Prediction of fatigue crack growth rates under variable loading using a simple crack closure model

A model for the prediction of growth rates of fatigue cracks in aluminium alloys subjected to service loading conditions is presented. The model is based on an approximate description of the crack opening behaviour and was applied successfully in an analysis of effects of crack growth acceleration and retardation observed experimentally under variable amplitude loading. Effects of the plane strain to plane stress transition and of relatively high overloads on the growth rate of the crack are described by the model. The effect of multiple overloads will be discussed. To evaluate the capabilities of crack growth models a data base of experimental results has been established at NLR. These results were used to analyse the effect of certain load spectrum variations on the growth rate of a central crack in sheet specimens cut from 7075-T6 and 2024-T3 materials (the sheet thickness is 2 mm). Using the crack growth model the effects of these load spectrum variations were predicted for the same specimen configuration and materials.