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Synchrotron X-ray microtomography has been utilized for the in-situ observation of steady state plane strain fatigue crack growth. A high resolution experimental configuration and phase contrast imaging technique have enabled the reconstruction of crack images with an isotropic voxel with a 0.7 µm edge. The details of a crack are readily observed, along with evidence of the incidence and mechanical influence of closure. After preliminary investigations of the achievable accuracy and reproducibility, a variety of measurement methods are used to quantify crack opening displacement (COD) and closure from the tomography data. Utilization of the physical displacements of microstructural features is proposed to obtain detailed COD data, and its feasibility is confirmed. Loss of fracture surface contact occurs gradually up to the maximum load. This is significantly different from tendencies reported where a single definable opening level is essentially assumed to exist. The closure behaviour is found to be attributable mainly to remarkable generation of mode III displacement which may be caused by local crack topology. Many small points of closure still remain near the crack-tip, suggesting that the near-tip contact induces crack growth resistance. The effects of overloading are also discussed.

X-ray microtomography was used to observe hydrogen micropores and their growth behavior at high temperatures in several aluminum alloys. High-density micropores were observed in highpurity Al-Mg alloys, but their density and volume fraction were much lower in pure aluminum. Our results have revealed that the growth behavior of micropores is dominated by Ostwald ripening. About 53 % of hydrogen is trapped in micropores in Al-Mg alloy with low hydrogen content, making micropores the predominant hydrogen trap site. Although total hydrogen content is similar to that in the alloy, the ratio of hydrogen trapped in micropores is below 7 % in pure aluminum. This difference is attributable to the lack of hydrogen precipitation sites in pure aluminium.Although the overall amounts of hydrogen at dislocations and grain boundaries are small in all the materials, the occupancies for these trap sites were concluded to be very high.

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The three-dimensional strain map is useful to elucidate the relationships between microstructures and locally caused deformation and fracture. However, a robust tracking method, which enables error-free tracking in synchrotoron radiation computed tomography (SR-CT) images with more than ten-thousand microstructural features, is not currently available. In this study, a model sample was subjected to a tensile test and scanned by the SR-CT technique in order to develop a new tracking method. The developed tracking methods indicated a high tracking ratio and tracking success ratio of nearly 100% in a wide strain range, which included the assumed strain in a practical experiment. It was confirmed that tracking errors produce an incorrect strain distribution in three-dimensional strain mapping. This study verified the validity of the developed tracking method. The application of this method to high-resolution SR-CT images will make measurement and visualization of the strain distribution possible in three dimensions in bulk materials.