Pinning and Depinning of Crack Fronts in Heterogeneous Materials

The fatigue fracture surfaces of a metallic alloy and the stress corrosion fracture surfaces of a silicate glass are investigated as a function of crack velocity. It is shown that in both cases there are two self-affine fracture regimes. At large enough length scales, the universal roughness index ζ0.78 is recovered. At smaller length scales, the roughness exponent is close to ζc0.50. The crossover length ξc separating these two regimes strongly depends on the material, and exhibits a power-law decrease with the measured crack velocity ξc∝v^-φ, with φ1. The exponents ν and β characterizing the dependence of ξc and v upon the pulling force are shown to be close to ν2 and β2.