Constraint effect on the brittle-to-ductile transition of single-crystal iron induced by dislocation mobility

The brittle-to-ductile transition (BDT), no matter what the dominated mechanism is, dislocation nucleation or dislocation motion, is not an intrinsic phenomenon of material, and depends not only on the strain rate but also on the constraint at crack tip. However, few work has been performed on studying the effect of constraint on BDT. In this study, a dislocation mobility based continuum model is employed to model the BDT behavior of single-crystal iron under different loading rates. Two scenarios of T-stress implementation in the model has been adopted to investigate the effect of constraint on BDT. It is found that the change of the stress distribution ahead of crack tip due to the T-stress dictates the fracture toughness of single-crystal iron in the BDT transition region. Lower constraint leads to a higher fracture toughness in the transition region, a smoother transition curve and a lower critical BDT temperature, and also a higher fracture toughness at the critical BDT temperature. A quantitative relation between fracture toughness and T-stress has been established such that the BDT curve with constraint can be estimated from a reference BDT curve. Moreover, a solution to build a temperature-dependent effective surface energy law is also introduced, which could facilitate the cleavage fracture assessment