Closure assessment and overload transient behaviour in damage tolerant airframe materials

Xu, Yigeng (2001) Closure assessment and overload transient behaviour in damage tolerant airframe materials. University of Southampton, Doctoral Thesis.

Record type: Thesis (Doctoral)

Abstract

Most engineering structures are subjected to variable amplitude fatigue (VA) loading. For ductile engineering alloys, various mechanisms have been proposed to rationalise VA fatigue behaviour. Their influences may vary with loading conditions and materials characteristics. Whilst considerable knowledge has been built up in the area of fatigue analysis of aerospace Al-alloys, quantitative understanding of VA fatigue behaviour remains disappointing, with basic understanding remaining controversial in a number of areas. Crack closure effects in particular remain open to debate, even for simple single overload conditions and constant amplitude (CA) loading.

A modified strip-yield analytical model has been developed to simulate plasticity-induced crack closure (PICC). The importance of the crack closure concept in controlling near-up material behaviour is demonstrated by the analytic results. The incidence of two transition points on post-overload local compliance curves (as noted experimentally) is seen to be consistent with the model's behaviour. The model has been extended into a multi-mechanistic form by introducing an effective surface roughness concept. Further understanding of the effect of the stress state, roughness-induced crack closure (RICC), and oxide-induced crack closure (OICC) on fatigue behaviour under CA and VA loading has thus been obtained. It is found analytically that RICC has a stronger influence in plane strain than in plane stress. Overall closure levels and prolonged post-overload retardation distances under plane strain loading have been successfully rationalised via first order approximations of how RICC effects may vary with crack tip conditions during an overload.