Generic design procedures for repair of acoustic fatigue damage

Acoustic fatigue is the result of high frequency lateral vibration of an aircraft panel due to time varying pressure waves caused by engine and/or aerodynamic effects. For example, acoustically induced cracks have been recorded in the lower external surface of the nacelle skin of the F/A-18 aircraft and aft fuselage. 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 methods of bonded repair developed for in-plane loads were made. However, the cracks continued to grow at a similar rate. While the repair of cracked aircraft structures subjected to in-plane loads using bonded repairs has resulted in considerable aircraft life time extension and hence cost savings, the use of bonded patches to repair panels with acoustically induced cracks (acoustic fatigue) is only recent. In this report a generic design procedure is presented for the repair of panels containing acoustically induced cracks using constrained layer damping (CLD). The application of bonded repairs to acoustically-induced cracks requires analytical tools that take into account high frequency out-of-plane vibration. The analytical tools described in the report will enable the rapid design of effective repairs using closed form solutions. A potential outcome for this study could be application to the repair of the batch 11 F/A-18 aircraft which suffer from acoustic fatigue cracking on the aft fuselage. Over a period of ten years this could result in cost savings of one order of magnitude improvement over those for metallic repairs