DISCRETE DISLOCATION MODELING OF EDGE CRACKS IN SINGLE CRYSTALS

摘要

Monotonic and cyclic loading crack growth behavior of 90° edge cracks is studied using discrete dislocation dynamics. The study is motivated by the apparently anomalous fatigue behavior of mechanically short cracks. Plastic deformation is modeled through the motion of edge dislocations in an elastic solid with the lattice resistance to dislocation motion, dislocation nucleation, dislocation interaction with obstacles and dislocation annihilation being incorporated through a set of constitutive rules and the fracture properties specified through an irreversible cohesive relation. In accord with experimental data we find that the fatigue threshold △K_(th) decreases with decreasing crack size below a critical crack length. Examination of the numerical results shows that the dislocation densities near the crack tip increase with decreasing crack length for the same applied remote stress intensity factor range △K_I. This acts to increase the stresses locally which results in crack growth at lower applied remote △K_I.