In-situ synchrotron X-ray microtomography has been employed to investigate the three-dimensional (3D) closure behavior of short fatigue crack in Ti-6Al-4V alloy. At a spatial resolution level of 1 µm, tomography datasets were taken at two short crack fronts to observe the time evolution behavior of closure (sometimes called 4D imaging). Owing to the high spatial resolution level, the complex 3D crack morphology consisting of deflections, facet paths and crack branching/overlap caused by the 3D variation in the microstructural features were readily observed together with the occurrence of crack closure. This study separated the contributions of near-tip and behind-tip crack closure and observed their behavior with crack growth. It was deduced that the evolution of behind-tip closure with increase in crack length predominantly affects the overall closure behavior in the short crack regime. Crack front closure behavior variation was found to be attributable to the existence of different crack path morphologies which showed varied degree of fracture surface roughness due to Mode II and Mode III displacements. The relationship between closure, crack tip opening displacement range (ΔCTOD) and crack growth rate are also discussed.

原位同步加速器X射线显微照相技术已被用于研究Ti-6Al-4V合金中短疲劳裂纹的三维（3D）闭合行为。在1 µm的空间分辨率级别上，在两个短裂纹前沿处获取了层析成像数据集，以观察闭合的时间演化行为（有时称为4D成像）。由于具有较高的空间分辨率，很容易观察到由挠曲，刻面路径和由微观结构特征中的3D变化引起的裂纹分支/重叠的复杂3D裂纹形态，并且出现了裂纹闭合。这项研究分离了近端和后端裂纹闭合的贡献，并观察了它们随裂纹扩展的行为。可以推断，随着裂纹长度的增加，尖端后部封闭的演变主要影响短裂纹状态下的整体封闭行为。发现裂纹前闭合行为的变化归因于存在不同的裂纹路径形态，这些裂纹路径形态由于模式II和模式III位移而显示出不同程度的断裂表面粗糙度。还讨论了闭合，裂纹尖端张开位移范围（ΔCTOD）和裂纹扩展速率之间的关系。