Plasticity Under Pressure Using a Windowed Pressure-Shear Impact Experiment

Many experimental techniques have been developed to determine the compressive strength or flow stress of a material under high strain rate or shock loading conditions. In addition, pressure-shear techniques have been developed that allow for the measurement of the shearing response of materials under pressure. The technique described is similar to the traditional pressure-shear plate-impact experiments except that window interferometry is used to measure both the normal and transverse particle velocities at a sample-window interface. The velocities are measured using the normal displacement interferometer (NDI) for the normal velocity, and the transverse displacement interferometer (TDI) for the transverse velocity. A schematic of the experiment is shown in Figure 1. For our experiment, the diameters of the impactor, sample and window are 31.75 mm. A 3 mm thick Ta-10W flyer is used for all the experiments, and the window material is a 10 mm thick c-cut sapphire. Two types of samples, both polycrystalline Cu and V have been tested and have a nominal thickness of 1.5 mm. For the TDI measurement a 1200 lines/mm grating is etched in the sapphire and a thin ({approx}120 nm) metallic film is deposited over the grating. The sample attached to the film side of the window by application of glue at several locations on the outer circumference of the sample. The experimental result for a Cu experiment is shown in figure 2. Figure 2a shows an example of the interferomentric data from the NDI and TDI measurement taken at the sample/window interface. For the TDI measurement (pink) each fringe is equivalent to a the transverse displacement of 416 nm. The velocity is calculated by taking the derivative as a function of time, and the experimental velocity data are shown in Figure 2b. The flow stress of the material is extracted by using a LLNL hydrodynamics code, ALE3D, and an appropriate strength model. For the Cu experiments, the Mechanical Threshold Model (MTS) is used and can be matched to the transverse velocity profile with parameters that calculate a flow stress of 180 MPa. Similar experiments have also been performed on polycrystalline V and calculate a flow stress of approximately 600 MPa