The hold period activity on channel 1 was caused by

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the pressure hold period. The hold period activity on channel 1 was caused by an air leak whereas the pressure hold activity on channels 65, 66 and 67 was caused by movement of the cockpit windows and ceased as the pressure decayed. The cracking of the tensile test object corresponded to amplitude bursts when the cracking probably occurred. The detection of cracking in real time was not as difficult during the actual test as might be thought from the preceding example, both because the operator had many more acoustic emission test graphs available on the computer and because all of the acoustic emission test plots showed dynamic changes rather dramatically. The cracking of the tensile test object can be seen when looking at a dynamic replay of amplitude and hits versus channel. The occurrence of cracking was detectable while the aircraft was being loaded. The test provided a straightforward real time means of differentiating harmful cracking from benign discontinuities, such as leaks, frets and the movement of composite window structures. One important aspect of the acoustic emission testing of the transport aircraft that should be noted was that the Royal Air Force considered acoustic emission testing to be only a sort of electronic safety net, there only to help conduct proof pressurization in safety. Officially, the Royal Air Force had no interest in locating acoustic emission sources that did not require stopping proof pressurization. However, a little ad hoc effort was spent to determine the source of some location clusters seen in the posttest analysis, clusters consisting of location plots over the entire aircraft, filtered by pressure, amplitude and energy criteria. In a fuselage area where a location cluster was observed in one fuselage, this cluster had not shown up until pressures in excess of 63 kPa (9 lb f ·in. –2 ) were attained. Figure 5 shows a closeup of a site in this area. The intercostal has a crack in it, right at the acoustic emission cluster. Several other location clusters were investigated in other aircraft. For example, in one a problem with the rear pressure bulkhead was found whereas in another skin cracks were found coming from the main deck cargo door. 8 The development of signal processing for this in-flight fatigue monitoring technique is discussed in detail in the next part of this chapter. Signal Processing Development Other approaches take advantage of advances in pattern recognition to automate the process of crack detection and recognition. Automatic detection and classification are addressed by applying 364 Acoustic Emission Testing F IGURE 4. Acoustic data obtained during pressurization. Leak noise Cracks T T T T 400 300 200 100 0 –100 –100 140 380 620 860 1100 Vertical index (arbitrary unit) Horizontal index (arbitrary unit) Legend T = transducer F IGURE 5. Cracked intercostal in fuselage.
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signal processing and pattern recognition techniques, including neural networks, to actual acoustic emission waveforms.
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  • Fall '19
  • Fighter aircraft, Nondestructive testing, Acoustic Emission, Acoustic Emission Testing

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