Crack propagation in MgO during in-situ deformation in the high-voltage electron microscope

The crack propagation in MgO is observed by in-situ straining experiments inside the high-voltage elecron microscope. Even though the material is relatively brittle and has a high flow stress a plastic zone is formed in front of the crack tip by extensive dislocation multiplication on {110} 〈110〉 slip systems. The width of the plastic zone agrees relatively well with that theoretically expected from the stress field of the elastic crack model and the flow stress of the material. The stress distribution within the plastic zone is determined by analysing the curvature of stressed screw dislocation segments. The results show that the properties of the crack are essentially determined by plastic processes.