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，当变形几乎不在低温和高负载速率下发生变形时，材料很容易失效。裂纹尖端塑性变形的温度和加载速率依赖性应与断裂韧性相关。在这项研究中，在温度强度因子的两个速率下，在温度范围为100 K至570 K的温度范围内对碱（KCL纯和KCl-KBR固体溶液）进行了断裂韧性测试。高于室温，碱卤化物单晶的断裂韧性随温度增加。在晶体和钢上观察到了脆性的过渡。脆性转变的解释是由裂纹尖端处的脱位发射的动态模型和裂纹尖端前的脱位运动的动态模型给出的，因为发出的脱位的堆积掩盖了裂纹尖端的外部施加的应力场。在碱卤化物的单晶中，使用光学双折射和蚀刻蚀刻技术在从裂解裂纹尖端产生的滑动带中看到了位错阵列。已经发现，滑动条带的长度和发射位错的数量随温度增加了。这些关系表明，位错的发射和运动都由热激活机制控制。在这项研究中，已经使用动态模型进行了对脆性转变的计算机模拟，其中认为裂纹尖端的位错的发射是由热激活过程引起的。计算结果似乎可以通过实验观察到脆性到脱胆的转变。因此，断裂机理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。