Publications

Simulation and analysis of Magnetically-Applied Pressure-Shear (MAPS) experiments

Haill, Thomas A.; Alexander, Charles S.

A new experimental technique to measure material shear strength at high pressures has been developed for use on magnetohydrodynamic (MHD) drive pulsed power platforms. The technique is referred to as Magnetically-Applied Pressure-Shear (MAPS). By applying an external static magnetic field to the sample region, the MHD drive directly induces a shear stress wave in addition to the usual longitudinal stress wave. Strength is probed by passing this shear wave through a sample material where the transmissible shear stress is limited to the sample strength. The magnitude of the transmitted shear wave is measured via a transverse velocity interferometry system (VISAR) from which the sample strength is determined. The strength of materials is defined as the ability of a material to sustain deviatoric (shear) stresses. Strength is an important aspect of the response of materials subjected to compression to high pressure. Beyond the elastic response, material strength will govern at what pressure and to what extent a material will plastically deform. The MAPS technique cleverly exploits the property that, for a von Mises yield criterion at a given longitudinal stress, the maximum amplitude shear wave that can be transmitted is limited by the strength at that stress level. Successful fielding of MAPS experiments to measure shear stresses relies upon correct numerical simulation of the experiment. Complex wave interactions among forward and reflected longitudinal and shear waves, as well as the advancing magnetic diffusion front of the MHD drive, can make the design of the experiment complicated. Careful consideration must be given to driver, sample, and anvil materials; to the thicknesses of the driver, sample and anvil layers; as well as to the timing of the interacting waves. This paper will present and analyze the 2D MHD simulations used to design the MAPS experiments. The MAPS experiments are modeled using Sandia's ALEGRA-MHD simulation code. ALEGRA-MHD is an operator-split, multi-physics, multi-material, arbitrary lagrangian-eulerian code developed to model magnetic implosion, ceramic fracture, and electromagnetic launch. We will detail the numerical investigations into MHD shear generation, longitudinal and shear stress coupling, timing of wave interactions, and transmission of shear at material interfaces.