Life extension of acoustically fatigued panels using add-on dampers

Secondary aircraft structures vibrate as a result of very high intensity pr essure waves caused by engine and/or aerodynamic effects, This type of exci tation can result in acoustically-induced cracking, also known as resonant high-cycle, acoustic or sonic fatigue. In modern military aircraft, high so und pressure levels are quite often encountered, for example, the overall s ound pressure levels of the order of 170 db have been recorded. Currently a coustic fatigue cracking has been encountered in the lower external skin of the nacelle of the F/A 18. Efforts to repair these acoustically-induced cr acks with standard boron/epoxy patches have not been successful. This paper addresses the repair and life enhancement of these acoustically-induced cr acks and investigates the use of constrained layer damping in order to alle viate such cracking, Experimental studies have shown that the use of constr ained layered damping alone can be extremely successful in alleviating this problem. However, if a reinforcement is required to restore the stiffness of the cracked structure, this reinforcement may reduce the effectiveness o f the damping treatment. (C) 1999 Published by Elsevier Science Ltd. All ri ghts reserved.