3D Crack-tip Microscopy: Illuminating Micro-Scale Effects on Crack-Tip Behavior


  • The author acknowledges funding from the EPSRC and beamtime made available at the ESRF. He is especially grateful for the support of beamline staff at the ID15 beamline at the ESRF, particularly Dr. Marco di Michiel and Dr. Thomas Buslaps. He is also grateful for discussions with Prof. P.D. Lee and those attending the Workshop on Characterization of Crack Tip Stress Fields held in Forni di Sopra, March 2011. Supporting Information is available online from Wiley Online Library or from the author.


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.