The influence of crack tip dislocation emission on the fracture toughness

Crack-tip dislocation emission is often considered to be the key mechanism that controls the so-called "intrinsically ductile" fracture behaviour. Yet, high fracture toughness and ductility in metals are determined by extensive plastic deformation that dissipates much more energy than solely due to the crack-tip emission process. Thus, there is a gap between intrinsically ductile behaviour and large toughness. Here, we implement the dislocation emission process within a 2D discrete dislocation plasticity (DDP) framework. The framework, which includes anisotropic elasticity and a cohesive-zone model to simulate crack propagation, enables to investigate the interplay between dislocation emission and near-crack-tip plasticity associated with activation of dislocation sources. Guided by dimensional analysis and a sensitivity study, we identify the main variables controlling the fracture process, including dislocation source and obstacle density, dislocation emission strength and the associated dwelling time-scales. DDP simulations are conducted with a range of parameters under mode-I loading. The initiation fracture toughness and the crack-growth resistance curve (R-curve) are calculated accounting for the statistics of dislocation and obstacle distributions. Comparison is performed with cases where no dislocation emission is enabled. Our findings show that dislocation emission can slow down crack growth considerably, resulting in a significant increase in slope of the R-curve. This phenomenon is due to crack-tip shielding caused by the emitted dislocations. Thus, intrinsic ductility can enhance crack-growth resistance and fracture toughness. However, we find that the extent of shielding can also be negligible for some emission planes, making the connection between intrinsic ductility and fracture toughness not straightforward.