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 ζ ≃ 0.78 is recovered. At smaller length scales, the roughness exponent is close to ζc ≃ 0.50. 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 φ ≃ 1. The exponents ν and β characterising the dependence of ξc and v upon the pulling force are shown to be close to ν ≃ 2 and β ≃ 2.

The long distance roughness of fatigue fracture surfaces of a nickel-based superalloy is reported for two samples of different grain size. Statistical analysis over a wide range of length scales, from a few nanometers to a few millimeters, using scanning electron microscopy and atomic force microscopy allows to obtain accurately the self-affine correlation length. Long distance fracture profiles of 14,000 points were obtained and digitized from overlapping electron micrographs at a resolution of 0.22 micrometers/point. We have also analyzed the long distance roughness of the mirror zone on a soda-lime glass using atomic force microscopy. In the case of the nickel superalloy, correlation lengths are found to correspond well to the grain size. This result gives information about the mechanism of crack propagation in heterogeneous materials and shows that the correlation length of fracture surfaces is of the order of the largest microstructural heterogeneity.

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