Crack closure effect on fatigue crack growth threshold by load-reduction tests

The fatigue crack growth behavior in the near-threshold regime, which is typically characterized using load-reduction test methods, significantly influences the high-cycle fatigue life of engineering structures, but the effect of load-amplitude changes has often been overlooked. In this study, we analyze the fatigue crack growth rate d a /d N versus the stress intensity factor range (Δ K) obtained from load-reduction tests using a three-dimensional variable-amplitude fatigue crack growth model. By explicitly modeling the evolution of crack closure during load-reduction histories and recalculating the effective Δ K (Δ K _eff), a parametric model describing the crack closure ratio U (=Δ K _eff/Δ K) and the percentage reduction of Δ K relative to its initial value at the start of load reduction is developed. This model enables convenient correction of the increases, at least, of the plasticity-induced crack closure caused by load reduction, and mitigates the fanning behavior commonly observed in near-threshold crack growth. The recommended percentage reduction ranges of Δ K for typical aerospace alloys under high stress ratios are finally proposed to make better use of the load-reduction method within ASTM Standard E647 for fatigue crack growth threshold determination.