Damage evolution and damage model validation in spalled tantalum

Abstract
The dynamic damage resistance of two grades of very clean tantalum is compared. The materials were loaded to incipient failure using an 80 mm gas gun equipped with soft recovery using a parallel flyer plate experimental configuration. Both loading time and applied shock pressure was varied from 1.1-2 µs and 5.6-9.8 GPa, respectively. A VISAR recorded the back surface velocity. The recovered samples were metallographically sectioned, polished, and etched. Image analysis and optical profilometry quantified the resulting damage. Longitudinal porosity distributions of the tests are compared. A calibrated hydrocode based damage model was used to simulate the tests. We compare predicted and experimental measurements of free surface velocity and porosity. The model also produced free surface pullback signals suggesting high spall strength with little evolved porosity. This is further confirmation that in clean, high Peierls energy materials the VISAR pullback signal is associated with the creation of a region of enormous hydrostatic tensile stresses in the material, but is relatively distinct from the void coalescence process. A transition in coalescence mechanism, from primarily void growth impingement to long-range plastic instabilities, occurs with increasing hydrostatic tension and increasing stress pulse duration.