Development of Nano-scale Dynamic Observation System on Fatigue Crack Growth Behavior using Image Processing Technique

为了研究疲劳裂纹扩展机理，设计并制造了可安装原子力显微镜（AFM）载物台的疲劳试验机。利用该装置对黄铜进行了恒幅疲劳裂纹扩展试验。本计画所取得的主要成果如下：开发一套新设计的测试系统，安装在原子力显微镜上，使奈米尺度的疲劳裂纹成长行为的线上观察成为可能。在疲劳裂纹驱动力较小的情况下，一个施密特因子较高的滑移系优先起作用，疲劳裂纹沿着该滑移线扩展。然而，循环应变硬化发生在该滑移线上，导致扩展速率降低或裂纹止裂。此时，二次滑移系的影响变大，疲劳裂纹向二次滑移方向偏转。疲劳裂纹扩展一小段距离后，主滑移系开始活动，疲劳裂纹沿主滑移系分支沿着扩展。结果表明，在低裂纹扩展速率区，疲劳裂纹呈锯齿形扩展。另一方面，在高裂纹驱动力区，从裂纹边缘发出的离散滑移线沿沿着两个优先滑移方向被清楚地观察到。这意味着两个优先滑移同时以几乎相同的程度起作用，疲劳裂纹以近I型扩展。在加载和卸载过程中连续观察到滑移变形，发现疲劳裂纹尖端后的滑移作用导致裂纹闭合，而裂纹尖端的滑移作用导致裂纹扩展。

In order to investigate fatigue crack growth mechanism, fatigue testing machine which can be installed the stage of An atomic force microscope (AFM) was designed and manufactured. Fatigue crack growth test under constant amplitude loading was conducted on an cc-brass using this testing apparatus. The knowledge obtained in this project is as follows: Nano-scale on-line observation of fatigue crack growth behavior becomes possible through the development of new designed testing system installed on AFM. In the case that the fatigue crack driving force is relatively small, one slip system, Schmitt factor of which is relatively high, operated preferentially and fatigue crack grew along this slip line. However, cyclic strain hardening occurs on that slip line, resulting in the decrease of growth rate or crack arrest. At this point, the effect of secondary slip system become larger and fatigue crack deflected to the secondary slip direction. After fatigue crack grew a short distance, the primary slip system become operates, and fatigue crack branched and grew along that. It was found that fatigue crack grew a zig-zag manner in low crack growth rate region. On the other hand, in the high crack driving force region, discrete slip lines which emanated from crack edges were observed clearly along the two preferred slip directions. It implies that two preferential slips operated to almost identical extent simultaneously and fatigue crack grew in near mode I. The slip deformation during loading and unloading was observed successively and it was found that the operation of slips behind the fatigue crack tip resulted in crack closure and that at the crack tip brought about crack growth.