Low-cycle fatigue damage accumulation near the cold-expanded hole by crack compliance data

Abstract

Novel destructive method is implemented for quantitative assessment of fatigue damage accumulation in the stress concentration zone near the cold-expanded hole, which is characterized by residual stress influence. Damage accumulation is reached by preliminary push–pull loading of plane specimens with cold-expanded holes. Artificially introduced narrow notches, emanating from the hole edge at different stages of low-cycle fatigue, serve to manifest a damage level. Deformation response to local material removing is measured by electronic speckle pattern interferometry (ESPI) in terms of in-plane displacement components. Notches are inserted under constant external load to increase the measurement accuracy by reaching optimal fringe density. Normalized values of the notch mouth open displacement (NMOD), in-plane displacement component acting in the notch direction (U), the stress intensity factor (SIF), the T-stress and the biaxiality ratio are used as current damage indicator. Numerical integration of curves, describing an evolution of each fracture mechanics parameter, produces the damage accumulation function in an explicit form. It is established that all five fracture mechanics parameters give very close results. Developed approach is the essential link in predictable joints design to reach maximal beneficial effect of cold hole expansion on the fatigue life improvement of advanced aerospace structures.