Crack resistance of hydrostatically compressed alkaline halide single crystals

Conclusions

• 1. The effective fracture toughness of the investigated alkali halide single crystals increases with an increase in pressure.

• 2. This effect is described well by Eq. (11), which was derived on the basis of the assumption of the addition to the effective surface energy caused by the action of high hydrostatic pressure on the elastic tip of the crack and of plastic relaxation of the stresses as the basic controlling factors.

• 3. The model obtained predicts the same slope of the K ^*_Ic (P) relationship with small values of P for all brittle and semibrittle crystalline bodies in the geometry of the double cantilever beam specimen. In intermediate cases investigations of the dislocation structure are necessary for description of this relationship.

• 4. For LiF single crystals a transition from bittle cleavage to semibrittle failure, which is controlled by the appearance and elongation of the slip lines (dislocation accumulations) originated by the crack, was observed. The dislocation structure in cleavage of NaCl and KCl makes it possible to include these crystals among initially semibrittle bodies.

• 5. One of the main reasons for the change in mechanical behavior of crystals and the increase in dislocation activity under high hydrostatic pressure conditions is the increase in the rate of liberation of elastic energy for accomplishment of the work for formation of the elastic tip of the moving crack.