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Temperature Dependence of Fracture Toughness and Dislocation Emission at Crack Tip

断裂韧性和裂纹尖端位错发射的温度依赖性

基本信息

批准号：
01550046
负责人：
KISHIDA Keizo
金额：
$ 1.47万
依托单位：
Osaka University
依托单位国家：
日本
项目类别：
Grant-in-Aid for General Scientific Research (C)
财政年份：
1989
资助国家：
日本
起止时间：
1989 至 1990
项目状态：
已结题

项目摘要

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.

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。裂纹尖端塑性变形的温度和加载速率依赖关系应该与断裂韧性的依赖关系有关。在本研究中，在两种应力强度因子速率下，在100K到570K的温度范围内，对碱卤化物(纯KCl和KCl-KBr固溶体)单晶进行了断裂韧性测试。在室温以上，碱卤化物单晶的断裂韧性随温度的升高而增加。在晶体和钢上都观察到了脆性到延性的转变。由于位错的堆积屏蔽了裂纹尖端的外加应力场，用裂纹尖端位错发射和裂纹尖端前方位错运动的动力学模型解释了脆韧转变的原因。在碱金属卤化物单晶中，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观察到位错阵列。结果表明，滑移带长度和发射位错数随温度升高而增大。这些关系表明位错的发射和运动都受热激活机制的控制。在本研究中，使用动力学模型对脆性-韧性转变进行了计算机模拟，其中裂纹尖端的位错发射被认为是由热激活过程引起的。计算结果似乎重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位错的热激活发射和运动。