Fracture strength of MgO bicrystals

The stress to form a crack, [sgrave]F, at the tip of a glide band blocked by the grain boundary of a MgO bicrystal has been measured as a function of glide band length D. The relationship [sgrave]F = [sgrave]0 + KD −1/2 has been found to hold at room temperature in direct quantitative confirmation of Zener's model. The constant [sgrave]0 is the stress for dislocation multiplication and K provides a qualitative measure of the effective cohesive strength at the glide band tip which in practice is dominated by porosity in the grain boundary. In addition, the effect of test temperature on [sgrave]F, for a constant value of D, is described.