Pinning/depinning of crack fronts in heterogeneous materials
The fatigue fracture surfaces of a metallic alloy, and the stress corrosion fracture surfaces of glass are investigated as a function of crack velocity. It is shown that in both cases, there are two fracture regimes, which have a well defined self-affine signature. At high enough length scales, the universal roughness index 0.78 is recovered. At smaller length scales, the roughness exponent is close to 0.50. The crossover length $ξ_c$ separating these two regimes strongly depends on the material, and exhibits a power-law decrease with the measured crack velocity $ξ_c \propto v^{-ϕ}$, with $ϕ\simeq 1$. The exponents $ν$ and $β$ characterising the dependence of $ξ_c$ and $v$ upon the pulling force are shown to be close to $ν\simeq 2$ and $β\simeq 2$.