Ndt Report

8/13/2019 Ndt Report

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Idaho Dakota State University Krage

Pocatello ID, 83201 Bengal ID# 827943

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NE 5599

Advanced Material Science

Non-Destructive Testing Review

Written by: Eric S. Krage

Instructors: Dr. Maria Okuniewski & Dr. Sebastien Teysseyre


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Abstract

Non Destructive testing is a set of analysis techniques used by industry and scientists to evaluate

and quantify material properties. The use of acoustic emission is a passive technique that takes

advantage of waves generated in a material by a microstructural change. The analysis of thegenerated waveform provides information about what types of changes are occurring; and by the

use of multiple sensors the location of the event can be quantified. There are multiple different

types of wave that are able to be generated but not limited to longitudinal, shear, and surfacewaves. Although, not all waves are supported in all types of media such as a longitudinal wave is

the only type of wave supported in a liquid. While all types of waves are supported in a

continuous solid. The use of surface waves is a way to actively detect surface defects in a

material. Surface waves can be guided along at a prescribed depth and subsequently received bya receiver. The effect of a rubber wedge in contact with a metal surface was evaluated close to

the transducer and receiver and was found to have different effects on the signal attenuation.

Surface waves were also used to show that they can be generated on the more complex geometry

of a curved surface and still resolve a surface inclusion. The future of non-destructive testing isvast and has the ability to actively and passively monitor for stress corrosion cracking events that

are prevalent in the nuclear industry.


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Background

Non-destructive Testing (NDT) is a wide group of analysis techniques used in science research

and industry to evaluate the properties of a material, component or system without causing

damage.1Because NDT does not permanently alter the article being inspected, it is a highly

valuable technique that can save both money and time in product evaluation, troubleshooting and

research. Common nondestructive testing methods include ultrasonic, magnetic-particle, liquid

penetrant, radiographic, and eddy-current testing. The majority of these techniques can be used

actively to scan and test the material as well as passively evaluate a material under testingconditions. The focus of this work is to implement active and passive ultrasonic testing to search

for stress corrosion cracking events.

The ultrasonic testing method was developed by Dr. Floyd Firestone between 1940 and 1944.

His original patent pertained to a device for detecting the presence of inhomogeneities of densityor elasticity in materials. His device allowed the presence of a flaw to be detected and its position

located, even though the flaw lies entirely within a casting and no portion of it extends out of the

surface. The general principal consisted of sending high frequency vibrations in the part to beinspected, and determination of the time intervals of arrival of the direct and reflected vibrations

at one or more stations on the surface of the part.2

Acoustic emission is the sound waves produce when a material undergoes stress, as a result of an

external force applied. The ultrasonic elastic waves generated from such an event occur because

of a small surface displacement of a material produced due to stress waves. The subsequent wave

generated by the source is of practical interest in methods used to simulate and capture acousticemissions in a controlled fashion, for study and or use in inspections, quality control, system

feedback, and process monitoring.

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Stress corrosion cracking (SCC) is defined as the growth and generation of cracks in a corrosiveenvironment. Stress corrosion cracking can lead too sudden failure of normally ductile metals

subjected to a tensile stress, especially at elevated temperature in the case of metals. This failure

mechanism is caused by the simultaneous contribution of three factors. The first factor is the

environment in which the material is placed such as a chlorinated or ammonia bearingenvironment. The second contribution to stress corrosion cracking is the susceptible material.

The most prominent material is alloyed materials when compared to pure metals with their

natural impurities because of their electronic configuration.5 The third contribution to stress

corrosion cracking is the tensile stress applied to the material. The stresses present in the material

can be the result of crevice loads due to stress concentration, or can be cause by the type ofassembly which may result in residual stress, this residual stress then can be subsequently

reduced by annealing the material.


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Theoretical Information

Acoustic Emission

As previously discussed an acoustic emission can result from deformation of a material under

stress or strain. Crack growth, slip and dislocations movements as well as twinning and phase

transformations in metals can generate an acoustic event. When a stress is impinged on amaterial, a strain is subsequently introduced into the material as well. The magnitude of thestress applied and the property of the material, the object may return to its original dimensions or

be permanently deformed after the stress is removed. These two types of conditions are known as

elastic and plastic deformation, respectively. Other effects during subsequent plasticdeformations can change the threshold to which an acoustic event will be generated. The most

evident acoustic emissions take place when loaded material undergoes plastic deformation or

when a material is loaded at or near its yield stress. The yield stress is defined by the amount of

stress applied to a material that causes a permanent deformation of the material. In observation ofplastic deformation at the microscopic level atomic planes slip past one another through the

movement of the dislocations. The atomic-scale deformations release energy in the form of

elastic waves which are essentially naturally generated ultrasonic waves that travel through theobject. When cracks exist in a crystalline material, the stress levels present in front of the cracktip can be several times higher than the surrounding area. This difference will cause acoustic

emission activity to be observed when the material of the crack tip undergoes plastic deformation

otherwise thought of as micro-yielding. Two sources of fatigue cracks also cause acousticemissions. The first is emissive particles such as non-metallic inclusions at the origin of the crack

tip. Since these particles are less ductile than the surrounding material, they tend to break more

readily when the material is strained which results in an acoustic emission signal. The secondsource is the propagation of the crack tip that occurs through the movement of dislocations and

small-scale cleavage produced by triaxial stresses. The energy released by an acoustic emission

and the amplitude of the resolved waveform are directly related to the magnitude and velocity of

the source event. The amplitude of the emission is proportional to the velocity of crackpropagations and amount of surface area created. Large, discrete crack jumps will produce large

acoustic emission signals than cracks that propagate slowly over the same distance. The

detection and conversion of these elastic waves to electrical signals that can be analyzed is thecomplicated part of the acoustic process. In depth analysis of the signals produced yield

information regarding the origin and importance of a discontinuity in a material.

Acoustic emission events can be generated by different loading situations and can provide

information regarding the structural integrity of the material under evaluation. Loading levels

that have been previously observed by the material do not produce acoustic emission activity.Therefore, the discontinuities created in a material do not expand or move until the previous

stress has been exceeded by the subsequent stress known as the Kaiser effect. The Kaiser effectcan be seen in the load versus acoustic emission plot shown in the figure below. As loading is

applied on the material acoustic emission events occur in a linear fashion represented in thefigure by segment AB. The segment shown in C represents loading being removed and reapplied

which exhibits no effect until the previous loading potential is exceeded. As the load applied to

the material is increased from the region of BD acoustic events are generated and stop when theload is removed. However there is an exception to the rule at which can be seen at F where the

load is high enough to cause significant emissions even though the previous maximum load at


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point D had been reached called the Felicity effect. The Felicity effect is used in terms of the

Felicity ration which is described as the load where a considerable increase in acoustic emissions

resume divided by the maximum applied load (F/D). Using information gained from the Felicityand Kaiser Effect during material testing and evaluation can be used to determine if significant

material defects are present. To measure a constant load is applied to the material with respect to

the specific material design and application and continuously search for acoustic emissionevents. The acoustic emissions continue when the prescribed loading tension is held as shown bysection GH of the curve. In this region it can be assumed that substantial structural defects are

present in the material. The reapplication of such a load may cause critical failures of the

material.

Figure 1: Loading Potential vs. Acoustic Emission Event Rate.

Some known issues with acoustic emission testing for stress corrosion cracking evaluation is that

it is a passive measurement. So, in other words it is not useful to actively look for a crack that

has already started but not currently propagating. This leads to issues with implementation in realworld settings that were not originally fitted with acoustic emission listening devices. Other suchshortfalls occur when hydrogen embrittlement occur in the material as it causes poor support for

waves to propagate through the medium. This occurs because the crystal structure breaks down

and causes the shear modulus to change in such a way that significant dampening of the acousticevent occurs in the material. Other known problems that occur are noise from external sources.

As with most testing the signal to noise ratio dominates the performance of the device and as the

ratio increases the confidence in the value measured increases. These undesirable signals


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detected by the sensors include; frictions sources (loose bolts or moving parts), impact sources

(falling objects or particulate fluid flow). Other mechanical vibrations from external components

of the system such as motors and fans may be seen in the acoustic background. To mitigate theeffects of background noise electronics improvements can be made to the sensor signal

processing. The addition of coincidence circuits that are in coincidence with a noise source along

with signal decomposition to eliminate elements of them are some of the more complex ways toanalyze the signals. The simplest method to reduce noise when searching for acoustic emissionevents is to move the sensors as far away from the sources as possible.

One of the key advantages of acoustic emission testing over other nondestructive test methods is

the fact that it detects signals in real time that are emanating from the materials themselves.

Upon fracture due to the loading potential an acoustic emission stress wave travels from the

source through the structure, and can be detected by an acoustic emission senor placed upon thestructure. The sensors location may be located near or far from the source and still detect the

signal after taking into account the possible self-attenuation in the sample medium. If multiple

sensors are placed on the same structure, one may be able to determine the location of the source

by analyzing the time difference of arrivals from multiple sensors that have been in place andsolving the triangulation calculations.

Determining the location in one dimension described by a line a minimum of two sensor arrivals

are required to pinpoint the signals origin. While in two dimensions that can be described as a

plane and or surface a minimum of three sensors are required. The three dimensional case require

a signal reaching a minimum of four sensors to be analyzed. The severity of an event can beanalyzed also by monitoring multiple acoustic emission events in the same area and applying a

clustered layout of detectors. The advantage to this is large structures can be monitored with a

minimal number of acoustic sensors. This is advantages when used in vessels, especially whenthey are insulated, since few access holes are needed for placement of acoustic emission sensors

to determine the structural integrity of the vessel in question. Other nondestructive testing

techniques require the complete removal of all the insulation and material around the

containment vessel to complete inspections which requires more time to complete whileincreasing operating costs.

Active Ultrasonic Testing

Wave Propagation FundamentalsIn order to describe waves traveling through media we must first describe the fundamental

propagation of a wave on a string. These waves are governed by the wave equation for wavepropagation in a taught spring expressed in Equation 1. The equation of motion in the direction

of the string is a second-order hyperbolic partial differential equation (PDE) that ishomogeneous. This one dimensional, homogeneous, simple wave equation can solve for the

velocity of wave propagation.

Use of Newtons second law and assuming small deflections in the string describe the following.


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( )

Which allows us to solve for cowhich is the velocity of wave propagation?

u = displacement, F = Tension, q(x, t) = body force or external loading per unit length, and =

mass density / length

Understanding how a wave travels across a taunt string we can apply this information to more

complex systems. This development of more complex wave propagation will be these equationsof motion for isotropic media are defined by the Naviers governing equations.

Longitudinal Wave Velocity

In order to examine the longitudinal velocity of steel block a 3.5MHz 0.25 diameter.Longitudinal transducer was used. The pulsar/receiver mode was used and set to 3.5MHz tocorrespond to the signal required by the transducer applying no filter. Using the following

equation the longitudinal wave velocity was found to be 6012 m/s where d = 15.40mm.

Shear Wave VelocityNow the need arises to experimentally measure and calculate the velocity in a steel specimen

using the shear wave transducer 5MHz 0.25 dia. The signal was sent and measured using thepulsar/receiver mode with a nominal velocity of 3306 m/s as calculated by the followingequation.

Surface Wave VelocityThe surface wave velocity was determined experimentally implementing a 0.5 * 1.0 1MHz

angle beam transducer used on pitch and catch mode. The pitch and catch mode causes a shortburst of acoustic energy in the sample and waits for a response back until it sends another signal.

The signal was received by the longitudinal transducer used in the longitudinal wave velocity

measurements. The second transducer was inserted into an angle block to act as a second anglebeam transducer to receive the signal. The angle beam transducer was set to generate surface

waves and the time between the initial pulse and the received pulse was recorded using the

computer display. Using the following equations we calculated the surface wave velocity andcritical angle.6

Poissons ratio is found using the following equation.

We can now rearrange the equation to solve for Poissons ratio.


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Now we need solve for using Viktorovs approximation which is less than 1% as shown in

homework #3. We now will calculate for the velocity of the surface wave using the following equation.

We now need to solve for the critical angle in which we generate surface waves.

( )To experimentally measure the velocity of the surface wave the angle of the angle beam

transducer was set to that of the critical angle for surface wave generation. Figure 2 exhibits theexperimental procedure and allows for us to utilize two transducers one a transmitter and the

other a receiver.7

Figure 2: Surface Wave Velocity Measurement Setup6

Current Research and Uses

Acoustic Emission

Currently acoustic emission nondestructive evaluation is used for crack detection, weld analysis,

vessel inspection, leak detection, and nuclear lift rig examination. Crack detection is one of the

most successful applications and has been accepted and used the longest; with the ability todetect any type of cracking in various materials during bonding, forming, or working the material

in some way. When a crack occurs, the acoustic emission system provides a failure output for


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part rejections. Many acoustic emission detection systems currently operating provide continual

real time monitoring. Acoustic emission has also been successfully applied to welding

applications for many years. Acoustic emission systems have addressed solutions to real timeweld quality, analysis, and weld control, including resistance spot welding. Every burst of an

acoustic emission received from the welding process has a logical source and meaning. By

focusing in on the expected occurrence of given weld phenomena through time, frequency andfeature discrimination. Utilizing this feedback the welding process can be modified to meet therequired or requesting weld requirements. Acoustic emission has been able to detect oxidation

burn off, nugget formation, hot cracking, expulsion, and post weld tracing. All of these

phenomena are very important in weld quality analysis and feedback control for decision makingin stopping the weld at the appropriate time.

14,15The welding acoustic emission setups are

commonly used in industry in conjunction with robotic welders to be able to reduce errors and

further manage the quality control and quality assurance of the facilities. Another very successful

application of acoustic emission testing is in vessel inspection for the chemical industry in bothmetal and composite vessels. Vessel testing is performed every day by many different service

companies around the world. Sensors are placed on the vessel in an array to monitor the entire

pressure boundary. The vessel is then subjected to pressures typically 10 percent above previousoperating levels still below the operating temperature. The test pressures the vessel is being

subjected to be done in an increase pressure hold increase and hold again to the peak stress level,

while simultaneously monitoring the acoustic emission activity during each of these

pressurization segments. This method is the most cost effective and sensitive way to testcontainment vessels and adheres to the standards set forth by the American Society of Testing

and Materials and the American Society of Mechanical Engineers.10

Using acoustic emission instrumentation acoustic events may be detected from turbulent or

cavitational flow through a crack, valve, seal or hole. The energy is transmitted as a longitudinal

wave through the fluid, and through the air or the structure to the piezo electric sensor mountedon the material. The signal is subsequently processed filtered and compared to a leak profile then

located using triangulation techniques. Existing installations include monitoring of pipelines in

nuclear and chemical plants as wells as boilers vessels and through valves. These systems offer

the capability to connect and monitor multiple sensors throughout a plant the systems can beoperated in a standalone mode, interfaced to programmable controllers or tied into plant-wide

distribution control systems. They also offer ability to plot plant piping and overlay acoustic

event information on top of them to improve repair time.12

One of the most significant nuclear applications is the use of acoustic emission testing on nuclearlift rig examinations in pressure water reactors in containment lift rigs that are utilized during

refueling and inspection outages. The internal lift rig is often contaminated so that exposure time

to the radioactive material must be minimized which gives rise to an advantage over active forms

of testing. The reduction of potential hazard is evident in the time spent in contact with the rig tojust place the sensors or to actually physically make measurements in the presence of the

potential contamination.


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Using this fundamental idea it is possible to actively search for defects between the transducer

and receiver and the setup previously discussed. Since the waves are traveling on the surface to

observe an interruption a rubber dampening wedge was placed on the sample block. The wedgewas placed in two locations on the sample block one close to the transmitter and the other close

to the receiver. A decrease was noticed in the received amplitude of the signal in both cases.

However, a bigger decrease was observed when the wedge was placed closer to the receiver asdepicted in Figure 3 and 4 which can be prepared to observe this phenomenon we can use Figure4 as the control image. This phenomenon can be explained that the waves have not fully

propagate into the surface traveling as deeper Raleigh waves and thus are more easily attenuated

at the transmitter side of the apparatus.

Figure 3: Wedge near the transmitter note the amplitude ~ 40


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Figure 4: Wedge near the receiver note the amplitude ~ 60

Understanding this phenomenon of surface waves between two transducers allows its applicationin more complex geometries. That can be used for finding defects or obstructions on plate

surfaces by scanning between the two surfaces. Or on a cylinder that is representative of a pipe.

Experimental Parameters

Sample DescriptionThe test specimen used was a curved aluminum cylinder, shown in Figure 5. No quantitative

calculations were done for this specimen; it was used solely to test if we could see surface waveson a curved surface refracted by the flaw.


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Figure 5: Curved aluminum block contains a surface flaw

Using the Olympus Omni Scan MX the shear, longitudinal, and adjustable angle wave

transducers were used at 5MHz, 3.5MHz, and 1MHz respectively. The angle beam transducerwas placed at the critical angle for the propagation of surface waves calculated in the theory

section.

Generating Surface Waves on a Curved Surface

Using the longitudinal beam transducer with the wedge placed against the curved surface to see

if a surface wave is able to be generated on a curved object with a flaw in it and reflected back.Figure 6 shows the reflection of the surface wave back from the defect. The distance from the

surface wave to the minor crack is proportional to the time from the main bang to the start of the

second reflection that partially overlaps the initial pulse. So in conclusion we are able to generate

surface waves along a curved surface just as we can generate they can be generated in a more

simple situation such as a plate.

Figure 6: Surface Wave on Curved Surface with a Surface Defect Reflection


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Conclusions

In conclusion it can be ascertained that the use of ultrasonic waves is a good method for

detection of stress corrosion cracking events both passively and actively. The acoustic emission

events generated by a cracking event are able to be quantitatively identified and located. Thistype of application has been prominently used in the petrochemical industry and many specific

material defects and stress corrosion cracking events have been identified; although, the nuclear

industry is behind in this regard. Future research needs to focus on improving the technology forhydrogen bubbles that cause a loss of signal in a material as a specific example. More recent

studies show that by further analysis of the signal received from the acoustic emission event is

allowing the ability to detect hydrogen penetration inside metallic structures.13

Conclusions of Wedge Touch Test

Since the waves are traveling on the surface to observe an interruption a wedge was placed on

the sample block. The wedge was subsequently placed in two locations on the sample block oneclose to the transmitter and the other close to the receiver. A noticeable decrease in the received

amplitude of the signal in both cases was observed. However, a bigger decrease is observedwhen the wedge was placed closer to the receiver as depicted in Figure 3 and 4. Thisphenomenon can be explained in the sense that the waves have not fully propagated into the

surface traveling as deeper Raleigh waves and thus, are more easily attenuated at the transmitter

side of the apparatus.

Detecting Surface Defects Using Surface Waves

High frequency waves were used to locate surface and subsurface defects in the metal material.Ultrasonic testing is superior to many other methods for testing for flaws near the surface of a

material, however, in order to take advantage of the ultrasonic testing, the surface must be

accessible to probe and a couplant.

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In the experiment, this wasn't a problem, due to having asmall material that could be easily manipulated, however, in some instances direct access to thesurface may not be so accessible and another method should be used. The surface waves move

in an elliptical pattern, and span a wide frequency range. When applying this to a real life

application such as the pipe segment depicted in Figure 5 it can be shown that these types of

surface waves may be generated on multiple geometries.


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References

1 Cartz, Louis (1995).Nondestructive Testing. A S M International.ISBN978-0-87170-517-4.

2 U.S. Patent 3,260,105 for Ultrasonic Testing Apparatus and Method to James F. McNulty

3 Sotirios J. Vahaviolos (1999).Acoustic Emission: Standards and Technology Update. STP-1353. Philadelphia, PA:ASTMInternational(publishing).ISBN0-8031-2498-8.

4 ASM International,Metals Handbook (Desk Edition), American Society for Metals, (1997)

5 Metals Handbook, 10th Ed., v.2, ASM Handbook Committee, American Society for Metals, USA,1990.

6 Rose, Joseph L. Ultrasonic Waves in Solid Media. Cambridge [ u.a.: Cambridge Univ. 2004]

7 D. E. Chimenti "Guided waves in plates and their use in materials characterization", Appl. Mech. Rev.,

vol. 50, no. 5, pp.247 -284 1997

8 I. A. Viktorov Rayleigh and Lamb Waves: Physical Theory and Applications, 1967 :Plenum

9 P. Fromme and M. B. Sayir "Measurement of the scattering of a Lamb wave by a through hole in a

plate", J. Acoust. Soc. Amer., vol. 111, no. 3, pp.1165 -1170 2002

10 NDT Education Resource Center, 2001-2012, The Collaboration for NDT Education, Iowa StateUniversity,www.ndt-ed.org

11. S. Q. Qang, D. K. Zhang, D. G. Wang , L. M. Xul and S. R. Ge, Int J. Electrochemical Science, 72012

12 Mazille H and Rotha R, The use of acoustic emission for the study and monitoring of localized

corrosion phenomena. In: K.R. Tretheway, P.R. Roberge, Modelling aqueous corrosion, Kluwer

Academic Publishers, Netherlands, (1994).

13. Djeddi, R. Khelif, International Journal of Electrochemical Science, 8 2013

14 D. Fang and A. Berkovits, "Fatigue Design Model Based on Damage Mechanisms Revealed byAcoustic Emission Measurements," Trans. of ASME, 117 (1995)

15 A. Berkovits and D. Fang, "Study of Fatigue Crack Characteristics by Acoustic Emission," Eng.Fracture Mech., 51 (3) (1995
http://en.wikipedia.org/wiki/International_Standard_Book_Numberhttp://en.wikipedia.org/wiki/International_Standard_Book_Numberhttp://en.wikipedia.org/wiki/Special:BookSources/978-0-87170-517-4http://en.wikipedia.org/wiki/Special:BookSources/978-0-87170-517-4http://en.wikipedia.org/wiki/Special:BookSources/978-0-87170-517-4http://en.wikipedia.org/wiki/ASTM_Internationalhttp://en.wikipedia.org/wiki/ASTM_Internationalhttp://en.wikipedia.org/wiki/ASTM_Internationalhttp://en.wikipedia.org/wiki/ASTM_Internationalhttp://en.wikipedia.org/wiki/International_Standard_Book_Numberhttp://en.wikipedia.org/wiki/International_Standard_Book_Numberhttp://en.wikipedia.org/wiki/Special:BookSources/0-8031-2498-8http://en.wikipedia.org/wiki/Special:BookSources/0-8031-2498-8http://en.wikipedia.org/wiki/Special:BookSources/0-8031-2498-8http://www.ndt-ed.org/http://www.ndt-ed.org/http://www.ndt-ed.org/http://www.ndt-ed.org/http://en.wikipedia.org/wiki/Special:BookSources/0-8031-2498-8http://en.wikipedia.org/wiki/International_Standard_Book_Numberhttp://en.wikipedia.org/wiki/ASTM_Internationalhttp://en.wikipedia.org/wiki/ASTM_Internationalhttp://en.wikipedia.org/wiki/Special:BookSources/978-0-87170-517-4http://en.wikipedia.org/wiki/International_Standard_Book_Number

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