On the effect of crack face contact and friction due to fracture surface roughness in edge cracks subjected to external shear

An elastic-plastic model predicting the reduction of mode III stress intensity factors by fracture surface roughness is presented. Normal contact forces arise when shear displacements cause the crack faces to be wedged open due to mismatch of the fracture surface asperities. These normal forces, in turn, generate frictional forces which act in opposition to the applied shear stresses and reduce the effective stress intensity factor. The fracture surface asperities are proposed to have a maximum amplitude (crack opening) and an average wavelength. This leads to three regions of behavior: region 1, where the shear displacement at the crack mouth is less than the wavelength of the asperities; region 2, where the crack mouth shear displacement is greater than the asperity wavelength; and region 3, where the crack tip displacement is greater than the wavelength of the asperities thereby wedging the crack open to the maximum amplitude over the entire fracture surface. The magnitude of the normal stresses are proposed to be limited by the yield stress which implies that weak materials will offer less frictional resistance to shear crack growth. The dependence of the frictional stress intensity factor on crack length, applied stress intensity factor, asperity height, wavelength of the fracture surface roughness, and yield strength are discussed. The model will be used to analyse the results of two published experimental studies—monotonie mode III fracture toughness testing of Al 2O 3 and mode III fatigue crack growth in steel.