Life extension techniques for aircraft structures-Extending durability andpromoting damage tolerance through bonded crack retarders

摘要

This paper explores the viability of the bonded crack retarder concept as adevice for life extension of damage tolerant aircraft structures. Fatigue crackgrowth behaviour in metallic substrates with bonded straps has been determined.SENT and M(T) test coupons and large scale skin-stringer panels were tested atconstant and variable amplitude loads. The strap materials were glass fibrepolymer composites, GLARE, AA7085 and Ti-6Al-4V. Comprehensive measurements weremade of residual stress fields in coupons and panels. A finite element model topredict retardation effects was developed. Compared to the test result,predicted crack growth life had an error range of -29% to 61%. Mechanisms andfailure modes in the bonded strap reinforced structures have been identified.The strap locally reduces substrate stresses and bridges the crack faces,inhibiting crack opening and reducing crack growth rates. In the absence ofresidual stress, global stiffness ratio accounts for effects of both strapmodulus and strap cross section area. In elevated temperature cure adhesives,retardation performance was best in aluminium and GLARE strap materials, whichhave the closest thermal expansion coefficient to the substrate. Strap materialsof high stiffness and dissimilar thermal expansion coefficient such as titaniumhad poor retardation characteristics.