the test piece, a layer of liquid (referred to as ‗couplant‘), usually oil, water or

grease, is applied to the surface. As mentioned previously, the crystal does not oscillate

continuously but in short pulses, between each of which it is quiescent. Piezo electric materials

not only convert electrical pulses to mechanical oscillations, but will also transducer mechanical

oscillations into electrical pulses; thus we have not only a generator of sound waves but also a

detector of returned pulses. The crystal is in a state to detect returned pulses when it is quiescent.

The pulse takes a finite time to travel through the material to the interface and to be reflected

back to the probe

10.2 Basic Principles of Ultrasonic Testing

Ultrasonic Testing (UT) uses high frequency sound energy to conduct examinations and make

measurements. Ultrasonic inspection can be used for flaw detection/evaluation, dimensional


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measurements, material characterization, and more. To illustrate the general inspection principle,

a typical pulse/echo inspection configuration as illustrated below will be used.

A typical UT inspection system consists of several functional units, such as the pulser/receiver,

transducer, and display devices. A pulser/receiver is an electronic device that can produce high

voltage electrical pulses. Driven by the pulser, the transducer generates high frequency ultrasonic

energy. The sound energy is introduced and propagates through the materials in the form of

waves. When there is a discontinuity (such as a crack) in the wave path, part of the energy will

be reflected back from the flaw surface. The reflected wave signal is transformed into an

electrical signal by the transducer and is displayed on a screen. In the applet below, the reflected

signal strength is displayed versus the time from signal generation to when a echo was received.

Signal travel time can be directly related to the distance that the signal traveled. From the signal,

information about the reflector location, size, orientation and other features can sometimes be

gained.

Ultrasonic Inspection is a very useful and versatile NDT method. Some of the advantages of

ultrasonic inspection that are often cited include:

It is sensitive to both surface and subsurface discontinuities.

The depth of penetration for flaw detection or measurement is superior to other NDT

methods.

Only single-sided access is needed when the pulse-echo technique is used.

It is highly accurate in determining reflector position and estimating size and shape.

Minimal part preparation is required.

Electronic equipment provides instantaneous results.

Detailed images can be produced with automated systems.

It has other uses, such as thickness measurement, in addition to flaw detection.

As with all NDT methods, ultrasonic inspection also has its limitations, which include:

Surface must be accessible to transmit ultrasound.

Skill and training is more extensive than with some other methods.


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It normally requires a coupling medium to promote the transfer of sound energy into the

test specimen.

Materials that are rough, irregular in shape, very small, exceptionally thin or not

homogeneous are difficult to inspect.

Cast iron and other coarse grained materials are difficult to inspect due to low sound

transmission and high signal noise.

Linear defects oriented parallel to the sound beam may go undetected.

Reference standards are required for both equipment calibration and the characterization

of flaws.

The above introduction provides a simplified introduction to the NDT method of ultrasonic

testing. However, to effectively perform an inspection using ultrasonics, much more about the

method needs to be known. The following pages present information on the science involved in

ultrasonic inspection, the equipment that is commonly used, some of the measurement

techniques used, as well as other information.

10.3 History of Ultrasonics

Prior to World War II, sonar, the technique of sending sound waves through water and observing

the returning echoes to characterize submerged objects, inspired early ultrasound investigators to

explore ways to apply the concept to medical diagnosis. In 1929 and 1935, Sokolov studied the

use of ultrasonic waves in detecting metal objects. Mulhauser, in 1931, obtained a patent for

using ultrasonic waves, using two transducers to detect flaws in solids. Firestone (1940) and

Simons (1945) developed pulsed ultrasonic testing using a pulse-echo technique.

Shortly after the close of World War II, researchers in Japan began to explore the medical

diagnostic capabilities of ultrasound. The first ultrasonic instruments used an A-mode

presentation with blips on an oscilloscope screen. That was followed by a B-mode presentation

with a two dimensional, gray scale image.

Japan's work in ultrasound was relatively unknown in the United States and Europe until the

1950s. Researchers then presented their findings on the use of ultrasound to detect gallstones,


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breast masses, and tumors to the international medical community. Japan was also the first

country to apply Doppler ultrasound, an application of ultrasound that detects internal moving

objects such as blood coursing through the heart for cardiovascular investigation.

Ultrasound pioneers working in the United States

contributed many innovations and important

discoveries to the field during the following decades.

Researchers learned to use ultrasound to detect

potential cancer and to visualize tumors in living

subjects and in excised tissue. Real-time imaging,

another significant diagnostic tool for physicians,

presented ultrasound images directly on the system's

CRT screen at the time of scanning. The introduction

of spectral Doppler and later color Doppler depicted

blood flow in various colors to indicate the speed and direction of the flow..

The United States also produced the earliest hand held "contact" scanner for clinical use, the

second generation of B-mode equipment, and the prototype for the first articulated-arm hand

held scanner, with 2-D images.

10.4 Present State of Ultrasonics

Ultrasonic testing (UT) has been practiced for

many decades. Initial rapid developments in

instrumentation spurred by the technological

advances from the 1950's continue today.

Through the 1980's and continuing through the

present, computers have provided technicians

with smaller and more rugged instruments with

greater capabilities.

Thickness gauging is an example application


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where instruments have been refined make data collection easier and better. Built-in data

logging capabilities allow thousands of measurements to be recorded and eliminate the need for a

"scribe." Some instruments have the capability to capture waveforms as well as thickness

readings. The waveform option allows an operator to view or review the A-scan signal of

thickness measurement long after the completion of an inspection. Also, some instruments are

capable of modifying the measurement based on the surface conditions of the material. For

example, the signal from a pitted or eroded inner surface of a pipe would be treated differently

than a smooth surface. This has led to more accurate and repeatable field measurements.

Many ultrasonic flaw detectors have a trigonometric function that allows for fast and accurate

location determination of flaws when performing shear wave inspections. Cathode ray tubes, for

the most part, have been replaced with LED or LCD screens. These screens, in most cases, are

extremely easy to view in a wide range of ambient lighting. Bright or low light working

conditions encountered by technicians have little effect on the technician's ability to view the

screen. Screens can be adjusted for brightness, contrast, and on some instruments even the color

of the screen and signal can be selected. Transducers can be programmed with predetermined

instrument settings. The operator only has to connect the transducer and the instrument will set

variables such as frequency and probe drive.

Along with computers, motion control and robotics have contributed to the advancement of

ultrasonic inspections. Early on, the advantage of a stationary platform was recognized and used

in industry. Computers can be programmed to inspect large, complex shaped components, with

one or multiple transducers collecting information. Automated systems typically consisted of an

immersion tank, scanning system, and recording system for a printout of the scan. The

immersion tank can be replaced with a squirter systems,

which allows the sound to be transmitted through a water

column. The resultant C-scan provides a plan or top view of

the component. Scanning of components is considerably faster

than contact hand scanning, the coupling is much more

consistent. The scan information is collected by a computer

for evaluation, transmission to a customer, and archiving.


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Today, quantitative theories have been developed to describe the interaction of the interrogating

fields with flaws. Models incorporating the results have been integrated with solid model

descriptions of real-part geometries to simulate practical inspections. Related tools allow NDE to

be considered during the design process on an equal footing with other failure-related

engineering disciplines. Quantitative descriptions of NDE performance, such as the probability

of detection (POD), have become an integral part of statistical risk assessment. Measurement

procedures initially developed for metals have been extended to engineered materials such as

composites, where anisotropy and inhomogeneity have become important issues. The rapid

advances in digitization and computing capabilities have totally changed the faces of many

instruments and the type of algorithms that are used in processing the resulting data. High-

resolution imaging systems and multiple measurement modalities for characterizing a flaw have

emerged. Interest is increasing not only in detecting, characterizing, and sizing defects, but also

in characterizing the materials. Goals range from the determination of fundamental

microstructural characteristics such as grain size, porosity, and texture (preferred grain

orientation), to material properties related to such failure mechanisms as fatigue, creep, and

fracture toughness. As technology continues to advance, applications of ultrasound also advance.

The high-resolution imaging systems in the laboratory today will be tools of the technician

tomorrow.

10.5 Future Direction of Ultrasonic Inspection

Looking to the future, those in the field of NDE see an

exciting new set of opportunities. The defense and

nuclear power industries have played a major role in

the emergence of NDE. Increasing global competition

has led to dramatic changes in product development

and business cycles. At the same time, aging

infrastructure, from roads to buildings and aircraft,

present a new set of measurement and monitoring

challenges for engineers as well as technicians.


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Among the new applications of NDE spawned by these changes is the increased emphasis on the

use of NDE to improve the productivity of manufacturing processes. Quantitative nondestructive

evaluation (QNDE) both increases the amount of information about failure modes and the speed

with which information can be obtained and facilitates the development of in-line measurements

for process control.

The phrase, "you cannot inspect in quality, you must build it in," exemplifies the industry's focus

on avoiding the formation of flaws. Nevertheless, manufacturing flaws will never be completely

eliminated and material damage will continue to occur in-service so continual development of

flaw detection and characterization techniques is necessary.

Advanced simulation tools that are designed for inspectability and their integration into

quantitative strategies for life management will contribute to increase the number and types of

engineering applications of NDE. With growth in engineering applications for NDE, there will

be a need to expand the knowledge base of technicians performing the evaluations. Advanced

simulation tools used in the design for inspectability may be used to provide technical students

with a greater understanding of sound behavior in materials. UTSIM, developed at Iowa State

University, provides a glimpse into what may be used in the technical classroom as an interactive

laboratory tool.

As globalization continues, companies will seek to develop, with ever increasing frequency,

uniform international practices. In the area of NDE, this trend will drive the emphasis on

standards, enhanced educational offerings, and simulations that can be communicated

electronically. The coming years will be exciting as NDE will continue to emerge as a full-

fledged engineering discipline.

10.6 Wavelength and Defect Detection

In ultrasonic testing, the inspector must make a decision about the frequency of the transducer

that will be used. As we learned on the previous page, changing the frequency when the sound

velocity is fixed will result in a change in the wavelength of the sound. The wavelength of the

ultrasound used has a significant effect on the probability of detecting a discontinuity. A general


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rule of thumb is that a discontinuity must be larger than one-half the wavelength to stand a

reasonable chance of being detected.

Sensitivity and resolution are two terms that are often used in ultrasonic inspection to describe a

technique's ability to locate flaws. Sensitivity is the ability to locate small discontinuities.

Sensitivity generally increases with higher frequency (shorter wavelengths). Resolution is the

ability of the system to locate discontinuities that are close together within the material or located

near the part surface. Resolution also generally increases as the frequency increases.

The wave frequency can also affect the capability of an inspection in adverse ways. Therefore,

selecting the optimal inspection frequency often involves maintaining a balance between the

favorable and unfavorable results of the selection. Before selecting an inspection frequency, the

material's grain structure and thickness, and the discontinuity's type, size, and probable location

should be considered. As frequency increases, sound tends to scatter from large or course grain

structure and from small imperfections within a material. Cast materials often have coarse grains

and other sound scatters that require lower frequencies to be used for evaluations of these

products. Wrought and forged products with directional and refined grain structure can usually

be inspected with higher frequency transducers.

Since more things in a material are likely to scatter a portion of

the sound energy at higher frequencies, the penetrating power

(or the maximum depth in a material that flaws can be located)

is also reduced. Frequency also has an effect on the shape of the

ultrasonic beam. Beam spread, or the divergence of the beam

from the center axis of the transducer, and how it is affected by

frequency will be discussed later.

It should be mentioned, so as not to be misleading, that a number of other variables will also

affect the ability of ultrasound to locate defects. These include the pulse length, type and voltage

applied to the crystal, properties of the crystal, backing material, transducer diameter, and the

receiver circuitry of the instrument. These are discussed in more detail in the material on signal-

to-noise ratio.


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10.7 Sound Propagation in Elastic Materials

In the previous pages, it was pointed out that sound waves propagate due to the vibrations or

oscillatory motions of particles within a material. An ultrasonic wave may be visualized as an

infinite number of oscillating masses or particles connected by means of elastic springs. Each

individual particle is influenced by the motion of its nearest neighbor and both inertial and elastic

restoring forces act upon each particle.

A mass on a spring has a single resonant frequency determined by its spring constant k and its

mass m. The spring constant is the restoring force of a spring per unit of length. Within the

elastic limit of any material, there is a linear relationship between the displacement of a particle

and the force attempting to restore the particle to its equilibrium position. This linear dependency

is described by Hooke's Law.

In terms of the spring model, Hooke's Law says that

the restoring force due to a spring is proportional to

the length that the spring is stretched, and acts in the

opposite direction. Mathematically, Hooke's Law is

written as F =-kx, where F is the force, k is the

spring constant, and x is the amount of particle

displacement. Hooke's law is represented graphically

it the right. Please note that the spring is applying a

force to the particle that is equal and opposite to the force pulling down on the particle.

10.8 Speed of Sound

Hooke's Law, when used along with Newton's Second Law, can explain a few things about the

speed of sound. The speed of sound within a material is a function of the properties of the

material and is independent of the amplitude of the sound wave. Newton's Second Law says that

the force applied to a particle will be balanced by the particle's mass and the acceleration of the

the particle. Mathematically, Newton's Second Law is written as F = ma. Hooke's Law then says

that this force will be balanced by a force in the opposite direction that is dependent on the


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amount of displacement and the spring constant (F = -kx). Therefore, since the applied force and

the restoring force are equal, ma = -kx can be written. The negative sign indicates that the force

is in the opposite direction.

Since the mass m and the spring constant k are constants for any given material, it can be seen

that the acceleration a and the displacement x are the only variables. It can also be seen that they

are directly proportional. For instance, if the displacement of the particle increases, so does its

acceleration. It turns out that the time that it takes a particle to move and return to its equilibrium

position is independent of the force applied. So, within a given material, sound always travels at

the same speed no matter how much force is applied when other variables, such as temperature,

are held constant.

10.9 Properties of material affect its speed of sound

Of course, sound does travel at different speeds in different materials. This is because the mass

of the atomic particles and the spring constants are different for different materials. The mass of

the particles is related to the density of the material, and the spring constant is related to the

elastic constants of a material. The general relationship between the speed of sound in a solid and

its density and elastic constants is given by the following equation:

Where V is the speed of sound, C is the elastic constant, and p is the material density. This

equation may take a number of different forms depending on the type of wave (longitudinal or

shear) and which of the elastic constants that are used. The typical elastic constants of a materials

include:

Young's Modulus, E: a proportionality constant between uniaxial stress and strain.

Poisson's Ratio, n: the ratio of radial strain to axial strain


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Bulk modulus, K: a measure of the incompressibility of a body subjected to hydrostatic

pressure.

Shear Modulus, G: also called rigidity, a measure of a substance's resistance to shear.

Lame's Constants, l and m: material constants that are derived from Young's Modulus

and Poisson's Ratio.

When calculating the velocity of a longitudinal wave, Young's Modulus and Poisson's Ratio are

commonly used. When calculating the velocity of a shear wave, the shear modulus is used. It is

often most convenient to make the calculations using Lame's Constants, which are derived from

Young's Modulus and Poisson's Ratio.

It must also be mentioned that the subscript ij attached to C in the above equation is used to

indicate the directionality of the elastic constants with respect to the wave type and direction of

wave travel. In isotropic materials, the elastic constants are the same for all directions within the

material. However, most materials are anisotropic and the elastic constants differ with each

direction. For example, in a piece of rolled aluminum plate, the grains are elongated in one

direction and compressed in the others and the elastic constants for the longitudinal direction are

different than those for the transverse or short transverse directions.

Examples of approximate compressional sound velocities in materials are:

Aluminum - 0.632 cm/microsecond

1020 steel - 0.589 cm/microsecond

Cast iron - 0.480 cm/microsecond.

Examples of approximate shear sound velocities in materials are:

Aluminum - 0.313 cm/microsecond

1020 steel - 0.324 cm/microsecond

Cast iron - 0.240 cm/microsecond.

When comparing compressional and shear velocities, it can be noted that shear velocity is

approximately one half that of compressional velocity. The sound velocities for a variety of


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materials can be found in the ultrasonic properties tables in the general resources section of this

site.

10.10 Piezoelectric Transducers

The conversion of electrical pulses to mechanical vibrations and the conversion of returned

mechanical vibrations back into electrical energy is the basis for ultrasonic testing. The active

element is the heart of the transducer as it converts the electrical energy to acoustic energy, and

vice versa. The active element is basically a piece of polarized material (i.e. some parts of the

molecule are positively charged, while other parts of the molecule are negatively charged) with

electrodes attached to two of its opposite faces. When an electric field is applied across the

material, the polarized molecules will align themselves with the electric field, resulting in

induced dipoles within the molecular or crystal structure of the material. This alignment of

molecules will cause the material to change dimensions. This phenomenon is known as

electrostriction. In addition, a permanently-polarized material such as quartz (SiO2) or barium

titanate (BaTiO3) will produce an electric field when the material changes dimensions as a result

of an imposed mechanical force. This phenomenon is known as the piezoelectric effect.

Piezoelectric Transducer


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Additional information on why certain materials produce this effect can be found in the linked

presentation material, which was produced by the Valpey Fisher Corporation.

The active element of most acoustic transducers used today is a piezoelectric ceramic, which can

be cut in various ways to produce different wave modes. A large piezoelectric ceramic element

can be seen in the image of a sectioned low

frequency transducer. Preceding the advent

of piezoelectric ceramics in the early 1950's,

piezoelectric crystals made from quartz

crystals and magnetostrictive materials were

primarily used. The active element is still

sometimes referred to as the crystal by old

timers in the NDT field. When piezoelectric

ceramics were introduced, they soon became

the dominant material for transducers due to

their good piezoelectric properties and their

ease of manufacture into a variety of shapes

and sizes. They also operate at low voltage

and are usable up to about 300oC. The first piezoceramic in general use was barium titanate, and

that was followed during the 1960's by lead zirconate titanate compositions, which are now the

most commonly employed ceramic for making transducers. New materials such as piezo-

polymers and composites are also being used in some applications.


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The thickness of the active element is determined by the desired frequency of the transducer. A

thin wafer element vibrates with a wavelength that is twice its thickness. Therefore, piezoelectric

crystals are cut to a thickness that is 1/2 the desired radiated wavelength. The higher the

frequency of the transducer, the thinner the active element. The primary reason that high

frequency contact transducers are not produced is because the element is very thin and too

fragile.

10.11 Characteristics of Piezoelectric Transducers

The transducer is a very important part of the ultrasonic instrumentation system. As discussed on

the previous page, the transducer incorporates a piezoelectric element, which converts electrical

signals into mechanical vibrations (transmit mode) and mechanical vibrations into electrical

signals (receive mode). Many factors, including material, mechanical and electrical construction,

and the external mechanical and electrical load conditions, influence the behavior of a

transducer. Mechanical construction includes parameters such as the radiation surface area,

mechanical damping, housing, connector type and other variables of physical construction. As of

this writing, transducer manufacturers are hard pressed when constructing two transducers that

have identical performance characteristics.


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A cut away of a typical contact transducer is shown above. It was previously learned that the

piezoelectric element is cut to 1/2 the desired wavelength. To get as much energy out of the

transducer as possible, an impedance matching is placed between the active element and the face

of the transducer. Optimal impedance matching is achieved by sizing the matching layer so that

its thickness is 1/4 of the desired wavelength. This keeps waves that were reflected within the

matching layer in phase when they exit the layer (as illustrated in the image to the right). For

contact transducers, the matching layer is made from a material that has an acoustical impedance

between the active element and steel. Immersion transducers have a matching layer with an

acoustical impedance between the active element and water. Contact transducers also incorporate

a wear plate to protect the matching layer and active element from scratching.

The backing material supporting the crystal has a great influence on the damping characteristics

of a transducer. Using a backing material with an impedance similar to that of the active element

will produce the most effective damping. Such a transducer will have a wider bandwidth

resulting in higher sensitivity. As the mismatch in impedance between the active element and the

backing material increases, material penetration increases but transducer sensitivity is reduced.

10.12 Radiated Fields of Ultrasonic Transducers

The sound that emanates from a piezoelectric transducer does not originate from a point, but

instead originates from most of the surface of the piezoelectric element. Round transducers are

often referred to as piston source transducers because the sound field resembles a cylindrical

mass in front of the transducer. The sound field from a typical piezoelectric transducer is shown

below. The intensity of the sound is indicated by color, with lighter colors indicating higher

intensity.


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Since the ultrasound originates from a number of points along the transducer face, the ultrasound

intensity along the beam is affected by constructive and destructive wave interference as

discussed in a previous page on wave interference. These are sometimes also referred to as

diffraction effects. This wave interference leads to extensive fluctuations in the sound intensity

near the source and is known as the near field. Because of acoustic variations within a near field,

it can be extremely difficult to accurately evaluate flaws in materials when they are positioned

within this area.

The pressure waves combine to form a relatively uniform front at the end of the near field. The

area beyond the near field where the ultrasonic beam is more uniform is called the far field. In

the far field, the beam spreads out in a pattern originating from the center of the transducer. The

transition between the near field and the far field occurs at a distance, N, and is sometimes

referred to as the "natural focus" of a flat (or unfocused) transducer. The near/far field distance,

N, is significant because amplitude variations that characterize the near field change to a

smoothly declining amplitude at this point. The area just beyond the near field is where the

sound wave is well behaved and at its maximum strength. Therefore, optimal detection results

will be obtained when flaws occur in this area.

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For a piston source transducer of radius (a), frequency (f), and velocity (V) in a liquid or solid

medium, the applet below allows the calculation of the near/far field transition point.

10.13 Transducer Types

Ultrasonic transducers are manufactured for a

variety of applications and can be custom

fabricated when necessary. Careful attention

must be paid to selecting the proper

transducer for the application. A previous

section on Acoustic Wavelength and Defect

Detection gave a brief overview of factors that affect defect detectability. From this material, we

know that it is important to choose transducers that have the desired frequency, bandwidth, and

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focusing to optimize inspection capability. Most often the transducer is chosen either to enhance

the sensitivity or resolution of the system.

Transducers are classified into groups according to the application.

10.13.1 Contact transducers

are used for direct contact inspections, and are generally hand manipulated. They have elements

protected in a rugged casing to withstand sliding contact with a variety of materials. These

transducers have an ergonomic design so that they are easy to grip and move along a surface.

They often have replaceable wear plates to lengthen their useful life. Coupling materials of

water, grease, oils, or commercial materials are used to remove the air gap between the

transducer and the component being inspected.

10.13.2 Immersion transducers

These transducers do not contact the component. These

transducers are designed to operate in a liquid environment and all

connections are watertight. Immersion transducers usually have an

impedance matching layer that helps to get more sound energy

into the water and, in turn, into the component being inspected.

Immersion transducers can be purchased with a planer,

cylindrically focused or spherically focused lens. A focused

transducer can improve the sensitivity and axial resolution by

concentrating the sound energy to a smaller area. Immersion

transducers are typically used inside a water tank or as part of a squirter or bubbler system in scanning applications.

10.13.3 More on Contact Transducers

Contact transducers are available in a variety of configurations to improve their usefulness for a

variety of applications. The flat contact transducer shown above is used in normal beam

inspections of relatively flat surfaces, and where near surface resolution is not critical. If the

surface is curved, a shoe that matches the curvature of the part may need to be added to the face

of the transducer. If near surface resolution is important or if an angle beam inspection is needed,

one of the special contact transducers described below might be used.


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10.13.4 Dual element transducer

It contains two independently operated elements in a single

housing. One of the elements transmits and the other

receives the ultrasonic signal. Active elements can be chosen

for their sending and receiving capabilities to provide a

transducer with a cleaner signal, and transducers for special

applications, such as the inspection of course grained

material. Dual element transducers are especially well suited

for making measurements in applications where reflectors

are very near the transducer since this design eliminates the

ring down effect that single-element transducers experience

(when single-element transducers are operating in pulse echo mode, the element cannot start

receiving reflected signals until the element has stopped ringing from its transmit function). Dual

element transducers are very useful when making thickness measurements of thin materials and

when inspecting for near surface defects. The two elements are angled towards each other to

create a crossed-beam sound path in the test material.

10.13.5 Delay line transducers

These provide versatility with a variety of

replaceable options. Removable delay line, surface

conforming membrane, and protective wear cap

options can make a single transducer effective for a

wide range of applications. As the name implies, the

primary function of a delay line transducer is to

introduce a time delay between the generation of the

sound wave and the arrival of any reflected waves.

This allows the transducer to complete its "sending" function before it starts its "listening"

function so that near surface resolution is improved. They are designed for use in applications

such as high precision thickness gauging of thin materials and delamination checks in composite


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materials. They are also useful in high-temperature measurement applications since the delay

line provides some insulation to the piezoelectric element from the heat.

10.13.6 Angle beam transducers

These are typically used to introduce a refracted shear wave

into the test material. Transducers can be purchased in a

variety of fixed angles or in adjustable versions where the

user determines the angles of incidence and refraction. In the

fixed angle versions, the angle of refraction that is marked

on the transducer is only accurate for a particular material,

which is usually steel. The angled sound path allows the

sound beam to be reflected from the backwall to improve

detectability of flaws in and around welded areas. They are

also used to generate surface waves for use in detecting defects on the surface of a component.

10.13.7 Normal incidence shear wave transducers

These transducers are unique because they allow the introduction of shear waves directly into a

test piece without the use of an angle beam wedge. Careful design has enabled manufacturing of

transducers with minimal longitudinal wave contamination. The ratio of the longitudinal to shear

wave components is generally below -30dB.

10.13.7 Paint brush transducers

It is used to scan wide areas. These long and narrow transducers are made up of an array of

small crystals that are carefully matched to minimize variations in performance and maintain

uniform sensitivity over the entire area of the transducer. Paint brush transducers make it

possible to scan a larger area more rapidly for discontinuities. Smaller and more sensitive

transducers are often then required to further define the details of a discontinuity.


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10.14 Couplant

A couplant is a material (usually liquid) that facilitates the

transmission of ultrasonic energy from the transducer into

the test specimen. Couplant is generally necessary because

the acoustic impedance mismatch between air and solids

(i.e. such as the test specimen) is large. Therefore, nearly all

of the energy is reflected and very little is transmitted into

the test material. The couplant displaces the air and makes it

possible to get more sound energy into the test

specimen so that a usable ultrasonic signal can be

obtained. In contact ultrasonic testing a thin film of oil,

glycerin or water is generally used between the

transducer and the test surface.

When scanning over the part or making precise

measurements, an immersion technique is often used.

In immersion ultrasonic testing both the transducer and

the part are immersed in the couplant, which is typically

water. This method of coupling makes it easier to maintain

consistent coupling while moving and manipulating the

transducer and/or the part.

10.15 Pulser-Receivers

Ultrasonic pulser-receivers are well suited to general

purpose ultrasonic testing. Along with appropriate

transducers and an oscilloscope, they can be used for flaw detection and thickness gauging in a

wide variety of metals, plastics, ceramics, and composites. Ultrasonic pulser-receivers provide a

unique, low-cost ultrasonic measurement capability.


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The pulser section of the instrument

generates short, large amplitude electric

pulses of controlled energy, which are

converted into short ultrasonic pulses

when applied to an ultrasonic transducer.

Most pulser sections have very low

impedance outputs to better drive transducers. Control functions associated with the pulser

circuit include:

Pulse length or damping (The amount of time the pulse is applied to the transducer.)

Pulse energy (The voltage applied to the transducer. Typical pulser circuits will apply

from 100 volts to 800 volts to a transducer.)

In the receiver section the voltage signals produced by the transducer, which represent the

received ultrasonic pulses, are amplified. The amplified radio frequency (RF) signal is available

as an output for display or capture for signal processing. Control functions associated with the

receiver circuit include

Signal rectification (The RF signal can be viewed as positive half wave, negative half

wave or full wave.)

Filtering to shape and smooth return signals

Gain, or signal amplification

Reject control

The pulser-receiver is also used in material characterization work involving sound velocity or

attenuation measurements, which can be correlated to material properties such as elastic

modulus. In conjunction with a stepless gate and a spectrum analyzer, pulser-receivers are also

used to study frequency dependent material properties or to characterize the performance of

ultrasonic transducers.


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10.16 Angle Beams I

Angle Beam Transducers and wedges are typically used to introduce a refracted shear wave into

the test material. An angled sound path allows the sound beam to come in from the side, thereby

improving detectability of flaws in and around welded areas.

10.17 Angle Beams II

Angle Beam Transducers and wedges are typically used to introduce a refracted shear wave into

the test material. The geometry of the sample below allows the sound beam to be reflected from

the back wall to improve detectability of flaws in and around welded areas.


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10.18 Calibration Methods

Calibration refers to the act of evaluating and adjusting the precision and accuracy of

measurement equipment. In ultrasonic testing, several forms of calibration must occur. First, the

electronics of the equipment must be calibrated to ensure that they are performing as designed.

This operation is usually performed by the equipment manufacturer and will not be discussed

further in this material. It is also usually necessary for the operator to perform a "user

calibration" of the equipment. This user calibration is necessary because most ultrasonic

equipment can be reconfigured for use in a large variety of applications. The user must

"calibrate" the system, which includes the equipment settings, the transducer, and the test setup,


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to validate that the desired level of precision and accuracy are achieved. The term calibration

standard is usually only used when an absolute value is measured and in many cases, the

standards are traceable back to standards at the National Institute for Standards and Technology.

In ultrasonic testing, there is also a need for reference standards. Reference standards are used to

establish a general level of consistency in measurements and to help interpret and quantify the

information contained in the received signal. Reference standards are used to validate that the

equipment and the setup provide similar results from one day to the next and that similar results

are produced by different systems. Reference standards also help the inspector to estimate the

size of flaws. In a pulse-echo type setup, signal strength depends on both the size of the flaw and

the distance between the flaw and the transducer. The inspector can use a reference standard with

an artificially induced flaw of known size and at approximately the same distance away for the

transducer to produce a signal. By comparing the signal from the reference standard to that

received from the actual flaw, the inspector can estimate the flaw size.

This section will discuss some of the more common calibration and reference specimen that are

used in ultrasonic inspection. Some of these specimens are shown in the figure above. Be aware

that there are other standards available and that specially designed standards may be required for

many applications. The information provided here is intended to serve a general introduction to

the standards and not to be instruction on the proper use of the standards.

10.19 Weldments (Welded Joints)

The most commonly occurring defects in welded joints are porosity, slag inclusions, lack of side-

wall fusion, lack of inter-run fusion, lack of root penetration, undercutting, and longitudinal or

transverse cracks.

With the exception of single gas pores all the defects listed are usually well detectable by

ultrasonics. Most applications are on low-alloy construction quality steels, however, welds in

aluminum can also be tested. Ultrasonic flaw detection has long been the preferred method for

nondestructive testing in welding applications. This safe, accurate, and simple technique has

pushed ultrasonics to the forefront of inspection technology.


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Ultrasonic weld inspections are typically performed using a straight beam transducer in

conjunction with an angle beam transducer and wedge. A straight beam transducer, producing a

longitudinal wave at normal incidence into the test piece, is first used to locate any laminations

in or near the heat-affected zone. This is important because an angle beam transducer may not be

able to provide a return signal from a laminar flaw.

The second step in the inspection involves using an angle beam transducer to inspect the actual

weld. Angle beam transducers use the principles of refraction and mode conversion to produce

refracted shear or longitudinal waves in the test material. [Note: Many AWS inspections are

performed using refracted shear waves. However, material having a large grain structure, such as

stainless steel may require refracted longitudinal waves for successful inspections.] This

inspection may include the root, sidewall, crown, and heat-affected zones of a weld. The process

involves scanning the surface of the material around the weldment with the transducer. This

refracted sound wave will bounce off a reflector (discontinuity) in the path of the sound beam.


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With proper angle beam techniques, echoes returned from the weld zone may allow the operator

to determine the location and type of discontinuity.

To determine the proper scanning area for the weld, the inspector must first calculate the location

of the sound beam in the test material. Using the refracted angle, beam index point and material

thickness, the V-path and skip distance of the sound beam is found. Once they have been

calculated, the inspector can identify the transducer locations on the surface of the material

corresponding to the crown, sidewall, and root of the weld.

10.20 Advantages of Ultrasonic Flaw Detection

Thickness and lengths up to 30 ft can be tested.

Position, size and type of defect can be determined.

Instant test results.

Portable.

Extremely sensitive if required.

Capable of being fully automated.

Access to only one side necessary.

No consumables.


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10.21 Disadvantages of Ultrasonic Flaw Detection

No permanent record available unless one of the more sophisticated test results and data

collection systems is used.

The operator can decide whether the test piece is defective or not whilst the test is in

progress.

Indications require interpretation (except for digital wall thickness gauges).

Considerable degree of skill necessary to obtain the fullest information from the test.

Very thin sections can prove difficult.

11 Applications of Non-Destructive Testing

11.1 Aerospace Industry

Testing components including aero-engine, Landing gear and air frame parts during production

11.2 Aircraft Overhaul

Testing components during overhaul including aero-engine and landing gear components

11.3 Automotive Industry

Testing Brakes-Steering and engine safety critical components for flaws introduced during

manufacture. Iron castings – material quality. Testing of diesel engine pistons up to marine

engine size.

11.4 Petrochemical & Gas Industries

Pipe-Line and tank internal corrosion measurement from outside. Weld testing on new work.

Automotive LPG tank testing

11.5 Railway Industry

Testing locomotive and rolling stock axles for fatigue cracks. Testing rail for heat induced

cracking. Diesel locomotive engines and structures.

11.6 Mining Industry

Testing of pit head equipment and underground transport safety critical components.

11.7 Agricultural Engineering

Testing of all fabricated, forged and cast components in agricultural equipment including those

in tractor engines.


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11.8 Power Generation

Boiler and pressure vessel testing for weld and plate defects both during manufacturing and in

subsequent service. Boiler pipe work thickness measurement and turbine alternator component

testing.

11.9 Iron Foundry

Testing ductile iron castings for metal strength on 100% quality control basis.

11.10 Shipbuilding Industry

Structural and welding testing. Hull and bulkhead thickness measurement. Engine components

testing.

11.11 Steel Industry

Testing of rolled and re-rolled products including billets, plate sheet and structural sections.

11.12 Pipe & Tube Manufacturing Industry

Raw plate and strip testing. Automatic ERW tube testing. Oil line pipe spiral weld testing

12. Conclusion

In Bangladesh NDT is a new technology and system for inspection and testing. But many

developed countries use it because of its huge benefits.

Modern NDT methods are becoming ever more quantitative and non-intrusive. This is valid for

NDT of new construction and for maintenance inspections.

For NDT of new construction this implies that, the more one knows about the material properties

and operational conditions, the better the acceptance criteria for weld defects can be based on the

required weld integrity and fine-tuned to a specific application. In pipeline industry, this is

already going to happen.

In plant maintenance, the availability of quantitative and non-invasive screening NDT methods

will reduce the time needed for shutdowns and increase the intervals between them. Modern

NDT methods will become just as important a tool for Risk Based Inspection approaches and

maintenance planning as operational parameters and degradation mechanisms already are.


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In both these NDT application fields, new construction and maintenance, these

tendencies can lead to rationalization, with cost reduction as a result, maintaining existing

safety levels.

13. Recommendation

The Non-Destructive Testing (NDT) is a new technology in Bangladesh. Saj Engineering &

Trading Company is the first Company that bring this technology in Bangladesh. The NDT

equipments are very expensive. Although recently some others company come in market to give

this service. For a developing country like Bangladesh, NDT is very needed for industrial

development. The company should make so more focus about this technology in industrial level.

Al the engineers should know about this technology, especially in industrial level. The

Bangladesh Government has the only one institution that only offers the NDT courses. That is

the Bangladesh Atomic Energy Commission – NDT Division. Another Institution, Bangladesh

Boiler Association also use this technology for the boiler testing and inspection. All companies

should have the NDT division with NDT practitioner.

14. References

1. ASNDT- American Society of Non-Destructive Testing.

2. Bangladesh Atomic Energy Commission- NDT Division.

3. Bangladesh Atomic Energy Commission – NDT Division ( NDT fundamental course

Handbook)

4. American Airlines. Nondestructive training manual: qualifications programDrury CG,

Watson J. (2000). Human factors good practices in borescope inspection. FAA/Office of

Aviation Medicine, Washington, D.C.. @ URL: http://hfskyway.faa.gov. Oct 2002. 16

Drury CG. (1999).

5. Human factors good practices in fluorescent penetrant inspection. Human factors in

aviation maintenance - phase nine, progress report,FAA/Human Factors in Aviation

Maintenance. @ URL: http://hfskyway.faa.gov.Oct 2002.


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6. Adams LK, Czepiel EJ, Krulee GK, Watson J. (1999). Job task analysis of the aviation

maintenance technician. FAA/Office of Aviation Medicine, Washington,D.C. @ URL:

http://hfskyway.faa.gov. Oct 2002.

7. Allen D. (1970). Phase III Report: A national study of the aviation mechanics occupation.

FAA, Washington, DC. [Cited by Adams et al. (1999). Job task analysis of the aviation

maintenance technician. FAA/Office of Aviation Medicine, Washington, D.C. @ URL:

http://hfskyway.faa.gov. Oct 2002.]

8. Bray, Don E. and Don McBride: ―Nondestructive Testing Techniques,‖ John Wiley & Sons,

Inc., 1992.

9. McMaster, Robert C.: ―Nondestructive Testing Handbook,‖ Volume II, The Ronald Press

Company, New York 1963.

10. Metals Handbook, Volume 17: ―Nondestructive Inspection and Quality Control,‖ pp. 89-

128, ASM International, Metals Park, OH, 1989.

11. ASTM E 1444-93: ―Standard Practice for Magnetic Particle Examination,‖ American

Society for Testing and Materials, 1916 Race St. Philadelphia, PA 18103. MSFC-STD-

1249: ―Standard NDE Guidelines and Requirements for Fracture Control Programs,‖

Marshall Space Flight Center, AL 35812, September 1985.

12. R.A. Quinn and C.C. Sigl ―Radiography in Modern Industry,‖ 4th Edition, , Eastman Kodak

Company, 1980.

13. ―Industrial Radiography Radiation Safety Personnel,‖ ASNT Practice No. ASNT-CP-

IRRSP-1A, 2001 Edition, American Society for Nondestructive Testing.

14. ASNT Level III Study Guide and Supplement on Visual and Optical Testing, American

Society for Nondestructive Testing, Columbus, OH, 2005.

15. Reliability of Visual Inspection for Highway Bridges, Publication Nos. FHWA-RD-01-020

and FHWA-RD-01-021, June 2001.

16. Reliability of Visual Inspection for Highway Bridges, Publication Nos. FHWA-RD-01-020

and FHWA-RD-01-021, June 2001.

17. F.A. Iddings, Visual Inspection, Materials Evaluation, Vol. 62,No. 5, May 2004, pp. 500-

501.

18. ASTM International, ASTM Volume 03.03 Nondestructive Testing

19. ASNT, Nondestructive Testing Handbook


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20. Bray, D.E. and R.K. Stanley, 1997, Nondestructive Evaluation: A Tool for Design,

Manufacturing and Service; CRC Press, 1996.

21. Hellier, C., Handbook of Nondestructive Evaluation, McGraw-Hill Professional; 2001

22. Shull, P.J., Nondestructive Evaluation: Theory, Techniques, and Applications, Marcel

Dekker Inc., 2002.

23. Albert S. Birks, Robert E. Green, Jr., technical editors ; Paul McIntire, editor. Ultrasonic

testing, 2nd ed. Columbus, OH : American Society for Nondestructive Testing, 1991.

ISBN 0931403049.

24. Josef Krautkrämer, Herbert Krautkrämer. Ultrasonic testing of materials, 4th fully rev.

ed. Berlin; New York: Springer-Verlag, 1990. ISBN 3540512314.

25. J.C. Drury. Ultrasonic Flaw Detection for Technicians, 3rd ed., UK: Silverwing Ltd.

2004. (See Chapter 1 online (PDF, 61 kB)).

26. Nondestructive Testing Handbook, Third ed.: Volume 7, Ultrasonic Testing. Columbus,

OH: American Society for Nondestructive Testing.

27. Detection and location of defects in electronic devices by means of scanning ultrasonic

microscopy and the wavelet transform measurement, Volume 31, Issue 2, March 2002,

Pages 77–91, L. Angrisani, L. Bechou, D. Dallet, P. Daponte, Y. Ousten

28. Cartz, Louis (1995). Nondestructive Testing. A S M International.

29. Blitz, Jack; G. Simpson (1991). Ultrasonic Methods of Non-Destructive Testing.

Springer-Verlag New York, LLC.

30. www.asndt.com- (NDT Course Material)

http://en.wikipedia.org/wiki/Special:BookSources/0931403049

http://en.wikipedia.org/wiki/Special:BookSources/3540512314

http://www.silverwinguk.com/en/technical%20pdfs/ultrasonics_pdf/article_1.pdf

http://www.asndt.com-/


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Shahin Internship Report on Non-Destructive Testing in Saj Engineering and Trading Company

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