Dislocation-based model of plasticity and roughness-induced crack closure

Along with external forces and the macro-geometry of cracked bodies, the local stress intensity factors Delta k and k(max) at fatigue crack fronts are also determined by internal stress fields and the crack front microgeometry (extrinsic shielding). This means that a description of the crack-tip stress field by two external (remote) parameters Delta K (K-max) and Delta T (T-max) is not sufficient. The paper presents a discrete dislocation model of contact shielding effects in the case of small-scale yielding under plane-strain conditions. The model is physically transparent and, unlike continuum-based models for plane stress, it enables us to directly assess the magnitude of both plasticity and roughness-induced components of crack closure. Moreover, it reflects an influence of microstructure on the roughness-induced term. The closure components can be simply extracted from experimentally measured values of the remote Delta K using standard data on mechanical properties and microstructure. Thus, the effective threshold Delta K-eff,K-th can be obtained as nearly independent of microstructure coarseness and applied cyclic ratio as shown for several important engineering materials. (C) 2011 Elsevier Ltd. All rights reserved.