A combination of X-ray phase contrast and diffraction imaging techniques has been used to study the crack initiation and propagation in an austenitic / ferritic stainless steel. The small difference in electron density between austenite and ferrite enables 3D characterization of the two-phase micro-structure inside the material by means of propagation based phase contrast imaging (Cloetens 1996). The same technique can also be used to visualize the 3D shape and growth of fatigue cracks in the bulk of the material.
X-ray diffraction contrast tomography on the other hand can be used to map out the 3D grain micro-structure of the material and gives access to crystallographic orientation and 3D shape of the grains (Ludwig 2009, Vigano 2016). Merging the information obtained from both imaging modalities, one can analyze the impact of crystallographic orientation and grain/phase boundaries on the initiation and progression of fatigue damage in the material (Herbig 2011).
After a brief review of the challenges encountered with each of these characterization steps, we will present the results obtained during a recent experiment at the European Synchrotron, in which we succeed in acquiring a small series of tomographic images of a growing fatigue crack inside a miniature specimen fatigued in the Gigacycle regime.