Nanoscale rotational deformation effect on dislocation emission from an elliptically blunted crack tip in nanocrystalline materials

A grain size-dependent model is theoretically established to describe the effect of a special physical micromechanism of plastic flow on the dislocation emission from an elliptical blunt crack tip in nanocrystalline solids. The micromechanism represents the fast nanoscale rotational deformation (NRD) occurring through collective events of ideal nanoscale shear near crack tips, which as a stress source is approximately equivalent to a quadrupole of wedge disclinations. By the complex variable method, the grain size-dependent criterion for the dislocation emission from an elliptical blunt crack tip is derived. The influence of the grain size and the features of NRD on the critical stress intensity factors for dislocation emission is evaluated. The results indicate that NRD releases the high stresses near the crack tip region and thereby enhances the critical stress intensity factor for dislocation emission. The NRD has great influence on the most probable angle for dislocation emission. The critical stress intensity factor will increase with the increment of the grain size, which means the emission of the dislocation becomes more difficult for larger grain size due to the effect of NRD.