Modeling of Three-Dimensional Effects on Fatigue Crack Closure Processes in Small-Scale Yielding

摘要

In ductile metals, plasticity-induced closure of fatigue cracks often retards significantly measured crack growth rates in the Paris regime and contributes strongly to the observed .R-ratio effect in experimental data. This work describes a similarity scaling relationship based on the 3D small-scale yielding framework wherein the thickness, B, defines the only geometric length-scale of the model. Dimensional analysis suggests a scaling relationship for the crack opening loads relative to the maximum cyclic loads (K_(op)/K_(max)) governed by the non-dimensional load parameter K = K_(max) / σ_0 B~(1/2), i.e., a measure of the in-plane plastic zone size normalized by the thickness. Both K_(op) and K_(max) refer to remotely applied values of the mode I stress-intensity factor. Large-scale, 3D finite element analyses described here demonstrate that K_(op)/K_(max) values vary strongly across the crack front in thin sheets but remain unchanged when K_(max), B, and σ_0 vary to maintain K = constant. The paper also includes results to demonstrate that the scaling relationship holds for non-zero values of the T-stress (which affect the K_(op)/K_(max) values) and for an overload interspersed in the otherwise constant amplitude cycles. The present results focus on R = K_(min)/K_(max) = 0 loading, although the scaling relationship has been demonstrated to hold for other R > 0 loadings as well. The new similarity scaling relationship makes possible more realistic estimates of crack closure loads for a very wide range of practical conditions from just a few analyses of the type described here.