Modelling the nucleation and propagation of cracks at twin boundaries

Fracture arising from cracks nucleating and propagating along twin boundaries is commonly observed in metals that exhibit twinning as a plastic deformation mechanism.  This phenomenon affects the failure of macroscopic mechanical components, but it is not fully understood.

In this paper 'Modelling the nucleation and propagation of cracks at twin boundaries', published in the International Journal of Fracture, researchers from this department and Engineering present simulations in which a continuum model for discrete twins and a cohesive zone model are coupled to aid the understanding of fracture at twin boundaries.

The interaction between different twin systems is modelled using a local term that depends on the continuum twin variables.  Simulations reveal that the resolved shear stress necessary for an incident twin to propagate through a barrier twin can be up to eight times the resolved shear stress for twin nucleation.  

Interface elements are used at the interfaces between all bulk elements to simulate arbitrary intragranular cracks.  An algorithm to detect twin interfaces is developed and their strength has been calibrated to give good agreement with the experimentally observed fracture path.  

The elasto-plastic deformation induced by discrete twins is modelled using the crystal plasticity finite element method and the stress induced by twin tips is captured.  The tensile stress caused by the tip of an incident twin on a barrier twin is sufficient to nucleate a crack.

A typical staircase fracture path, with cracks propagating along the twin surfaces, is reproduced only if the strength of the twin interfaces is decreased to about one-third of the strength of the bulk material.  

This model can be used to help understand fracture caused by the activation of multiple twin systems in different materials.