Published online by Cambridge University Press: 03 September 2012
Experiments aiming at the measurement of the roughness index ζ of “rapid” fracture surfaces are briefly reviewed. For rapid crack propagation, measured values of ζ are close to 0.8, which seems to be a universal exponent. However, it is argued, by re-writing the Griffith criterion for a self-affine crack, that the self-affine correlation length ξ might depend upon the microstructure, and hence on 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.