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 ate two self-affine fracture regimes. At large enough length scales, the universal roughness index zeta similar or equal to 0.78 is recovered. At smaller length scales, the roughness exponent is close to zeta(c) similar or equal to 0.50. The crossover length xi(c) separating these two regimes strongly depends on the material, and exhibits a pou er-Iau decrease with the measured crack velocity xi(c) proportional to v(-phi), with phi similar or equal to 1. The exponents v and beta characterizing the dependence of xi(c) and v upon the pulling force are shown to be close to v similar or equal to 2 and beta similar or equal to 2.