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.