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Abstract

Traditionally the driving force for crack growth is described in terms of global parameters which, for continua, nevertheless reflect the local conditions at the crack-tip. Mimicking nature, materials microstructures are now being designed at the microscale, or even the nanoscale, employing interfaces and heterogeneities to shield the propagating crack for improved resistance to sub-critical crack growth. In such cases global approaches simply will not do. Information is needed about intrinsic damage occurring ahead, and extrinsic shielding mechanisms behind, the crack. Here, high resolution X-ray imaging and diffraction modes analogous to those in 2D electron microscopy are combined to form a 3D “crack-tip microscope”. In this way one can quantify the effect of microstructural scale events on the crack-tip environment. The technique is applied to study overload phenomena under fatigue, to characterize the bridging ligaments under stress corrosion cracking and fiber bridging during fatigue of a metal matrix composite. Besides identifying shielding mechanisms, it provides three methods for calculating the effective stress intensity at the crack tip; namely from the near tip stress field, from crack face tractions and from crack opening displacements. The wider opportunities opened up by this approach to study self-healing, transformation toughening and other microstructural toughening mechanisms are also discussed.