AE11.pdf

Discontinuity continued to energetically emit sound

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discontinuity continued to energetically emit sound during a pressure hold period, it was an obvious indication that harmful damage was occurring. Conversely, if the discontinuity stopped emitting or its emission characteristics changed drastically when the pressure was held constant, it was an indication that the discontinuity was benign. To establish that very small or growing cracks could be detected in the structure of the VC-10, tensile test objects containing fatigue cracks were fastened to the outer surface of a test aircraft. The test objects were attached to the aircraft using epoxy and rivets so that they spanned the space between adjacent stringer members. Thus, when the aircraft was pressurized to 59.3 kPa (8.6 lb f ·in. –2 ), the fatigue cracks were subjected to tensile forces. The dimensions of the test objects were chosen such that the pressure loading would cause the fatigue cracks to extend. In this manner, known cracks were made physically and acoustically part of the airframe so that the sensitivity of acoustic emission testing could be readily checked. Sensitivity was confirmed by showing that the standard transducer arrays proposed for use on the transport aircraft could locate the growing cracks and by showing that acoustic emission test data from the cracks were present during load hold periods. Figure 4 shows locations obtained during the pressurization of the transport aircraft. The locations at the site of the test objects are caused by cracking during actual pressure loading. Figure 4 shows the acoustic emission test activity from the cracking of these test objects during pressurization, plus a few signals from a small leak source. It is clear that the cracking of the two tensile test objects was detectable with acoustic emission testing through posttest source location. For acoustic emission testing to be successful in preventing serious damage to a transport aircraft during proof pressurization, however, it was necessary to determine that cracking was occurring while the aircraft was actually being loaded. This cracking was made by using channel activity indicators and a series of plots updated in real time. During the test, the operator kept graphs 363 Aerospace Applications of Acoustic Emission Testing F IGURE 3. Acoustic emission monitoring on British VC-10 military jet aircraft.
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of cumulative acoustic emission hits versus channels on the computer screen and scanned the channel activity indicators. Additional plots and quantitative criteria were used to investigate any channel that showed activity characteristic of leaks or cracks. During the test, mechanics followed up on any suspicious channels by investigating the aircraft itself. The amplitude of the signals being received is a function of time and aircraft pressure. In a representative test, four large amplitude bursts were received before the pressure hold and the amplitude dropped off markedly during the load hold. The activity on some channels remained constant throughout the pressure hold period. The hold period
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  • Fall '19
  • Fighter aircraft, Nondestructive testing, Acoustic Emission, Acoustic Emission Testing

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