Toward an understanding of Mode II fatigue crack growth

The Mode II crack flank displacement and crack growth responses of three precracked specimens made from structural steel were measured, using plastic replicas and a crack-tip compliance gage. Crack surface interaction was found to dominate behavior: at low stress intensity range (ΔKIInom = 9 MPa √m) the precracks did not suffer reversed slip to their tips and no crack growth occurred, while at high stress intensity range (ΔKIInom = 19 MPa √m) the effective stress intensity range was less than half that nominally applied. Three sources of crack flank frictional attenuation were identified: compressive residual stresses due to precracking, Mode I wedging over asperities, and gross plastic deformation of interlocking asperities. The measured unlocking response was modeled successfully by assuming that crack flank frictional stresses obeyed a constant interfacial shear stress friction law.