Discrete dislocation dynamics study on fatigue fracture

In the present study, the 2-dimensional discrete dislocation dynamics (DDD) simulation is applied to crack tip field under cyclic loading. The crack is located on bcc (body centered cubic) iron subjected to the mode I crack opening load. The crack front is taken in [1 (1) over bar 0] direction and the plane strain assumption confines the number of active slip system to two (((1) over bar (1) over bar 2)[111], (112)[11 (1) over bar]) of the 48 proper bcc slip systems. The angles theta between the slip direction and the direction of the crack extension are set 54.7 degrees and 35.3 degrees. The plastic deformations, obtained by different computational methods (molecular dynamics, DDD, and FEM based on crystal plasticity) in different scales are compared mutually. The shapes of plastic region, obtained by the DDD and the FEM based on crystal plasticity qualitatively correspond with each other. The simulation results show that the system changes to a dynamical steady state with the formation of dislocation structure. Under cyclic loading, the number of nucleated dislocations is almost balanced to the number of annihilated/disappearing dislocation so that the number of current dislocations is almost constant through the simulation. The cleavage stress at crack tip is increasing during this steady state. This seems to give some useful informations of ductile-brittle transition in fatigue crack extension by the DDD approach.