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2 units in a distance of only 50 mm 2 in this ω ak 2

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unit to approximately 0.2 units in a distance of only 50 mm (2 in.). This ω = ak 2 c ak = ± c a = ± ω y Be i t x c = ω a y y xxxx tt 2 0 , , + = 89 Fundamentals of Acoustic Emission Testing
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decrease is solely caused by the effect of dispersion. Notice also that the duration of the wave increases as a result of dispersion, a fact that complicates location of discontinuities using acoustic emission signals. These same effects accompany waves generated by localized sources in thin plates. In addition to the attenuation because of dispersion, the wave in a plate would also be attenuated by the geometric spreading of the beam, scattering and diffraction, and energy loss mechanisms. Dispersion effects are significant in the waves that comprise typical acoustic emission hits. An example of the effect of dispersion on waves in bars has been documented. 121 Short duration waves were generated in a bar by pulsing a piezoelectric transducer. The resulting waveforms were monitored at several locations downstream from the source and the peak amplitudes showed rapid attenuation with distance from the source. Attenuation from Scattering and Diffraction Waves that propagate through media having complex boundaries and discontinuities (such as holes, slots, cavities, cracks and inclusions) interact with these geometric anomalies. In general, a wave encountering any type of interface, including a bounded one, will be reflected or at least partially reflected. A plane wave encountering a finite interface such as a cylindrical or spherical void will be partially reflected and partially transmitted past the void. Actually, some energy is found propagating in all directions in the general case of a plane wave encountering a finite void or inclusion. Thus, the wave is said to be scattered . Likewise, a wave encountering a sharp edge such as a crack or a groove in a solid is reflected from the discontinuity. Some of the energy propagating past the sharp edge will be bent or diffracted down into the shadow region of the crack. These diffraction effects are very difficult to compute; introductions to the subject of diffraction effects can be found in the literature. 116,117 Both scattering and diffraction of a wave can lead to a decrease (or in some cases an increase) in the amplitude of a wave and both phenomena can cause attenuation. A commonly encountered cause of scattering is the heterogeneities of grain boundaries in some materials. For example, waves in the megahertz frequency range will be significantly scattered by the coarse grain structure in cast iron. Thus, the attenuation caused by scattering is considerable in this case. The amplitude decrease (attenuation) of a wave subject to scattering or diffraction is usually so difficult (if possible) to calculate that it becomes impractical except in the cases of simplest geometry. This leads to empirical approaches for dealing with attenuation, as discussed below.
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  • Fall '19
  • Nondestructive testing, Acoustic Emission, Acoustic Emission Testing

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