Microscopic Fracture Mechanics Study on Mechanisms of Small Fatigue Crack Propagation by Micro Beam X-ray Stress Evaluation

1. 采用高强度x射线发生器和位置敏感探测器，研制了一种新型的微束应力测量系统。该系统测量了约100 μm范围内的应力，测量精度较高。该系统还用于测量亚微米厚度薄膜表面的面内应力。利用同步辐射源SPring-8的高强度x射线，成功地测量了钢、陶瓷和复合材料裂纹尖端附近区域的应力，并用于预测裂纹的扩展行为。利用电子背散射图和原子力显微镜分析了细晶钢疲劳裂纹萌生的晶体学条件。在疲劳裂纹扩展初期，裂纹的挠曲和分支增加了裂纹扩展的阻力。裂纹的挠曲和分支决定了疲劳裂纹的闭合程度，裂纹扩展速率是有效应力强度因子范围的唯一函数。晶界和第二相对疲劳裂纹扩展的影响通过裂纹尖端附近的阻塞滑移带来模拟。在模型中进一步考虑了裂纹闭合。研究了微观组织障碍和裂纹闭合对小疲劳裂纹阈值条件的相对作用。采用预裂空心圆柱体，研究了轴扭混合加载对预裂空心圆柱体疲劳裂纹扩展行为的影响。疲劳裂纹的扩展方向受裂纹尖端附近切向应力最大范围的控制。

1. A new system of microbeam stress measurement was developed by using a high intensity X-ray generator and a position sensitive detector. The stress in the area of about 100 μm was measured with a high precision by the new system. The system was also used to measure the in-plane stress in the surface region of thin films with submicrometer thickness.2. With the use of a high intensity X-rays from a synchrotron source, SPring-8, the stresses in the near crack-tip region in steel, ceramics and composites were successfully measured and used to predict the propagation behavior of cracks.3. Using electron back scattering pattern and atomic force microscopy, the crystallographic condition of fatigue crack initiation in fine-grained steels is proposed. In the early stage of fatigue crack propagation, the crack deflection and branching increase the resistance of crack propagation.4. The crack deflection and branching induce the fatigue crack closure level, and the crack propagation rate was a unique function of the effective stress intensity factor range.5. The effects of the grain boundaries and second phase on fatigue crack propagation was modeled by the blocked slip band near the crack tip. The crack closure is further included in the model. The relative role of microstructural barriers and crack closure on the threshold condition of small fatigue cracks were calarified.6. The effect of mixed mode loading on the fatigue crack propagation behavior was studied by using a precracked hollow cylinder under mixed torsional and axial loading. The fatigue crack propagation direction is controlled by the maximum range of tangential stress near the crack tip.