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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.
Signal Evaluation: Raise Time/ Event Counts/ Signal Duration
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ring-down counting
Signal Evaluation: Raise Time/ Event Counts/ Signal Duration
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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
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