Tensile Microcrack Formation During Experimental Dynamic Shear Rupture Under Uniaxial Loading

Motivated by the occurrence of high-angle pseudotachylyte injection veins along exhumed faults, we studied secondary tensile microcrack formation during dynamic shear rupture in the laboratory. Shear ruptures were induced by an exploding wire embedded along a frictionally held and glued interface in Homalite. During the experiments, the samples were held under a uniaxial load P applied at an angle α to the normal to the rupture interface. Test values of α varied between 30° and 70°, and P varied between 15MPa and 30MPa. The dynamic stress fields produced by the propagating shear rupture were recorded using photoelasticity and high-speed digital photography. Observed isochromatic fringe patterns were similar to contours of maximum shear stress evaluated using a solution for dynamic propagation of a mode II crack with a velocity-weakening endzone. Rupture velocities during experiments ranged between 60% and 90% of the shear wave velocity. Secondary microcracks were produced in the Homalite samples during rupture under multiple loading configurations. In all cases, the region of shear stress concentration was spread over a cohesive endzone (10-20 mm in length) behind the traveling rupture tip. In some cases, microcracks were observed to grow at a finite distance behind the shear rupture tip. We observed several interesting behaviors: (i) some tensile microcracks appear to form due to transient stress concentrations associated with the mode II rupture and are roughly periodic; (ii) some tensile microcracks are associated with rupture termination and are concentrated at the final rupture tips; and (iii) a correlation between rupture velocity and microcrack orientation appears to exist. Future work will address the correlation of secondary tensile fracture orientation and spacing with rupture parameters including velocity, directivity, and remote stress state.