Practical application of state-of-the-art NDE techniques: evaluation of graphite-epoxy composite wing covers

Abstract

The X-29 experimental aircraft, based at the NASA Dryden Flight Research Facility, Edwards Air Force Base, California, represents a demonstrator for several state-of-the-art aerospace technologies. The most obvious of these is the forward-swept wing configuration, made possible in this high-performance aircraft by the use of graphite fiber-reinforced epoxy composite laminate wing surfaces. During a routine inspection of the aircraft, a delamination was found in the wing on the right underside of the airplane. A NASA review board investigated the damage and recommended to repair the delamination and to closely monitor its integrity during the post-repair period. The local strain on the surface of the repaired part was measured with strain gages monitored in real time during subsequent flights and the area was periodically inspected nondestructively in a reproducible manner to test for failure of the repair or growth of the delamination. The Materials Characterization Instrumentation Section of the NASA Langley Research Center was called upon to acquire quantitative ultrasonic NDE data from the repaired delamination and to analyze it using the advanced techniques available at that facility. These measurements were in addition to more subjective conventional ultrasonic pulse-echo inspections. THE DELAMINATIONThe delamination occurred on the inboard leading edge of the lower right wing cover, on a tab containing nut plates fastened by rivets for mounting the adjacent boot paneL Fig. 1 shows a sketch of the affected composite part. The composite is 48 plies thick at the edge, with the delamination occurring approximately 4 plies from the upper (interior to the wing) surface. The composite part is separated from a structural titanium spar by a layer of silicon-loaded epoxy, referred to as liquid shim, to reduce vibrations.The delamination was repaired according to the following procedure. First, the cured liquid shim material was removed from between the composite and titanium spar behind the delamination. The crack was pried open slightly to aid in the injection of a liquid epoxy, Epon 934, into the crack. This epoxy is similar, but not identical, to the matrix material of the composite. A wedge was inserted between the composite and the titanium spar to close the crack during cure of the epoxy. After cure, the wedge was removed and the gap between composite and titanium was filled with liquid shim.Conventional pulse-echo ultrasonic inspection was performed, and the 995