Multi-modal strain mapping of steel crack tips with micrometre spatial resolution

Due to their superior fatigue strength, martensitic steels are the material of choice for high cyclic loading applications such as coil springs. However, crack propagation is influenced by residual stresses and their interaction is poorly understood. In fact, linear elastic fracture mechanics predicts unphysical singularities in the strain around the crack tip. In this study, we have combined synchrotron-based X-ray diffraction, X-ray fluorescence and optical microscopy to map the factual strain fields around crack tips with micrometre spatial resolution. X-ray fluorescence and optical images were co-registered to locate the crack in the X-ray diffraction maps. Observed crystal recovery close to cracks confirmed that the diffraction signal originates at least in part from the cracks. The retrieved local strain field around the crack was further improved by averaging information over carefully selected diffraction peaks. This procedure provided strain maps around crack tips with a spatial resolution of about 1 µm and may enable heuristic predictions of further crack growth.