The scaling properties of post-mortem fracture surfaces of brittle (silica
glass), ductile (aluminum alloy) and quasi-brittle (mortar and wood) materials
have been investigated. These surfaces, studied far from the initiation, were
shown to be self-affine. However, the Hurst exponent measured along the crack
direction is found to be different from the one measured along the propagation
direction. More generally, a complete description of the scaling properties of
these surfaces call for the use of the 2D height-height correlation function
that involves three exponents zeta = 0.75, beta = 0.6 and z = 1.25 independent
of the material considered as well as of the crack growth velocity. These
exponents are shown to correspond to the roughness, growth and dynamic
exponents respectively, as introduced in interface growth models. They are
conjectured to be universal.Comment: 12 page
