These postprocessed hits of cracks alone were sorted

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These postprocessed hits of cracks alone were sorted in Fig. 17 according to channel. Figures 17a to 17c show data from channel 1, the transducer closest to the shaker table and closer to the crack. Figures 17d to 17f show data from channel 2, the transducer closer to the fixed end of the test object and farther from the crack. The plots of counts versus energy (Figs. 17c and 17f) gave a very interesting and unexpected picture of the crack region as it grew through file 20. First, as the crack was initiated and began to grow in the aluminum test object (Fig. 18), it propagated vertically downward for 65 s. Then, the crack stopped its vertical propagation and changed to a 0.79 rad (45 deg) orientation and continued propagating in this new direction for the last 25 s of file 20. The plot of counts versus energy for channel 2 (Fig. 17f) gives an interesting picture of the crack growth as a second region developed during the last 25 s of the test. On initiation of the crack at 372 Acoustic Emission Testing F IGURE 17. Laboratory file filtered to contain only crack data: (a) channel 1 amplitude; (b) channel 1 counts; (c) channel 1 energy; (d) channel 2 amplitude; (e) channel 2 counts; (f) channel 2 energy. 8 6 4 2 0 0 20 40 60 80 100 Duration (ms) Amplitude (dB) (a) 8 6 4 2 0 0 20 40 60 80 100 Duration (ms) Amplitude (dB) (d) 8 6 4 2 0 0 200 400 600 800 1000 Duration (ms) Counts (b) 8 6 4 2 0 0 200 400 600 800 1000 Duration (ms) Counts (e) 1000 800 600 400 200 0 0 200 400 600 800 1000 1200 14001600 Energy (arbitrary unit) Counts (c) 1000 800 600 400 200 0 0 200 400 600 800 1000 1200 14001600 Energy (arbitrary unit) Counts (f)
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120 s into the file, acoustic emission energy in the form of extensional lamb waves emanated from the source. Then at 185 s into the file, the crack source changed from an extensional mode to a flexural mode. The individual characteristics of these two wave types, coupled with the fact that transducer 2 was farther from the source than transducer 1, explains the second region of cracking in the channel 2 plot of counts versus energy. Lamb waves are acoustic emission waves that propagate through thin plates. One distinct characteristic common to both lamb wave modes, extensional and flexural, is that their speed of propagation depends on frequency. The wave speed of the extensional waves in this test was calculated to be 5500 m·s –1 (1.8 × 10 4 ft·s –1 ) and the wave speed of the flexural waves was 2500 m·s –1 (8 × 10 3 ft·s –1 ). 15 Unlike other acoustic emission waves whose velocities are not frequency dependent, lamb waves exhibit a large amount of dispersion as they propagate through a medium. This dispersion acts to separate the different wave groups, as their frequency variations cause them to propagate at different velocities with respect to one another. It turns out that this physical phenomenon of dispersion was, in part, responsible for the development of the second region on the counts versus energy plot (Fig. 17f).
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  • Fall '19
  • Fighter aircraft, Nondestructive testing, Acoustic Emission, Acoustic Emission Testing

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