AET-2006 Reading 10A

1. My Pre-Exam R eading on Acoustic Emission Testing Literature Reading 10 2016-10: For my ASNT Level III Examination on coming 2016 August. 26thJuly 2016 Fion Zhang/ Charlie Chong

2. Acoustic Emission Testing Fion Zhang/ Charlie Chong

3. Acoustic Emission Testing Charlie Chong/ Fion Zhang

4. Acoustic Emission Testing Charlie Chong/ Fion Zhang

5. Acoustic Emission Testing Fion Zhang/ Charlie Chong

6. Charlie Chong/ Fion Zhang

7. Fion Zhang at Copenhagen Harbor 26stJuly 2016 Charlie Chong/ Fion Zhang

8. SME- Subject Matter Expert http://cn.bing.com/videos/search?q=Walter+Lewin&FORM=HDRSC3 https://www.youtube.com/channel/UCiEHVhv0SBMpP75JbzJShqw Charlie Chong/ Fion Zhang

9. http://www.yumpu.com/zh/browse/user/charliechong http://issuu.com/charlieccchong http://independent.academia.edu/CharlieChong1 Charlie Chong/ Fion Zhang

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

11. Charlie Chong/ Fion Zhang

12. The Magical Book of Tank Inspection ICP Charlie Chong/ Fion Zhang

13. ASNT Certification Guide NDT Level III / PdM Level III AE - Acoustic Emission Testing Length: 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) Charlie Chong/ Fion Zhang

14. 2 Equipment and Materials • Transducing processes • Sensors • Sensor attachments • Sensor utilization • Simulated acoustic emission sources • Cables • Signal conditioning • Signal detection • Signal processing • Source location • Advanced signal processing • Acoustic emission test systems • Accessory materials • Factors affecting test equipment selection Charlie Chong/ Fion Zhang

15. 3 Techniques • Equipment calibration and set up for test • Establishing loading procedures • Precautions against noise • Special test procedures • Data displays 4 Interpretation and Evaluation • Data interpretation • Data evaluation • Reports 5 Procedures 6 Safety and Health 7 Applications • Laboratory studies (material- characterization) • Structural applications Charlie Chong/ Fion Zhang

16. References & Catalog Numbers  NDT Handbook, Second Edition: Volume 5, Acoustic Emission Testing Catalog Number 130  Acoustic Emission: Techniques and Applications Catalog Number 752 Charlie Chong/ Fion Zhang

17. 数字签名者：Fion Zhang DN：cn=Fion Zhang, o=Technical, ou=Academic, email=fion_zhang @qq.com, c=CN 日期：2016.07.29 10:35:30 +08'00' Charlie Chong/ Fion Zhang

18. 闭门练功 Charlie Chong/ Fion Zhang

19. Chapter2 Fundamental Principles of Acoustic Emission Testing P. Kalyanasundaram, C.K. Mukhopadhyay, S.V Subba Rao Series Editor: Baldev Raj B. Venkatraman Charlie Chong/ Fion Zhang

20. 2.1 Introduction Fundamental Principles of Acoustic Emission Testing It is well known that when a solid is subjected to stress at certain levels, discrete acoustic waves are generated, which can be detected using appropriate detectors placed in contact. The phenomenon of sound generation in materials under stress is termed as acoustic emission. This phenomenon has also been referred to as stress wave emission. The American Society for Testing of Materials (ASTM) formally defines acoustic emission as 'the class of phenomena where transient elastic waves are generated by the rapid release of energy from localized sources within a material, or the transient elastic waves so generated'. In its less conventional forms, acoustic emission can be so loud that it is audible to the unaided ear. Familiar examples of this are the creakingofwood such as timber subjected to loads near failure or the crackling of twigs, rocks and bones before breaking. The purpose of the present book is to discuss the basic concepts of acoustic emission principles and its application as a non destructive testing and evaluation.(NDT & E) tool. This chapter provides an overview of the physical principles, advantages and applications of AE. Charlie Chong/ Fion Zhang

21. 2.2 H istory of Acoustic Emission Acoustic emission and microseismic activity are naturally occurring phenomena which man has observed from early times. Although it is not known exactly when the first acoustic emissions were heard, fracture processes such as the snapping of twigs, the cracking of rocks, and the breaking of bones were no doubt among the earliest. But the first non- estructive application of acoustic emission can be attributed to the ancient potters. Potters observed the sounds of cracking of clay vessels cooling too quickly in the kin. Through these audible acoustic emissions, the potter knew that his creation wa11 defective and structurally failing. The oldest variety ofhard fired pottery dates back to as early as 6500 B.C. In metals it would be reasonable to assume that the first true acoustic emission heard was the cry of tin the audible emission produced by mechanical twinning of pure tin during plastic deformation. This then would have occurred only after pure tin was smelted, which would be around 2500 BC. In the 19th and 20th century, incidental observations of audible sounds emitted by metals during the course of studying metallurgical phenomena, such as twinning and martensitic phase transformation studies, are reported in the literature as early as 1916 by J. Czochralski and others. Charlie Chong/ Fion Zhang

22. Since then in the period 1923-1950, a number of observations relating to AE release during mechanical testing and deformation have been reported. However, all these were just passing observations and no detailed investigations had been attempted. The first systematic studies and the genesis oftoday's technology in acoustic emission can be considered as the outcome of the work of Josef Kaiser at the Technische Hochule Munchen in Germany. In 1950 he published his Ph. D.thesis which reported the first comprehensive investigation into the phenomena of acoustic emission. The objectives of Kaiser's research was to determine from tensile tests of conventional engineering materials such as tin, lead, duralumin, copper, brass, gray iron, steel etc what noises are generated from within the specimen, the acoustic processes involved and the relation between the stress-strain curve and the frequencies noted for the various stresses to which the specimens were subjected. He made two major discoveries. The first was the near universality of the acoustic emission phenomenon by observing emissions in all the materials including dry wood he studied and second is the irreversibility phenomenon which now bears his name. He also proposed a distinction between burst-type and continuous emission. Charlie Chong/ Fion Zhang

23. Following the pioneering work of Kaiser, many others such as Bradford H. Schofield, Tatro and Harold L. Dunegan initiated research in the middle 1950s and 1960s and did much to improve the instrumentation, clarify the source of acoustic emission and worked extensively in this area. In the decade of the 1960s, many engineers and scientists became interested in this method and utilized it in studies relating to materials research and characterization, nondestructive testing and structural evaluation. By the mid 1960's, Acoustic Emission Testing (AET) started to move out from the laboratory into the field environment tool primarily as an NDT tool for structural integrity evaluations of pressure vessels. Charlie Chong/ Fion Zhang

24. Today AET is a matured NDT tool. Its applications are widespread and range from fundamental studies directed at clarifying the mechanisms of AE generation, correlating AE signals to physical or mechanical processes, extending the knowledge of material behavior to non-destructive testingand evaluation of industrial components and structures. With the advancement in electronics and computer technology, AE instrumentation has also improved and the applications have spread to different fields such as nuclear, aerospace, chemical plan is and process industries etc. Today, two of the most successful practical applications in which AE has proved to be a robust NDE method include periodic and continuous monitoring of pressure vessels and pressure containment systems to detect dangerous defects such as cracks and detection of incipient fatigue failures in aerospace and engineering structures. Charlie Chong/ Fion Zhang

25. 2.3 Principles of Acoustic Emission Testing and AE Phenomenon Acoustic Emission refers to the class of phenomenon where transient elastic waves are generated due to the rapid release of energy from localized source or sources (origin of emission) within a material. The generation of AE is a mechanical phenomenon, and can originate from a number of different mechanisms. Mechanical deformation and fracture are the primary sources of AE, but phase transformation, corrosion, friction and magnetic processes among others also give rise to AE. The energy thus released travels as a spherical wave front and can be picked up from the surface of a material using highly sensitive transducers, usually piezoelectric type placed on the surface ofthe component. Sensors are coupled to the structure by means of a fluid couplant and are secured with tape, adhesive bonds or magnetic hold- owns. The output of each piezoelectric sensor (during structure loading) is amplified through a low-noise preamplifier, filtered to remove any extraneous noise and further processed by suitable electronic equipment and analysed to reveal valuable information about the source causing the energy release. Charlie Chong/ Fion Zhang

26. Various types of other sensors are strain gages, accelerometers, electromagnetic acoustic transducers and optical or fibre-optic interferometers. The frequency range of acoustic emission phenomena extends from the infrasonic(< 16Hz) into the ultrasonic range. This is shown in Fig. 2.1. The largest and therefore the longest events such as earthquakes are found at the lowest end of the scale while frequencies in the audible range occur predominantly in micro seismology i.e. during fracture phenomena in rocks. Acoustic emission in the proper sense covers the audible frequencies and the ultrasonic range. In some cases, frequencies of 30 MHz and higher have been recorded. The measurements of Kaiser were in the audible range while today, measurements are carried out in the ultrasonic range between 50 kHz and 1.5 MHz. At higher frequencies, the acoustic emission is not intense enough in most cases and the material also absorbs large parts of the signal. The lower frequency limit is primarily set by the background noise. Charlie Chong/ Fion Zhang

27. Fig. 2.1 Frequency range of "acoustic emission" phenomena. 500Hz? Charlie Chong/ Fion Zhang

28. Acoustic Emission refers to the class of phenomenon where transient elastic waves are generated due to the rapid release of energy from Localized source or sources (origin of emission) within a material. ■ It is a mechanical phenomenon, and can originate. from a number of different mechanisms. Charlie Chong/ Fion Zhang

29. 2.4 Sources of AE Sources of AE include many different mechanisms of deformation and fracture. The largest naturally occurring sources are the earthquakes and rock bursts while the smallest sources are the dislocations, slip, twinning, etc. The other typical sources are initiation and propagation of cracks, sudden reorientation of grain boundaries and bubble formation during boiling or martensitic phase transformation. Thus acoustic emission sources can be classified as macroscopic and microscopic sources. The term macroscopic refers to circumstances where a relatively large part of the test material is contributing to the emission phenomenon whereas in the case of microscopic sources, it is the individual events or micro-mechanisms such as dislocation motion, slip formation, microcleavage fracture etc. that act as sources of AE. Charlie Chong/ Fion Zhang

30. Apart from these, there are other mechanisms such as (1) leaks and (2) cavitations, (3) friction in rotating equipments, (4) growth or realignment of magnetic domains (magnetic barkhausen noise effect) all of which release emissions. These also fall under the definition of acoustic emission. However, such sources are termed as secondary sources or pseudo sources, to distinguish them from the classical acoustic emissions arising due to mechanical deformation of materials that are subjected to stressing. Table 2.1 below summarizes the various mechanisms which act as sources of AE. Some of the source characteristics that affect the acoustic waves are the time response of the change in internal stress, its magnitude and the area or volume in which it is active. Orientation of the source also plays a part. However, accurate determination of source parameters is a complex task. The same source mechanism can have different characteristics in different materials. It can also significantly differ for different loading conditions within the same material. Charlie Chong/ Fion Zhang

31. Sources of AE include : earthquakes and rock bursts, dislocations, slip, twinning, • initiation and propagati'on of cracks, sudden reorientation of grain boundaries and bubble formation during boiling or martensitlc phase transformation. Thus acoustic emission sources can be classified as macroscopk and microscopic sources. Charlie Chong/ Fion Zhang

32. 2.5 AE Signals The emissions from various sources outlined above are released as acoustic energy impulses. The energy thus released travels through the structure as a spherical elastic wave to a detector, normally a piezo electric transducer which converts this acoustic impulse into an electrical signal. This electrical signal is then suitably processed and analysed to reveal information about the source causing the energy release. Two types of signals can be recognized in general acoustic emission. These are (a) Burst Emission (b) Continuous Emission Charlie Chong/ Fion Zhang

33. ? barkhausen noise effect Charlie Chong/ Fion Zhang

34. Burst Emission: Burst emissions are discrete type of signals of very short duration (ranging from a few microseconds to a few milliseconds) and hence of broad frequency domain spectrum (broad frequency domain spectrum). On the screen or monitor, they appear as individual signals or single needles well separated in time. Although these signals are rarely simple needle like, they usually rise rapidly to a maximum amplitude and decay in an exponential way to the level of background noise. Fig. 2.2 shows a typical AE burst signal. Burst signals are characteristic of crack growth and propagation and are also observed during twin formation as with the tin cry and micro-yielding. Charlie Chong/ Fion Zhang

35. Fig. 2.2 Typical AE burst signal Charlie Chong/ Fion Zhang

36. Continuous emission: If the acoustic impulses are emitted close to one another or if the burst rate is very high then the signals occur very close and sometimes even overlap (hits not discernible). In such cases, the emissions are termed as continuous. In this type of emission, one cannot observe the individual signals (hit? Count?) separately. A typical continuous emission is shown in Fig 2.3. Charlie Chong/ Fion Zhang

37. Fig. 2.3 Typical AE continuous signal Charlie Chong/ Fion Zhang

38. 2.6 Factors affecting Acoustic Emission Acoustic emission is the elastic energy spontaneously released by materials undergoing deformation. AE is thus a wave phenomenon and AE testing uses the attribute or characteristics of these waves to characterize the material/process. Acoustic emission waveform is the convolution result of three effects; generation at the source, propagation and measurement. Two of the most common waveform parameters are frequency and amplitude. As indicated earlier, AE is a wide band phenomenon and frequencies can range from audible range to about 50 MHz. The observed frequency spectrum of the AE signals greatly depends on the geometry and acoustic properties of the specimen and characteristics of the sensor. In general practical applications, the background noise governs the lower frequency limit which is normally about 10 kHz while the upper frequency limit is dictated by wave attenuation. Most of the practical applications of AE testing are carried out in the frequency range of about 100 kHz to 300 kHz. The sensitivity of AE method is primarily governed by the background noise. For the AE signal to be discernible, its amplitude should be clearly above the noise level. Charlie Chong/ Fion Zhang

39. AE from metals, wood, plastic and other sources can generate signals ranging from fraction of micro-volts to more than hundred volts. The dynamic range ofthe signal amplitude from a test object may be 120 dB (V). When the signal amplitudes are very low, appropriate amplification using preamplifiers and signal conditioning would be required to visualize and interpret the AE siguals reliably. Apart from this, prior to any experimentation, the noise sources should be identified and then removed or inhibited or limited. Table 2.2 lists the factors that affect the relative amplitude of AE levels. These factors are just indicative and should not be considered as absolute. Charlie Chong/ Fion Zhang

40. Acoustic emission waveform is the convolution result of three effects; generation at the source, propagation and measurement. • Two of the most common waveform parameters are: frequency and amplitude. Charlie Chong/ Fion Zhang

41. Table 2.2 Factors Affecting Emission levels Toward lower fracture toughness Charlie Chong/ Fion Zhang

42. 2.7 Characteristic of AE Technique Acoustic emission is primarily a passive NDT method (passive in term that no intentional added stimuli) that monitors the dynamic redistribution of stresses within the material or component. Hence the method is effective only when the structure or component is loaded or subjected to stresses. Simple examples ofthese loading or stresses include tensile or bend testing and pressurizing the component. 2. 7.1 Kaiser Effect Josef Kaiser observed that when materials are stressed, emissions occur and when the stress is relaxed the emissions cease and no new emissions will occur until the previous maximum stress level has been exceeded. This effect of irreversibility has been named as Kaiser Effect in honor of Kaiser who discovered it and has proved to be very useful in many of the AE applications. The degree to which this effect is present varies between metals. In some alloys and materials, the effect cannot be measured. This effect is effectively utilized in assessing structural integrity of components and diagnosing damage in pressure vessels and other engineering structures during proof testing. Charlie Chong/ Fion Zhang

43. Should the vessel suffer no damage during a particular working period, the Kaiser effect dictates that no emission will be observed during the subsequent proof loading. In the event of discontinuity growth during a working period, subsequent proof loading would subject the discontinuity in the material to higher stresses than before, and the discontinuity would emit. Emission during the proof loading is therefore a measure of damage experienced during the preceding working period. (Dunegan corollary) Because of Kaiser effect, each signal can occur only once and hence any AE inspection has to be carefully planned and signals recorded and interpreted reliably. Charlie Chong/ Fion Zhang

44. When materials are stressed, emissions occur and when the stress i's relaxed the emission cease and no new emissions will occur until the previous maximum stress level has been exceeded. This effect of irreversibility is known as Kaiser’s Effect. Charlie Chong/ Fion Zhang

45. 2.7.2 Felicity Effect The concept of Kaiser effect in composite materials is slightly different from that observed in metallic materials. Specifically in fiber reinforced plastic components, emission is often observed at loads lower than the previous maximum, especially when the material is in poor condition or close to failure. This breakdown of the Kaiser effect has been successfully used for predicting failure loads in composite pressure vessels. This appearance of significant acoustic emission at a stress level below the previous maximum applied stress level has been exceeded is defined as Felicity Effect. Thus in successive loading cycles in composite materials, the acoustic emission activity starts when the stress value is a fraction of the previous high value usually ranging from 85% to 95%. The ratio between the applied load or pressure at which the acoustic emission reappears during the next application of loading and the previous maximum load applied is termed as the felicity ratio. This ratio is always less than one. It has been observed that Kaiser effect fails most noticeably in situations where time dependent mechanisms control the deformation processes. Apart from composite materials, other cases where the Kaiser effect has been observed to fail are corrosion processes and hydrogen embrittlement. Charlie Chong/ Fion Zhang

46. In fiber reinforced plastic components, emission is often observed at Loads Lower than the previous maximum, especially when the material is in poor conditjon or close to failure. This breakdown of the Kaiser Effect Is called Felicity Effect. Charlie Chong/ Fion Zhang

47. 2.8 Comparison of AE with other NOT .Methods In the conventional non-destructive methods, some form of energy is fed into the material (active instead of passive) , which interacts with the flaws and defects in the material and provides evidence of the same. In acoustic emission, we detect the energy that is released from the flaw or defect when the same is stressed (of course obeying the Kaiser effect). AE is thus a process in which dynamic or active flaws are detected. This technique also provides us with the dynamic characteristics of a flaw or defect such as its growth, growth rate, critically and intensity. Acoustic Emission inspection is a powerful aid to materials testing and the study of deformation, fracture and corrosion. It gives an immediate indication of the response and behaviour of a material under stress, intimately connected with strength, damage and failure. Charlie Chong/ Fion Zhang

48. AET - This technique also provides us with the dynamic characteristics of a flaw or defect such as its:  growth,  growth rate,  critically and  intensity. Charlie Chong/ Fion Zhang

49. 2.9 Merits of Acoustic Emission Technique 1. AE technique gives dynamic characteristics of active defects. 2. It is a volume technique i.e. the entire structure can be covered in single inspection. 3. AE data gives real time record of progressing damage. 4. AET can be used for location of active flaws in large components. 5. It can distinguish different types of active defects i.e. source characterisation is possible. (to a certain extent? Limited characterization capability?) 6. AE is non-directional and hence a sensor located anywhere on the test object can detect emission. 7. Though initial cost is comparable with other NDT techniques, operational cost is a minimum for AET. Charlie Chong/ Fion Zhang

50. AET- Volume Method Charlie Chong/ Fion Zhang