Digital volume correlation (DVC) of laboratory X-ray tomography images has been used to characterize the closure of artificial internal fatigue cracks in nodular graphite cast iron. DVC allows to produce crack opening displacement (COD) map from 3D tomography images at the sub-voxel scale thanks to the graphite nodules which can be used as internal natural markers. Fatigue samples containing controlled internal defects have been produced in a nodular cast iron by diffusion bonding plates containing laser-machined sharp notches. Specimens with internal cracks and surface cracks were respectively characterized by laboratory tomography at different loads during fatigue crack propagation tests. The local stress intensity factors (SIFs) have been extracted from the measured displacement fields by using Williams series, based on those results the closure levels of internal cracks and surface cracks were comparatively analyzed and discussed. Crack closure levels were found to be linearly related to the location of the surface crack front, and the level of internal crack closure was comparable to that of the surface crack in bulk.

中文翻译：

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使用实验室 X 射线断层扫描图像的数字体积相关技术对球墨铸铁的表面和内部疲劳裂纹闭合进行 3D 表征

实验室 X 射线断层扫描图像的数字体积相关 (DVC) 已用于表征球墨铸铁中人工内部疲劳裂纹的闭合。由于石墨结节可用作内部自然标记，DVC 允许根据 3D 断层扫描图像以亚体素尺度生成裂纹张开位移 (COD) 图。通过含有激光加工锋利凹口的扩散结合板，在球墨铸铁中生产了含有受控内部缺陷的疲劳样品。疲劳裂纹扩展试验中不同载荷下的实验室断层扫描分别对具有内部裂纹和表面裂纹的试样进行了表征。利用Williams级数从实测位移场中提取局部应力强度因子(SIF)，并根据这些结果对内部裂纹和表面裂纹的闭合程度进行对比分析和讨论。发现裂纹闭合水平与表面裂纹前沿的位置线性相关，并且内部裂纹闭合水平与整体表面裂纹的闭合水平相当。