IIIIIII IIIIIIIII iium 1000165779

F'usat K. n1Ulnc: 1 wu; k; ub; iat r%r. auýiuin UNIVERtiIT1 MALAYSIASARAWAK.

CRACK DETECTION IN WELDED JOINT

ABDUL MUTALIB FAISAL BIN LAMPONG

This report is submitted to Faculty of Engineering University Malaysia Sarawak

(UNIMAS) as to fulfil the requirements of Bachelor Degree Program

Mechanical Engineering and Manufacturing System

Faculty of Engineering UNIVERSITI MALAYSIA SARAWAK

2005

DEDICATION

I could not have done this alone without your help and support. I would like to

express my greatest gratitude to my lecturer, colleagues, friends and my loving

family. Thanks a million.

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APPRECIATION

This thesis was completed with the helps and supports from so many individuals by

means of advice, participation, and morally.

First of all, I would like to acknowledge and great thank to my supervisor, Dr.

Mohd. Shahril b. Osman of University Malaysia Sarawak for the excellent

patience, careful supervision and encouragement rendered to me in preceding my final year project successfully. Formidable paper works were handled with precise

administrative skills and effective communication system by you at through the

protocol process and I appreciate your efforts in the whole process. I am especially

thankful for the advice given by you when there were some bottlenecks in my

project.

My next thanks are expressed to the technicians of Mechanical and Manufacturing

System, Mr. Ryhier, Mr. Masri, Mr. Sabariman, Mr. Eiman, Mr. Zaidi and Mrs.

Hasmiza. They don't hesitate in giving their helping hands to help me solves

problems and difficulties which I faced during the period at the workshop and

laboratory.

Finally, I would like to thank my family and friends who always stand by me giving

me supports mentally and financially.

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ABSTRACT

As in all manufacturing process, the quality of the welded joint is established by

testing. Welded joint may be tested either destructively testing (DT) or non-

destructively testing (NDT). Non-destructively testing technique for welded joint

usually consist of visual, radiography, magnetic-particle, liquid-penetrant and

ultrasonic testing. Non-destructive testing is carried out in such a manner that

integrity and surface structure remain unchanged. In this experimental study on

specimen of mild steel pipe, non-destructive testing method of ultrasonic will be used

base on acoutoelastic. Four specimen of mild steel pipe were used, i. e. not welded,

half welded and 2 fully welded. Each specimen was then tested by using panametric,

Epoch III flaw detector. This equipment is used to measure the variation of ultrasonic

wave speed in which represented in form of A-scan graph. In this experiment study,

ultrasonic amplitude is measure and use as parameter. Graph collected from crack

welding specimen will be compared with graph collected with no crack welding

specimen. The different will be analysed to understand the difference between crack

and no crack welding in mild steel pipe.

IV

ABSTRAK

Di dalam proses pembuatan, kualiti sambungan kimpalan boleh diuji dengan

menjalankan beberapa ujian. Sambungan kimpalan ini boleh diuji samaada ujian

musnah ataupun ujian tanpa musnah. Ujian tanpa musnah untuk sambungan

kimpalan terdiri daripada ujian visual, radiografi, partikal magnetik, penembusan

cecair dan ultrasonik. Ujian tanpa musnah ini dilaksanakan tanpa mengubah ciri-ciri

serta permukaan struktur bahan yang diuji. Dalam pembelajaran ujikaji ke atas

spesimen paip keluli ini, cara ujian tanpa musnah ultrasonik akan digunakan

berdasarkan prinsip `acoutoelastic'. Empat specimen paip keluli digunakan, iaitu

tidak dikimpal, dikimpal separuh dan 2 paip keluli dikimpal sepenuhnya. Kemudian,

setiap spesimen diuji dengan menggunakan `Panametric, Epoch III flaw detector'.

Alat ini digunakan untuk menguji kepelbagaian laju gelombang ultrasonik dalam

bentuk graf A-scan. Dalam ujikaji ini, amplitut ultrasonik diukur dan digunakan

sebagai parameter. Graf yang diperolehi dari specimen yang mempunyai keretakan

kimpalan dibandingkan dengan graf yang diperolehi dari spesimen yang tidak

mempunyai keretakan kimpalan. Perbezaan yang diperolehi akan dianalisis dengan

lebih lanjut untuk memahami perbezaan diantara keretakan dan tiada keretakan

kimpalan pada paip keluli.

V

Yusat Khidrnat MaklurDal Akaaernuc UNIVEKSITI MALAYSIA SARAWAK.

TABLE OF CONTENTS

CONTENTS

Dedication

Appreciation

Abstract

Abstrak

Table of Contents

List of Tables

List of Figures

Chapter 1:

1.1

1.2

Introduction

Introduction

Project Objectives

Chapter 2: Literature Review

2.1 Ultrasonic Basic Principle

2.1.1 Beam Spread

2.1.2 Acoustic Impedance

2.1.3 Reflection and Refraction: Mode Conversion

2.1.4 Decibel Notation

2.15 Attenuation

2.2 Fracture Mechanics

PAGE

ii

111

iv

V

vi

ix

X

I

4

5

9

11

II

16

16

17

vi

2.2.1 Modes of Crack Displacement

2.2.2 Linear-Elastic Fracture Mechanics

Chapter 3: Methodology

3.1 Introduction

3.2 Specimen Material for This Experiment Study

3.3 Joint Design for Specimen Material

3.4 Shielded Metal-Arc Welding (SMAW)

3.5 Equipment for Welding Work

3.6 Safety Aspect

3.7 Specimens Material Welding Procedures

3.8 Ultrasonic Testing Instrument

3.9 Calibration

Chapter 4: Result and Discussion

4.1 Result

4.1.1 Specimen 1

4.1.2 Specimen 2

4.1.3 Specimen 3

4.1.4 Specimen 4

4.2 Discussion

Chapter 5: Conclusion and Recommendation

5.1 Conclusion

5.2 Recommendation

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18

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20

21

22

23.

24

25

26

29

29

32

34

36

38

41

42

Vll

References

Appendices

Appendix A

Appendix B

43

viii

LIST OF TABLES

TABLE PAGE

2.1 Typical ultrasonic wave velocities in different

materials, m/s.

4.1 Result of specimen 1

4.2 Result of specimen 2

4.3 Result of specimen 3

4.4 Result of specimen 4

4.5 Result of specimen 1, 2, 3 and 4

6

31

33

35

37

38

ix

LIST OF FIGURES

FIGURE PAGE

2.1 The principle of ultrasonic flaw detection, using a

single wave probe and A-scan on oscilloscope

display screen.

2.2 Ultrasonic wave incident on interface between two

media, 1 and 2 at angel: compressional wave.

2.3 Ultrasonic wave incident on interface between two

media, 1 and 2 at angel: shear wave.

2.4 Typical shear waves probe showing construction and

mode of operation.

2.5 Crack displacement modes. (a) Mode I, (b) Mode II,

(c) Mode III

3.1 Dimension of specimen material.

3.2 Dimension and Single-V butt joint design.

3.3 Welding equipment.

3.4 Structure of welding process.

3.5 Specimen being welded.

3.6 Panametrics Epoch III model 2300 digital ultrasonic

flaw detector.

3.7 Initial setup for calibration.

7

13

13

14

18

20

21

23

24

25

25

27

X

3.8 Tranducer located at the specimen.

4.1 Specimen 1

4.2 28 point location on specimen.

4.3 Graph of point versus amplitude for specimen 1.

28

29

30

30

4.4 Specimen 2 32

4.5 Graph of point versus amplitude for specimen 2.

4.6 Specimen 3

4.7 Graph of point versus amplitude for specimen 3.

4.8 Specimen 4

4.9 Graph of point versus amplitude for specimen 4.

32

34

34

36

36

X1

CHAPTER 1

INTRODUCTION

1.1 Introduction

Identification of damage in a structure is an important research in engineering

communities. Damage present in a structure reduces its in-service capability,

degrades its performance, and even could contribute to the loss of enormous wealth

and human lives. A number of methods have been developed for the identification of

damage in a structure. The available conventional methods for the identification of

damage in a structure are visual inspection, magnetic particle inspection (MPI),

ultrasonic, radiography and alternating current potential difference (ACPD), eddy

current, alternating current field measurement (ACFM) and acoustic emission.

All of these methods require the vicinity of the damage to be known and the

portions of the structure being inspected to be readily accessible. To overcome these

drawbacks, researchers have tried to develop a global damage detection method so

that proper information about the damage of the structure could be easily obtained. In

recent years, ultrasonic-based damage detection technique has been used to give

information about the global behavior of the structure. Numerous research studies

I

have been carried out to establish this method. Sophisticated experimental techniques

also have been developed to enhance crack detection in the high frequency ranges.

Crack commonly occurs in welded structures, bridges, ships, aircraft, land

vehicles, pressure vessel, etc. Several metallurgy investigations indicated that brittle

fractures are resulted from a combination of factor. This cannot be eliminated in

structures because of the interrelation among materials, design, fabrication, and

loading. Although brittle fractures are not as common as fatigue, yielding, or

buckling failures, there are most costly in terms of human life and/or property

damage. These lead to researcher in seeking better failure theories, since the ones

that are available could not adequately explained the observed phenomena.

To cope with these problems, it is import that the fracture behavior of

materials is characterized. Studied and researches of the phenomenon as well as the

properties of material are still being carried out and are going on. In addition,

precautions are to be taken to avoid or minimize damages that have been

recommended, criteria for choice of material and methods for estimation of

durability of stressed components that have been developed.

Hence, periodic structural-safety inspections for crack are required to verify

the validity of structures, as in bridges, aircraft, etc. These inspections can be by X-

ray, ultrasonic, or just visual inspection. When crack are discovered, an engineering

judgment must be made either to repair or replace the flawed part, retire the

assembly, or to continue it in service for a further time subject to more frequent

inspection.

2

To ensure the satisfactory performance of a welded structure, the quality of

the welds must be determined by adequate testing procedures. Therefore, they are

proof tested under conditions that are the same or more severe than those

encountered by the welded structures in the field. These tests reveal weak or

defective sections that can be corrected before the materiel is released for use in the

field. The tests also determine the proper welding design for ordnance equipment and

forestall injury and inconvenience to personnel.

An ultrasonic method is one of Non-Destructive Testing (NDT) technique to

detect flaw in welded joint. Most ultrasonic flaw detection in welds is done by

moving a small probe (the transducer) over the surface of the parent metal adjacent

to the weld hand, and watching a display on an oscilloscope screen. The probe needs

to be `coupled' to the surface of the metal by layer of liquid (water, oil, grease) and

produces a beam of ultrasound that passes into the metal and it reflected back from

any flaw or other discontinuity. The pulse of reflected ultrasound is then picked up

by the same probe, which acts as both transmitter and receiver (transceiver). This is

possible because of the pulsed nature of the signals. This is the `pulse echo'

technique and its success depends upon an accurate knowledge of the beam direction,

its size and the physical principles involved.

3

1.2 Project Objectives

The objective for this project is to determine the crack in welded joint by

using Non-Destructive Testing (NDT) technique. The result is then analyzed and

correlate with the crack in mild steel. The entire data collect from crack welding will

be compared with the data collected from the non-crack welding. The differences

will be analysed to understand between the crack and the non-crack welding in mild

steel. Finally it is hope the ultrasonic can be used through this project to determine

correlation of crack detection and ultrasonic wave speed as an inspection tool.

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CHAPTER 2

LITERATURE REVIEW

2.1 Ultrasonic Basic Principles

As the name implies, ultrasonic waves are mechanical vibrations having the

same characteristics as sound waves, but having a frequency so high that they cannot

be detected by the human ear, i. e. greater than about 20 kHz. For weld examination

in metals the ultrasonic waves usually have a frequency in the range 500 kHz to 10

MHz, most applications using a frequency between 2 and 5MHz. It is important to

realize that ultrasonic waves are not electromagnetic radiation passing through the

specimen, but are the result of induced particle vibration in the specimen, and are

possible because of the elastic properties of the material of the specimen. For this

reason, the wave velocity is different in different materials (Table 2.1). From the

basic velocity law:

V = (X) (n) (2.1)

Where:

V= Velocity of waves

X = wavelength

n = frequency

5

Table 2.1: Typical ultrasonic wave velocities in different materials, m/s.

Material Compressional Shear Surface (Rayleigh)

Steel 5.9 x 10 3.2 x 103 3 x 101 Aluminum 6.2 x 103 3.1 x 103 2.8 x 103Water (20°C) 1.5 x 103 -Perspex 2.7 x 103 1.1 X 103 1.0 X 103 Oil 1.2 x 103 - -

There are several types of ultrasonic wave, the most important being

compressional (longitudinal), shear (transverse) and surface waves. In compressional

waves the particles of the transmitting material move in the direction of wave

propagation, and these waves can be transmitted in solids and liquids. In shear waves

the particles of the material vibrate at right angles to the direction of travel of the

waves, and shear waves cannot be propagated in liquids. They have a velocity

approximately half that of compressional waves in the same material.

The particle movement is very small, within the elastic limits of the material,

so that there is no change in the specimen due to the ultrasonic energy. There are

several different types of surface wave. Rayleigh waves can be propagated and are

somewhat analogous to water waves, in which the motion of the particles is both

transverse and longitudinal in a plane containing the direction of propagation and the

normal to the surface. If the specimen thickness is comparable to the wavelength, a

type of surface wave known as a Lamb wave can be generated, and as the velocity of

propagation is a function of plate thickness and frequency there can be an infinite

number of Lamb wave modes. Waves which travel on the surface without any

vertical component are known as Love waves. Rayleigh, Lamb and Love waves are

6

used for special applications of ultrasonic testing, but by far the most important types

of wave for ultrasonic flaw detection in welds are compressional and shear waves.

Nearly all methods of ultrasonic flaw detection use the pulse echo technique

in which a short ultrasonic pulse is propagated from a transmitter probe, through a

coupling medium, into the material under test. During its travel this pulse is partially

reflected from any discontinuities in its path and the 'echoes' produced are picked up

by a receiver probe, which may be the transmitter probe itself (used as a transceiver)

or a separate probe. It is quite feasible and common for the transmitter probe to act as

a receiver between successive transmission pulses. Any surface where there is a

change in elastic properties (cavity, inclusion and segregation) can act as a reflecting

surface and give rise to an echo. The echo from the discontinuity provides

information about its position and size, the usual method of information presentation

being the so-called 'A scan' on an oscilloscope display screen.

Probe on specimen Y Cathode ray screen

Figure 2.1: The principle of ultrasonic flaw detection, using a single wave probe and A-scan on oscilloscope display screen.

7

Figure 2.1 is shown a schematic illustration of an ultrasonic probe generating

a compressional wave. The probe is placed on a flat plate and coupled to the surface

by a thin film of grease. When the probe is pulsed, the electronic circuitry produces

an indication on the screen at X; the circuit then measures the time the pulse takes to

travel through the specimen to the bottom and back and produces a screen indication

at Y. If some of the ultrasonic beam strikes a cavity, it obviously reflects back to the

probe in a shorter time, and a corresponding indication is produced on the screen at

Z. If the electronic circuitry has a linear time-base, distance XZ is a measure of the

distance of the cavity from the probe, and the height of the indication can be used as

some measure of the size of the cavity.

Piezoelectric crystals are almost universally used as the transducer element in

ultrasonic probes. This is a material which, when given an electrical pulse, changes

its thickness, i. e. it vibrates and produces ultrasonic waves (the transmission mode).

Conversely, when it is caused to vibrate by an incident ultrasonic wave, it produces

an electrical pulse (the receiver mode). Piezoelectric materials are used in the form of

thin discs with metallised surfaces, the thickness of the disc being related to the

natural vibration frequency. In ultrasonic flaw detection probe the piezoelectric disc

is usually damped either mechanically or electrically, as it is desirable to have a very

short train of ultrasonic waves in each pulse. Thus, the probe emits a wave packet

which has a dominant frequency equal to the natural frequency of the crystal, but

with a rapidly decaying train of vibrations.

There are some natural piezoelectric materials such as quartz, but most

modern probes use synthetic materials such as lead zirconate titanate (PZT) or lead

8

niobate, which have superior and more reproducible properties. One problem for

many years in ultrasonic testing has been the variability of performance of ultrasonic

probes of the same nominal construction, and some large inspection authorities have

produced detailed acceptance codes for probe performance. 1-2 If ultrasonic flaw

detection is to be executed with a high degree of reliability the operator must have a

precise knowledge of the performance of the equipment, and the equipment must be

accurately calibrated.

There are other types of ultrasonic transducer for special applications.

Piezoelectric plastic film transducers (electrets) made from PVDF can be shaped to

fit curved surfaces for focusing purposes; they are mostly used for high frequency

applications. Composite transducers, in which a PZT element is embedded into

plastic resin, can produce much larger outputs with lower noise levels, for use on

strongly attenuating materials. Transducer arrays and EMATS

(electromagnetic-acoustic transducers) will be mentioned later. Laser transducers, in

which the laser pulses produce thermal shock vibrations which transform into

mechanical vibrations, are used for non-contact applications (e. g. hot or soft

surfaces).

2.1.1 Beam Spread

Although, because of their short wavelength, ultrasonic waves travel

essentially in a straight line, there is always some spread of the beam. The angle of

spread, 0, is given by:

9

0 1.22?, sin - -

2 D

Where:

A = ultrasonic wavelength

D = diameter

(2.2)

Thus, a higher frequency probe produces less beam spread. An ultrasonic

beam from a probe is usually described as having a 'near zone' (or Fresnel region)

and a 'far zone' (or Fraunhofer region). The near zone is an approximately

parallel-sided beam and its length, N, is given by:

-Dz N 4k

Where:

N = length

it = ultrasonic wavelength

D = diameter

(2.3)

In this region there are marked variations of maximum and minimum

ultrasonic intensities which can cause serious problems in flaw size estimation if the

flaw is in this region of the beam. In the far zone the beam diverges and the

ultrasonic intensity decreases according to the inverse square law, and it is to this

region that equation 2.2 for beam width applies.

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2.1.2 Acoustic Impedance

When an ultrasonic wave strikes an interface between two media, at right

angles, some of the energy is reflected and some transmitted.

E - 4Z1 Z2 E. EI

IZi

+z2 2 .

Z, Z2 Z, +Z2

2

"E r

Where:

E; = incident energy

E, = transmitted energy

E, = reflected energy

Z = acoustic impedances

2.1.3 Reflection and Refraction: Mode Conversion

(2.4)

(2.5)

When an ultrasonic beam strikes an interface between two different materials

at any angle but normal it can produce both reflected and refracted compressional

and shears waves. The two cases of an incident compressional wave (Figure 2.2) and

an incident shear wave (Figure 2.3) are shown. Simple relationships describe the

angles and velocities of these various waves, the general equation being known as

Snell's Law. In Figure 2.2:

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sin a sin b sin c sin dVL

I Vsi VL2 Vs2

Where:

VL1= longitudinal (compressional) wave velocity in material 1,

VL2 = compressional wave velocity in material 2,

VSj = shear wave velocity in material 1 and

Vs2 = shear wave velocity in material 2.

(2.6)

Exactly the same relationships apply to an incident shear wave (Fig. 32) and can

applied analogously for a wave traveling from medium 2 to medium 1.

sin a sin b sin c sin dVsi VLI VL2 Vsz

Where:

VL, = longitudinal (compressional) wave velocity in material 1,

VL2 = compressional wave velocity in material 2,

Vs, = shear wave velocity in material I and

VS2 = shear wave velocity in material 2.

(2.7)

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Figure 2.2: Ultrasonic wave incident on interface between two media, I and 2 at angel: compressional wave.

Figure 2.3: Ultrasonic wave incident on interface between two media, I and 2 at angel: shear wave.

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