understanding acoustic emission testing- reading i

Charlie Chong/ Fion Zhang

Understanding Acoustic Emission Testing, AET- Reading 1My Pre-exam ASNT Self Study Notes3rd September 2015


E&P Applications


Concrete Offshore structure


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Refinery Applications


E&P Applications

http://wins-ndt.com/oil-chem/spherical-tanks/Charlie Chong/ Fion Zhang

Refinery Applications

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


The Magical Book of Neutron Radiography



ASNT Certification GuideNDT Level III / PdM Level IIIAE - Acoustic Emission TestingLength: 4 hours Questions: 135

1 Principles and Theory Characteristics of acoustic emission testing Materials and deformation Sources of acoustic emission Wave propagation Attenuation Kaiser and Felicity effects, and Felicity ratio Terminology (refer to acoustic emission glossary, ASTM 1316)


Signal conditioning Signal detection Signal processing Source location Advanced signal processing Acoustic emission test systems Accessory materials Factors affecting test equipment selection

2 Equipment and Materials Transducing processes Sensors Sensor attachments Sensor utilization Simulated acoustic emission sources Cables


4 Interpretation and Evaluation Data interpretation Data evaluation Reports5 Procedures6 Safety and Health7 Applications Laboratory studies (material-characterization)

Structural applications

3 Techniques Equipment calibration and set up for test

Establishing loading procedures Precautions against noise Special test procedures Data displays


Reference Catalog NumberNDT Handbook, Second Edition: Volume 5, Acoustic Emission Testing 130Acoustic Emission: Techniques and Applications 752

Fion Zhang at Shanghai3rd September 2015

http://meilishouxihu.blog.163.com/Charlie Chong/ Fion Zhang

Greek Alphabet


Charlie Chong/ Fion Zhang http://greekhouseoffonts.com/

Video on - Leak Detection on Buried Water Piping using Acoustic Emission

https://www.youtube.com/watch?v=9kq6JxIJDik


Contents:AE Codes and Standards

ASTM ASME V

1. Reading 01- www.geocities.ws/raobpc/AET.html2. Reading 02- Sidney Mindess University of British Columbia Chapter 16:

Acoustic Emission Methods3. Reading 03- AET ndt-ed.org4. Reading 04- Terms & Definitions ASTM E13165. Reading 05- Q&A 25 items6. Reading 06- High Strength Steel- TWIP Steel7. Reading 07- AET- optimum solution for leakage detection of water pipeline8. 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) AE Article 12 (Metallic)AE Article 13 (Continuous)Qualif. Article 14ACFM Article 15


ASTM StandardsE569 - 07Standard Practice for Acoustic Emission Monitoring of StructuresDuring Controlled Stimulation E650 97 (2007)Standard Guide for Mounting Piezoelectric Acoustic Emission SensorsE749 - 07Standard Practice for Acoustic Emission Monitoring DuringContinuous WeldingE750 - 04Standard Practice for Characterizing Acoustic EmissionInstrumentationE751 - 07Standard Practice for Acoustic Emission Monitoring During ResistanceSpot-Welding


ASTM StandardsE976 - 05Standard Guide for Determining the Reproducibility of AcousticEmission Sensor ResponseE1067 - 07Standard Practice for Acoustic Emission Examination of FiberglassReinforced Plastic Resin (FRP) Tanks/VesselsE1106 - 07Standard Test Method for Primary Calibration of Acoustic EmissionSensorsE1118 - 05Standard Practice for Acoustic Emission Examination of ReinforcedThermosetting Resin Pipe (RTRP)E1139 - 07Standard Practice for Continuous Monitoring of Acoustic Emissionfrom Metal Pressure Boundaries


ASTM StandardsE1211 - 07Standard Practice for Leak Detection and Location Using Surface-Mounted Acoustic Emission SensorsE1419 - 09Standard Practice for Examination of Seamless, Gas-Filled, PressureVessels Using Acoustic EmissionE1495 - 02(2007)Standard Guide for Acousto-Ultrasonic Assessment of Composites,Laminates, and Bonded JointsE1736 - 05Standard Practice for Acousto-Ultrasonic Assessment of Filament-Wound Pressure Vessels


ASTM StandardsE1781 - 08Standard Practice for Secondary Calibration of Acoustic EmissionSensorsE1888 /E1888M 07 Standard Practice for Acoustic Emission Examination of PressurizedContainers Made of Fiberglass Reinforced Plastic with Balsa WoodCoresE1930 07 Standard Practice for Examination of Liquid-Filled Atmospheric andLow-Pressure Metal Storage Tanks Using Acoustic EmissionE1932 - 07 Standard Guide for Acoustic Emission Examination of Small PartsE2075 05 Standard Practice for Verifying the Consistency of AE-SensorResponse Using an Acrylic Rod


ASTM StandardsE2076 - 05Standard Test Method for Examination of Fiberglass Reinforced PlasticFan Blades Using Acoustic EmissionE2191 - 08Standard Practice for Examination of Gas-Filled Filament-WoundComposite Pressure Vessels Using Acoustic EmissionE2374 - 04Standard Guide for Acoustic Emission System PerformanceVerificationE2478 - 06aStandard Practice for Determining Damage-Based Design Stress forFiberglass Reinforced Plastic (FRP) Materials Using AcousticEmissionE2598 - 07Standard Practice for Acoustic Emission Examination of Cast IronYankee and Steam Heated Paper Dryers


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

Charlie Chong/ Fion Zhang 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 (1) mechanical, (2) thermal or (3) 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 (piezoelectric) . In this way information about the existence and location (triangulation by multi-transducers) 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.


Applications:Static subjectsDynamic subjects

Acoustic EmissionAcoustic 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.

:,,,,-


AE TechniqueThe 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 AETAcoustic emissions are broadly classified into two major types namely;

continuous type (associated with lattice dislocation) burst type. (twinning, micro yielding, development of crack)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



What is Normal (Gaussian) distributionIn probability theory, the normal (or Gaussian) distribution is a very common continuous probability distribution. Normal distributions are important in statistics and are often used in the natural and social sciences to represent real-valued random variables whose distributions are not known.[1][2]

The normal distribution is remarkably useful because of the central limit theorem. In its most general form, under mild conditions, it states that averages of random variables independently drawn from independent distributions are normally distributed. Physical quantities that are expected to be the sum of many independent processes (such as measurement errors) often have distributions that are nearly normal.[3] Moreover, many results and methods (such as propagation of uncertainty and least squares parameter fitting) can be derived analytically in explicit form when the relevant variables are normally distributed.

https://en.wikipedia.org/wiki/Normal_distribution

Charlie Chong/ Fion Zhang https://en.wikipedia.org/wiki/Normal_distribution

The normal distribution is sometimes informally called the bell curve. However, many other distributions are bell-shaped (such as Cauchy's, Student's, and logistic). The terms Gaussian function and Gaussian bell curve are also ambiguous because they sometimes refer to multiples of the normal distribution that cannot be directly interpreted in terms of probabilities.

The probability density of the normal distribution is:

Here is the mean or expectation of the distribution (and also its median and mode). The parameter is its standard deviation with its variance then 2. A random variable with a Gaussian distribution is said to be normally distributed and is called a normal deviate.

If = 0 and = 1, the distribution is called the standard normal distributionor the unit normal distribution denoted by N(0,1) and a random variable with that distribution is a standard normal deviate.


Probability density function for the normal distribution



Cumulative distribution function of an acoustic emission



DiscussionSubject: What is the difference between an Gaussian random noise and an engineering acoustic emission?

Answer: The waveform of continuous type AE signal is similar to Gaussian random noise, but the amplitude varies with acoustic emission activity.



Crystal TwinningCrystal twinning occurs when two separate crystals share some of the same crystal lattice points in a symmetrical manner. The result is an intergrowth of two separate crystals in a variety of specific configurations. A twin boundary or composition surface separates the two crystals. Crystallographers classify twinned crystals by a number of twin laws. These twin laws are specific to the crystal system. The type of twinning can be a diagnostic tool in mineral identification.

Twinning can often be a problem in X-ray crystallography, as a twinned crystal does not produce a simple diffraction pattern.

https://en.wikipedia.org/wiki/Crystal_twinning


Twin boundaries occur when two crystals of the same type intergrow, so that only a slight misorientation exists between them. It is a highly symmetrical interface, often with one crystal the mirror image of the other; also, atoms are shared by the two crystals at regular intervals. This is also a much lower-energy interface than the grain boundaries that form when crystals of arbitrary orientation grow together.

Twin boundaries are partly responsible for shock hardening and for many of the changes that occur in cold work of metals with limited slip systems or at very low temperatures. They also occur due to martensitic transformations: the motion of twin boundaries is responsible for the pseudoelastic and shape-memory behavior of nitinol, and their presence is partly responsible for the hardness due to quenching of steel. In certain types of high strength steels, very fine deformation twins act as primary obstacles against dislocation motion. These steels are referred to as 'TWIP' steels, where TWIP stands for TWinning Induced Plasticity



What is Crystal TwinningCrystal twinning occurs when two separate crystals share some of the same crystal lattice points in a symmetrical manner.

Crystal-A

Crystal-B



Crystal Twinning



Fivefold twinning in a gold nano-particle (electron microscope image).



Crystal Twinning- Diagram of twinned crystals of Albite. On the more perfect cleavage, which is parallel to the basal plane (P), is a system of fine striations, parallel to the second cleavage (M).



Crystal Twinning- Martensitic Formation


AET Set-up


Continuous type- Gaussian random noise


Continuous type


Discrete Burst Type


DiscussionSubject: explains on the weak damages signal w.r.t the severe damage in term of the recorded peak signal.


Discrete Burst Type (Kaiser effect)


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'. (?)

Comments:Kaiser effect- when the load is released and later applied, AE will not be emitted until the previous maximum is reaches.Falicity effect- an effect that deviate from Kaiser 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



Felicity effect is an effect in acoustic emission that reduces Kaiser effectat high loads of material. Under Felicity effect the acoustic emission resumes before the previous maximum load was reached

https://en.wikipedia.org/wiki/Felicity_effect

Kaiser effect

Felicity effect


Basic AE history plot showing Kaiser effect (BCB), Felicity effect (DEF), and emission during hold (GH) 2


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), 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 Effect- The 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


Felicity Effect 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.

(D)(F)

Felicity Ratio= F/D

AE ParametersVarious 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.


AE ParametersVarious 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 IdentificationThe 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 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 and other 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 .

AET AdvantagesAE 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 LimitationsOn the negative side;

AET requires stimulus. (process stimulus or externally test 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, (1) signal processing and (2) 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 Compared to conventional inspection methods the advantages of the Acoustic Emission Testing technique are:

Tank bottom Testing without removal of product. Inspection of Insulated Piping & Vessels Real time monitoring during cool-down & start-ups Real Time Monitoring Saves Money Real Time Monitoring Improves Safety


Tank AET


End of Reading 1


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


16Acoustic EmissionMethods


Dam

http://www.boomsbeat.com/articles/116/20140118/tianzi-mountains-china.htmCharlie Chong/ Fion Zhang

Content: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 (ultrasonic?, subsonic?) , 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 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 (magnitude or event count?) coincided with the point at which Poissons ratio also began to increase (i.e., at the onset of significant matrix cracking in the concrete).


Poisson's ratio, named after Simon Poisson, is the negative ratio of transverse to axial strain. When a material is compressed in one direction, it usually tends to expand in the other two directions perpendicular to the direction of compression. This phenomenon is called the Poisson effect. Poisson's ratio (nu) is a measure of this effect. The Poisson ratio is the fraction (or percent) of expansion divided by the fraction (or percent) of compression, for small values of these changes.

Conversely, if the material is stretched rather than compressed, it usually tends to contract in the directions transverse to the direction of stretching. This is a common observation when a rubber band is stretched, when it becomes noticeably thinner. Again, the Poisson ratio will be the ratio of relative contraction to relative expansion, and will have the same value as above. In certain rare cases, a material will actually shrink in the transverse direction when compressed (or expand when stretched) which will yield a negative value of the Poisson ratio.

Charlie Chong/ Fion Zhang https://en.wikipedia.org/wiki/Poisson%27s_ratio

Figure 1: A cube with sides of length L of an isotropic linearly elastic material subject to tension along the x axis, with a Poisson's ratio of 0.5. The green cube is unstrained, the red is expanded in the x direction by L due to tension, and contracted in the y and z directions by L'.Poisson Ratio = L/ L

Charlie Chong/ Fion Zhang https://en.wikipedia.org/wiki/Poisson%27s_ratio

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.

Burst emission is a qualitative description of the discrete signal related to an individual emission event occurring within the material, 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 SignalsA 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, 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.

Comments:20log (I/Io) = 80, (I/Io) = 1000020log(I/Io) = 100, (I/Io) = 100000


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.)


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.


since the most recent crossing of the threshold

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 helpdiscriminate 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.


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


The Fourier transform- (Deconvolution - of Frequency)The Fourier transform decomposes a function of time (a signal) into the frequencies that make it up, similarly to how a musical chord can be expressed as the amplitude (or loudness) of its constituent notes. The Fourier transform of a function of time itself is a complex-valued function of frequency, whose absolute value represents the amount of that frequency present in the original function, and whose complex argument is the phase offset of the basic sinusoid in that frequency. The Fourier transform is called the frequency domain representation of the original signal. The term Fourier transform refers to both the frequency domain representation and the mathematical operation that associates the frequency domain representation to a function of time. The Fourier transform is not limited to functions of time, but in order to have a unified language, the domain of the original function is commonly referred to as the time domain. For many functions of practical interest one can define an operation that reverses this: the inverse Fourier transformation, also called Fourier synthesis, of a frequency domain representation combines the contributions of all the different frequencies to recover the original function of time.


Fourier-Transform (FT)The Fourier theorem states that any waveform can be duplicated by the superposition of a series of sine and cosine waves. As an example, the following Fourier expansion of sine waves provides an approximation of a square wave. The three curves in the plot show the first one term (black line), four terms (blue line), and sixteen terms (red line) in the Fourier expansion. As more terms are added the superposition of sine waves better matches a square wave.

Charlie Chong/ Fion Zhang http://www.tissuegroup.chem.vt.edu/chem-ed/data/fourier.html

Fourier-Transform (FT) of FrequencyTo understand any complicated signal, one of the first step is to generate the Fourier transform of that signal. Fourier transform is a mathematical function that decomposes a time varying signal, as shown in figure to the right, into several sinusoidal waves. These sinusoidal waves will have different frequency, amplitude and phases but when you add them all together, the original waveform is magically recreated. The fundamental idea here is complexity reduction by splitting a waveform into manageable chunks. For reasons that initially baffled me, the powers there be chose sinusoidal waves as this manageable chunk.

Charlie Chong/ Fion Zhang https://ranabasheer.wordpress.com/2014/03/16/why-do-we-use-fourier-transform/

Signal Evaluation: Analysis of the wave-form

http://sirius.mtm.kuleuven.be/Research/NDT/AcousticEmissions/index.htmlCharlie Chong/ Fion Zhang

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

Ring-down count= 13


Signal Evaluation: Raise Time/ Event Counts/ Signal Duration

Raise time mV/s

Signal duration s

Event counts = 3 in unit time


Signal Evaluation: Amplitude Distribution- Triangulation to locate source

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

Signal Evaluation: Amplitude Distribution- Triangulation to locate source

http://iopscience.iop.org/0964-1726/21/3/035009;jsessionid=DE0B79359A6ADDA1365CAC54ABA381A2.c2Charlie Chong/ Fion Zhang

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.



ring-down counting


16.5 Instrumentation and Test ProceduresInstrumentation (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.




Raw Display?

Selective Display?

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 100V. 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.

Keywords: Since these transducers are high impedance devices, they yield relatively

low signals, typically less than 100V. Wide band & Narrow Band


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

1. Transducers: Piezoelectric transducers (generally made of lead zirconate titanate, 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 100V. 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.

Keywords: Since these transducers are high impedance devices, they yield relatively low signals, typically less than 100V. Wide band & Narrow Band

Question:Band pass (selective, High, Low?) as part of transducer constructions? Or post transducer electronic?


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.htmlCharlie Chong/ Fion Zhang

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 = 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/Charlie Chong/ Fion Zhang

AETTransducerIn 0.1KHz~2.0KHz


UT Transducers 2.0~5.0 MHz ( AET Transducer)


2. Preamplifier: Because of the low voltage output (100V) , 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 (x100, x1000). (Note: The decibel scale measures only relative amplitudes. Using this scale:

where V is 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.(high pass, low pass, selective pass)

4. The main amplifier: further amplifies the signals, typically with a gain of an additional 20 to 60 dB.

5. The threshold discriminator: is used to set the threshold voltage above which signals are counted (or analyze) .

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 (or more) 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. (i.e., hydrostatic testing)

3. Inservice monitoring where the loadings are the service loading? (e.g., monitoring of crack growth in a inservice coke drum)

4. Other?


16.6 Parameters Affecting Acoustic Emissions from Concrete16.6.1 The Kaiser EffectThe 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 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. (felicity effect)

Keypoints: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.

Comments:The continual curing of concrete matrix repair the previous loading induced effects (microcracks, disbonding etc.) and return the concrete back to almost preloading condition.


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 DevicesAs 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 AttenuationThe 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. Related to this, Mindess 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 GeometryIt 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 AggregateIt 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; 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 StrengthIt 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 EmissionBy 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 StudiesA number of studies have shown that acoustic emission can be related to crack growth or fracture mechanics parameters in cements, mortars, and concretes. Evans et al. showed that acoustic emission could be correlated with crack velocity in mortars. Morita and Kato and Nadeau, Bennett, and Mindess 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. (K6?) 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.


16.7.1 Fracture Mechanics StudiesA number of studies have shown that acoustic emission can be related to crack growth or fracture mechanics parameters in cements, mortars, and concretes. Evans et al. showed that acoustic emission could be correlated with crack velocity in mortars. Morita and Kato and Nadeau, Bennett, and Mindess 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. (K6?) 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 CracksA 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., 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 Kobayashi 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. 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) LocationPerhaps 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. 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. and Lenain and Bunsell 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., 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 Robert and Rossi 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 Robert 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. developed deconvolutiontechniques to analyze acoustic waveforms, in order to provide a stress-time history of the source of an acoustic event. Similar deconvolutiontechniques were subsequently used by Maji and Shah 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 RelationshipsSince 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 Kobayashi 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, Fertis had concluded that acoustic emissions could be used to determine not only strength, but also static and dynamic material behavior. Rebic, 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. 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. As an example of the scatter in data. Figure 16.6 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 ShrinkageAcoustic emission has been used to try to monitor shrinkage in cement pastes and mortars. Nadeau et al. found that, in hardened pastes, theacoustic emission resulted from cracking due to the unequal shrinkage of thehydration products. Mortar gave less acoustic emission than hardened paste,suggesting that the fracture processes at the sand/cement paste interface arenot an important source of acoustic emission. Jeong et al. also suggested that, in autoclaved aerated concrete, the acoustic emissions during drying could be related to microcracking. Again, however, it is unlikely that acoustic emission measurements will be able to be used as a means of predicting the shrinkage as a function of time.


16.7.6 Fiber Reinforced Cements and ConcretesA number of acoustic emission studies have been carried out on fiber reinforced cements and concretes. Lenain and Bunsell, in a study of asbestos cement, found that acoustic emissions resulted both from cracking of the matrix and fiber pullout.

They noted that the Kaiser effect was not found for this type of fiber-reinforced composite, since on unloading of a specimen the partially pulled out fibers were damaged, and particles of cement attached to them were crushed, giving rise to acoustic emissions on unloading. Because these damaged fibers were then less able to resist crack growth, on subsequentreloading cracks started to propagate at lower stress levels than on the previous cycle, thus, giving off acoustic emissions below the previouslyachieved maximum load.

Akers and Garrett also studied asbestos cement; they found that acoustic emission monitoring could be used to detect the onset and development of prefailure cracking.


However, they concluded that there is no basis whatsoever for usingamplitude discrimination in acoustic emission monitoring for distinguishing between the various failure modes which occur in this material. On the other hand, Faninger et al. argued that in fiber-reinforced concrete the amplitude pattern of the acoustic emission signals did make it possible to distinguish whether fracture had occurred in the fibers or between them. Similarly, Jeong et al. stated that acoustic emission frequency analysis could distinguish between different micro-fracture mechanisms in fiber-reinforced autoclaved aerated concrete.


Fiber Reinforced Cements and Concretes


16.7.7 High Alumina CementIn concretes made with high alumina (calcium aluminate) cement, the conversion from CAH10 * to C3AH6* on prolonged aging can lead to a largeincrease in porosity and therefore a large decrease in strength. There hasthus been considerable interest in finding a nondestructive technique tomonitor high alumina cement concrete (HAC) members. Parkinson andPeters concluded that the conversion process itself is not a source of acoustic emission activity, since no acoustic emissions were generated during the accelerated conversion of pastes at the critical w/c ratio of 0.35. However, at the high w/c ratio of 0.65, conversion was accompanied by a high level of acoustic emission activity, due to the fracture processes taking place during conversion, associated perhaps with the liberation of excess water. Arrington and Evans suggested that the structural integrity of HAC could be evaluated from the shape of the acoustic emission vs. load plot, the emissions recorded while the specimens were held under a constant load, and the decay of emission activity with time.

*Note that cement chemistry notation is being used: C= CaO; A= Al2O3; H= H2O.


Perhaps the most extensive series of tests on HAC, carried out at the Fulmer Research Institute in the U.K., was reported by Williams. Apart fromobserving that the Kaiser effect existed up to the point at which the beamscracked, some tentative suggestions were made for monitoring HAC beamswith acoustic emissions:

1. If, on loading a beam, no acoustic emission is noted, then the applied load is still less than about 60% of the ultimate load; if acoustic emission occurs,then this percentage of the ultimate load has been exceeded.

If, upon unloading such a beam, further acoustic emission activity is recorded,then the beam is cracked. The amount of acoustic emission during this unloading could indicate the degree to which the cracking load had been exceeded.


2. If a beam is under its service load, it would behave similarly on application of a superimposed load. The presence or absence of acoustic emissionsduring this further loading and unloading might indicate the condition of thebeam.

3. If a beam under service load showed no acoustic emission activity during further loading, but did so at a later date when loaded to the same level, then the strength must have decreased during that time interval.

As well, Williams noted similar behavior on testing of ordinary prestressed concrete beams, and suggested that these techniques could be used to evaluate any type of concrete structure, as long as acoustic emissions not connected with beam damage could be eliminated.


16.7.8 Thermal CrackingRelatively little work has been carried out on acoustic activity when concrete is subjected to high temperatures, such as those that may be encountered infires. However, Hinrichsmeyher et al. carried out tests up to temperatures of 900C. They claimed that acoustic emission analysis during heating enabled them to distinguish the different types of thermally induced cracking that occurred. They noted a thermal Kaiser effect in the temperature range 300 to 600C, which might help in determining the maximum temperature reached in a previous heating cycle. The technique was even sensitive enough torecord the acoustic emissions from the quartz inversion at 573C.


16.7.9 Bond in Reinforced ConcreteA number of acoustic emission studies of debonding of reinforcing bars in reinforced concrete have been carried out. Kobayashi et al. tested simulated beam-column connections with a 90 hooked reinforcing bar subjected to various cyclic loading histories. They found that the penetration of a surface crack down to the level of the bar gave rise to only one or two acoustic events; most acoustic emission signals were generated by the internal cracking around the bar due to fracture at the lugs (ribs) of the bars. Acoustic emission signals were able to indicate, with reasonable accuracy, the degree of debonding. They suggested that acoustic emission techniques could be used to determine the amount of bond deterioration in concrete structures during proof testing, or due to overloads. In addition, several studies of bond degradation at elevated temperatures have been carried out. Royles et al. studied simple pullout specimens at temperatures up to 800C.


They found that acoustic emissions were associated with the adhesive failureat the steel-concrete interface, followed by local crushing under the ribs of thereinforcing bars. They suggested that acoustic emissions could be used toidentify the point of critical slip. In further work, Royles and Morley suggested that acoustic emission techniques might be useful in estimating the quality of the bond in reinforced concrete structures that had been subjected to fires.


16.7.10 Corrosion of Reinforcing Steel in ConcreteThe deterioration of concrete due to corrosion of the reinforcing steel is a major problem, which is usually detected only after extreme cracking hasalready taken place. Weng et al. found that measurable levels of acousticemission occurred even during the corrosion of unstressed reinforcedconcrete. They suggested that, at least in the laboratory, acoustic emissionmonitoring would assist in characterizing corrosion damage. In subsequentwork, Dunn et al. developed a relationship between the observed damageand the resulting acoustic emissions. Damage could be detected in its earlystages, and by a combination of total counts and amplitude measurements,the nature of the corrosion damage could be determined.


Corrosion of Reinforcing Steel in Concrete

16.8 Field Studies of Acoustic EmissionAs 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; 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 ConclusionsFrom 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. 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.


Concrete Structures


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


1.0 IntroductionAcoustic 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 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.


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 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 from loud service environments which contribute extraneous noise to the signals. For successful applications, signal discrimination and noise reduction are crucial.


2.0 A Brief History of AE TestingAlthough 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,

understanding acoustic emission testing- reading i

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