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Abstract
Classical dynamic fracture theories predict the Rayleigh surface wave speed (cR) to be the limiting speed of propagation for mode-I cracks in constitutively homogeneous, isotropic, linear elastic materials subjected to remote loading. For mode-II cracks, propagating along prescribed straight line paths, the same theories, while excluding the possibility of crack growth in the speed regime between cR and the shear wave speed, cs, do not exclude intersonic (cs<υ<cl) crack tip speeds. In the present study, we provide the first experimental evidence of intersonic crack growth in such constitutively homogeneous and isotropic solids, ever recorded in a laboratory setting. Intersonic shear dominated crack growth, featuring shear shock waves, was observed along weak planes in a brittle polyester resin under far-field asymmetric loading. The shear cracks initially propagate at speeds just above cs and subsequently accelerate rapidly to the longitudinal wave speed (cl) of the solid. At longer times, when steady state conditions are attained, they propagate at speeds slightly higher than √2cs. The experimental results compare well with existing asymptotic theories of intersonic crack growth, and the significance of the preferred speed of √2cs is discussed.