A J2 - J3 approach in plastic and damage description of ductile materials

This paper illustrates a methodology to improve the description of the plastic behavior and the fracture prediction for ductile materials under complex loading conditions. To this purpose, a plasticity model and a damage estimation model are proposed. The former, differently from the classic J2 plasticity theory, takes into account the effect of the third deviatoric invariant on the plastic flow. The latter assumes that damage accumulation is governed by both stress triaxiality and deviatoric parameters, and takes advantage of the new plasticity formulation. The two models rely on the same theoretical foundation, where a specific function is invoked to describe the subsequent yield surfaces and the damage accumulation up to fracture. Both have been implemented into a commercial finite element code via user subroutines. Three steel alloys have been tested under very different stress states: tensile tests on smooth and round notched bars, plane strain tests, torsion tests, and combined tension-torsion tests on hollow and solid cylindrical bars have been executed. For the last ones, several tension - torsion-loading ratios have been applied. These kinds of tests allow to explore a wide domain of the governing parameters for both models. The experimental results from tensile and torsion tests are used to calibrate the proposed plasticity model and the damage model; combined tests are used for validation purposes. The experimental-numerical comparison of global quantities made by using a standard plasticity approach confirms the need for a more accurate plastic description in the large strain range. The proposed plasticity model is able to provide a very good match until fracture for all tests available. Moreover, the damage model has the potential to take into account the experimental evidence, predicting the fracture initiation accurately. In particular, its validation by using tension-torsion tests shows to be really significant.