Predictions of Fatigue Crack Growth Behavior Using a Crack Closure Ligament Model

A ligament model, which performs a simplified, one-dimensional elastic-plastic analysis to predict fatigue crack closure behavior, is described. The model is a modification and extension of a similar one developed by Newman and has the ability to predict the influence of pre-existing residual stress fields on crack closure. Different aspects of the model are examined and, where possible, compared with experimental data and with results from theoretical studies in the literature. For specimens without pre-existing residual stresses, the aspects examined include prediction of plasticity and closure induced residual stresses near the crack tip and prediction of the influence of applied mean stress on crack opening stress. For specimens with pre-existing residual stresses, aspects examined include prediction of residual stress redistribution caused by crack growth and prediction of the influence of such stresses on crack growth behavior, using the closure concept of an effective stress intensity range. Predictions of crack growth are also compared to those based on the superposition approach, for both compressive and tensile residual stresses.