65. Crack-Dislocation Interactions Ahead of a Crack Tip

An overview of interactions of crack and dislocations emitted from the crack is provided with particular emphasis on theoretical approaches. Several existing models, such as due to Bilby, Cottrell and Swinden, Burns and Majumdar, Lin and Thomson, and Pande and Masamura, are presented. These models provide a formalism that gives the equilibrium positions of dislocations in an array ahead of crack tip, from which important parameters such as plastic zone size, dislocation-free zone, and dislocation stress intensity factor can be determined, which are useful in discussing the phenomenon of fatigue. Following these overviews, the experimental results on the dislocation-free zone and plastic zone observed ahead of the cracks in aluminum using transmission electron microscopy are presented and compared with the existing models. The experimentally measured values of these zones are shown to be in reasonably good agreement with theoretical models of crack-dislocation configuration based on a continuum distribution of dislocations ahead of the crack. However, these models fail to predict the total number of emitted dislocations, underlying the need for better analytical models. We also provide a brief overview on crack tip dislocation behavior under fatigue loading.