Phase contrast stereometry: Fatigue crack mapping in 3D

Plate-like samples are particularly challenging to reconstruct with computed tomography (CT) while preserving sensitivity to very small features within the sample. Specifically, quantifying fatigue crack openings of ≤ 2.5 μm in compact tension samples with maximum cross-sections of 25 mm is impractical with conventional microCT. If one is constrained to use plate-like samples, then an alternative approach to conventional microCT is needed. Imaging with X-ray phase contrast offers increased sensitivity compared to X-ray absorption-based techniques. Synchrotron X-ray phase contrast microradiographs (propagation method) coupled with multiple-angle stereometry are used to map the 3D position of fatigue crack surfaces within aluminum samples. The method is briefly outlined, and crack positions obtained with phase stereometry are found to agree with those determined from absorption microCT. Preliminary calculations of phase contrast derived from a sample fractured in fatigue are compared with phase micrographs of the same sample: at present agreement is only approximate.