Strain-rate dependent brittle deformation of rocks during impact cratering: Linking mechanical data and microstructure

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During an impact event, the deformation of minerals and rocks is highly dynamic and reaches strain rates of >1000000 per second. The hemispherical attenuation of propagating shock waves causes strain rates and pressures to decrease with distance. A significant volume of the rock -is deformed at elevated strain rates larger than 10 to 100 per second at which the behavior of brittle materials likely becomes rate-sensitive. The aim of this project is to understand and quantify the role of strain rate sensitive brittle deformation of rocks for typical loads experienced during impact cratering in the range from quasi-static to highly dynamic conditions and to associate it with corresponding microstructures within the deformed rock.To accomplish this, we will systematically determine mechanical data (strength, elastic moduli, Poisson ratio) for limestone, claystone, granite, and basalt at strain rates ranging from quasi-static (~0.1 per second) to highly dynamic (1000000 per second) conditions under both compressive and tensile loading. Data are gathered with a triaxial loading frame (0.1-1 per second), a Split-Hopkinson-Pressure-Bar (1-1000 per second), and a Flyer-plate facility (>>1000 per second). We will derive the dynamic increase factors (DIF) of tensile and compressive strength for a given strain rate. The mechanical analyses will be combined with a comprehensive microstructural investigation of the experimentally deformed samples, in particular with respect to fracture topography, crack branching characteristics, fracture density, and the fracture/fragment size-frequency distribution. This will lead to an under-standing of failure mechanisms, energy dissipation and stress resistance of brittle materials at diverse strain rates conditions. We expect to identify a strain-rate dependent structural fingerprint of rocks that can be applied to naturally deformed rocks. This is key to narrow down the physical boundary conditions of deformation in natural settings. We will test and apply our results to samples from Barringer and Chicxulub impact craters.The inferred strain rate dependent dynamic increase factor (DIF) will be implemented into existing numerical models (e.g., the iSALE shock physics code). We will particularly evaluate the role of strain rate dependent deformation for the efficiency of impact cratering. The results of this study can potentially have a great relevance also for other geo-hazards involving rock deformation at medium to high strain rates, in particular seismogenic faulting.