Role of stress triaxiality in elastoplastic characterization and ductile failure prediction

The triaxiality of the stress state is known to greatly influence the amount of plastic strain which a material may undergo before ductile failure occurs. During tensile load histories, the necking induces significant stress triaxiality modifications which in turn affect the experimental stress–strain measurements needed for the characterization of ductile metals. In this paper, the recently proposed "MLR" model of necking effect is used to obtain the flow curves of various metals by correcting the experimental data of tensile tests. Finite elements simulations of the experimental tests are performed to calculate the stress triaxiality evolution on various notched and unnotched specimens. A ductile failure criterion, due to Bao and Wierzbicki, is then applied to evaluate the material damage and predict failure. This procedure is applied to a set of 20 specimens series made of six metals with 10 different notch shapes. The damage calculations also indicate the material points where failure initiates. These predictions are confirmed by micrographic observation of voids on polished fragments of the broken specimens.