Using free surface velocimetry to infer hole closure in Tantalum under dynamic compression

The flow stress in a metal is dependent on a variety of factors such as strain, strain rate, microstructure, and temperature. Experiments (i.e. quasi-static tensile testing or Kolsky bar testing) with well characterized stress states have been used to determine the relation between flow stress and these many factors. However, for higher strain rates (>105/s) there is a dearth of high-fidelity data at high plastic strains. Here, we present results of a recent in-situ gas-gun experimental technique that can probe strength effects at strain rates of >105/s. By measuring the diameter of a long cylindrical hole using x-ray imaging in conjunction with back surface velocimetry while the sample is subjected to controlled dynamic loading, the factors affecting flow stress can be inferred. Materials with higher dynamic flow stresses tend to exhibit less diameter reduction than materials with lower flow stresses all else being equal. Typically, the hole size is measured through imaging, but if the hole is blocked, it becomes necessary to infer the amount of closure from another diagnostic like velocimetry. We present preliminary analysis and results on the velocimetry traces from a suite of hole closure experiments that indicates hole size information may be embedded in velocimetry traces.