Modeling damage and deformation in impact simulations

Numerical modeling is a powerful tool for investigating the formation of large impact craters but is one that must be validated with observational evidence. Quantitative analysis of damage and deformation in the target surrounding an impact event provides a promising means of validation for numerical models of terrestrial impact craters, particularly in cases where the final pristine crater morphology is ambiguous or unknown. In this paper, we discuss the aspects of the behavior of brittle materials important for the accurate simulation of damage and deformation surrounding an impact event and the care required to interpret the results. We demonstrate this with an example simulation of an impact into a terrestrial, granite target that produces a 10 km-diameter transient crater. The results of the simulation are shown in terms of damage (a scalar quantity that reflects the totality of fragmentation) and plastic strain, both total plastic strain (the accumulated amount of permanent shear deformation, regardless of the sense of shear) and net plastic strain (the amount of permanent shear deformation where the sense of shear is accounted for). Damage and plastic strain are both greatest close to the impact site and decline with radial distance. However, the reversal in flow patterns from the downward and outward excavation flow to the inward and upward collapse flow implies that net plastic strains may be significantly lower than total plastic strains. Plastic strain in brittle rocks is very heterogeneous; however, continuum modeling requires that the deformation of the target during an impact event be described in terms of an average strain that applies over a large volume of rock (large compared to the spacing between individual zones of sliding). This paper demonstrates that model predictions of smooth average strain are entirely consistent with an actual strain concentrated along very narrow zones. Furthermore, we suggest that model predictions of total accumulated strain should correlate with observable variations in bulk density and seismic velocity.