An Introduction to AET Reading 2004-01

7/25/2019 An Introduction to AET Reading 2004-01

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An Introduction to

Acoustic Emission Testing, AET

2014-JuneFacilitators: Fion Zhang/ Charliechong


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http://wins-ndt.com/oil-chem/spherical-tanks/


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http://www.smt.sandvik.com/en/search/?q=stress+corrosion+cracking


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Speaker: Fion Zhang2014/6/13


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Contents:

1. AE Codes and Standards ASTM

ASME V

2. Reading 01,

3. Reading 02,4. Reading 03,

5. Others reading.


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ASME V Article Numbers:

Gen Article 1RT Article 2

Nil Article 3

UT Article 4 for welds

UT Article 5 for materialsPT Article 6

MT Article 7

ET Article 8

Visual Article 9

LT Article 10

AE Article 11 (FRP) /Article 12 (Metallic) / Article 13 (Continuous)

Qualif. Article 14

ACFM Article 15


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ASTM Standards

1. ASTM E 1930 Standard Practice for Examination of Liquid-FilledAtmospheric and Low-Pressure Metal Storage Tanks Using Acoustic

Emission

2. ASTM E 569 Standard Practice for Acoustic Emission Monitoring of

Structures During Controlled Stimulation3. ASTM E 749-96 is a standard practice of AE monitoring of continuous

welding

4. ASTM F914 governs the procedures for examining insulated aerial

personnel devices.

5. ASTM E 1932 for the AE examination of small parts,

6. ASTM E1419-00 for the method of examining seamless, gas-filled,

pressure vessels.


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Others Reading

http://www.globalspec.com/reference/63985/203279/Chapter-10-Acoustic-Emission-Testing

http://www.corrosionsource.com/(S(vf34kqncr0uklwzu0ioy5dz2))/FreeContent/3/Combatting+Liq

uid+Metal+Attack+by+Mercury+in+Ethylene+and+Cryogenic+Gas+PlantsTask+1+-+Non-

Destructive+Testing

http://www.ndt.net/ndtaz/index.php?id=2


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Typical AET Signal

https://dspace.lib.cranfield.ac.uk/bitstream/1826/2196/1/Acoustic%20Emission%20Waveform%20Changes%202006.pdf


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Typical AET Signal


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Study Note 1:

http://www.geocities.ws/raobpc/AET.html


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What is AE

Acoustic emission is the technical term for the noise emitted by materials andstructures when they are subjected to stress. Types of stresses can be

mechanical, thermal or chemical. This emission is caused by the rapid

release of energy within a material due to events such as crack initiation and

growth, crack opening and closure, dislocation movement, twinning, and

phase transformation in monolithic materials and fiber breakage and fiber-

matrix debonding in composites.

The subsequent extension occurring under an applied stress generates

transient elastic waves which propagate through the solid to the surfacewhere they can be detected by one or more sensors. The sensor is a

transducer that converts the mechanical wave into an electrical signal. In this

way information about the existence and location of possible sources is

obtained. Acoustic emission may be described as the "sound" emanatingfrom regions of localized deformation within a material.


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Until about 1973, acoustic emission technology was primarily employed in the

non-destructive testing of such structures as pipelines, heat exchangers,

storage tanks, pressure vessels, and coolant circuits of nuclear reactor plants.However, this technique was soon applied to the detection of defects in

rotating equipment bearings.


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Acoustic Emission

Acoustic Emission (AE) refers to generation of transient elastic waves

during rapid release of energy from localized sources within a material.

The source of these emissions in metals is closely associated with the

dislocation movement accompanying plastic deformation and with the

initiation and extension of cracks in a structure under stress.,

/()..

Other sources of AE are: melting, phase transformation, thermal stresses,

cool down cracking and stress build up, twinning, fiber breakage and fiber-matrix debonding in composites.

:

,,,,-

http://www.geocities.ws/raobpc/AET.html


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AE Technique

The AE technique (AET) is based on the detection and conversion of high

frequency elastic waves emanating from the source to electrical signals. This

is accomplished by directly coupling piezoelectric transducers on the surface

of the structure under test and loading the structure. The output of the

piezoelectric sensors (during stimulus) is amplified through a low-noise

preamplifier, filtered to remove any extraneous noise and further processedby suitable electronics. AET can non-destructively predict early failure of

structures. Further, a whole structure can be monitored from a few locations

and while the structure is in operation. AET is widely used in industries for

detection of faults or leakage in pressure vessels, tanks, and piping systemsand also for on-line monitoring welding and corrosion. The difference

between AET and other non-destructive testing (NDT) techniques is that AET

detects activities inside materials, while other techniques attempt to examine

the internal structures of materials by sending and receiving some form ofenergy.


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Types of AET

Acoustic emissions are broadly classified into two major types namely;

continuous type and

burst type.

The waveform of continuous type AE signal is similar to Gaussian random

noise, but the amplitude varies with acoustic emission activity. In metalsand alloys, this form of emission is considered to be associated with the

motion of dislocations. Burst type emissions are short duration pulses and

are associated with discrete release of high amplitude strain energy. In

metals, the burst type emissions are generated by twinning, micro yielding,development of cracks.

Continuos type (Gaussian random noise) Motion of dislocation,

Burst type (discrete high amplitude strain energy)

twinning, microyielding, development of cracks


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AET Set-up


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Continuous type- Gaussian random noise


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Continuous type


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Discrete Burst Type


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Discrete Burst Type


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Kaiser Effect

Plastic deformation is the primary source of AE in loaded metallic structures.

An important feature affecting the AE during deformation of a material isKaiser Effect, which states that additional AE occurs only when the stress

level exceeds previous stress level. A similar effect for composites is termed

as 'Falicity effect'.

Key words:

Kaiser effect

Falicity effect


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Kaiser Effect- which states that additional AE occurs only when the stress

level exceeds previous stress level. A similar effect for composites is termed

as 'Falicity effect'.

http://www.ndt.net/ndtaz/content.php?id=476


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AE Parameters

Various parameters used in AET include: AE burst, threshold, ring down

count, cumulative counts, event duration, peak amplitude, rise time, energyand rms voltage etc. Typical AE system consists of signal detection,

amplification & enhancement, data acquisition, processing and analysis units.


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Sensors / Source Location Identification

The most commonly used sensors are resonance type piezoelectric

transducers with proper couplants. In some applications where sensorscannot be fixed directly, waveguides are used. Sensors are calibrated for

frequency response and sensitivity before any application. The AE

technique captures the parameters and correlates with the defect

formation and failures. When more than one sensors is used,

AE source can be located based by measuring the signals arrival time to

each sensor. By comparing the signals arrival time at different sensors,

the source location can be calculated through triangulation andother methods.

AE sources are usually classified based on activity and intensity

. A source is considered to be active if its event count continues to

increase with stimulus. A source is considered to be critically active if the rate of change of its

count or emission rate consistently increases with increasing stimulation

.


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AET Advantages

AE testing is a powerful aid to materials testing and the study of deformation,

fatigue crack growth, fracture, oxidation and corrosion. It gives an immediateindication of the response and behaviour of a material under stress, intimately

connected with strength, damage and failure. A major advantage of AE

testing is that it does not require access to the whole examination area. In

large structures / vessels permanent sensors can be mounted for periodicinspection for leak detection and structural integrity monitoring.

Typical advantages of AE technique include:

1. high sensitivity,2. early and rapid detection of defects, leaks, cracks etc.,

3. on-line monitoring,

4. location of defective regions,

5. minimization of plant downtime for inspection,6. no need for scanning the whole structural surface and

7. minor disturbance of insulation.


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AET Limitations

On the negative side;

AET requires stimulus.

AE technique can only (1) qualitatively estimate the damage and predict (2)

how long the components will last. So,

other NDT methods are still needed for thorough examinations and forobtaining quantitative information.

Plant environments are usually very noisy and the AE signals are usually

very weak. This situation calls for incorporation of signal discrimination and

noise reduction methods. In this regard, signal processing and frequencydomain analysis are expected to improve the situation.


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A Few Typical Applications

Detection and location of leak paths in end-shield of reactors (frequencyanalysis)

Identification of leaking pressure tube in reactors

Condition monitoring of 17 m Horton sphere during hydro testing (24

sensors) On-line monitoring of welding process and fuel end-cap welds

Monitoring stress corrosion cracking, fatigue crack growth

Studying plastic deformation behaviour and fracture of SS304, SS316,

Inconel, PE-16 etc Monitoring of oxidation process and spalling behaviour of metals and

alloys

A ti E i i T ti li ti t it bl f


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Acoustic Emission Testing applications are most suitable for:

1. Aboveground Storage Tank Screening for Corrosion & Leaks

2. Pressure Containment Vessels (Columns, Bullets, Cat Crackers)3. Horton Spheres & legs

4. Fiberglass Reinforced Plastic Tanks and Piping

5. Offshore Platform Monitoring

6. Nuclear components inspection7. Tube Trailers

8. Railroad tank cars

9. Bridge Critical Members monitoring

10. Pre- & Post-Stressed Concrete Beams11. Reactor Piping

12. High Energy Seam Welded Hot Reheat Piping Systems in Power Plants.

13. On-Stream Monitoring

14. Remote Long Term Monitoring

http://www.techcorr.com/services/Inspection-and-Testing/Acoustic-Emission-Testing.cfm

A ti E i i T ti Ad t


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Acoustic Emission Testing Advantages

1. Compared to conventional inspection methods the advantages of the

Acoustic Emission Testing technique are:2. Tank bottom Testing without removal of product.

3. Inspection of Insulated Piping & Vessels

4. Real time monitoring during cool-down & start-ups

5. Real Time Monitoring Saves Money6. Real Time Monitoring Improves Safety

T k AET


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Tank AET

E d f R di


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End of Reading

Study Note 2:


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Study Note 2:

Sidney Mindess

University of British Columbia

Chapter 16: Acoustic Emission Methods

16


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16

Acoustic Emission

Methods

http://unina.stidue.net/Politecnico%20di%20Milano/Inge

gneria%20Strutturale/Corsi/Felicetti%20-

%20Structural%20assessment%20and%20residual%20

bearing%20capacity/books/Handbook%20of%20NDT%

20of%20Concrete/1485_C16.pdf

Dam


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Dam

http://www.boomsbeat.com/articles/116/20140118/tianzi-mountains-china.htm

Dam


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Dam

16 1 Introduction


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16.1 Introduction

16.2 Historical Background

16.3 Theoretical Considerations16.4 Evaluation of Acoustic Emission Signals

16.5 Instrumentation and Test Procedures

16.6 Parameters Affecting Acoustic Emissions from Concrete

The Kaiser Effect Effect of Loading Devices SignalAttenuation Specimen Geometry Type of aggregate Concrete Strength

16.7 Laboratory Studies of Acoustic Emission

Fracture Mechanics Studies Type of Cracks Fracture Process

Zone (Crack Source) Location Strength vs. Acoustic Emission

Relationships Drying Shrinkage Fiber Reinforced Cements

and Concretes High Alumina Cement Thermal Cracking

Bond in Reinforced Concrete Corrosion of Reinforcing Steel

in Concrete

16.8 Field Studies of Acoustic Emission

16.9 Conclusions

Foreword:


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Foreword:

Acoustic emission refers to the sounds, both audible and sub-audible, that are

generated when a material undergoes irreversible changes, such as thosedue to cracking. Acoustic emissions (AE) from concrete have been studied for

the past 30 years, and can provide useful information on concrete properties.

This review deals with the parameters affecting acoustic emissions from

concrete, including discussions of the Kaiser effect, specimen geometry, andconcrete properties. There follows an extensive discussion of the use of AE to

monitor cracking in concrete, whether due to (1) externally applied loads, (2)

drying shrinkage, or (3) thermal stresses. AE studies on reinforced concrete

are also described. While AE is very useful laboratory technique for the studyof concrete properties, its use in the field remains problematic.

16 1 Introduction


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16.1 Introduction

It is common experience that the failure of a concrete specimen under load is

accompanied by a considerable amount of audible noise. In certaincircumstances, some audible noise is generated even before ultimate failure

occurs. With very simple equipment a microphone placed against the

specimen, an amplifier, and an oscillograph subaudible sounds can be

detected at stress levels of perhaps 50% of the ultimate strength; with thesophisticated equipment available today, sound can be detected at much

lower loads, in some cases below 10% of the ultimate strength. These sounds,

both audible and subaudible, are referred to as acoustic emission. In general,

acoustic emissions are defined as the class of phenomena whereby transientelastic waves are generated by the rapid release of energy from localized

sources within a material. These waves propagate through the material, and

their arrival at the surfaces can be detected by piezoelectric transducers.

Keywords: Audible & Sub-audible sounds

Acoustic emissions, which occur in most materials, are caused by irreversible


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, , y

changes, such as (1) dislocation movement, (2) twinning, (3) phase

transformations, (4) crack initiation, and propagation, (5) debonding betweencontinuous and dispersed phases in composite materials, and so on.

In concrete, since the first three of these mechanisms do not occur, acoustic

emission is due primarily to:

1. Cracking processes

2. Slip between concrete and steel reinforcement

3. Fracture or debonding of fibers in fiber-reinforced concrete

16.2 Historical Background


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g

The initial published studies of acoustic emission phenomena, in the early

1940s, dealt with the problem of predicting rockbursts in mines; this techniqueis still very widely used in the field of rock mechanics, in both field and

laboratory studies. The first significant investigation of acoustic emission from

metals (steel, zinc, aluminum, copper, and lead) was carried out by Kaiser.

Among many other things, he observed what has since become known as the

Kaiser effect: the absence of detectable acoustic emission at a fixed

sensitivity level, until previously applied stress levels are exceeded. While

this effect is not present in all materials, it is a very important observation, and

it will be referred to again later in this review. The first study of acoustic

emission from concrete specimens under stress appears to have been carried

out by Rsch, who noted that during cycles of loading and unloading below

about 70 to 85% of the ultimate failure load, acoustic emissions were

produced only when the previous maximum load was reached (the Kaiser

effect). At about the same time, but independently, LHermite also measured

acoustic emission from concrete, finding that a sharp increase in acoustic

emission coincided with the point at which Poissons ratio also began to

increase (i.e., at the onset of significant matrix cracking in the concrete).

In 1965, however, Robinson used more sensitive equipment to show that


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acoustic emission occurred at much lower load levels than had been reported

earlier, and hence, could be used to monitor earlier microcracking (such asthat involved in the growth of bond cracks in the interfacial region between

cement and aggregate). In 1970, Wells built a still more sensitive apparatus,

with which he could monitor acoustic emissions in the frequency range from

about 2 to 20 kHz. However, he was unable to obtain truly reproducible

records for the various specimen types that he tested, probably due to the

difficulties in eliminating external noise from the testing machine. Also in 1970,

Green reported a much more extensive series of tests, recording acoustic

emission frequencies up to 100 kHz. Green was the first to show clearly that

acoustic emissions from concrete are related to failure processes within the

material; using source location techniques, he was also able to determine the

locations of defects. It was this work that indicated that acoustic emissions

could be used as an early warning of failure. Green also noted the Kaiser

effect, which suggested to him that acoustic emission techniques could be

used to indicate the previous maximum stress to which the concrete had been

subjected. As we will see below, however, a true Kaiser effect appears not to

exist for concrete.

Green also noted the Kaiser effect, which suggested to him that acoustic


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emission techniques could be used to indicate the previous maximum stress

to which the concrete had been subjected. As we will see below, however, a

true Kaiser effect appears not to exist for concrete.

Nevertheless, even after this pioneering work, progress in applying acoustic


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emission techniques remains slow. An extensive review by Diederichs et al.

(et al means: and others), covers the literature on acoustic emissions from

concrete up to 1983. However, as late as 1976, Malhotra noted that there was

little published data in this area, and that acoustic emission methods are in

their infancy. Even in January, 1988, a thorough computer-aided search of

the literature found only some 90 papers dealing with acoustic emissions from

concrete over about the previous 10 years; while this is almost certainly not a

complete list, it does indicate that there is much work to be carried out before

acoustic emission monitoring becomes a common technique for testing

concrete. Indeed, there are still no standard test methods which have even

been suggested for this purpose.

16.3 Theoretical Considerations


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When an acoustic emission event occurs at a source with the material, due to

(1) inelastic deformation or (2) to cracking, the stress waves travel directly

from the source to the receiver as body waves. Surface waves may then arise

from mode conversion. When the stress waves arrive at the receiver, the

transducer responds to the surface motions that occur.

It should be noted that the signal captured by the recording device may be

affected by:

the nature of the stress pulse generated by the source,

the geometry of the test specimen, and

the characteristics of the receiver,

making it difficult to interpret the recorded waveforms.

Two basic types of acoustic emission signals can be generated (Figure 16.1):


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Continuous emission is a qualitative description of the sustained signal

level produced by rapidly occurring acoustic emission events. These are

generated by events such as plastic deformations in metals, which occur

in a reasonably continuous manner.

Burt emission is a qualitative description of the discrete signal related to

an individual emission event occurring within the matrial,1 such as that

which may occur during crack growth or fracture in brittle materials.

These burst signals are characteristic of the acoustic emission events

resulting from the loading of cementitious materials.


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FIGURE 16.1 The two basic types of acoustic emission signals. (A) Continuous

emission. (B) Burst emission.

16.4 Evaluation of Acoustic Emission Signals


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A typical acoustic emission signal from concrete is shown in Figure 16.2.12

However, when such acoustic events are examined in much greater detail, asshown in Figure 16.3,13 the complexity of the signal becomes even more

apparent; the scatter in noise, shown in Figure 16.3, makes it difficult to

determine exactly the time of arrival of the signal; this means that very

sophisticated equipment must be used to get the most information out of theacoustic emission signals. In addition, to obtain reasonable sensitivity, the

acoustic emission signals must be amplified. In concrete, typically, system

gains in the range of 80 to 100 decibels (dB) are used.

FIGURE 16.2 A typical acoustic emission signal from concrete. (From

Berthelot J M et al private communication 1987 With permission )


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Berthelot, J.M. et al., private communication, 1987. With permission.)

FIGURE 16.3 Typical view of an acoustic emission event as displayed in an

oscilloscope screen (Adapted from Maji A and Shah S P Exp Mech 26


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oscilloscope screen. (Adapted from Maji, A. and Shah, S.P., Exp. Mech., 26,

1, 1988, p. 27.)

FIGURE 16.2 A typical acoustic emission signal from concrete. (From

Berthelot J M et al private communication 1987 With permission )


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Berthelot, J.M. et al., private communication, 1987. With permission.)

There are a number of different ways in which acoustic emission signals may

be evaluated


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be evaluated.

Acoustic Emission Counting (ring-down counting)

This is the simplest way in which an acoustic emission event may be

characterized. It is the number of times the acoustic emission signal exceeds

a preset threshold during any selected portion of a test, and is illustrated in

Figure 16.4. A monitoring system may record:

FIGURE 16.4 The principle of acoustic emission counting (ring-down counting).

1. The total number of counts (e.g., 13 counts in Figure 16.4). Since the

shape of a burst emission is generally a damped sinusoid pulses of higher


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shape of a burst emission is generally a damped sinusoid, pulses of higher

amplitude will generate more counts.

2. The count rate. This is the number of counts per unit of time; it is

particularly useful when very large numbers of counts are recorded.

3. The mean pulse amplitude. This may be determined by using a root-mean

square meter, and is an indication of the amount of energy being

dissipated.

Clearly, the information obtained using this method of analysis depends upon

both the gain and the threshold setting. Ring-down counting is affected

greatly by the characteristics of the transducer, and the geometry of the

test specimen (which may cause internal reflections) and may not be

indicative of the nature of the acoustic emission event. In addition, there is

no obvious way of determining the amount of energy released by a single

event, or the total number of separate acoustic events giving rise to thecounts.

Event counting Circuitry is available which counts each acoustic emission

event only once by recognizing the end of each burst emission in terms of a


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event only once, by recognizing the end of each burst emission in terms of a

predetermined length of time since the last count (i.e., since the most recent

crossing of the threshold). In Figure 16.4, for instance, the number of events

is three. This method records the number of events, which may be very

important, but provides no information about the amplitudes involved.

Rise time This is the interval between the time of first occurrence ofsignals above the level of the background noise and the time at which the

maximum amplitude is reached. This may assist in determining the type of

damage mechanism.

Signal duration This is the duration of a single acoustic emission event;

this too may be related to the type of damage mechanism.

Amplitude distribution This provides the distribution of peak amplitudes.

This may assist in identifying the sources of the emission events that are


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This may assist in identifying the sources of the emission events that are

occurring.

Frequency analysis This refers to the frequency spectrum of individual

acoustic emission events. This technique, generally requiring a fast Fourier

transformation analysis of the acoustic emission waves, may help

discriminate between different types of events. Unfortunately, a frequencyanalysis may sometimes simply be a function of the response of the

transducer, and thus reveal little of the true nature of the pulse.

Energy analysisThis is an indication of the energy released by an

acoustic emission event; it may be measured in a number of ways, depending

on the equipment, but it is essentially the area under the amplitude vs. time

curve (Figure 16.4) for each burst. Alternatively, the area under the envelope

of the amplitude vs. time curve may be measured for each burst.

Defect location By using a number of transducers to monitor acoustic

emission events, and determining the time differences between the detection


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e ss o e e ts, a d dete g t e t e d e e ces bet ee t e detect o

of each event at different transducer positions, the location of the acoustic

emission event may be determined by using triangulation techniques. Work

by Maji and Shah, for instance, has indicated that this technique may be

accurate to within about 5 mm.

Analysis of the wave-form Most recently, it has been suggested that an

elaborate signals processing technique (deconvolution) applied to the wave-

form of an acoustic emission event can provide information regarding the

volume, orientation, and type of microcrack. Ideally, since all of these

methods of data analysis provide different information, one would wish to

measure them all. However, this is neither necessary nor economically

feasible. In the discussion that follows, it will become clear that the more

elaborate methods of analysis are useful in fundamental laboratory

investigations, but may be inappropriate for practical applications.

Signal Evaluation:Analysis of the wave-form


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http://sirius.mtm.kuleuven.be/Research/NDT/AcousticEmissions/index.html

Signal Evaluation:Acoustic Emission Counting (ring-down counting)


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Ring-down count= 13

Signal Evaluation: Raise Time/ Event Counts/ Signal Duration


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Raise time

mV/s

Signal duration s

Event counts = 3 in unit time

Signal Evaluation:Amplitude Distribution- Triangulation to locate source


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Signal Evaluation:Amplitude Distribution- Triangulation to locate source


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http://iopscience.iop.org/0964-1726/21/3/035009;jsessionid=DE0B79359A6ADDA1365CAC54ABA381A2.c2

Signal Evaluation: Frequency analysis


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Signal Evaluation:

Energy analysis- it is essentially the area under the amplitude vs. time curve


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Note: all areas under curves or only areas above threshold.



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ring-down counting



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16.5 Instrumentation and Test Procedures

Instrumentation (and, where necessary, the associated computer software) is


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( , y, p )

available, from a number of different manufacturers, to carry out all of themethods of signal analysis described above. It might be added that advances

in instrumentation have outpaced our understanding of the nature of the

elastic waves resulting from microcracking in concrete. The main elements of

a modern acoustic emission detection system are shown schematically inFigure 16.5.

FIGURE 16.5 The main elements of a modern acoustic emission detection system.


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A brief description of the most important parts of this system is as follows:

1. Transducers: Piezoelectric transducers (generally made of lead zirconate


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titanate, PZT) are used to convert the surface displacements into electricsignals. The voltage output from the transducers is directly proportional to

the strain in the PZT, which depends in turn on the amplitude of the

surface waves. Since these transducers are high impedance devices, they

yield relatively low signals, typically less than 100V. There are basicallytwo types of transducers. (a) Wide-band transducers are sensitive to

acoustic events with frequency responses covering a wide range, often

several hundred kHz. (b) Narrow-band transducers are restricted to a

much narrower range of frequencies, using bandpass filters. Of course, thetransducers must be properly coupled to the specimen, often using some

form of silicone grease as the coupling medium.

PZT:- If the p.d or the stress is changing the resulting effect also changes. Therefore if

an alternating potential difference with a frequency equal to the resonant frequency of

the crystal is applied across it the crystal will oscillate. A number of crystalline


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y pp y y

materials show this effect examples of these are quartz, barium titanate, lithiumsulphate, lead metaniobate, lead zirconate titanate (PZT) and polyvinylidine difluoride.

Piezoelectric transducers can act as both as a transmitter and a detector of vibrations.

However there are certain conditions. The crystal must stop vibrating as soon as the

alternating potential difference is switched off so that they can detect the reflected

pulse. For this reason a piece of damping material with an acoustic impedance the

same as that of the crystal is mounted at the back of the crystal. (See Figure 2).The

transducer is made with a crystal that has a thickness of one half of the

wavelength of the ultrasound, resonating at its fundamental frequency. A layer of

gel is needed between the transducer and the body to get good acoustic coupling (seeacoustic impedance).

http://www.schoolphysics.co.uk/age16-19/Medical%20physics/text/Piezoelectric_transducer/index.html

The transducer is made with a crystal that has a thickness of one half of the

wavelength of the ultrasound, resonating at its fundamental frequency.

Example: Frequency= 519Hz Wavelength = Speed/ frequency =


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Example: Frequency= 519Hz, Wavelength = Speed/ frequency =

5890/519=11.35mm. The thickness of the transducer= 5.7mm approx.

s= 5890m/s

http://www.olympus-ims.com/en/ndt-tutorials/thickness-gage/appendices-velocities/

AET

Transducer

In 0 1KHz~2 0KHz


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In 0.1KHz 2.0KHz

UT Transducers 2.0~5.0 MHz


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2. Preamplifier: Because of the low voltage output, the leads from the

transducer to the preamplifier must be as short as possible; often, the

preamplifier is integrated within the transducer itself. Typically, the gain in the


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preamplifier is integrated within the transducer itself. Typically, the gain in the

preamplifier is in the range 40 to 60 dB. (Note: The decibel scale measuresonly relative amplitudes. Using this scale:

where Vis the output amplitude and Vi is the input amplitude. That is, a gainof 40 dB will increase the input amplitude by a factor of 100; a gain of 60 dB

will increase the input amplitude by a factor of 1000, and so on.)

3. Passband filters are used to suppress the acoustic emission signals that

lie outside of the frequency range of interest.

4. The main amplifier further amplifies the signals, typically with a gain of an


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p p g , yp y g

additional 20 to 60 dB.5. The discriminator is used to set the threshold voltage above which signals

are counted.

The remainder of the electronic equipment depends upon the way in whichthe acoustic emission data are to be recorded, analyzed, and displayed.

Acoustic emission testing may be carried out in the laboratory or in the field.

Basically, one or more acoustic emission transducers are attached to thespecimen. The specimen is then loaded slowly, and the resulting acoustic

emissions are recorded.

There are generally two categories of tests:

1. To use the acoustic emission signals to learn something about the internal


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structure of the material, and how structural changes (i.e., damage) occurduring the process of loading. In this case, the specimens are generally

loaded to failure.

2. To establish whether the material or the structure meet certain design or

fabrication criteria. In this case, the load is increased only to somepredetermined level (proof loading). The amount and nature of the

acoustic emissions may be used to establish the integrity of the specimen

or structure, and may also sometimes be used to predict the service life.

16.6 Parameters Affecting Acoustic Emissions from Concrete

16.6.1 The Kaiser Effect


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The earliest acoustic emission studies of concrete, such as the work of Rsch,indicated that a true Kaiser effect (see above) exists for concrete; that is,

acoustic emissions were found not to occur in concrete that had been

unloaded until the previously applied maximum stress had been exceeded on

reloading. This was true, however, only for stress levels below about 75 to85% of the ultimate strength of the material; for higher stresses, acoustic

emissions began again at stresses somewhat lower than the previous

maximum stress. Subsequently, a number of other investigators have also

concluded that concrete exhibits a Kaiser effect, at least for stresses belowthe peak stress of the material.

Key points:

For concrete This was true, however, only for stress levels below about 75 to85% of the ultimate strength of the material

Spooner and Dougill confirmed that this effect did not occur beyond the peak

of the stress-strain curve (i.e., in the descending portion of the stress-strain

curve), where acoustic emissions occurred again before the previous


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maximum strain was reached. It has also been suggested that a form of theKaiser effect occurs as well for cyclic thermal stresses in concrete, and for

drying and wetting cycles. On the other hand, Nielsen and Griffin have

reported that the Kaiser effect is only a very temporary effect in concrete; with

only a few hours of rest between loading cycles, acoustic emissions are againrecorded during reloading to the previous maximum stress. They therefore

concluded that the Kaiser effect is not a reliable indicator of the loading

history for plain concrete. Thus, it is unlikely that the Kaiser effect could be

used in practice to determine the previous maximum stress that a structuralmember has been subjected to.

Kaiser Effect- Concrete that this effect did notoccur beyond the

peak of the stress-

strain curve (i.e., in

the descending


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For concrete This

was true, however,only for stress

levels below about

75 to 85% of the

ultimate strength

of the material

portion of the stress-strain curve), where

acoustic emissions

occurred again

before the previous

maximum strain was

reached.

Spooner and Dougill conclusion on Kaiser Effect- Concrete:

They therefore concluded that the Kaiser effect is not a


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y

reliable indicator of the loading history for plain concrete.

16.6.2 Effect of Loading Devices

As is well known, the end restraint of a compression specimen of concrete


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due to the friction between the ends of the specimen and the loading platenscan have a considerable effect on the apparent strength of the concrete.

These differences are also reflected in the acoustic emissions measured

when different types of loading devices are used. For instance, in

compression testing with stiff steel platens, most of the acoustic emissionappears at stresses beyond about half of the ultimate stress; with more

flexible platens, such as brush platens, significant acoustic emission appears

at about 20% of the ultimate stress. This undoubtedly reflects the different

crack patterns that develop with different types of platens, but it nonethelessmakes inter-laboratory comparisons, and indeed even studies on different

specimen geometries within the same laboratory, very difficult.

16.6.3 Signal Attenuation

The elastic stress waves that are generated by cracking attenuate as they


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propagate through the concrete. Thus, large acoustic emission events thattake place in the concrete far from a pick-up transducer may not exceed the

threshold excitation voltage due to this attenuation, while much smaller

events may be recorded if they occur close to the transducer. Very little

information is available on acoustic emission attenuation rates in concrete. Ithas been shown that more mature cements show an increasing capacity to

transmit acoustic emissions.20 Related to this, Mindess23 has suggested that

the total counts to failure for concrete specimens in compression are much

higher for older specimens, which may also be explained by the bettertransmission through older concretes.

As a practical matter, the maximum distance between piezoelectric

transducers, or between the transducers and the source of the acoustic

emission event, should not be very large. Berthelot and Robert required an


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array of transducers arranged in a 40-cm square mesh to locate acousticemission events reasonably accurately. They found that for ordinary concrete,

with a fifth transducer placed in the center of the 40 x 40-cm square mesh,

only about 40% of the events detected by the central transducer were also

detected by the four transducers at the corners; with high strength concrete,this proportion increased to 60 to 70%. Rossi also found that a 40-cm square

mesh was needed for a proper determination of acoustic emission events.

Although more distant events can, of course, be recorded, there is no way of

knowing how many events are lost due to attenuation. This is an area thatrequires much more study.

16.6.4 Specimen Geometry

It has been shown that smaller specimens appear to give rise to greater


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levels of acoustic emission than do larger ones. The reasons for this are notclear, although the observation may be related to the attenuation effect

described above. After an acoustic emission event occurs, the stress waves

not only travel from the source to the sensor, but also undergo (1) reflection,

(2) diffraction, and (3) mode conversions within the material. The basicproblem of wave propagation within a bounded solid certainly requires further

study, but there have apparently been no comparative tests on different

specimen geometries.

16.6.5 Type of Aggregate

It is not certain whether the mineralogy of the aggregate has any effect on

ti i i It h b t d th t t ith ll


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acoustic emission. It has been reported that concretes with a smallermaximum aggregate size produce a greater number of acoustic emission

counts than those with a larger aggregate size;10 however, the total energy

released by the finer aggregate concrete is reduced. This is attributed to the

observation that concretes made with smaller aggregates start to crack atlower stresses; in concretes with larger aggregate particles, on the other hand,

individual acoustic events emit higher energies. For concretes made with

lightweight aggregates, the total number of counts is also greater than for

normal weight concrete, perhaps because of cracking occurring in the

aggregates themselves.

16.6.6 Concrete Strength

It has been shown that the total number of counts to the maximum load is

t f hi h t th t H ti d li f


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greater for higher strength concretes. However, as was mentioned earlier, forsimilar strength levels the total counts to failure appears to be much higher for

older concretes.

16.7 Laboratory Studies of Acoustic Emission

By far the greatest number of acoustic emission studies of concrete have

b i d t i th l b t d h b l l th ti l i


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been carried out in the laboratory, and have been largely theoretical innature:

1. To determine whether acoustic emission analysis could be applied to

cementitious systems2. To learn something about crack propagation in concrete

16.7.1 Fracture Mechanics Studies

A number of studies have shown that acoustic emission can be related to

crack growth or fracture mechanics parameters in cements mortars and


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crack growth or fracture mechanics parameters in cements, mortars, andoncretes. Evans et al. showed that acoustic emission could be correlated with

crack velocity in mortars. Morita and Kato and Nadeau, Bennett, and

Mindess20 were able to relate total acoustic emission counts to Kc (the

fracture toughness). In addition, Lenain and Bunsell found that the number ofemissions could be related to the sixth power of the stress intensity factor, K.

Izumi et al. showed that acoustic emissions could also be related to the strain

energy release rate, G. In all cases, however, these correlations are purely

empirical; no one has yet developed a fundamental relationship between

acoustic emission events and fracture parameters, and it is unlikely that such

a relationship exists.

16.7.2 Type of Cracks

A number of attempts have been made to relate acoustic events of different

frequencies or of different energies to different types of cracking in concrete


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frequencies, or of different energies, to different types of cracking in concrete.For instance, Saeki et al.,31 by looking at the energy levels of the acoustic

emissions at different levels of loading, concluded that the first stage of

cracking, due to the development of bond cracks between the cement paste

and the aggregate, emitted high energy signals; the second stage, which they

termed crack arrest, emitted low energy signals; the final stage, in which

cracks extended through the mortar, was again associated with high energy

acoustic events. Similarly, Tanigawa and Kobayashi32 used acoustic

energies to distinguish the onset of the proportional limit, the initiation stress

and the critical stress. On the other hand, Tanigawa et al.11 tried to relate

the fracture type (pore closure, tensile cracking, and shear slip) to the power

spectra and frequency components of the acoustic events. The difficulty with

these and similar approaches is that they tried to relate differences in the

recorded acoustic events to preconceived notions of the

nature of cracking in concrete; direct cause and effect relationships were

never observed.

16.7.3 Fracture Process Zone (Crack Source) Location

Perhaps the greatest current interest in acoustic emission analysis is its use

in locating fracture processes and in monitoring the damage that concreted k Ok d t l 33 34 h d th t th l ti f


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in locating fracture processes, and in monitoring the damage that concreteundergoes as cracks progress. Okada et al.33,34 showed that the location of

crack sources obtained from differences in the arrival times of acoustic

emissions was in good agreement with the observed fracture surface. At

about the same time, Chhuy et al.35 and Lenain and Bunsell29 were able to

determine the length of the damaged zone ahead of the tip of a propagating

crack using one-dimensional acoustic emission location techniques. In

subsequent work, Chhuy et al.,36 using more elaborate equipment and

analytical techniques, were able to determine damage both before the

initiation of a visible crack and after subsequent crack extension. Berthelot

and Robert24,37 and Rossi25 used acoustic emission to monitor concrete

damage as well.

They found that, while the number of acoustic events showed the progression

of damage both ahead and behind the crack front, this technique alone could

not provide a quantitative description of the cracking. However, using more

elaborate techniques including amplitude analysis and measurements of


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elaborate techniques, including amplitude analysis and measurements ofsignal duration, Berthelot and Robert24 concluded that acoustic emission

testing is practically the only technique which can provide a quantitative

description of the progression in real time of concrete damage within test

specimens. Later, much more sophisticated signals processing techniqueswere applied to acoustic emission analysis. In 1981, Michaels et al.15 and

Niwa et al.38 developed deconvolution techniques to analyze

acoustic waveforms, in order to provide a stress-time history of the source of

an acoustic event. Similar deconvolution techniques were subsequently usedby Maji and Shah13,39 to determine the volume, orientation and type of

microcrack, as well as the source of the acoustic events. Such sophisticated

techniques have the potential eventually to be used to provide a detailed

picture of the fracture processes occurring within concrete specimens.

16.7.4 Strength vs. Acoustic Emission Relationships

Since concrete quality is most frequently characterized by its strength, many

studies have been directed towards determining a relationship betweenti i i ti it d t th F i t T i d


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studies have been directed towards determining a relationship betweenacoustic emission activity and strength. For instance, Tanigawa and

Kobayashi32 concluded that the compressive strength of concrete can be

approximately estimated by the accumulated AE counts at relatively low

stress level. Indeed, they suggested that acoustic emission techniques might

provide a useful nondestructive test method for concrete strength. Earlier,

Fertis40 had concluded that acoustic emissions could be used to determine

not only strength, but also static and dynamic material behavior. Rebic,41 too,

found that there is a relationship between the critical load at which the

concrete begins to be damaged, which can be determined from acoustic

emission measurements, and the ultimate strength; thus, acoustic emission

analysis might be used as a predictor of concrete strength. Sadowska-Boczar

et al.42 tried to quantify the strength vs. acoustic emission relationship using

the equation

Sadowska-Boczar et al.42 tried to quantify the strength vs. acoustic emission

relationship using the equation:


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Where:

Fr is the rupture strength,Fp is the stress corresponding to the first acoustic emission signal, and

a and b are constants for a given material and loading conditions.

Using this linear relationship, which they found to fit their data reasonably well,

they suggested that the observation of acoustic emissions at low stresses

would permit an estimation of strength, as well as providing some

characterization of porosity and critical flaw size.

Unfortunately, the routine use of

acoustic emissions as an

estimator of strength seems to be

an unlikely prospect in large part


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an unlikely prospect, in large partbecause of the scatter in the data,

as has been noted by Fertis.40 As

an example of the scatter in data.

Figure 16.623 indicates thevariability in the strength vs. total

acoustic emission counts

relationship; the within-batch

variability is even more severe, asshown in Figure 16.7.23

FIGURE 16.6 Logarithm of total acoustic emission counts vs.

compressive strength of concrete cubes. (From Mindess, S., Int.

J. Cem. Comp. Lightweight Concr., 4, 173, 1982. Withpermission.)


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FIGURE 16.7 Within-batch variability of total acoustic emission counts vs. applied compressive

stress on concretecubes. (From Mindess, S., Int. J. Cem. Comp. Lightweight Concr., 4, 173,

1982. With permission.)

16.7.5 Drying Shrinkage

16.7.6 Fiber Reinforced Cements and Concretes

16.7.7 High Alumina Cement

16.7.8 Thermal Cracking


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16.7.8 Thermal Cracking16.7.9 Bond in Reinforced Concrete

16.7.10 Corrosion of Reinforcing Steel in Concrete

Read text for details

http://unina.stidue.net/Politecnico%20di%20Milano/Inge

gneria%20Strutturale/Corsi/Felicetti%20-

%20Structural%20assessment%20and%20residual%20

bearing%20capacity/books/Handbook%20of%20NDT%

20of%20Concrete/1485_C16.pdf

16.8 Field Studies of Acoustic Emission

As shown in the previous section, acoustic emission analysis has been used

in the laboratory to study a wide range of problems. Unfortunately, its use

in the field has been severely limited; only a very few papers on field


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in the field has been severely limited; only a very few papers on field

application have appeared, and these are largely speculation on future

possibilities. The way in which acoustic emission data might be used to

provide information about the condition of a specimen or a structure has

been described by Cole;54 his analysis may be summarized as follows:

1. Is there any acoustic emission at a certain load level? If no, then no

damage is occurring under these conditions; if yes, then damage is

occurring.

2. Is acoustic emission continuing while the load is held constant at the

maximum load level? If no, no damage due to creep is occurring; if yes,

creep damage is occurring. Further, if the count rate is increasing, then

failure may occur fairly soon.

3. Have high amplitude acoustic emissions events occurred? If no, individual

fracture events have been relatively minor; if yes, major fracture events

have occurred.

4. Does acoustic emission occur if the structure has been unloaded and is? f


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then reloaded to the previous maximum load? If no, there is no damage or

crack propagation under low cycle fatigue; if yes, internal damage exists

and the damage sites continue to spread even under low loads.

5. Does the acoustic emission occur only from a particular area? Ifno, theentire structure is being damaged; if yes, the damage is localized.

6. Is the acoustic emission in a local area very localized? if no, damage is

dispersed over a significant area; if yes, there is a highly localized stress

concentration causing the damage.

16.9 Conclusions

From the discussion above, it appears that acoustic emission techniques may

be very useful in the laboratory to supplement other measurements of

concrete properties. However, their use in the field remains problematic.


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concrete properties. However, their use in the field remains problematic.

Many of the earlier studies held out high hopes for acoustic emission

monitoring of structures. For instance, McCabe et al.17 suggested that, if a

structure was loaded, the absence of acoustic emissions would indicate that it

was safe under the existing load conditions; a low level of acoustic emissions

would indicate that the structure should be monitored carefully, while a high

level of acoustic emission could indicate that the structure was unsafe. But

this is hardly a satisfactory approach, since it does not provide any help with

quantitative analysis. In any event, even the sophisticated (and expensive)

equipment now available still provides uncertain results when applied to

structures, because of our lack of knowledge about the characteristics of

acoustic emissions due to different causes, and because of the possibility of

extraneous noise (vibration, loading devices, and so on).

Another serious drawback is that acoustic emissions are only generated

when the loads on a structure are increased, and this poses considerable

practical problems. Thus, one must still conclude, with regret, that acoustic

emission analysis has not yet been well developed as a technique for thel ti f h t ki l i t i t t 18


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y y p qevaluation of phenomena taking place in concrete in structures.18

End of Reading


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Study Note 3:

Introduction to Acoustic Emission Testinghttp://www.ndt-ed.org/EducationResources/CommunityCollege/Other%20Methods/AE/AE_Intro.htm


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Acoustic Emission (AE) refers to the generation of transient elastic waves

produced by a sudden redistribution of stress in a material. When a

structure is subjected to an external stimulus (change in pressure, load, or

temperature), localized sources trigger the release of energy, in the form ofstress waves which propagate to the surface and are recorded by sensors


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stress waves, which propagate to the surface and are recorded by sensors.

With the right equipment and setup, motions on the order of picometers

(10-12 m) can be identified. Sources of AE vary from natural events like:

1. earthquakes and rock bursts to

2. the initiation and growth of cracks,

3. slip and dislocation movements,

4. melting,5. twinning, and

6. phase transformations

in metals. In composites, matrix cracking and fiber breakage and de-bondingcontribute to acoustic emissions.

AEs have also been measured and recorded in polymers, wood, and

concrete, among other materials. Detection and analysis of AE signals can

supply valuable information regarding the origin and importance of a

discontinuity in a material. Because of the versatility of Acoustic EmissionTesting (AET)


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Testing (AET),

It has many industrial applications e.g.

1. assessing structural integrity,

2. detecting flaws,

3. testing for leaks, or

4. monitoring weld quality and5. is used extensively as a research tool.

Twinning


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AET


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Acoustic Emission is unlike most other nondestructive testing (NDT)

techniques in two regards. The first difference pertains to the origin of the

signal. Instead of supplying energy to the object under examination, AET

simply listens for the energy released by the object.AE tests are oftenperformed on structures while in operation, as this provides adequate loading


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performed on structures while in operation, as this provides adequate loading

for propagating defects and triggering acoustic emissions.

The second difference is that AET deals with dynamic processes, or changes,

in a material. This is particularly meaningful because only active features (e.g.

crack growth) are highlighted. The ability to discern between developing and

stagnant defects is significant. However, it is possible for flaws to go

undetected altogether if the loading is not high enough to cause an acoustic

event.

Furthermore, AE testing usually provides an immediate indication relating to

the strength or risk of failure of a component. Other advantages of AET

include fast and complete volumetric inspection using multiple sensors,permanent sensor mounting for process control, and no need to disassemble

and clean a specimen.

Unfortunately, AE systems can only qualitatively gauge how much damage is

contained in a structure. In order to obtain quantitative results about size,

depth, and overall acceptability of a part, other NDT methods (often ultrasonic

testing) are necessary. Another drawback of AE stems from loud serviceenvironments which contribute extraneous noise to the signals. For


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environments which contribute extraneous noise to the signals. For

successful applications, signal discrimination and noise reduction are crucial.

A Brief History of AE Testing

Although acoustic emissions can be created in a controlled environment, they

can also occur naturally. Therefore, as a means of quality control, the origin of

AE is hard to pinpoint. As early as 6,500 BC, potters were known to listen for

dibl d d i th li f th i i i if i t t l


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audible sounds during the cooling of their ceramics, signifying structural

failure. In metal working, the term "tin cry" (audible emissions produced by the

mechanical twinning of pure tin during plastic deformation) was coined

around 3,700 BC by tin smelters in Asia Minor. The first documented

observations of AE appear to have been made in the 8th century by Arabian

alchemist Jabir ibn Hayyan. In a book, Hayyan wrote that Jupiter (tin) gives

off a harsh sound when worked, while Mars (iron) sounds much during

forging. Many texts in the late 19th century referred to the audible emissionsmade by materials such as tin, iron, cadmium and zinc. One noteworthy

correlation between different metals and their acoustic emissions came from

Czochralski, who witnessed the relationship between tin and zinc cry and

twinning. Later, Albert Portevin and Francois Le Chatelier observed AEemissions from a stressed Al-Cu-Mn (Aluminum-Copper-Manganese) alloy.

The next 20 years brought further verification with the work of Robert

Anderson (tensile testing of an aluminum alloy beyond its yield point), Erich

Scheil (linked the formation of martensite in steel to audible noise), and

Friedrich Forster, who with Scheil related an audible noise to the formation ofmartensite in high-nickel steel. Experimentation continued throughout the


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g p g

mid-1900s, culminating in the PhD thesis written by Joseph Kaiser entitled

"Results and Conclusions from Measurements of Sound in Metallic Materials

under Tensile Stress. Soon after becoming aware of Kaisers efforts,Bradford Schofield initiated the first research program in the United States to

look at the materials engineering applications of AE. Fittingly, Kaisers

research is generally recognized as the beginning of modern day acoustic

emission testing.

Theory - AE Sources

As mentioned in the Introduction, acoustic emissions can result from the

initiation and growth of cracks, slip and dislocation movements, twinning, or

phase transformations in metals. In any case, AEs originate with stress.

When a stress is exerted on a material a strain is induced in the material as


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When a stress is exerted on a material, a strain is induced in the material as

well. Depending on the magnitude of the stress and the properties of the

material, an object may return to its original dimensions or be permanently

deformed after the stress is removed. These two conditions are known aselastic and plastic deformation, respectively.

The most detectible acoustic emissions take place when a loaded material

undergoes plastic deformation or when a material is loaded at or near its yield

stress. On the microscopic level, as plastic deformation occurs, atomic planes

slip past each other through the movement of dislocations. These atomic-scale deformations release energy in the form of elastic waves which can be


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thought of as naturally generated ultrasound traveling through the object.

When cracks exist in a metal, the stress levels present in front of the crack tip

can be several times higher than the surrounding area. Therefore, AE activitywill also be observed when the material ahead of the crack tip undergoes

plastic deformation (micro-yielding).

Two sources of fatigue cracks also cause AEs. The first source is emissive

particles (e.g. nonmetallic inclusions) at the origin of the crack tip. Since these

particles are less ductile than the surrounding material, they tend to break

more easily when the metal is strained, resulting in an AE signal. The secondsource is the propagation of the crack tip that occurs through the movement


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of dislocations and small-scale cleavage produced by triaxial stresses.

The amount of energy released by an acoustic emission and the amplitude of

the waveform are related to the magnitude and velocity of the source event.The amplitude of the emission is proportional to the velocity of crack

propagation and the amount of surface area created. Large, discrete crack

jumps will produce larger AE signals than cracks that propagate slowly over

the same distance.Detection and conversion of these elastic waves to electrical signals is the

basis of AE testing. Analysis of these signals yield valuable information

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

discussed in the following section, specialized equipment is necessary todetect the wave energy and decipher which signals are meaningful.

http://www.nature.com/nmat/journal/v10/n11/full/nmat3167.html


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Activity of AE Sources in Structural Loading

AE signals generated under different loading patterns can provide valuable

information concerning the structural integrity of a material. Load levels that

have been previously exerted on a material do not produce AE activity. In

other words discontinuities created in a material do not expand or move until


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other words, discontinuities created in a material do not expand or move until

that former stress is exceeded. This phenomenon, known as the Kaiser Effect,

can be seen in the load versus AE plot to the right. As the object is loaded,

acoustic emission events accumulate (segment AB). When the load isremoved and reapplied (segment BCB), AE events do not occur again until

the load at point B is exceeded. As the load exerted on the material is

increased again (BD), AEs are generated and stop when the load is removed.

However, at point F, the applied load is high enough to cause significantemissions even though the previous maximum load (D) was not reached.

This phenomenon is known as the Felicity Effect. This effect can be

quantified using the Felicity Ratio, which is the load where considerable AE

resumes, divided by the maximum applied load (F/D).

Kaiser/Felicity effects


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Felicity effect F/D

Kaiser effect

Knowledge of the Kaiser Effect and Felicity Effect can be used to determine if

major structural defects are present. This can be achieved by applying

constant loads (relative to the design loads exerted on the material) and

listening to see if emissions continue to occur while the load is held. Asshown in the figure, if AE signals continue to be detected during the holding

f th l d (GH) it i lik l th t b t ti l t t l d f t t


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of these loads (GH), it is likely that substantial structural defects are present.

In addition, a material may contain critical defects if an identical load is

reapplied and AE signals continue to be detected. Another guidelinegoverning AEs is the Dunegan corollary, which states that if acoustic

emissions are observed prior to a previous maximum load, some type of new

damage must have occurred. (Note: Time dependent processes like corrosion

and hydrogen embrittlement tend to render the Kaiser Effect useless)

Dict:

Corollary: something that results from something else.

Emissions are observed prior to a previous maximum load;

Felicity effect,

Dunegan corollary

K d


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Keywords:

Kaiser effect,

Felicity effect,

Dunegan corollary

Noise

The sensitivity of an acoustic emission system is often limited by the amount

of background noise nearby. Noise in AE testing refers to any undesirable

signals detected by the sensors. Examples of these signals include frictionalsources (e.g. loose bolts or movable connectors that shift when exposed to


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( g p

wind loads) and impact sources (e.g. rain, flying objects or wind-driven dust)

in bridges. Sources of noise may also be present in applications where the

area being tested may be disturbed by mechanical vibrations (e.g. pumps).To compensate for the effects of background noise, various procedures can

be implemented. Some possible approaches involve fabricating special

sensors with electronic gates for noise blocking, taking precautions to place

sensors as far away as possible from noise sources, and electronic filtering(either using signal arrival times or differences in the spectral content of true

AE signals and background noise).

Pseudo Sources

In addition to the AE source mechanisms described above, pseudo source

mechanisms produce AE signals that are detected by AE equipment.

Examples include liquefaction and solidification, friction in rotating bearings,solid-solid phase transformations, leaks, cavitation, and the realignment or


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growth of magnetic domains (See Barkhausen Effect).

Wave Propagation

A primitive wave released at the AE source

is illustrated in the figure right. The

displacement waveform is a step-likefunction corresponding to the permanent


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change associated with the source process.

The analogous velocity and stress

waveforms are essentially pulse-like. Thewidth and height of the primitive pulse

depend on the dynamics of the source

process. Source processes such as

microscopic crack jumps and precipitatefractures are usually completed in a fraction

of a microsecond or a few microseconds,

which explains why the pulse is short in

duration. The amplitude and energy of theprimitive pulse vary over an enormous range

from submicroscopic dislocation movements

to gross crack jumps.

Primitive AE wave

released at a source. The

primitive wave is

essentially a stress pulsecorresponding to a

permanent displacement


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permanent displacement

of the material. The

ordinate quantities refer toa point in the material.

Waves radiates from the

source in all directions, often

having a strong directionality

depending on the nature of thesource process, as shown in

the second figure Rapid


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the second figure. Rapid

movement is necessary if a

sizeable amount of the elasticenergy liberated during

deformation is to appear as an

acoustic emission.

Angular dependence of acoustic emission radiated from a growing

microcrack. Most of the energy is directed in the 90 and 270o directions,

perpendicular to the crack surfaces.


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Angular dependence of acoustic emission radiated from a growing

microcrack. Most of the energy is directed in the 90 and 270o directions,

perpendicular to the crack surfaces.

As these primitive waves travel through a material, their form is changed

considerably. Elastic wave source and elastic wave motion theories are being

investigated to determine the complicated relationship between the AE

source pulse and the corresponding movement at the detection site. Theultimate goal of studies of the interaction between elastic waves and material

structure is to accurately develop a description of the source event from the


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structure is to accurately develop a description of the source event from the

output signal of a distant sensor.

However, most materials-oriented researchers and NDT inspectors are not

concerned with the intricate knowledge of each source event. Instead, they

are primarily interested in the broader, statistical aspects of AE. Because of

this, they prefer to use narrow band (resonant) sensors which detect only asmall portion of the broadband of frequencies emitted by an AE. These

sensors are capable of measuring hundreds of signals each second, in

contrast to the more expensive high-fidelity sensors used in source function

analysis. More information on sensors will be discussed later in theEquipment section.

The signal that is detected by a sensor is a combination of many parts of the

waveform initially emitted. Acoustic emission source motion is completed in a

few millionths of a second. As the AE leaves the source, the waveform travels

in a spherically spreading pattern and is reflected off the boundaries of theobject. Signals that are in phase with each other as they reach the sensor

produce constructive interference which usually results in the highest peak of


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p y g p

the waveform being detected. The typical time interval from when an AE wave

reflects around the test piece (repeatedly exciting the sensor) until it decays,ranges from the order of 100 microseconds in a highly damped, nonmetallic

material to tens of milliseconds in a lightly damped metallic material.

Decay Time:highly damped, nonmetallic material order of 100 microseconds (s-6)

lightly damped metallic material tens of milliseconds (s-3)

Decay time


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y

Decay Time:

highly damped, nonmetallic material order of 100 microseconds (s-6)lightly damped metallic material tens of milliseconds (s-3)

Attenuation

The intensity of an AE signal detected by a sensor is considerably lower than

the intensity that would have been observed in the close proximity of the

source. This is due to attenuation. There are three main causes of attenuation,beginning with geometric spreading. As an AE spreads from its source in a

plate like material its amplitude decays by 30% every time it doubles its


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plate-like material, its amplitude decays by 30% every time it doubles its

distance from the source. In three-dimensional structures, the signal decays

on the order of 50%. This can be traced back to the simple conservation ofenergy. Another cause of attenuation is material damping, as alluded to in the

previous paragraph. While an AE wave passes through a material, its elastic

and kinetic energies are absorbed and converted into heat. The third cause of

attenuation is wave scattering. Geometric discontinuities (e.g. twinboundaries, nonmetallic inclusions, or grain boundaries) and structural

boundaries both reflect some of the wave energy that was initially transmitted.

Attenuation:

Spread (30% for 2D, 50% for 3D for each doubling of distance from source),

Material damping,

Wave scattering at interfaces

Attenuation:

1. Spread (30% for 2D, 50% for 3D for each doubling of distance from

source),

2. Material damping,3. Wave scattering at interfaces


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1

2

3

3

Measurements of the effects of attenuation on an AE signal can be performed

with a simple apparatus known as a Hsu-Nielson Source. This consists of a

mechanical pencil with either 0.3 or 0.5 mm 2H lead that is passed through a

cone-shaped Teflon shoe designed to place the lead in contact with thesurface of a material at a 30 degree angle. When the pencil lead is pressed

and broken against the material, it creates a small, local deformation that is


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relieved in the form of a stress wave, similar to the type of AE signal produced

by a crack. By using this method, simulated AE sources can be created atvarious sites on a structure to determine the optimal position for the

placement of sensors and to ensure that all areas of interest are within the

detection range of the sensor or sensors.

http://www.ndt.net/ndtaz/content.php?id=474


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Wave Mode and Velocity

As mentioned earlier, using AE inspection in conjunction with other NDE

techniques can be an effective method in gauging the location and nature of

defects. Since source locations are determined by the time required for thewave to travel through the material to a sensor, it is important that the velocity

of the propagating waves be accurately calculated This is not an easy task


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of the propagating waves be accurately calculated. This is not an easy task

since wave propagation depends on the material in question and the wave

mode being detected. For many applications, Lamb waves are of primaryconcern because they are able to give the best indication of wave

propagation from a source whose distance from the sensor is larger than the

thickness of the material. For additional information on Lamb waves, see the

wave mode page in the Ultrasonic Inspection section.

Equipment- Probe

Case


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Damping

materials

Wear plate

Electrode

Piezoelectric element

Couplants

Specimen

Equipment- Probe


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Equipment

Acoustic emission testing can be performed in the field with portable

instruments or in a stationary laboratory setting. Typically, systems contain a

sensor, preamplifier, filter, and amplifier, along with measurement, display,and storage equipment (e.g. oscilloscopes, voltmeters, and personal

computers). Acoustic emission sensors respond to dynamic motion that is


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p ) p y

caused by an AE event. This is achieved through transducers which convert

mechanical movement into an electrical voltage signal. The transducerelement in an AE sensor is almost always a piezoelectric crystal, which is

commonly made from a ceramic such as Lead Zirconate Titanate (PZT).

Transducers are selected based on operating frequency, sensitivity and

environmental characteristics, and are grouped into two classes: resonantand broadband. The majority of AE equipment is responsive to movement in

its typical operating frequency range of 30 kHz to 1 MHz. For materials with

high attenuation (e.g. plastic composites), lower frequencies may be used to

better distinguish AE signals. The opposite holds true as well.

Key Points:

Two classes: resonant and broadband.

The majority of AE equipment is responsive to movement in its typicaloperating frequency range of 30 kHz to 1 MHz.

For materials with high attenuation (e.g. plastic composites), lower


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g ( g p p ),

frequencies may be used to better distinguish AE signals. The opposite

holds true as well.

Ideally, the AE signal that reaches the mainframe will be free of background

noise and electromagnetic interference. Unfortunately, this is not realistic.

However, sensors and preamplifiers are designed to help eliminate unwanted

signals. First, the preamplifier boosts the voltage to provide gain and cabledrive capability. To minimize interference, a preamplifier is placed close to the

transducer; in fact, many transducers today are equipped with integrated

preamplifiers Next the signal is relayed to a bandpass filter for elimination of


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preamplifiers. Next, the signal is relayed to a bandpass filter for elimination of

low frequencies (common to background noise) and high frequencies.Following completion of this process, the signal travels to the acoustic system

mainframe and eventually to a computer or similar device for analysis and

storage. Depending on noise conditions, further filtering or amplification at the

mainframe may still be necessary.

Schematic Diagram of a Basic Four-channel Acoustic Emission Testing

System


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FIGURE 16.5 The main elements of a modern acoustic emission detection system.


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After passing the AE system mainframe, the signal comes to a

detection/measurement circuit as shown in the figure directly above. Note that

multiple-measurement circuits can be used in multiple sensor/channel

systems for source location purposes (to be described later). At themeasurement circuitry, the shape of the conditioned signal is compared with a

threshold voltage value that has been programmed by the operator. Signals

are either continuous (analogous to Gaussian random noise with amplitudes


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are either continuous (analogous to Gaussian, random noise with amplitudes

varying according to the magnitude of the AE events) or burst-type. Each time

the threshold voltage is exceeded, the measurement circuit releases a digital

pulse. The first pulse is used to signify the beginning of a hit. (A hit is used to

describe the AE event that is detected by a particular sensor. One AE event

ca

An Introduction to AET Reading 2004-01

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