These results emphasize the importance of

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These results emphasize the importance of understanding the causal mechanisms of acoustic emission in real structures. Good design and material selection result in new structures that will not fail unless loaded beyond their nominal yield strength. Over time, microstructural and environmentally induced discontinuities result in circumstances that may lead to brittle fracture. Acoustic emission is able to detect the process of brittle crack growth before catastrophic failure takes place and is a powerful technique for determining structural integrity. 68 Acoustic Emission Testing T ABLE 5. Toughness criteria for compact tension specimens in transverse and longitudinal orientations of Unified Numbering System A97075, temper 651, wrought aluminum alloy. 94 Toughness Integral (MPa·m 0.5 ) Crack Length to Stress Intensity Factor _______________________________________ Specimen Width (N·mm –1 ) J E (see note a ) Before Fracture b ______________________ _________________ __________________ (ratio) J IC J IAE K IC a K IAE a K IC b K IAE b 0.430 8.39 7.71 25.8 24.7 26.5 24.7 0.543 7.66 7.06 24.6 23.6 24.4 23.4 0.623 8.27 7.89 25.6 25.3 26.3 25.0 0.040 9.83 27.9 27.8 0.538 8.73 8.40 26.3 25.5 26.1 25.3 0.545 9.15 8.37 26.9 25.8 26.6 25.5 0.597 8.88 8.28 26.5 25.6 26.0 25.2 0.647 10.36 9.18 28.5 27.0 27.8 26.3 a. K = (1.1) 0.5 J E , where E = Young’s modulus and J = integral. b. Integral K according to ASTM E 399, Test Method for Plane-Strain Fracture Toughness of Metallic Materials. F IGURE 25. Acoustic emission counts and load as a function of time for two identical compact tension specimens of Unified Numbering System A92219 wrought aluminum alloy, one tested under displacement control and the other tested under load control. 92 Summation of acoustic emission (10 –2 counts) 80 72 64 56 48 40 32 24 16 8 100 200 300 400 500 600 700 800 Time (s) Load, kN (10 3 lb f ) 45 (10.1) 40 (9.0) 35 (7.9) 30 (6.7) 25 (5.6) 20 (4.5) 15 (3.4) 10 (2.2) 5 (1.1) 0 Displacement control Load control Load Acoustic emission Acoustic emission Load Legend = load = acoustic emission
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Origination of Elastic Waves Acoustic emission can be thought of as the naturally generated ultrasound created by local mechanical instabilities within a material. Imagine that an object has been placed under load sometime in the past and is now in elastic equilibrium throughout. Suppose a small crack appears within the object at a point distant from that point where the loads were applied. The surfaces of the crack are able to move in such a manner that they become stress free. In doing so, they release some of the stored elastic energy in the object. This release of energy is in the form of elastic waves that propagate freely throughout the object, experiencing reflections or mode conversions at the object’s boundaries. In fact, the propagation of these elastic waves is the mechanism by which the changed elastic state in the immediate vicinity of the crack is transmitted to the rest of the object. The waves enable the entire object to change its shape and accommodate the crack, and each propagating wave front carries a component of this shape change.
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  • Fall '19
  • Nondestructive testing, Acoustic Emission, Acoustic Emission Testing

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