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A method of nondestructive testing and materials characterization that uses mechanical waves moving through materials. It's similar to seismology, except in being concerned with the scale of engineering structures, such as air craft, bridges, and chemical tanks. When a structure is subjected to external force (or stress), a defect (for example, a crack or welding flaw) on the structure is activated and enlarged dynamically, and thus generates waves, which spread through materials at a certain speed. Such waves, known as acoustic emission signals, are detected by sensors attached on the surfaces of the structure. Mechanical vibration due to acoustic emission signals is weak and requires high-sensitivity sensors and electronic amplification before it can be analyzed.
In nondestructive testing of structures, acoustic emission signals are typically evaluated in order to know if the failure of a structure is imminent; if cracks and other defects, presumed to be present in any structure, are active; the positions of such active defects; and whether a structure with such defects can be safely operated. In evaluating material behavior and quality, acoustic emission is used to assess how a material responds to mechanical stress, that is, when and how it changes shape permanently and how it proceeds to eventual fracture; how an alloy withstands repeated application of stress (known as fatigue); the level of stress and corrosive environment that lead to failure of a material; and the types of microscopic failure processes that arise in a material under stress.
Acoustic emission signals emanating from their sources contain information about the source, such as the direction and speed of crack opening. For example, the high-speed cracking of brittle materials (such as high-strength steels and ceramics) produces short, fast-varying acoustic emission signals, which are typically plotted against time measured in microseconds. In contrast, slow-growing defects in plastics result in longer, slowly varying signals, which are typically plotted on a time scale of milliseconds. Because of the distortion of waves during the propagation through a complex structure and detection by a sensor, however, much of the information is lost. Thus, the presence of detectable acoustic emission signals is the most important clue in assessing the integrity of the structure.
By detecting one such signal at multiple sensor positions, the location of its source can be determined from the timing of signal arrivals. The basic principle of triangulation is the same as practiced in seismology, except that the differences in signal arrival times are of the order of microseconds to milliseconds. The speed of wave propagation is a material constant, determined by the stiffness and mass density of the propagating medium.
A typical sensor uses a piezoelectric ceramic element, which converts mechanical vibration into an electrical signal, which can be amplified 1000 to 10,000 times. Various electrical measurement techniques are used to characterize and analyze the signals received. It's common to obtain and record several features of acoustic emission signals. These form the basis of real-time analysis and decision-making. In laboratory studies, the entire waveforms are also recorded for detailed analysis after testing.
