Realisation of 3D In-situ Assessment of Local Crack Driving Forces via Ultra-high Resolution Synchrotron X-ray CT

本研究采用亚微米超高分辨率同步辐射x射线计算机断层扫描（以下简称CT）技术，实现了三维、原位、高密度（间隔小于1微米）测量真实的局部裂纹驱动力（ⅰ、ⅱ、ⅲ型裂纹沿裂纹前缘驱动力）的方法。这已经通过数百个特征的微观结构跟踪实现，并且可以进一步扩展到数万个特征级别。过去只能通过光弹性、云纹干涉和焦散等特殊材料测试方法对试样表面进行测量，现在可以很容易地应用于材料内部。总的来说，局部裂纹驱动力不仅受到宏观外加应力的影响，还受到实际材料微观结构的各种裂纹屏蔽/反屏蔽效应的影响，如裂纹偏转、损伤演化和裂纹闭合现象。该方法已应用于铝合金的疲劳裂纹扩展。在SPring-8的成像光束线BL20B2和BL47XU以及ESRF的ID19上进行了实验，验证了其可行性。通过分析裂纹尖端各点的真实局部裂纹驱动力机理，可以定量地阐明裂纹偏转、损伤演化和裂纹闭合现象等各种裂纹屏蔽/反屏蔽效应的贡献。通过将Ga掺杂到晶界等纳米级结构中，可以实现晶界的可视化。在铸造材料、变形材料和多孔铝材料中成功实现了晶界可视化，表明了该技术的有效性。

The procedure, by which real local crack driving forces (Modes I,II and III crack driving forced along crack front) can be measured in 3D, in-situ and in high density (at intervals less than 1 micron) via the sub-micrometer ultra-high resolution synchrotron radiation X-ray computed tomography (CT, hereinafter), has been developed in this study. This has been realized by microstructural tracking of several hundred features, and this can be further extended to several ten thousand features level. Measurements, which had been available only on specimen surface by the special material test procedures such as photoelasticity, Moire interferometry and caustics, have been readily applied to material interior. In general, the local crack driving forces are intricately and remarkably affected not only by macroscopic applied stress, but also by various crack shielding/anti-shielding effects, such as crack deflection, damage evolution and crack closure phenomenon, due to microstructures of the actual materials. The procedure developed has been applied to fatigue cracks propagating in aluminum alloys. The experiments were performed at the imaging beam lines BL20B2 and BL47XU of the SPring-8 and ID19 of the ESRF, and its feasibility was verified. By analyzing mechanisms of real local crack driving forces at each crack-tip point, the contributions by various crack shielding/anti-shielding effects, such as crack deflection, damage evolution and crack closure phenomenon, could be quantitatively clarified.By doping Ga into nano-scopic structures such as grain boundaries, they could be visualized. The visualization of the grain boundary has been succeeded in casting materials, wrought materials and porous aluminum materials, which indicates the effectiveness of the technique developed.