In this study, characteristics of dislocation structures around a crack‐tip in both crystals of MgO and Si were investigated using high‐voltage electron microscopy (HVEM). The interaction between a crack and dislocations was analyzed to discuss the effect of crack‐tip plasticity on fracture toughness. The present paper first demonstrates a dislocation configuration around a crack‐tip in a MgO thin crystal, where the plastic zone called 45°‐shear type is dominant under the plane stress condition. Then, the slip systems necessary for brittle‐to‐ductile transition in ionic crystals with the rock‐salt structure are discussed based on the temperature dependence of stress‐strain relationships and the aspect of crack front observed in NaCl crystals. Second, crack‐tip dislocations near the surface of bulk Si crystals are exhibited based on the 3D analyses using HVEM tomography. We focus on a newfound plastic zone being perpendicular to the crack plane, which is different from either 45°‐shear type or the hinge type generally well‐known. The dislocation loops observed in this plastic zone fundamentally have a crack‐tip shielding effect to increase fracture toughness. Still, in addition, they contribute to activating crack‐tip dislocations by their localized antishielding field. These analyses reveal a fundamental toughening mechanism for crystalline materials. [ABSTRACT FROM AUTHOR]

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