Crack roughness of high-speed fracture in brittle single crystalline material

Dynamic crack involves instabilities promoted by either external perturbation or intrinsic front oscillation. In an effort to decipher fracture surface markings of brittle materials, crack roughness of Wallner lines promoted by shear waves and surface corrugations induced by spontaneous front oscillations was investigated with silicon single crystal. The statistical distribution of surface height variations and the corresponding roughness exponents were determined. The Wallner lines are found to be self-affine with a Gaussian distribution and a roughness exponent of 0.8, which are consistent with the surface flaws giving rise to shear waves. In contrast, the surface corrugations do not exhibit self-affinity, due to their scale invariant characteristic shape. Interestingly, specific instabilities, which appear at very high speed and exhibit similar physical properties as front waves, render the crack roughness self-affine with a Gaussian distribution and a roughness exponent of 0.5. The findings of the present work will help to understand the origin of fracture surface markings for brittle materials, especially for single crystalline ceramics.