Effect of nanoscale twin and dislocation pileup at twin boundary on crack blunting in nanocrystalline materials
A theoretical model is developed to investigate the effects of the nanoscale twin and the dislocation pileup at the twin boundary on crack blunting in nanocrystalline materials. In the model, the nanoscale twin as a stress source approximately equals a quadrupole of wedge disclination. Using the complex variable method, the complex form expressions of the stress field and the force field are derived. The critical stress intensity factors (SIFs) for the first dislocation emission from the crack tip are calculated. The effects of the dislocation pileup, disclination strength, twin size, twin orientation, twin position and crack length on the critical SIFs are discussed in detail. Moreover, the shielding/anti-shielding effect produced by the twin, the dislocation pileup at the twin boundary and the first dislocation emitted on the crack tip is discussed. The results show that both the twin and the dislocation pileup at the twin boundary would suppress the dislocation emission from the crack tip. The suppressive effect induced by the dislocation pileup at the twin boundary is much stronger that that by the twin. Meanwhile, the emission angle has a significant effect on the mode I shielding/anti-shielding effect on the crack tip a.
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