An Introduction to Acoustic Emission Testing, AET

2014-JuneFacilitators: Fion Zhang/ Charliechong

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

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

Speaker: Fion Zhang2014/6/13

Contents:1. AE Codes and Standards

■ ASTM ■ ASME V

2. Reading 01,3. Reading 02,4. Reading 03,5. Others reading.

ASME V Article Numbers:Gen Article 1RT Article 2Nil Article 3 UT Article 4 for weldsUT Article 5 for materialsPT Article 6MT Article 7ET Article 8Visual Article 9LT Article 10AE Article 11 (FRP) /Article 12 (Metallic) / Article 13 (Continuous)Qualif. Article 14ACFM Article 15

ASTM Standards1. ASTM E 1930 “Standard Practice for Examination of Liquid-Filled

Atmospheric and Low-Pressure Metal Storage Tanks Using Acoustic Emission”

2. ASTM E 569 “Standard Practice for Acoustic Emission Monitoring of Structures During Controlled Stimulation”

3. 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.

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

http://www.corrosionsource.com/(S(vf34kqncr0uklwzu0ioy5dz2))/FreeContent/3/Combatting+Liquid+Metal+Attack+by+Mercury+in+Ethylene+and+Cryogenic+Gas+PlantsTask+1+-+Non-Destructive+Testing

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

Typical AET Signal

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

Typical AET Signal

Study Note 1:http://www.geocities.ws/raobpc/AET.html

What is AEAcoustic emission is the technical term for the noise emitted by materials and structures 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 surface where 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" emanating from regions of localized deformation within a material.

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.

Acoustic Emission 声发射

Acoustic Emission (AE) refers to generation of transient elastic waves 瞬间弹性波 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. 应力作用下, 结构中的裂纹萌生/扩展(塑性变形)造成的位错运动. 这位错运动会引发瞬间的弹性波.

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.

其他会引起瞬间的弹性波 的因素:熔化,相变,热应力冷却裂纹和应力建立,孪晶,在复合材料中的纤维断裂和纤维-基体界面脱粘

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

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 processed by 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 systems and 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 of energy.

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 metals and 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, micro

yielding, development of cracks

AET Set-up

Continuous type- Gaussian random noise

Continuous type

Discrete Burst Type

Discrete Burst Type

Kaiser EffectPlastic deformation is the primary source of AE in loaded metallic structures. An important feature affecting the AE during deformation of a material is ‘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'.

Key words:Kaiser effectFalicity effect

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

AE Parameters

Various parameters used in AET include: AE burst, threshold, ring down count, cumulative counts, event duration, peak amplitude, rise time, energy and rms voltage etc. Typical AE system consists of signal detection, amplification & enhancement, data acquisition, processing and analysis units.

Sensors / Source Location Identification

The most commonly used sensors are resonance type piezoelectric transducers with proper couplants. In some applications where sensors cannot 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 defectformation and failures. When more than one sensors is used,

AE source can be located based by measuring the signal’s arrival time to each sensor. By comparing the signal’s arrival time at different sensors, the source location can be calculated through triangulation 三角测量 and other methods.

AE sources are usually classified based on activity 活动力 and intensity 强度. 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 变化率随着刺激增加不断提高.

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 immediate indication 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 periodic inspection 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.

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 for

obtaining 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 frequency domain analysis are expected to improve the situation.

A Few Typical Applications

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

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

Acoustic Emission Testing applications are most suitable for:

1. Aboveground Storage Tank Screening for Corrosion & Leaks2. Pressure Containment Vessels (Columns, Bullets, Cat Crackers)3. Horton Spheres & legs4. Fiberglass Reinforced Plastic Tanks and Piping5. Offshore Platform Monitoring6. Nuclear components inspection7. Tube Trailers8. Railroad tank cars9. Bridge Critical Members monitoring10. Pre- & Post-Stressed Concrete Beams11. Reactor Piping12. High Energy Seam Welded Hot Reheat Piping Systems in Power Plants.13. On-Stream Monitoring14. Remote Long Term Monitoring

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

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 & Vessels4. Real time monitoring during cool-down & start-ups5. Real Time Monitoring Saves Money6. Real Time Monitoring Improves Safety

Tank AET

End of Reading

Study Note 2:Sidney MindessUniversity of British ColumbiaChapter 16: Acoustic Emission Methods

16Acoustic EmissionMethods

http://unina.stidue.net/Politecnico%20di%20Milano/Ingegneria%20Strutturale/Corsi/Felicetti%20-%20Structural%20assessment%20and%20residual%20bearing%20capacity/books/Handbook%20of%20NDT%20of%20Concrete/1485_C16.pdf

Dam

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

Dam

16.1 Introduction16.2 Historical Background16.3 Theoretical Considerations16.4 Evaluation of Acoustic Emission Signals16.5 Instrumentation and Test Procedures16.6 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 EmissionFracture Mechanics Studies · Type of Cracks · Fracture ProcessZone (Crack Source) Location · Strength vs. Acoustic EmissionRelationships · Drying Shrinkage · Fiber Reinforced Cementsand Concretes · High Alumina Cement · Thermal Cracking ·Bond in Reinforced Concrete · Corrosion of Reinforcing Steelin Concrete

16.8 Field Studies of Acoustic Emission16.9 Conclusions

Foreword:Acoustic emission refers to the sounds, both audible and sub-audible, that are generated when a material undergoes irreversible changes, such as those due 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, and concrete 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 study of concrete properties, its use in the field remains problematic.

16.1 IntroductionIt is common experience that the failure of a concrete specimen under load is accompanied by a considerable amount of audible noise. In certain circumstances, 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 the sophisticated 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 transient elastic 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 changes, such as (1) dislocation movement, (2) twinning, (3) phase transformations, (4) crack initiation, and propagation, (5) debonding between continuous 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 processes2. Slip between concrete and steel reinforcement3. Fracture or debonding of fibers in fiber-reinforced concrete

16.2 Historical BackgroundThe initial published studies of acoustic emission phenomena, in the early 1940s, dealt with the problem of predicting rockbursts in mines; this technique is 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 carriedout by Rüsch, 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, L’Hermite also measured acoustic emission from concrete, finding that a sharp increase in acoustic emission coincided with the point at which Poisson’s 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 acoustic emission occurred at much lower load levels than had been reported earlier, and hence, could be used to monitor earlier microcracking (such as that 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 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 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 ConsiderationsWhen 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):

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.

FIGURE 16.1 The two basic types of acoustic emission signals. (A) Continuousemission. (B) Burst emission.

16.4 Evaluation of Acoustic Emission Signals

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, as shown 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 the acoustic 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.)

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, 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.)

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

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 the counts.

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 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 of signals 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 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 frequency analysis may sometimes simply be a function of the response of the transducer, and thus reveal little of the true nature of the pulse.

Energy analysis — This 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 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

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

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

Ring-down count= 13

Signal Evaluation: Raise Time/ Event Counts/ Signal Duration

Raise time mV/μs

Signal duration μs

Event counts = 3 in unit time

Signal Evaluation: Amplitude Distribution- Triangulation to locate source

Signal Evaluation: Amplitude Distribution- Triangulation to locate source

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

Signal Evaluation: Frequency analysis

Signal Evaluation:Energy analysis- it is essentially the area under the amplitude vs. time curveNote: all areas under curves or only areas above threshold.

Signal Evaluation: Raise Time/ Event Counts/ Signal Duration

ring-down counting

Signal Evaluation: Raise Time/ Event Counts/ Signal Duration

16.5 Instrumentation and Test Procedures

Instrumentation (and, where necessary, the associated computer software) is available, from a number of different manufacturers, to carry out all of the methods 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 in Figure 16.5.

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

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

1. Transducers: Piezoelectric transducers (generally made of lead zirconatetitanate, PZT) are used to convert the surface displacements into electric signals. 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 100μV. There are basically two 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, the transducers 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 materials show this effect – examples of these are quartz, barium titanate, lithium sulphate, 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 (see acoustic 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 λ = 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/

AETTransducerIn 0.1KHz~2.0KHz

UT Transducers 2.0~5.0 MHz

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 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 gain of 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 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 which the 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 the specimen. 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 structure of the material, and how structural changes (i.e., damage) occur during 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 some predetermined 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 Concrete16.6.1 The Kaiser Effect

The earliest acoustic emission studies of concrete, such as the work of Rüsch, 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 onreloading. This was true, however, only for stress levels below about 75 to 85% 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 below the peak stress of the material.

Key points:For concrete This was true, however, only for stress levels below about 75 to 85% 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 maximum strain was reached. It has also been suggested that a form of the Kaiser 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 again recorded 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 structural member has been subjected to.

Kaiser Effect- Concrete

For concrete This was true, however, only for stress levels below about 75 to 85% of the ultimate strength of the material

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 maximum strain was reached.

Spooner and Dougill conclusion on Kaiser Effect- Concrete:

They therefore concluded “that the Kaiser effect is not a 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 due to the friction between the ends of the specimen and the loading platens can 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 emission appears 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 nonetheless makes 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 propagate through the concrete. Thus, large acoustic emission events that take 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. It has 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 better transmission 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 array of transducers arranged in a 40-cm square mesh to locate acoustic emission 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 that requires much more study.

16.6.4 Specimen Geometry

It has been shown that smaller specimens appear to give rise to greater levels of acoustic emission than do larger ones. The reasons for this are not clear, 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 basic problem 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 acoustic emission. It has been reported that concretes with a smaller maximum 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 at lower 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 greater for higher strength concretes. However, as was mentioned earlier, for similar 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 been carried out in the laboratory, and have been largely “theoretical” in nature:

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

2. 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 oncretes. 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 of emissions 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. 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 先入为主的观念 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 concrete undergoes 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 signal 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 techniques were applied to acoustic emission analysis. In 1981, Michaels et al.15 and Niwa et al.38 developed deconvolution techniques 反褶积技术 to analyze acoustic waveforms, in order to provide a stress-time history of the source of an acoustic event. Similar deconvolution techniques were subsequently used by 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 between acoustic 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-Boczaret 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:

Where:Fr is the rupture strength, Fp is the stress corresponding to the first acoustic emission signal, anda 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 because 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 the variability in the strength vs. total acoustic emission counts relationship; the within-batch variability is even more severe, as shown 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. With permission.)

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 Shrinkage16.7.6 Fiber Reinforced Cements and Concretes16.7.7 High Alumina Cement16.7.8 Thermal Cracking16.7.9 Bond in Reinforced Concrete16.7.10 Corrosion of Reinforcing Steel in Concrete

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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 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 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? If no, the entire 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. 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 the evaluation of phenomena taking place in concrete in structures.”18

End of Reading

Study Note 3:Introduction to Acoustic Emission Testing http://www.ndt-ed.org/EducationResources/CommunityCollege/Other%20Methods/AE/AE_Intro.htm

Acoustic Emission (AE) refers to the generation of transient elastic wavesproduced 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 of 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-bonding contribute to acoustic emissions.

AE’s 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 Emission 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 and 5. is used extensively as a research tool.

Twinning

AET

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 often 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 逆 from loud service 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 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 emissions made 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 AE emissions 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 of martensite in high-nickel steel. Experimentation continued throughout the mid-1900’s, 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 Kaiser’s efforts, Bradford Schofield initiated the first research program in the United States to look at the materials engineering applications of AE. Fittingly, Kaiser’s 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, AE’s originate with stress. 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 as elastic 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 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 activity will also be observed when the material ahead of the crack tip undergoes plastic deformation (micro-yielding).

Two sources of fatigue cracks also cause AE’s. 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 second source is the propagation of the crack tip that occurs through the movement 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 to detect the wave energy and decipher which signals are meaningful.

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

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 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 is removed 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), AE’s are generated and stop when the load is removed. However, at point F, the applied load is high enough to cause significant emissions 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

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. As shown in the figure, if AE signals continue to be detected during the holding 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 guideline governing AE’s 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

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 frictional sources (e.g. loose bolts or movable connectors that shift when exposed to 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 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-like function corresponding to the permanent change associated with the source process. The analogous velocity and stress waveforms are essentially pulse-like. The width and height of the primitive pulse depend on the dynamics of the source process. Source processes such as microscopic crack jumps and precipitate fractures 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 the primitive 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 pulse corresponding to a permanent displacement of the material. The ordinate quantities refer to a point in the material.

Waves radiates from the source in all directions, often having a strong directionality depending on the nature of the source process, as shown in the second figure. Rapid movement is necessary if a sizeable amount of the elastic energy 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.

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. The ultimate goal of studies of the interaction between elastic waves and material 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 a small 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 the Equipment 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 the object. Signals that are in phase with each other as they reach the sensor produce constructive interference which usually results in the highest peak of 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

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 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 of energy. 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. twin boundaries, 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

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 the surface 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 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 at various 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

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 the wave 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 since wave propagation depends on the material in question and the wave mode being detected. For many applications, Lamb waves are of primary concern 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

Damping materials

Wear plate

ElectrodePiezoelectric elementCouplants

Specimen

Equipment- Probe

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 caused by an AE event. This is achieved through transducers which convert mechanical movement into an electrical voltage signal. The transducer element 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: resonant and 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 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.

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 cable drive 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 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

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

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 the measurement 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 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 can cause a system with numerous channels to record multiple hits.) Pulses will continue to be generated while the signal exceeds the threshold voltage. Once this process has stopped for a predetermined amount of time, the hit is finished (as far as the circuitry is concerned). The data from the hit is then read into a microcomputer and the measurement circuit is reset.

Hit Driven AE Systems and Measurement of Signal Features

Although several AE system designs are available (combining various options, sensitivity, and cost), most AE systems use a hit-driven architecture. The hit-driven design is able to efficiently measure all detected signals and record digital descriptions for each individual feature (detailed later in this section). During periods of inactivity, the system lies dormant. Once a new signal is detected, the system records the hit or hits, and the data is logged for present and/or future display.

Also common to most AE systems is the ability to perform routine tasks that are valuable for AE inspection. These tasks include quantitative signal measurements with corresponding time and/or load readings, discrimination between real and false signals (noise), and the collection of statistical information about the parameters of each signal.

AET

AET

AE Signal Features

With the equipment configured and setup complete, AE testing may begin. The sensor is coupled to the test surface and held in place with tape or adhesive. An operator then monitors the signals which are excited by the induced stresses in the object. When a useful transient, or burst signal is correctly obtained, parameters like amplitude, counts, measured area under the rectified signal envelope (MARSE), duration, and rise time can be gathered. Each of the AE signal feature shown in the image is described below.

Abbreviation:measured area under the rectified signal envelope (MARSE)

AET Signals

Amplitude, A, is the greatest measured voltage in a waveform and is measured in decibels (dB). This is an important parameter in acoustic emission inspection because it determines the detectability of the signal. Signals with amplitudes below the operator-defined, minimum threshold will not be recorded.

Rise time, R, is the time interval between the first threshold crossing and the signal peak. This parameter is related to the propagation of the wave betweenthe source of the acoustic emission event and the sensor. Therefore, rise time is used for qualification of signals and as a criterion for noise filter.

Duration, D, is the time difference between the first and last threshold crossings. Duration can be used to identify different types of sources and to filter out noise. Like counts (N), this parameter relies upon the magnitude of the signal and the acoustics of the material.

MARSE, E, sometimes referred to as energy counts, is the measure of the area under the envelope of the rectified linear voltage time signal from the transducer. This can be thought of as the relative signal amplitude and is useful because the energy of the emission can be determined. MARSE is also sensitive to the duration and amplitude of the signal, but does not use counts or user defined thresholds and operating frequencies. MARSE is regularly used in the measurements of acoustic emissions.

Counts, N, refers to the number of pulses emitted by the measurement circuitry if the signal amplitude is greater than the threshold. Depending on the magnitude of the AE event and the characteristics of the material, one hit may produce one or many counts. While this is a relatively simple parameter to collect, it usually needs to be combined with amplitude and/or duration measurements to provide quality information about the shape of a signal

Data Display

Software-based AE systems are able to generate graphical displays for analysis of the signals recorded during AE inspection. These displays provide valuable information about the detected events and can be classified into four categories:

■ location, ■ activity, ■ intensity, and ■ data quality (crossplots).

Location displays identify the origin of the detected AE events. These can be graphed by X coordinates, X-Y coordinates, or by channel for linear computed-source location, planar computed-source location, and zone location techniques.

Examples of each graph are shown to the right. Activity displays show AE activity as a function of time on an X-Y plot (figure below left).

Each bar on the graphs represents a specified amount of time. For example, a one-hour test could be divided into 100 time increments. All activity measured within a given 36 second interval would be displayed in a given histogram bar. Either axis may be displayed logarithmically in the event of high AE activity or long testing periods. In addition to showing measured activity over a single time period, cumulative activity displays (figure below right) can be created to show the total amount of activity detected during a test. This display is valuable for measuring the total emission quantity and the average rate of emission.

Intensity displays are used to give statistical information concerning the magnitude of the detected signals. As can be seen in the amplitude distribution graph to the near right, the number of hits is plotted at each amplitude increment (expressed in dB’s) beyond the user-defined threshold. These graphs can be used to determine whether a few large signals or many small ones created the detected AE signal energy. In addition, if the Y-axis is plotted logarithmically, the shape of the amplitude distribution can be interpreted to determine the activity of a crack (e.g. a linear distribution indicates growth).

The fourth category of AE displays, crossplots, is used for evaluating the quality of the data collected. Counts versus amplitude, duration versus amplitude, and counts versus duration are frequently used crossplots. As shown in the final figure, each hit is marked as a single point, indicating the correlation between the two signal features. The recognized signals from AE events typically form a diagonal band since larger signals usually generate higher counts. Because noise signals caused by electromagnetic interference do not have as many threshold-crossing pulses as typical AE source events, the hits are located below the main band. Conversely, signals caused by friction or leaks have more threshold-crossing pulses than typical AE source events and are subsequently located above the main band. In the case of ambiguous data, expertise is necessary in separating desirable

Amplitude/countsSignal Analysis

Because noise signals caused by electromagnetic interference do not have as many threshold-crossing pulses as typical AE source events, the hits are located below the main band

Conversely, signals caused by friction or leaks have more threshold-crossing pulses than typical AE source events and are subsequently located above the main band.

The recognized signals from AE events typically form a diagonal band since larger signals usually generate higher counts. Because noise signals caused by electromagnetic interference do not have as many threshold-crossing pulses as typical AE source events,

AE Source Location Techniques

Multi-Channel Source Location Techniques:

Locating the source of significant acoustic emissions is often the main goal of an inspection. Although the magnitude of the damage may be unknown after AE analysis, follow up testing at source locations can provide these answers. As previously mentioned, many AE systems are capable of using multiple sensors/channels during testing, allowing them to record a hit from a single AE event. These AE systems can be used to determine the location of an event source. As hits are recorded by each sensor/channel, the source can be located by knowing the velocity of the wave in the material and the difference in hit arrival times among the sensors, as measured by hardware circuitry or computer software. By properly spacing the sensors in this manner, it is possible to inspect an entire structure with relatively few sensors.

Source location techniques assume that AE waves travel at a constant velocity in a material. However, various effects may alter the expected velocity of the AE waves (e.g. reflections and multiple wave modes) and can affect the accuracy of this technique. Therefore, the geometric effects of the structure being tested and the operating frequency of the AE system must be considered when determining whether a particular source location technique is feasible for a given test structure.

Keywords:reflections and multiple wave modes

■ Linear Location Technique

Several source location techniques have been developed based on this method. One of the commonly used computed-source location techniques is the linear location principle shown to the right. Linear location is often used to evaluate struts on truss bridges. When the source is located at the midpoint, the time of arrival difference for the wave at the two sensors is zero. If the source is closer to one of the sensors, a difference in arrival times is measured.

To calculate the distance of the source location from the midpoint, the arrival time is multiplied by the wave velocity. Whether the location lies to the right or left of the midpoint is determined by which sensor first records the hit. This is a linear relationship and applies to any event sources between the sensors.

Because the above scenario implicitly assumes that the source is on a line passing through the two sensors, it is only valid for a linear problem. When using AE to identify a source location in a planar material, three or more sensors are used, and the optimal position of the source is between the sensors. Two categories of source location analysis are used for this situation: zonal location and point location.

■ Zonal Location Technique

As the name implies, zonal location aims to trace the waves to a specific zone or region around a sensor. This method is used in anisotropic materials or in other structures where sensors are spaced relatively far apart or when high material attenuation affects the quality of signals at multiple sensors. Zones can be lengths, areas or volumes depending on the dimensions of the array. A planar sensor array with detection by one sensor is shown in the upper right figure. The source can be assumed to be within the region and less than halfway between sensors.

When additional sensors are applied, (1) arrival times and (2) amplitudes help pinpoint the source zone. The ordered pair in lower right figure represents the two sensors detecting the signal in the zone and the order of signal arrival at each sensor. When relating signal strength to peak amplitude, the largest peak amplitude is assumed to come from the nearest sensor, second largest from the next closest sensor and so forth.

■ Point Location

In order for point location to be justified, signals must be detected in a minimum number of sensors: (1) two for linear, (2) three for planar, (3) four for volumetric. Accurate arrival times must also be available. Arrival times are often found by using peak amplitude or the first threshold crossing. The velocity of wave propagation and exact position of the sensors are necessary criteria as well. Equations can then be derived using sensor array geometry or more complex algebra to locate more specific points of interest.

AE Barkhausen Techniques

The Barkhausen effect

The Barkhausen effect refers to the sudden change in size of ferromagnetic domains that occur during magnetization or demagnetization. During magnetization, favorably oriented domains develop at the cost of less favorably oriented domains. These two factors result in minute jumps of magnetization when a ferromagnetic sample (e.g. iron) is exposed to an increasing magnetic field (see figure). Domain wall motion itself is determined by many factors like microstructure, grain boundaries, inclusions, and stress and strain. By the same token, the Barkhausen effect is too a function of stress and strain.

Barkhausen Noise

Barkhausen noise can be heard if a coil of wire is wrapped around the sample undergoing magnetization. Abrupt movements in the magnetic field produce spiking current pulses in the coil. When amplified, the clicks can be compared to Rice Krispies or the crumbling a candy wrapper. The amount ofBarkhausen noise is influenced by material imperfections and dislocations and is likewise dependent on the mechanical properties of a material. Currently, materials exposed to high energy particles (nuclear reactors) or cyclic mechanical stresses (pipelines) are available for nondestructive evaluation using Barkhausen noise, one of the many branches of AE testing.

Hysterisis Loop- magnetization or demagnetization.

Barkhausen noisegenerated if the magnetic field was induced on the areas with discontinuiies(throughout the whole loop)

Applications

Acoustic emission is a very versatile, non-invasive way to gather information about a material or structure. Acoustic Emission testing (AET) is be applied to inspect and monitor pipelines, pressure vessels, storage tanks, bridges, aircraft, and bucket trucks, and a variety of composite and ceramic components. It is also used in process control applications such as monitoring welding processes. A few examples of AET applications follow.

Weld Monitoring

During the welding process, temperature changes induce stresses between the weld and the base metal. These stresses are often relieved by heat treating the weld. However, in some cases tempering the weld is not possible and minor cracking occurs. Amazingly, cracking can continue for up to 10 days after the weld has been completed. Using stainless steel welds with known inclusions and accelerometers for detection purposes and background noise monitoring, it was found by W. D. Jolly (1969) that low level signals and more sizeable bursts were related to the growth of microfissures and larger cracks respectively. ASTM E 749-96 is a standard practice of AE monitoring of continuous welding.

Bucket Truck (Cherry Pickers) Integrity Evaluation

Accidents, overloads and fatigue can all occur when operating bucket trucks or other aerial equipment. If a mechanical or structural defect is ignored, serious injury or fatality can result. In 1976, the Georgia Power Company pioneered the aerial manlift device inspection. Testing by independent labs and electrical utilities followed. Although originally intended to examine only the boom sections, the method is now used for inspecting the pedestal, pins, and various other components. Normally, the AE tests are second in a chain of inspections which start with visual checks. If necessary, follow-up tests take the form of magnetic particle, dye penetrant, or ultrasonic inspections. Experienced personnel can perform five to ten tests per day, saving valuable time and money along the way. ASTM F914 governs the procedures for examining insulated aerial personnel devices.

AET Application

Gas Trailer Tubes

Acoustic emission testing on pressurized jumbo tube trailers was authorized by the Department of Transportation in 1983. Instead of using hydrostatic retesting, where tubes must be removed from service and disassembled, AET allows for in situ testing. A 10% over-pressurization is performed at a normal filling station with AE sensors attached to the tubes at each end. A multichannel acoustic system is used to detection and mapped source locations. Suspect locations are further evaluated using ultrasonic inspection, and when defects are confirmed the tube is removed from use. AET can detect subcritical flaws whereas hydrostatic testing cannot detect cracks until they cause rupture of the tube. Because of the high stresses in the circumferential direction of the tubes, tests are geared toward finding longitudinal fatigue cracks.

Bridges

Bridges contain many welds, joints and connections, and a combination of load and environmental factors heavily influence damage mechanisms such as fatigue cracking and metal thinning due to corrosion. Bridges receive a visual inspection about every two years and when damage is detected, the bridge is either shut down, its weight capacity is lowered, or it is singled out for more frequent monitoring. Acoustic Emission is increasingly being used for bridge monitoring applications because it can continuously gather data and detect changes that may be due to damage without requiring lane closures or bridge shutdown. In fact, traffic flow is commonly used to load or stress the bridge for the AE testing.

Aerospace Structures

Most aerospace structures consist of complex assemblies of components that have been design to carry significant loads while being as light as possible. This combination of requirements leads to many parts that can tolerate only a minor amount of damage before failing. This fact makes detection of damage extremely important but components are often packed tightly together making access for inspections difficult. AET has found applications in monitoring the health of aerospace structures because sensors can be attached in easily accessed areas that are remotely located from damage prone sites. AET has been used in laboratory structural tests, as well as in flight test applications. NASA's Wing Leading Edge Impact Detection System is partially based on AE technology. The image to the right (above) shows a technician applying AE transducers on the inside of the Space Shuttle Discovery wing structure. The impact detection system was developed to alert NASA officials to events such as the sprayed-on-foam insulation impact that damaged the Space Shuttle Columbia's wing leading edge during launch and lead to its breakup on reentry to the Earth's atmosphere.

Others

Fiber-reinforced polymer-matrix composites, in particular glass-fiber reinforced parts or structures (e.g. fan blades)

Material research (e.g. investigation of material properties, breakdown mechanisms, and damage behavior)

Inspection and quality assurance, (e.g. wood drying processes, scratch tests)

Real-time leakage test and location within various components (small valves, steam lines, tank bottoms)

Detection and location of high-voltage partial discharges in transformers Railroad tank car and rocket motor testing

There are a number of standards and guidelines that describe AE testing and application procedures as supplied by the American Society for Testing and Materials (ASTM). Examples are ASTM E 1932 for the AE examination of small parts and ASTM E1419-00 for the method of examining seamless, gas-filled, pressure vessels.