A local viewpoint for evaluating the influence of stress triaxiality and Lode angle on ductile failure and hardening

All damage and failure models, describing either the evolution of microvoids, the development of shear bands or local rupture, rely on the knowledge of the hardening function at large plastic strains which, then, becomes an essential prerequisite for any failure prediction. The phenomenon of ductile failure is analyzed here by focusing on its relationship with the variables for the stress--strain characterization, and by discussing the influence of plastic strain, stress triaxiality and Lode angle parameters on both the above aspects of materials behavior. Failure predictions are presented for different metals and different combinations of load-specimen geometry, according to three theories (the Tresca criteria and two models by Wierzbicki et al.) and to a procedure previously developed for the stress--strain characterization in the post-necking range. Experimental tests are performed by pulling tensile specimens and notched flat samples up to failure, then finite elements simulations are used to calculate the required failure-related variables within the volume of failing specimens; the results of the failure calculations are compared each other and with experimental data, and a discussion about the peculiarities of the methods used for predicting failure is also provided.