Defense Technical Information Center Compilation Part Notice ADP 014084 TITLE : Recognition and Correction of Sonic Fatigue Damage in Fighter Aircraft DISTRIBUTION : Approved for public release , distribution unlimited
High performance aircraft that fly aggressive low altitude or high-angle of attack maneuvers will experience high acoustic loading, especially in the presence of external stores. These high acoustic loadscan lead to rapid sonic fatigue in thin gage metallic structure. As a means of prolonging fatigue life, while at the same time restoring structural load carrying capability in damaged structure, composite bonded repairs which incorporate damping layers have been developed. These damped composite bonded repairs are installed on F-15 flight test aircraft. This paper describes ways to recognize sonic fatigue damage, and the design and flight test results of represeintative damped patches.
Durability Patch: Repair and Life Extension of High Cycle Fatigue Damage on Secondary Structure of Aging Aircraft
Abstract : The Durability Patch Program addresses the repair and life enhancement of nuisance cracks which have been induced into secondary structure by resonant high cycle fatigue from aerovibroacoustics. For this type of damage, safety of flight concerns are virtually non-existant, but maintenance and repair costs are high. Conventional repair techniques consist of mechanically fastened, single sided doublers. For significant static in-plane loads and/or for significant vibration levels due to out-of-plane dynamic loads, the repair does not last long because new cracks will form and emanate from the repair. Eventually, large areas of skin and substructure will have to be replaced. The Durability Patch consists of a bonded pair region which is an elastic elliptical laminate overlaid by and surrounded by a thoroughly integrated damping treatment. In some configurations the transition from the elastic repair region to the damping region is accomplished by the use of a viscoelastic material instead of a structural adhesive in one layer; thus, the other layers are multi-functional. The bonded repair does not introduce stress concentrations, does reduce static and dynamic stresses, and does reduce crack tip stress intensities. Damping is maximized within thickness and area constraints in order to enhance the life of adjoining structure with undetected damage. The life improvement goal is 600x. Finite element analysis results comparing static and vibratory stresses will be presented. High cycle fatigue and crack growth rates will be compared. The design and use of a miniature autonomous damage dosimeter to obtain service temperature and vibration environmental data at low cost will be described. Selection of structural materials and processes to attain a goal of field installation will be described. Comparison of analysis and laboratory results will be presented. Patch configurations will be described and compared using a numerical measure of merit system.
Durability patch: application of passive damping to high-cycle fatigue cracking on aircraft
Although high-cycle fatigue cracks in secondary structure are often termed 'nuisance cracks,' they are costly to repair. Often the repairs do not last long because the repaired part still responds in a resonant fashion to the environment. Although the use of visco-elastic materials for passive dampening applications is well understood, there have been few applications to high-cycle fatigue problems because the design information: temperature, resonant response frequency, and strain levels are difficult to determine. The damage dosimeter, and the durability patch are an effort to resolve these problems with the application of compact, off-the-shelf electronics, and a damped bonded repair patch. This paper presents the electronics, and patch design concepts as well as damping performance test data from a laboratory patch demonstration experiment.