Abstract
Fatigue crack closure has been identified as an important factor in determining crack growthrates. However, the methods of measurement of crack closure remain the subject of ongoingcontroversy. To date, computed finite element models, analytical models and widelyestablished compliance-based experimental methods have offered limited micromechanicalinsight and/or direct information on the active crack tip region within bulk material. Tounderstand the absolute contributions of crack closure mechanisms, such as plasticity-inducedand roughness-induced closure, to fatigue properties, an internal, three-dimensional insightinto 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 of0.7 µm to provide unique three-dimensional in situ observation of steady state plane strain fatiguecrack 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 fatiguecracks are readily observed, along with the occurrence of closure. A novel microstructuralcrack displacement gauging method is used to quantify the mixed mode character of crackopening displacement and the closure effect. A liquid gallium grain boundary wettingtechnique is used in conjunction with the microtomography to visualize the correlationbetween the three-dimensional structure of the grains and fatigue crack behaviour.Subsequently, electron backscattering diffraction assessment of the grain orientation on thesamples provides a uniquely complete 3D description of crack–microstructure interactions.
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