Abstract
The ratio (γ2 : γ1) of the work of fracture in crystals of sodium chloride containing known dispersions of one micron diameter cavities to that in normal crystals of sodium chloride, has been determined under dynamic conditions of cleavage. The method, based on measurements of the crack-velocity during cleavage, only permitted an order of magnitude calculation for γ21; nevertheless, the results show that dispersions of cavities with mean separations less than about 30 microns can increase the work of fracture of a crystal by at least two orders of magnitude. This increased work of fracture in the composite crystals can be correlated with the generation of cleavage steps when the crack intersects the cavities.

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