The influence of particle morphology on the dynamic densification of metal powders

Powders are well known for their dispersive properties, which derive from the many dissipative processes that occur during densification. While numerous studies have been devoted to understand these processes over a wide range of initial densities, the influence of particle morphology has been for the large part overlooked. In this paper, we discuss a new research campaign at the Institute of Shock Physics, to systematically investigate the role of starting configuration on the dynamic densification of metal powders. Multi-target gun loading experiments have been performed on both stainless steel and copper powders of equiaxed-and fiber-shaped morphology. Frequency-shifted PDV was employed to measure the structure and velocity of the dynamic densification wave, to yield the crush strength of the various powders. We find that while the crush strength for the stainless steel powders is reasonably described by a modifiedWu-Jing model, this model underpredicts the densification stress for the copper powder

Birefringence measurements in single crystal sapphire and calcite shocked along the a axis

Calcite and sapphire were shock compressed along the ⟨101⎯⎯0⟩ direction (a axis) in a plate impact configuration. Polarimetery and Photonic Doppler Velocimetery (PDV) were used to measure the change in birefringence with particle velocity in the shock direction. Results for sapphire agree well with linear photoelastic theory and current literature showing a linear relationship between birefringence and particle velocity up to 310 m s−1. A maximum change in birefringence of 5% was observed. Calcite however showed anomolous behaviour with no detectable change in birefringence (less than 0.1%) over the range of particle velocities studied (up to 75 m s−1)

A dynamic discrete dislocation plasticity method for the simulation of plastic relaxation under shock loading

In this article, it is demonstrated that current methods of modelling plasticity as the collective motion of discrete dislocations, such as two-dimensional discrete dislocation plasticity (DDP), are unsuitable for the simulation of very high strain rate processes (106 s-1 or more) such as plastic relaxation during shock loading. Current DDP models treat dislocations quasi-statically, ignoring the time-dependent nature of the elastic fields of dislocations. It is shown that this assumption introduces unphysical artefacts into the system when simulating plasticity resulting from shock loading. This deficiency can be overcome only by formulating a fully time-dependent elastodynamic description of the elastic fields of discrete dislocations. Building on the work of Markenscoff & Clifton, the fundamental time-dependent solutions for the injection and non-uniform motion of straight edge dislocations are presented. The numerical implementation of these solutions for a single moving dislocation and for two annihilating dislocations in an infinite plane are presented. The application of these solutions in a two-dimensional model of timedependent plasticity during shock loading is outlined here and will be presented in detail elsewhere. © 2013 The Author(s) Published by the Royal Society. All rights reserved