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

1.利用高强度X射线发生器和位置灵敏探测器研制了一种新型的微束应力测量系统。利用该系统可以高精度地测量约100μm区域的应力。并利用该系统测量了亚微米厚薄膜表面区的面内应力。利用同步加速器SPRING-8的高强度X射线，成功地测量了钢、陶瓷和复合材料中裂纹尖端附近区域的应力，并用于预测裂纹的扩展行为。利用电子背散射和原子力显微镜，提出了细晶粒钢疲劳裂纹萌生的结晶学条件。在疲劳裂纹扩展的早期阶段，裂纹的偏转和分支增大了裂纹扩展的阻力。裂纹偏转和分支导致疲劳裂纹闭合程度，裂纹扩展速率是有效应力强度因子范围的唯一函数。用裂纹尖端附近的受阻滑移带模拟了晶界和第二相对疲劳裂纹扩展的影响。模型中进一步考虑了裂纹闭合的影响。计算了微结构势垒和裂纹闭合对疲劳小裂纹门槛条件的相对作用。以预裂纹空心筒为研究对象，研究了在扭转和轴向混合载荷作用下，复合型载荷对疲劳裂纹扩展行为的影响。疲劳裂纹扩展方向受控于裂纹尖端附近的最大切向应力范围。

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.