Cleavage cracking along a Immaterial interface between Nb and sapphire was observed by Elssner et al. (1994) and Kora et al. (2002). The stress required for cleavage cracking is around the theoretical strength of the material. Classical plasticity models fall short to reach such a high stress level. We use the conventional theory of mechanism-based strain gradient plasticity (Huang et al., 2004), which has been established from the Taylor dislocation model, to investigate the stress level around the tip of an interface crack between Nb and sapphire. It is shown that the stress level significantly increases due to the strain gradient effect. At a distance of 0.1μm ahead of the interface crack tip, the tensile stress is high enough to trigger cleavage cracking.
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