High– and low–cycle fatigue crack initiation using polycrystal plasticity

A polycrystal plasticity finite–element model has been developed for nickel–base alloy C263. That is, a representative region of the material, containing about 60 grains, has been modelled using crystal plasticity, taking account of grain morphology and crystallographic orientation. With just a single material property (in addition to standard elastic properties), namely, the critical resolved shear stress, the model is shown to be capable of predicting correctly a wide range of cyclic plasticity behaviour in face–centred cubic nickel alloy C263. A fatigue crack initiation criterion is proposed, based simply on a critical accumulated slip. When this critical slip is achieved within the microstructure, crack initiation is taken to have occurred. The model predicts the development of persistent slip bands within individual grains with a width of ca. 10 μm. The model also predicts that crack initiation can occur preferentially at grain triple points under both low– (LCF) and high–cycle fatigue (HCF). For the case of HCF, this also corresponds to a free surface. The polycrystal plasticity model combined with the fatigue crack initiation criterion are shown to predict correctly the standard Basquin and Goodman correlations in HCF, and the Coffin–Manson correlation in LCF. The model predictions are based on just two material properties: the critical resolved shear stress and the critical accumulated slip. Just one experimental test is required to determine these properties, for a given temperature, which have been obtained for nickel alloy C263. Predictions of life for nickel alloy C263 are then made over a broad range of loading conditions covering both LCF and HCF. Good agreement with experiments is achieved, despite the simplicity of the proposed ‘two–parameter’ model. A simple three–dimensional form of the model has provided an estimate of the fatigue limit for HCF crack initiation in C263.