Plastic flow modelling under abrupt strain-path change and parameter identification method

Rational plastic flow modelling under abrupt strain-path change is crucial for accurate metal forming simulations. Previous experimental studies have revealed that many metals exhibit “corner effects” after abrupt strain-path change in the plastic stage, including a sharp vertex produced on the subsequent yield surface and apparent non-normality of the plastic flow direction. Some advanced models can give reasonable predictions on the above phenomenon, but they have not been widely used in metal forming simulations due to their complexity. A simple non-associated flow rule is developed here to describe corner effects. A stable stress update algorithm is developed for this rule based on the fully implicit backward Euler method, and its validation is verified by FE simulations of the nonlinear tension-torsion loading experiments. In order to determine the material parameters of these non-associated flow rules representing corner effects, a general identification method is proposed based on the equivalent tangential shear modulus reduction. The results show that, compared with the previous theories, the present rule performs better in predicting corner effects. In addition, although the apparent tangential shear modulus reduction is observed during the nonlinear experiments, the modulus does not seem to decrease to near zero.