Numerical study of ductile failure under non-proportional loading

This paper investigates two numerical methods for predicting the initiation of ductile failure under moderately and strongly non-proportional loading paths. Two distinct phenomena are considered as indicators for the initiation of ductile failure: (i) the localization of deformation into a narrow band and (ii) the coalescence of microscopic voids. Recent experimental data in the literature from various axisymmetric tension tests on a high strength steel are used to calibrate and validate the two methods. In the first method, which is based on the imperfection band approach, strain localization analyses are conducted using the deformation history extracted from finite element simulations of the tension tests. In the second method, axisymmetric unit cells are utilized to evaluate the onset of void coalescence using the stress history extracted from the same finite element simulations of the experiments. The various uniaxial tension tests yield different moderately and strongly non-proportional loading paths that are used to evaluate the predictive capabilities of the two methods. The numerical results are further used to discuss the similarities and differences between the two methods. Both the strain localization analyses and the coalescence analyses are found capable of predicting the initiation of failure in the experiments with good accuracy; however, the coalescence analyses are generally in closer agreement to the experiments.