A study of fatigue crack tip characteristics using discrete dislocation dynamics

• Crack tip stress field of a polycrystalline material has been modelled under cyclic loads using DDD.
• Both dislocation climb and grain boundary penetration are considered in the model.
• Strain ratchetting is found near the crack tip, to which dislocation climb is found to be mainly responsible.
• Multiple slip systems are found close to the crack tip.

The near-tip deformation of a transgrannular crack under cyclic loading conditions has been modelled using discrete dislocation dynamics (DDD) with both dislocation climb and dislocation-grain boundary (GB) penetration considered. A representative cell was built to model the constitutive behaviour of the material, from which the DDD model parameters were fitted against the experimental data. The near-tip constitutive behaviour was simulated for a transgranular crack in a polycrystalline nickel-based superalloy. A phenomenon of cyclic creep or strain ratchetting was reproduced, similar to that obtained using viscoplastic and crystal-plastic models in continuum mechanics. Ratchetting has been found to be associated with dislocation accumulation, dislocation climb and dislocation-GB penetration, among which dislocation climb seems to be the dominant mechanism for the cases considered at elevated temperature. Ratchetting behaviour seems to have a distinctive discrete characteristic in that more pronounced ratchetting occurred within slip bands than elsewhere. Multiple slip systems were activated in grains surrounding the crack tip, as opposed to single active slip system in grains away from the crack tip. The present DDD results show that, the near-tip ratchetting strain ahead of the crack tip seems to be a physical phenomenon, which may be of particular significance for developing a physical-based model of crack growth.