Temperature Dependence of Fracture Toughness and Dislocation Emission at Crack Tip

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It is well known that materials fail easily when deformation hardly occurs at low temperature and high loading rates. The temperature and loading rate dependences of plastic deformation at the crack tip should relate to those of fracture toughness. In this research, the fracture toughness tests has been performed on alkali halide (KCl pure and KCl-KBr solid solution) single crystals over the temperature range from 100 K to 570 K at two rates of stress intensity factor. Above room temperature, the fracture toughness of the alkali halide single crystals increases with temperature. The brittle-to-ductile transition has been observed on the crystals as well as steels. The explanation of the brittle-to-ductile transition has been given by the dynamic models of the dislocation emission at the crack tip and dislocation motion ahead of the crack tip, since the pile-up of emitted dislocations shields the externally applied stress field at the crack tip. In the alkali halide single crystals, the dislocation array has been seen within the slip bands generated from the cleavage crack tip using optical birefringence and etch-pitting techniques. It has been found that the length of slip bands an the number of emitted dislocations have increased with temperature. These relations imply that both emission and motion of dislocations are controlled by the thermal activation mechanism. In this research, the computer simulation of the brittle-to-ductile transition has been carried out using the dynamic model where the emission of dislocations from the crack tip is thought to arise from the thermally activated process. The computational results seem to reproduce the brittle-to-ductile transition observed experimentally. Therefore, the predominant process for the fracture mechanism is considered to be the thermally activated emission and motion of dislocations in the vicinity of the crack tip.