The influence of torsional and tensile pre-straining on the validity of ductile fracture criteria

Continuous loading and smooth strain paths are usually used to obtain data for empirical ductile fracture criteria or to verify such criteria. It is of interest to understand whether or not these ductile fracture criteria are valid for discontinuous paths. To this end, an experimental program is carried out and its results are combined with two conventional empirical ductile fracture criteria. It is shown that the criteria are not applicable for discontinuous loading paths. Based on the experimental results obtained and experimental results available in the literature it is hypothesized that a measure of rotation of the principal stress (or principal strain rate) axes relative to the material affects the initiation of ductile fracture.