A strip model for fatigue crack growth predictions under general load conditions

A strip crack closure model, based on the Dugdale-Barenblatt model but modified to leave plastically deformed material in the wake of the crack tip, was investigated for various aspects of fatigue crack growth behaviour. A constraint factor was introduced into the model to account for the 3D effect at the crack tip to extend the applicable range of the original Dugdale-Barenblatt model to the plane strain condition. Comparisons with experimental data show that the constraint factor in the strip model results in a fairly good prediction for the fatigue crack closure under the plane strain condition. A variable constraint factor makes it possible for the model to account for the gradual change of stress state from plane strain to plane stress when the crack grows large. The change of stress state occurs typically for the fatigue crack in plates where the stress state at the crack tip is mainly determined by the relative ratio for the crack tip plastic zone size and the plate thickness. The change of stress state has a significant influence on fatigue crack growth behaviour. Weight functions were combined in the modified Dugdale model because crack surface displacement solutions can be derived and the stress intensity factors for complex stress fields can be obtained from corresponding weight functions. It has been shown that a great many problems can be solved when approximate weight function techniques are used. Comparisons were made between predictions from the model and experimental data available in the literature. There was very good agreement for the fatigue crack growth under constant amplitude loading with different stress ratios ( R-ratios), for the load interaction effects in both plane stress and plane strain conditions, for the fatigue crack growth in plates under spectrum loading, for the fatigue crack growth in residual stress fields, and for the small crack growth behaviour.