On fatigue behavior of short cracks subjected to compressive underloads

This work explores the effects of underloads on physically short fatigue cracks propagating under nearthreshold zero-tension loading in various constraint conditions. A finite element model is employed to model the transient behavior of plasticity-induced crack closure and residual stress, from which propagation behavior can be inferred. The expected behavior of acceleration after an underload is mostly descriptive of the plane stress results, but in axisymmetric and plane strain conditions a post-underload deceleration is predicted with single or scarce underloads. Frequently repeated underloads, however, are found to reduce fatigue strength in all cases considered. Short cracks prove especially vulnerable to underload acceleration when initiated at notch-like defects. Three independent physical mechanisms are recognized, namely, the removal of load history, compressive notch plasticity, and Bauschinger effect, a combination of which explains the underload results. Additionally, tentative guidance for fatigue design in finite and infinite life underload applications is provided.