An experimental technique is presented for subjecting cracks to high rates of shear (mode-II) loading. By means of the shadow optical method of caustics, the dynamic crack tip loading conditions are investigated for both the phase of loading and the subsequent phase of failure. A failure mode transition is observed when the loading rate exceeds a certain limit. At low rates failure is controlled by fracture processes. At high rates, failure is controlled by the formation of adiabatic shear bands, a process not considered by fracture mechanics so far.

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