Prediction of effective mode II fatigue crack growth threshold for metallic materials

The fatigue crack growth threshold consists of the intrinsic component (effective threshold) and the extrinsic component (crack closure). Theoretical formulae for prediction of the mode II effective threshold in metallic materials are presented. The model is based on physical interpretation of experimental findings. For a given material the dominant local crack growth mode at the kinked crack front of a remote mode II loaded crack is identified and the local stress intensity factor is expressed, which represents a new concept for estimation of the mode II effective threshold. The local crack growth mode depends on microstructure and crystallographic structure of the material. Moreover, new experimental data for mode II effective thresholds of the Ti6Al4V alloy (ΔKIIeff,th = 1.8 MPa m1/2) and pure zirconium (ΔKIIeff,th = 1.3 MPa m1/2) are presented. These data extended the available database for metallic materials and confirmed a broad validity of the proposed theoretical relationships.