Abstract
Acoustic fatigue is the major damage phenomenon induced by the high frequency lateral vibration of structural panels, such as that of an aircraft skin, under time varying pressure waves generated by engine and/or aerodynamic effects. For instance, acoustically-induced cracks have been discovered in the lower external surface of the nacelle skin and aft fuselage of the F/A-18 aircraft. In the case of the inlet nacelle overall sound pressure levels of the order of 172 dB have been recorded. Attempts to repair these cracks by applying standard bonded repairs developed for in-plane loads were made. However the cracks continued to grow at a similar rate as before the application of repairs. While the repair of cracked aircraft structures subjected to in-plane loads using bonded repairs has resulted in considerable aircraft life extension and hence cost savings, the use of bonded patches to repair panels with acoustically-induced cracks (acoustic fatigue) is only recent. In this chapter a generic design procedure is presented for the repair of aircraft panels containing acoustically induced cracks by incorporating the constrained layer damping technique. The analytical tools described in this chapter will enable the rapid design of damped repairs using closed form solutions that account for the effects of high frequency out-of-plane vibration on crack extension. A case study is also undertaken of a design of a repair to prevent acoustic fatigue cracking on the aft fuselage of the F/A-18.
