Plasticity and roughness closure interactions near the fatigue crack growth threshold

An advanced model has been developed to calculate interactions between plasticity and roughness closure mechanisms resulting in the fatigue crack growth threshold. Near-threshold closure is complex because multiple closure mechanisms are likely, including roughness-, oxide-, and plasticity-induced crack closure. Results show closure first occurs either at the crack tip (tip contact), or at the fatigue-crack-surface asperity nearest the crack tip (asperity contact), in the absence of load history effects. Model calculations, verified by laboratory experiments, reveal a transition from tip contact to asperity contact produces fatigue crack growth threshold behavior; here, both model and experiment show that an abrupt increase in closure (asperity contact) results in the rapid decrease in fatigue crack growth rate at threshold. A comparison of an advanced crack-tip load displacement measurement technique, and standard global compliance methods show that global methods lack the sensitivity to accurately characterize crack closure at threshold.