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Fatigue crack closure has been identified as an important factor in determining crack growth rates. However, the methods of measurement of crack closure remain the subject of ongoing controversy. To date, computed finite element models, analytical models and widely established compliance-based experimental methods have offered limited micromechanical insight and/or direct information on the active crack tip region within bulk material. To understand the absolute contributions of crack closure mechanisms, such as plasticity-induced and roughness-induced closure, to fatigue properties, an internal, three-dimensional insight into crack behaviour during loading and unloading is clearly of value. In this work, synchrotron radiation x-ray microtomography is carried out at a high resolution of 0.7 µm to provide unique three-dimensional in situ observation of steady state plane strain fatigue crack growth in a 2024-type Al alloy (Al-Cu-Mg-Mn). Using such high resolution imaging (additionally exploiting the phase contrast effect in interface imaging), the details of fatigue cracks are readily observed, along with the occurrence of closure. A novel microstructural crack displacement gauging method is used to quantify the mixed mode character of crack opening displacement and the closure effect. A liquid gallium grain boundary wetting technique is used in conjunction with the microtomography to visualize the correlation between the three-dimensional structure of the grains and fatigue crack behaviour. Subsequently, electron backscattering diffraction assessment of the grain orientation on the samples provides a uniquely complete 3D description of crack-microstructure interactions.

The characterization and measurement of grain structures is of great importance to materials scientists because not only does grain size strongly affect the mechanical properties, but it also has an influence on physical properties, surface properties and [3,4,5,6,7]. In most cases, it is assumed that the microstructure features, especially the grain size obtained from light microscopy and EBSD are the same. However, there is very little information available in literature related to the

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