Large elastoplastic deformation of a sample under compression and torsion in a rotational diamond anvil cell under megabar pressures

Material behavior under high pressure and large plastic shear is usually studied in a rotational diamond anvil cell (rotational DAC, or RDAC). The first modeling results for three-dimensional plastic flow and evolution of the stress-strain state for a rhenium sample compressed and twisted in RDAC under megabar pressures are presented. Large elastic and plastic deformation theory is utilized. In addition to quantitative information about evolution of the stress-strain fields, the following unexpected results are obtained. During initial compression followed by torsion under a fixed axial force and for beveled anvils, the sample thickness reduces by a factor of ninety. Pressure, as well as the pressure gradient, grow drastically at the central part, and this new phenomenon is called the pressure self-focusing effect. This effect allows a much higher maximum pressure to be reached in a RDAC than in a DAC under the same force. The effects of diamond and sample shape, yield strength and its pressure dependence, the friction coefficient, and applied force on the pressure distribution are analyzed. The obtained results open new ways to increase maximum achievable pressure and develop new intuition in mechanical responses at megabar pressures which will be beneficial to the optimum design of geometry and loading conditions in the experiment.