Abstract
A system for measuring the interface crack velocity is presented, and applied to photolithographically-generated 90–100 μm long cracks between a thin (1 μm) Nb strip of 60 μm width and a sapphire substrate. The cracks were loaded using a laser-generated stress wave. The duration of crack advance used for determining the average crack velocity was taken as the time during which the interface tensile stress exceeded that needed for crack initiation. This was numerically calculated using the interferometrically-measured free surface velocity of the sapphire as an input. Equi-spaced Al lines of 200 Å thickness were deposited in front of the crack tip using photolithography and used as markers to determine the total crack advance by viewing the shock-loaded samples in a scanning electron microscope. The maximum crack propagation speed of 11,346 m/s was determined which is close to the dilatational wave speed of 11,090 m/s in the stiffer sapphire along its basal plane axis. These observations are the first report of crack speeds approaching the dilatational wave speed of the stiffer bimaterial component.
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