A Mechanistic Description Of Strain Hardening And Softening In Quartz Sand

M. Hernandez 1, L. Cruz 1, G. Hilley 1, W.A. Take 2 1 Department of Geological and Environmental Sciences, Stanford University, Stanford, CA 2 Department of Civil Engineering, Queen's University, Kingston, Canada Dry quartz (beach) sand has been used in geodynamic models in the last 60 years as a proxy for the Earth’s upper crust (upper 10-15 km). Its use has been justified, in part, by the fact that quartz sand appears to deform in a similar manner to brittle upper crustal rocks, which includes a period of strain hardening before rupture, followed by strain softening after rupture. In this study we take advantage of our state-of-the-art experimental (sandbox) setting to document the link between the kinematics and dynamics of deforming sand in the verge of rupture (shear band or fault generation) using high resolution imaging techniques and a displacement-and-load controlled system that allows us to track the changes in load (pushing force) required to maintain constant displacement rates that result from strain hardening or softening while the faults (discontinuities) are generated. This experimental setting includes three cameras located at different positions outside the sandbox and connected to a computer so that they acquire high-resolution photographs of the experimental models during the experimental runs. We then process the images using a Particle Image Velocimetry (PIV) matlab code to derive strain and analyze it in combination with the displacement-and-load system data. Our results indicate that before the formation of a coherent (continuous) shear band, small areas or “pockets” of concentrated strain develop, and the main drop in load occurs during this initial period of strain localization and not when the shear band or fault is fully developed.