Fatigue crack growth under variable-amplitude loading is usually accompanied by the load interaction effect. One of the significant phenomena of load interaction effects is retardation induced by an overload or multiple overloads. To predict the fatigue crack growth rate under constant-amplitude loading with single and multiple overload, an improved constitutive model based on partial crack closure due to crack-tip plasticity is proposed in this paper. In this model, both the maximum stress intensity factor at crack opening level and the threshold value of effective stress intensity factor range are expressed as the explicit functions of stress ratio and the threshold stress intensity factor range when stress ratio is zero. It is assumed in the improved constitutive model that the crack closure level can rise due to a larger plastic zone resulting from the overload effect, and a modified coefficient based on the Wheeler model is introduced to correct the amount of the maximum stress intensity factor at crack opening level during the recovering period after an overload. By a cycle-by-cycle integration procedure, the fatigue crack growth rate under constant-amplitude loading with single and multiple overload is predicted quantitatively with this model. Comparison is made between the results of analytical predictions and experimental data and good agreements are found. This indicates that the proposed constitutive model is able to explain the load interaction effect under variable-amplitude loading.
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