A Study on Applicability of Microscopic Approach to clear the Fracture Behavior of Materials

利用扫描电子显微镜(SEM)和分子动力学(MD)从微观角度对疲劳裂纹扩展行为的本质进行了研究。利用CT试件对铜单晶(面心立方材料)和3%硅钢(体心立方材料)进行了疲劳试验。对于每个试件，晶体取向和预裂纹方向的组合是系统地不同的。结果表明，两种材料的疲劳裂纹都是在各自晶系特有的活动滑移面上微观扩展，而不是在机械主应力面上扩展。这种机制不能用连续介质力学中被认为合适的Laird模型来解释，而可以用修正的Neumann模型来解释。在这些材料中，活动滑移系统与其他系统相互干扰，疲劳裂纹扩展速率在很大程度上依赖于这一机制，因此巴黎定律在这些情况下是不适用的。在与试验条件相对应的MD模拟中，两种材料的疲劳裂纹扩展行为与试验结果和断口的扫描电子显微镜观察结果相似。模拟了循环载荷下原子的变化，反映了原子的不可逆变化(疲劳损伤)。以3%硅钢为例，研究了晶界对断裂行为的影响，并与试验结果进行了比较。当裂纹穿过晶界时，断裂行为将在很大程度上取决于相邻两个晶粒的晶体方向。可以说，分子动力学方法对于分析金属断裂的微观机制是一种有用的工具。

We studied on essential aspects of fatigue crack propagation behavior from a microscopic view by means of Scanning Electoron Microscope (SEM) and Molecular Dynamics (MD). Fatigue tests of Cu single cristals (fcc material) and 3%Si steels (bcc material) were conducted using CT specimens. Combinations of crystallographic orientation and pre-crack direction are varied systematically for each specimens. It is shown that fatigue crack of both materials propagates microscopically on the active slip plane which is characteristic of each crystal system, but not on the mechanical principal stress plane. This mechanism can not be explained by the Laird model, which is thought to be appropriate in continuum mechanics, and can be explained by the modefied Neumann model. In these materials an active slip system interferes intricately with the others, and fatigue crack propagation rates depend much on this mechanism, therfore Paris law are not useful in these cases. In MD simulations corresponding to test conditions, it is shown that the propagation behavior of fatigue crack of both materials is similar to tests resuls and SEM observations of fractured surfaces. Changes of atomcs under cyclic loading are also simulated and represent the irreversible changes of atoms (fatigue damages). In case of 3%Si steels, we investigated effects of grain boundary on fracture behavior and comperared with test resuls. When a crack across the grain boundary, fracture behavior will be changed largely upon the crystallographic direction of two adjoining grains. It can be said that MD method may be a useful tool for analyzing a microscopical mechanism of fracture in metals.