Temperature Dependence of Fracture Toughness and Dislocation Emission at Crack Tip

众所周知，当材料在低温和高加载速率下几乎不发生变形时，材料容易失效。裂纹尖端塑性变形对温度和加载速率的依赖关系应与断裂韧性的依赖关系相关。在100 ~ 570 K的温度范围内，以两种应力强度因子速率对纯卤化碱（KCl）和KCl- kbr固溶体单晶进行了断裂韧性试验。室温以上，碱卤化物单晶的断裂韧性随温度升高而增大。在晶体和钢中都观察到脆性到延性的转变。裂纹尖端的位错发射和裂纹尖端之前的位错运动的动态模型给出了脆性到韧性转变的解释，因为发射的位错的堆积掩盖了裂纹尖端的外部应力场。在碱卤化物单晶中，利用光学双折射和蚀刻点蚀技术，在解理裂纹尖端产生的滑移带内发现了位错阵列。结果表明，滑移带的长度和位错的数量随温度的升高而增加。这些关系表明，位错的发射和运动都受热活化机制的控制。在本研究中，使用动态模型进行了脆性到延性转变的计算机模拟，其中从裂纹尖端发射的位错被认为是由热激活过程引起的。计算结果似乎再现了实验观察到的脆性到韧性的转变。因此，断裂机制的主要过程被认为是裂纹尖端附近的热激活发射和位错运动。

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.