Viscoplastic constitutive theory for brittle to ductile damage in polycrystalline materials under dynamic loading

For every dynamic problem an ad hoc model can be developed to reproduce the damage levels in materials. This work provides a unified model to capture low to high strain rate and ductile to brittle damage processes in dynamic problems with different dynamic energy densities. First a viscoplastic model for low to high strain rate responses of polycrystalline materials is developed. The established viscoplastic model shows excellent correlation between the simulations and experimental results for a wide range of strain rates while it can be readily calibrated with two set of the experimental results. The pressure and temperature dependent elastic material properties are incorporated to account for the effect of the pressure and temperature variations in dynamic problems. The microscale degradation processes, i.e., microcracking and microvoiding, are then correlated to the macroscale failure modes in which the transition between the ductile to brittle microfracture modes is attributed to stress triaxiality, shielding and blunting effects in microcracking, and rate-sensitivity. Also, as a complementary to previously developed ductile void growth models, a novel fracture mechanics based damage model is developed to describe the microcracking process. While microvoiding models, such as Johnson void model (Johnson, 1981), assume the hydrostatic part of the applied stress dominates the deformation mechanism, the developed microcracking model is suited for the problems with the dominant deviatoric stress. The develop model takes into account the effects of the ductile crack bunting and brittle crack cleavage and also the effect of the formation and propagation of the dislocation trails at the microcrack tip. The microscale damage mechanisms are then correlated to the macrofracture modes within the Continuum Damage Mechanics (CDM) framework. The developed multiscale damage framework shows excellent flexibility to reproduce a wide variety of experimental results. Also the developed damage framework is capable of incorporating the stochastic analyses which can take into account the inherent uncertainties in the dynamic problems. (C) 2013 Elsevier Ltd. All rights reserved.