The Ohio State University

Modeling the large deformation, damage, and failure response of polycrystalline metallic materials is very important to defense related applications.  The failure process of materials which behave in a ductile manner involves shear localization, pore nucleation, pore growth and pore coalescence to lead to ultimate failure.  The combination of these mechanisms which leads to ultimate failure is strongly dependent on material and loading.  The characteristic length scale of these events also demonstrates the importance of a materials microstructure to their damage and failure response.  Shear localization and adiabatic shear band formation is an important component of the damage and failure process.  A common sample geometry used to study shear localization is the "tophat"; an axi-symmetric sample with an upper "hat" portion and a lower "brim" portion with the gage section between the hat and brim.  The gage section length is generally on the order of 0.9 mm with deformation imposed through a Split-Hopkinson Pressure Bar system at maximum top-to-bottom velocity in the range of 10-25 m/sec.  Metallographic analysis, nano-indentation and OIM measurements have been performed on sections of the samples to quantify the topology and hardness state of the material after large deformation shear.  We have modeled these polycrystalline tantalum experiments using polycrystal plasticity finite element models.  A Voronoi tessellation based microstructural model and a rate and temperature dependent crystal plasticity model was used.  The crystal plasticity model allowed for slip to occur on the twelve {110}á111ñ and twelve {112}á111ñ slip systems.  The statistics of the metallographic analysis, nano-indentation measurements and OIM crystallographic texture are compared directly to the numerical simulation results for one of the experimental conditions.  The numerical results suggest a maximum strain rate on the order of 10⁵ sec.^-1 in the gage section.  The results also suggest that for an initial temperature of 25 ^oC, a temperature in the neighborhood of 700 ^oC is reached within the gage section due to the substantial plastic deformation (up to 5.0).  The shear stress results within the gage section suggest a factor of three differences between high and low values due to intergranular interactions.