Plasticity of olivine at extreme strain rates characterized with ball-drop experiments

A variety of geophysical processes, such as seismogenic slip on frictional surfaces and shock associated with meteorite impact, involve plasticity at strain rates >10 ³ s ^-1 . Previous work on metals has demonstrated that strain rates >10 ³ s ^-1 result in significantly different rheological behavior than plasticity at lower strain rates. However, plasticity at these extreme strain rates remains largely uncharacterized in geological materials.

Here we present a series of ball-drop experiments that induce extreme strain rates in a single crystal of forsterite. The sample was mounted on an alumina substrate and laterally surrounded by a thin-walled tube of low-carbon steel. An induction furnace was used to heat the steel tube, and therefore the forsterite sample, allowing us to explore sample temperatures up to 450°C. A thermocouple was placed in contact with the exposed surface of the sample, which was prepared with a high-quality polish. A steel ball bearing was then dropped on the surface, and the rebound height was measured with high-speed video.

Based on these measurements, we calculate the dynamic hardness, contact time, and average strain rate as a function of temperature and impact velocity. Measured values are approximately 1.5 GPa, 5 μs, and 10 ⁴ s ^-1 , respectively. In contrast to plasticity at lower rates, our results reveal a strong sensitivity of hardness to strain rate, approaching a linear viscous rheology. We also demonstrate that the hardness increases as temperature increases above 300°C.

These observations are consistent with plasticity being limited by the drag of dislocations. Our measurements are in quantitative agreement with models of phonon-dislocation interactions, assuming reasonable initial dislocation densities. Because strength increases with increasing temperature in this regime, we predict that adiabatic shear heating and related instabilities will be less effective, or even stabilized, during high-rate deformation at elevated temperatures.