ABSTRACT
Abstract Experiments aiming at the measurement of the roughness index £ of " rapid"
fracture surfaces are briefly reviewed. It is argued, by re-writing the Griffith criterion for a self-affine crack, that these results are compatible with the assumption that £ is universal, i.e. independant of the fracture mode and of the material. Only the relevant lengthscales of the structure, especially the selfaffine correlation length f, might depend upon the fracture toughness. More recent experiments are also described, which reveal at smaller lengthscales the existence of a quasi-static fracture regime separated from the previously studied rapid fracture regime by a crossover length which decreases with increasing crack velocity.
