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J. Lachambre et alii, Frattura ed Integrità Strutturale, 25 (2013) 50-53; DOI: 10.3221/IGF-ESIS.25.08 50 Special Issue: Characterization of Crack Tip Stress Field In situ 3D characterization of fatigue cracks displacement fields Joel Lachambre, Jean-Yves Buffiere MATEIS Lab. UMR CNRS 5510 INSA Lyon France Julien Réthoré LAMCOS Lab. UMR CNRS 5259 INSA Lyon France Arnaud Weck Univ. of Ottawa Canada A BSTRACT . The three dimensional growth of fatigue cracks in samples of nodular graphite cast iron is characterized using laboratory X-ray computed tomography. The cracks grow from laser machined artificial defects, their development is monitored in situ using laboratory X-ray computed tomography (lab. CT) and Digital Volume Correlation (DVC). The combination of both techniques gives access to the 3D displacement field at the tip of the crack (mainly mode I opening). K EYWORDS . Cast iron; Digital volume correlation; Crack tip displacement; In situ. I NTRODUCTION n their early stage of propagation, fatigue cracks have a strong three dimensional character. This is because, in structural materials, crack initiation occurs at heterogeneities such as holes, surface scratches, harder or softer second phase particles, well oriented grain etc. Once initiated, such small cracks strongly interact with the first surrounding grains and, at low stress levels, a large number of fatigue cycles is necessary before the propagation rate of a crack at the specimen surface is comparable to the propagation rate in the bulk of the specimen. Fatigue models tend to ignore this three dimensional aspect of crack growth partly because reliable experimental data was (and still is) lacking. This is a crucial issue because a very large fraction of the fatigue life of real components correspond to the growth of such small 3D cracks [1]. Various 3D-imaging techniques have been used in previous studies to describe the 3D propagation of fatigue cracks. - SEM/FIB observations: this destructive approach can provide very high resolution images of a restricted area (typically 20 µm 3 ) around the crack tip but in situ experiment are obviously impossible. - Laboratory X ray computed tomography (lab. CT)is a non-destructive technique useful for long cracks. The resolution is still rather low for direct imaging of short cracks even under load. - Synchrotron radiation computed tomography (SR CT) has been used for in situ characterisation of crack initiation and growth; very good detectors coupled to phase contrast offer a better resolution than in the case of lab. CT but the price to pay is the small (sub millimetric) sample size and limited availability of relevant synchrotron beamlines. It has been shown in the past that lab. CT can be used to map displacement fields at the tip of through thickness fatigue cracks [2]. However so far, the studied cracks have been characterized in specimens carved out from larger samples and the influence of sample preparation (relaxation of stress) could not be excluded. In this work a method has been developed to initiate and grow a crack in a single sample thus providing a more reliable picture of the displacement field at the tip of a crack grown in situ. I