Concept of Acoustic Emission
The rapid release of energy from localized sources within a material produces a transient elastic wave, which is called Acoustic Emission (AE), sometimes referred to stress wave emission. The deformation and crack propagation of materials under stress are the important mechanisms of structural failure. This source, directly related to deformation and fracture mechanisms, is called an acoustic emission source. In recent years, other types of elastic wave sources, such as fluid leakage, friction, impact and combustion, which are not directly related to deformation and fracture mechanisms, are called other or secondary acoustic emission sources.
Acoustic emission is a common physical phenomenon. Range of acoustic emission signal frequency of a variety of materials is very wide, from a few Hz infrasound frequency, 20 Hz ~ 20K Hz audio frequency to several MHz ultrasonic frequency; range of acoustic emission signal amplitude is also very large, from a microscopic dislocation of 10-13m to a seismic wave of 1m. If the acoustic emission releasing strain energy is large enough, it can produce the the sound to be heard by human ear. Most materials have acoustic emission when they are deformed and broken. However, the acoustic emission signal intensity of many materials is so weak that the human ear can not hear directly and needs sensitive electronic equipment to detect it. The technology of detecting, recording and analyzing acoustic emission signals with instruments and using acoustic emission signals to deduce acoustic emission sources is called acoustic emission technology. People call acoustic emission instruments materials stethoscopes.
Development of Acoustic Emission Technology
Acoustic emissions and micro-vibrations are natural phenomena that occur naturally in nature. Although it is impossible to verify when people first hear acoustic emission, the rupture process such as breaking tree branches, rock breaking and breaking bones is undoubtedly the earliest known acoustic emission signal. It is quite certain to conclude that "tin" is the first acoustic emission phenomenon observed in metals because pure tin produces audible acoustic emission during plastic deformation and smelting of copper and tin can be traced back to 3700 BC.
The beginning of modern acoustic emission technology was marked by the research done by Kaiser in Germany in the early 1950s. He observed that metals and alloys such as copper, zinc, aluminum, lead, tin, brass, cast iron and steel had acoustic emission during the deformation process. His most interesting discovery is the irreversible effect of material deformation acoustic emission: "no acoustic emission signal is generated before the stress reaches the last maximum stress loaded, during material reloading". Now people call this material irreversible phenomenon "Kaiser effect." Kaiser also proposed the concept of continuous and burst acoustic emission signals.
In the late 1950s, Schofield and Tatro of the United States, after extensive research, found that the acoustic emission of metal plastic deformation was mainly caused by a large number of dislocation motions , and also got an important conclusion that the acoustic emission is mainly the volume effect rather than surface effect. Tatro did research on the physical mechanism leading to acoustic emission phenomena. For the first time, he proposed that acoustic emission could be used as a tool to study difficult problems of engineering materials and predicted the unique potential advantages of acoustic emission in nondestructive testing.
In the early 1960s, Green et al. first started the application of acoustic emission technology in nondestructive testing. For the first time, Dunegan applied acoustic emission technology to pressure vessel research. Throughout the 1960s, the United States and Japan began extensive research on acoustic emission. In addition to basic research on acoustic emission phenomena, people applied this technique to materials engineering and nondestructive testing. The United States established the Acoustic Emission Task Force in 1967, and Japan established the Acoustic Emission Association in 1969.
In the early 1970s, Dunegan et al. started the development of modern acoustic emission devices. They increased the experimental frequency to the range of 100 KHz-1 MHz. This is a significant development of acoustic emission experiment technology. The success of modern acoustic emission devices development set the stage for the structural integrity of acoustic emission technology from the material research phase in the laboratory to monitoring the large components at the production site.
With the advent of modern acoustic emission devices, extensive and systematic research has been conducted in the 1970s and early 1980s from the mechanism of acoustic emission sources, the propagation of waves to the analysis of acoustic emission signals. It has also been widely used at the production site, with particular success in the control of chemical containers, nuclear containers and welding processes. Drouillard published in 1979 the world's catalog of acoustic emission papers prior to 1979. According to his statistics, more than 5,000 papers on acoustic emission were published worldwide by the end of 1986.
In the early 1980s, PAC Company of USA introduced modern microprocessor technology into the acoustic emission detection system, designed the second generation of acoustic emission source detection instrument with smaller volume and weight, and developed a series of multifunctional advanced detection and data analysis software, through control of the microprocessor computer, which can monitor in real-time acoustic emission source positioning of detected components and display data analysis. The second generation of acoustic emission devices are easy to carry because of their small volume and weight, which pushed wide use of onsite detection of acoustic emission technology in the 1980s. On the other hand, due to the use of 286 and higher microprocessor and multi-functional detection & analysis software, the speed at which the instrument collects and processes acoustic emission signals is greatly increased, and the information storage capacity of the instrument is huge, thereby improving the acoustic emission source positioning function and the defect detection accuracy of the acoustic emission detection technology.
In the 1990s, the American companies PAC and DW, the German company Vallen Systeme and China Shenghua Industrial Technology Co., Ltd. has developed separately the third generation of digital multi-channel acoustic emission detection and analysis system, which is more highly computerized with smaller volume and weight. In addition to the real-time measurement of acoustic emission parameters and the positioning of acoustic emission sources, these systems can directly observe, display, record and analyze acoustic emission waveforms.
In the early 1970s, China began to study the acoustic emission characteristics of metals and composites. In the mid-1980s, acoustic emission technology was applied to the detection of pressure vessels and metal structures. Acoustic emission detection devices have been widely used in manufacturing, signal processing, metal materials, composite materials, magnetoacoustic emission, rock, process monitoring, pressure vessels and aircraft.
In 1978, China Nondestructive Testing Society established the Acoustic Emission Professional Committee and held the first national academic conference on acoustic emission in 1979 in Huangshan. In recent years, it has held an academic conference every two years and has held so far 11 sessions.
Basic principle of acoustic emission
Elastic waves emitted from an acoustic emission source eventually propagate to the surface of the material, causing surface displacement that can be detected with acoustic emission sensors that convert the mechanical vibration of the material into an electrical signal that is then amplified, processed and recorded. The variation of internal stress in solid materials produces acoustic emission signals. During material processing, handling and use, many factors can induce the variation of internal stress, such as dislocation motion, twins, crack initiation and propagation, fracture,
non-diffusive phase transition can be induced by many factors , magnetic domain wall motion, thermal expansion and contraction, external load changes and so on. People analyze the observed acoustic emission signals and infer to understand the mechanism of acoustic emission from materials.
The main purpose of acoustic emission detection is to: 1) determine the location of acoustic emission source; 2) analyze the nature of acoustic emission sources; 3) determine the time or load of acoustic emission; 4) assess the severity of acoustic emission sources. In general, for the excessive acoustic emission source, we shall use other nondestructive testing methods for local re-examination to accurately determine the nature and size of defects.
Characteristics of Acoustic Emission Technology
The acoustic emission detection method is different from other conventional nondestructive testing methods in many aspects. Its advantages are as follows:
(1) acoustic emission is a dynamic inspection method. The energy detected by acoustic emission comes from the tested object itself, rather than the ultrasonic or radiographic inspection method, which is provided by the nondestructive testing instrument.
(2) acoustic emission detection method is sensitive to linear defects. It can detect the activities of these defects under the applied structural stress, and stable defects do not produce acoustic emission signals.
(3) in the process of one test, acoustic emission test can detect and evaluate the state of the defect in the whole structure.
(4) it can provide real-time or continuous information of defects which is changed with external variables such as load, time and temperature, so it is suitable for on-line monitoring and early or near failure prediction of industrial processes.
(5) because of the low requirements for approaching the tested parts, it is suitable for the special circumstance, such as high and low temperature, nuclear radiation, flammable, explosive and extreme poison circumstance, where other methods are difficult or impossible to be used.
(6) for regular inspection of in-service pressure vessels, acoustic emission test can shorten the idling period for inspection, or do not need to stop production.
(7) for the pressure test of pressure vessels, acoustic emission test method can prevent catastrophic failure caused by unknown discontinuous defects and limit the maximum working pressure of the system.
(8) because of the insensitivity to the geometric shape of the component, it is suitable for detecting complex shaped components while other methods are restrictly used.
Because acoustic emission detection is a dynamic detection method, and detecting mechanical waves, it has the following characteristics:
(1) Acoustic emission is very sensitive to materials and is easily disturbed by electromechanical noise. Therefore, the correct interpretation of data should have more extensive database and on-site inspection experience.
(2) Acoustic emission detection generally requires proper loading procedures. In most cases, off-the-shelf loading conditions are available, but sometimes special preparation is required;
(3) At present, acoustic emission testing can only provide the location, activity and intensity of acoustic emission sources. The nature and size of defects in acoustic emission sources can not be given, which requires other nondestructive testing methods.