Crack tip dislocations and their shielding effect in MgO thin crystals

Crack tip shielding by dislocations has been studied using the method of 3-dimensional stress analyses in connection with dislocation structures observed by HVEM in MgO thin crystals. A screw dislocation array corresponding to the plastic zone of 45°-shear-type is commonly observed along the [100] direction ahead of the (010) crack in (001) thin plate crystals. The 3-dimensional stress analyses show that the main component of local stress intensity factor induced by the crack tip dislocations corresponds to mode I while mode II and III components are negligible in spite that they are screw dislocations. The mode I component induced is negative, which implies that the dislocations observed are of shielding type so as to accommodate the tensile stress concentration near the crack tip. The value of fracture toughness of MgO crystals is also discussed based on the stress analyses.