Plasticity-induced crack closure: A contribution to the debate

The premature contact of crack surfaces attributable to the near-tip plastic strains under cyclic loading, which is commonly referred to as plasticity-induced crack closure (PICC), has long been focused as supposedly controlling factor of fatigue crack growth (FCG). Nevertheless, when the plane-strain near-tip constraint is approached, PICC lacks of straightforward evidence, so that its significance in FCG, and even the very existence, remain debatable. To add insight into this matter, large-deformation elastoplastic simulations of plane-strain crack under constant amplitude load cycling at different load ranges and ratios, as well as with an overload, have been carried out. Performed modelling visualises the Laird-Smith conceptual mechanism of FCG by crack-tip blunting and re-sharpening. The simulations reproduce experimental trends of FCG concerning the roles of ΔK and overload, whilst PICC has not occurred yet. Near-tip deformation patterns discard the filling-in a crack with material stretched out of its plane in the wake behind the tip as the origin of PICC. Despite the absence of closure, load-deformation curves appear bent, which raises doubts about the trustworthiness of closure assessment from the compliance variation. This points out the ambiguities of PICC as a supposedly ubiquitous factor that controls FCG.