Fatigue crack growth model including load sequence effects – Model development and calibration for railway axle steels

Beside well-known influence factors like residual stresses, component and crack geometry also load sequence effects significantly impact the crack propagation rate. In this paper two major load sequence effects, namely overloads and small loads, and their particular influence on the fatigue crack growth are investigated in detail. Both effects lead to a retardation in the crack propagation rate by increasing the plasticity-induced (overload) or the oxide-induced (small loads) crack closure. To develop a holistic fatigue crack growth model including the short crack behaviour as well as load sequence effects, the NASGRO equation is modified accordingly. This improved fatigue crack growth approach is calibrated for two common material grades for railway axles, namely EA4T and EA1N, and implemented into crack growth assessment software tools.