Numerical modelling of combined roughness and plasticity induced crack closure effects in fatigue

The incidence of roughness induced fatigue crack closure has been studied by finite element modelling. Closure analyses in the literature have been reviewed and been shown to lack a reasonable treatment of: (a) propagating elastic-plastic cracks, and (b) the influence of the characteristically inhomogeneous plastic deformation associated with near-threshold crack growth in many materials. Based on an analysis of both overall specimen compliance and node behaviour along the crack path, the present modelling shows: (a) an increasing effect of crack path angle on roughness induced closure levels in keeping with the simple analytical model of Suresh and Ritchie; (b) the mechanism by which closure occurs is more strongly dependent on residual plastic strains in the wake than global shear displacements of the fracture surfaces due to mixed-mode behaviour at the crack tip; and (c) the closure levels are relatively low compared to experimental data, consistent with the absence of environmental irreversibility in the finite element models and the idealised crack path morphologies that were studied. Slip band simulations show a significant increasing effect of inhomogeneous deformation on closure levels, improving the apparent accuracy of the modelling results.