A theoretical model describing the effect of nano-twin and dislocation pileup at twin boundary on the lattice dislocation from an elliptical blunt crack tip in nanocrystalline materials is established. The nano-twin can be represented by a wedge disclination quadrupole and some dislocations accumulate on a twin boundary. The emission criterion of the first edge dislocation emitting from the blunt crack tip are considered, and the analytical solution of stress intensity factor is obtained by applying complex variable function method. Then the influence of the dislocation emission angle, the position and orientation of the twin,the strength of nano-twin,the length and curvature radius of elliptical blunt crack on the critical stress intensity factor for the emission of the first dislocation from blunt crack tip is analyze in detail. The results shows that the effect of nano-twin on dislocations emitting from crack tip depends on the position and azimuth of the nano-twin. There is an optimal position of nano-twin to make the dislocations easiest to emit from the blunt crack tip. The dislocation pileup at the twin boundary will increase the critical stress intensity factors for dislocation emission, making the dislocation emission from crack tip difficult, thus reducing the toughness of the material contributed by dislocation emission.

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An impact analysis model is built to describe the effect of nanoscale amorphization on dislocation emission from a surface semi-elliptical crack tip in nanocrystalline materials. The nanoscale amorphization is formed by the splitting transformation of grain boundary(GB)disclinations caused by the motion of GBs. The analytical solution of the model is obtained by using the complex method, and the influence of nanoscale amorphization, dislocation emission angle, crack length, and curvature radius of surface crack tip on the critical stress intensity factor (SIF) of the first dislocation emission is investigated through numerical analysis. The numerical analysis shows that the impact of nanoscale amorphization on the critical SIF corresponding to dislocation emission depends on the dislocation emission angle, the position and the size of the nanoscale amorphous, the curvature radius, and the length of surface crack. As the curvature radius of surface crack tip and the crack length increase, the normalized critical SIF increases. When the nanoscale amorphization size is small, it has a great impact on the critical SIF for dislocation, but when the size is relatively large, the effect becomes small. The effect of the increasing strength of the nanoscale amorphization on dislocation emission from the surface crack tip is related to the distance between the nanoscale amorphization and the crack tip, and there is a critical crack-junction for which the increase of dislocation strength has little effect on dislocation emission.