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Formally defined acoustic emission is the class of

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Formally defined, acoustic emission is 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. 2 This definition embraces both the process of wave generation and the wave itself. Source Mechanisms Sources of acoustic emission include many different mechanisms of deformation and fracture. Earthquakes and rockbursts in mines are the largest naturally occurring emission sources. Sources that have been identified in metals include crack growth, moving dislocations, slip, twinning, grain boundary sliding and the fracture and decohesion of inclusions. In composite materials, sources include matrix cracking and the disbonding and fracture of fibers. These mechanisms typify the classical response of materials to applied load. Other mechanisms fall within the definition and are detectable with acoustic emission equipment. These include leaks and cavitation; friction (as in rotating bearings); the realignment or growth of magnetic domains (barkhausen effect); liquefaction and solidification; and solid-to-solid phase transformations. Sometimes these sources are called secondary sources or pseudo sources to distinguish them from the classic acoustic emission caused by mechanical deformation of stressed materials. Acoustic Emission Nondestructive Testing Acoustic emission testing is a nondestructive method with demonstrated capabilities for monitoring structural integrity, for detecting leaks and incipient failures in mechanical equipment and for characterizing material behavior. Applications of acoustic emission to engineering structures have been documented since the 1960s. Comparison with Other Techniques Acoustic emission testing differs from most other nondestructive methods in two significant respects. First, the energy that is detected is released from within the test object rather than being supplied by the test method, as in radiographic or ultrasonic testing. Second, the acoustic emission method can detect the dynamic processes associated with the degradation of structural integrity. Crack growth and plastic deformation are major sources of acoustic emission. Latent discontinuities that enlarge under load and are active sources of acoustic emission by virtue of their size, location or orientation are also likely to be significant in terms of structural integrity. Usually, certain areas within a structural system will develop local instabilities long before the structure fails. These instabilities result in minute dynamic movements such as plastic deformation, slip or crack initiation and propagation. Although the stresses in a metal part may be well below the elastic design limit, the region near a crack tip may undergo plastic deformation as a result of high local stresses. In this situation, the propagating discontinuity acts as a source of stress waves and becomes an active acoustic emission source.
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  • Fall '19
  • Nondestructive testing, Acoustic Emission, Acoustic Emission Testing

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