Crack growth behavior of U71MnG rail steel under overload conditions described using a dislocation correction model

Abstract

The single-peak overload test based on DIC technology was carried out in this study, and U71MnG steel was used to explore the influence of dislocation motion on crack propagation during overload. The changes in the shape and size of the plastic zone during the overload fatigue cycle are tracked and recorded, and the trends in the stress intensity factors of the Christopher–James–Patterson (CJP) and dislocation correction models are compared. The degree of influence of the dislocation motion on the variation in the stress intensity factors is evaluated, and the variation pattern of the plastic flow factor is derived (the amount of crack tip blunting, ρ). The results showed that the dislocation correction model increased the accuracy of the solution of the coefficient set, and the predicted size of the plastic zone of the correction model was more consistent with the experimental data. A better match with the crack tip was observed in the experimental plastic zone, the dislocation correction model error with the theoretical plastic zone fluctuates within 10%, whereas the CJP model can reach a maximum of 36.75%, demonstrating the insensitivity of the dislocation correction model. The plastic flow factor ρ follows the same pattern as that of the plastic zone area and stress intensity factor amplitude, ρ increases slowly with the increase of crack length before overload, ρ increases significantly after overload and then decreases sharply, and it recovers to be stable with the disappearance of the overload hysteresis effect of crack propagation.