The minimum plastic zone radius criterion for crack initiation direction applied to surface cracks and through-cracks under mixed mode loading

The theories for crack initiation orientation under biaxial loading based on the singular elastic field may become inaccurate as the crack tip plasticity increases. In this study, a criterion based on the relative minimum plastic zone radius (MPZR) is investigated for prediction of fatigue crack initiation angles. The MPZR criterion is applied to inclined surface crack specimens and through-crack specimens under mixed mode loading, and its performance is compared with those of the maximum tensile stress theory (MTS) and the minimum strain energy density theory (MSED). It is found that the MPZR criterion provides somewhat better overall prediction of the crack initiation angle than the MTS and MSED theories. The approach is also applied to predict crack initiation under mixed monotonic loading for through-crack specimens. The MPZR criterion is found to give conservative results, while the MTS criterion yields best correlation.