Phase contrast stereometry: fatigue crack mapping in three dimensions

The work reported below describes development of a novel method, termed X-ray phase contrast stereometry, for reconstructing the three-dimensional (3D) positions of features within a specimen. The approach takes measured positions of a given feature in phase microradiographs recorded at different viewing angles and refines that feature's position numerically. The approach is designed to address limitations in conventional laboratory or synchrotron X-ray absorption micro computed tomography (microCT), particularly those inherent in studying plate-like samples. The phase stereometric reconstruction technique was applied to a small compact tension sample of aluminium alloy AA 2090 (20.3?mm wide from notch tip to back face and 2.7?mm thick) containing a fatigue crack and was used to map the 3D crack surface non-invasively. Up to ten viewing angles were used for each of 2269 points mapped on the approximately 2.7?mm?×?6.0?mm surface. The 3D stereometry-derived crack surface agreed with the surface map from absorption microCT of the same sample. The phase technique was superior in terms of its crack sensitivity; the phase images allowed the crack to be followed to its tip, even with zero applied load while absorption microCT could only detect about 5?mm of the crack 6?mm long when loaded to 40?kgf. The origin of the contrast used for stereometric mapping was confirmed to be from the peaks on the crack face; phase microradiographs from a second (intact) sample showed a pattern of features identical with those revealed on the surface when the sample was split open. Crack opening as a function of in-situ load was measured for the same points on the crack surface using both phase stereometry and absorption microCT; the openings were in good agreement.