Discrete dislocation dynamics study of fracture mechanism and dislocation structures formed in mesascopic field near crack tip under cyclic loading

In the present study, the 2-dimensional discrete dislocation dynamics (DDD) simulation is applied to crack tip field under cyclic loading. The crack i s located on bcc (body centered cubic) iron subjected to the mode I crack o pening load. The crack front is taken in [1(1) over bar 0] direction and th e 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 pr oper 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 (molecul ar dynamics, DDD, and FEM based on crystal plasticity) in different scales are compared mutually. The shapes of plastic region, obtained by the DDD an d the FEM based on crystal plasticity qualitatively correspond with each ot her. Under cyclic loading, the number of nucleated dislocations is almost b alanced to the number of annihilated/disappearing dislocation so that the n umber of current dislocations is almost constant through the Simulation. Th e cleavage stress at crack tip is increasing during this steady state. This seems to give some useful informations of ductile-brittle transition in fa tigue crack extension by the DDD approach.