Strain-rate dependent brittle deformation of rocks during impact cratering: Linking mechanical data and microstructure
Final Report Abstract
The projects aim was to characterize the behavior of geological materials under dynamic loading conditions as experienced during meteorite impact events. This objective was to be addressed through a combination of experimental mechanical experiments, microstructural characterization, and numerical modelling. More specifically, the strength and fragmentation of a variety of rock types under compressive uniaxial loads was investigated. Numerical impact modelling was then used both to demonstrate that the conditions for dynamic compressive behavior are reached in impact events and to improve currently implemented models of rock strength. A total of 7 lithologies were characterized and further investigated with 156 high-rate (split-Hopkinson pressure bar; SHPB, flyer-plate facility) deformation experiments and 48 low-rate (hydraulic loading frame) experiments. Analysis of the failed rock samples included: determination of fragment size distributions, determination of fragment shape distributions, and mapping of fracture surface topography. Finally, the iSALE shock physics code was used and developed to include the effects seen in our experiments. We found that rock strength is significantly affected at strain rates greater than 15-30 s-1. These strain rates are readily achieved during meteorite impact events. The observed increase in rock strength with strain rate was implemented in the iSALE shock physics code. We also found that fragment size is strongly dependent upon the strain rate and propose an empirical scaling law for the average fragment size as a function of strain rate under uniaxial loading conditions. Furthermore, we found that the fragment shape under uniaxial loading conditions appears to be independent of the strain rate, but shows topographical differences as the strain rate increases. With insight from geometric fragmentation algorithms, we suggest that uniform fragment shapes as a function of strain rate is a consequence of fragmentation as a stochastic process where the growth of fractures is limited by the presence of other growing fractures, i.e. the behavior of individual fractures is fundamentally the same under low- and high-rate deformation. One implication of our study is that fragment size may be used as a diagnostic indicator of the strain rate at rock failure while fragment shape cannot be used.
Publications
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Dynamic Compressive Strength and Fragmentation in Felsic Crystalline Rocks. Journal of Geophysical Research: Planets, 125(10).
Rae, Auriol S. P.; Kenkmann, Thomas; Padmanabha, Vivek; Poelchau, Michael H. & Schäfer, Frank
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Stress and strain during shock metamorphism. Icarus, 370, 114687.
Rae, Auriol S.P.; Poelchau, Michael H. & Kenkmann, Thomas
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Dynamic compressive strength and fragmentation in sedimentary and metamorphic rocks. Tectonophysics, 824, 229221.
Rae, Auriol S.P.; Kenkmann, Thomas; Padmanabha, Vivek; Poelchau, Michael H.; Schäfer, Frank; Dörfler, Matthias A. & Müller, Louis
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Dynamic Split Tensile Strength of Basalt, Granite, Marble and Sandstone: Strain Rate Dependency and Fragmentation. Rock Mechanics and Rock Engineering, 56(1), 109-128.
Padmanabha, Vivek; Schäfer, Frank; Rae, Auriol S. P. & Kenkmann, Thomas
