Near Threshold Fatigue Curve Based on the Dislocation Shielding Model of Fracture

Abstract

Dislocation pileup along an inclined direction from a crack tip has been observed in metals during tensile deformation in an electron microscope. Dislocations are emitted from the crack tip, move through a dislocation free zone and pile up in the plastic zone. The problem of inclined pileup of screw dislocations at a crack tip with a dislocation free zone has been solved by applying the Wiener-Hopf method. The result can be used to calculate the crack tip blunting and the local stress intensity K for the applied stress intensity K3. According to the fatique theory of Laird or Neumann, the crack propagation per cycle has the order of magnitude of the blunting. The blunting versus K3 is then plotted. It has the shape of the near threshold fatigue curve. To plot this curve, the local stress intensity K has been assumed to be the critical value Kg for the dislocation emission at the crack tip.