Dislocation pile-up and cleavage: effects of strain gradient plasticity on micro-crack initiation in ferritic steel

Abstract

Micro-cracks in ferritic steel often originate from broken or debonded carbide particles, or from cleavage of the ferrite matrix. Experiments in literature show that dislocation pile-ups at grain boundaries or particles predominantly induce micro-cracks in ferritic steel. On the other hand, the ferrite can also arrest nucleated micro-cracks owing to local plastic deformations which reduce the stresses at the crack tip. In the present study, the competition between these mechanisms is investigated by cell model simulations using effective gradient plasticity (scalar gradient plasticity) for the ferrite. This theory allows to model the dislocation pile-up by suitable interface conditions. Potential cleavage of the ferrite or failure of the carbide is modelled by a cohesive zone. Parameter studies are performed with respect to the size of the particle and the strengths of ferrite and carbide.