A physically based model for dynamic failure in ductile metals

It is well established that high rate failure of structural materials takes place by rate processes occurring at the microscopic level and involving nucleation, growth, and coalescence of voids or cracks. At the submicroscopic level, the mechanism of failure in polycrystalline metals is often dislocation controlled. In the present work, we propose a physically based model describing these processes at high deformation rate such as under the planar impact test. This model combines the mechanical threshold stress (MTS) theory for the evolution of the flow stress and a mechanistic model for failure behavior by cumulative nucleation and growth of voids. This paper describes the approach used to obtain the constitutive equations and the resulting computational modeling for predicting dynamic failure in ductile metals. The model formulation is three-dimensional and is suitable for a general state of stress and strain. Results from the simulations of the planar impact problems for different configurations are presented and compared with the experimental results for OFHC copper and HY-100 steel. This comparison shows the model capabilities in predicting the experimentally measured free surface velocity profile as well as the observed spall pattern respectively in the copper target of a planar impact test and in the steel target of a plate-conic impact. We have also compared the results from this model to those of the phenomenological model of Johnson-Cook.