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Quasi-static rock strength is a nonconservative property, as fatigue during cyclic loading reduces the macroscopic strength. When strain rate under compressive loading increases above a lithology-specific threshold, the primary failure mechanism transitions from localized failure along discrete fractures to distributed fracturing. However, the role of load path under high strain rate conditions has not been explored in any detail. We examine the effect of rapid stress cycles on the dynamic compressive strength of Westerly Granite using a modified split Hopkinson pressure bar approach and explore the implications of our results for the formation of pulverized fault zone rocks. Under cyclic loading conditions, the compressive strength can be reduced by a factor of 2, demonstrating that, like the quasi-static strength, the dynamic rock strength is also a nonconservative property. Therefore, traditional high strain rate experimental approaches utilizing simple load paths may overestimate strength when rocks are subjected to complex load paths.

Plain Language Summary Rock strength increases with the rate at which loads are applied to them and at extremely rapidly applied loads, the style of rock failure changes from simple fracture to fragmentation. Understanding the mechanisms by which rock materials fail under rapid loading events has broad implications in numerous geologic and engineering applications, such as earthquake mechanics and mining and tunneling blasting. In this work, we adapt an innovative high strain rate experimental approach to investigate the effect of multiple compressive stress cycles that occur within hundreds of microseconds on rock deformation and strength. Our results indicate that the dynamic strength of granite can be reduced nearly twofold from that observed in traditional tests in which rocks are exposed to one simple load cycle. Therefore, traditional experimental approaches may overestimate the true dynamic rock strength.