13th International Conference on Fracture June 16–21, 2013, Beijing, China Figure 2: optical micrograph of a DP600 cut-edge profile (after 0.2% nital etching) The proportion of these 4 characteristic zones are heavily dependent on cutting parameters such as the material nature, clearance, cutting edge radius and cutting speed [3,4,9]. The fracture zone displays the highest damage and presents a high roughness. Some zones of decohesion at the ferritemartensite interfaces can be observed in the fracture zone and are aligned along the flow lines [1]. 2.3. Experiment The experimental technique used in this study was synchrotron radiation computed laminography (SRCL). It is a non-destructive technique similar to synchrotron radiation computed tomography (SRCT), for three dimensions imaging of objects that are extended in two dimensions. It provides a unique opportunity to observe internal damage mechanisms in three dimensions during extended crack propagation in sheet materials [12]. Unlike SRCT which is especially adapted to compact or one-dimensionally elongated objects which stay in the field of view of the detector system under rotation, SRCL is optimized to image regions of interest (ROIs) out of flat, sheet-like specimens. For this, the specimen rotation axis is inclined at an angle of θ < 90° with respect to the beam direction (θ = 90° corresponds to the case of CT). For sheet-like specimens, this enables a relatively constant average X-ray transmission over the entire scanning range of 360°, which allows reliable projection data to be acquired [12]. Although the 3D Fourier domain of the specimen is not sampled completely [13], which leads to imaging artefacts, these artefacts are often less disruptive than the ones produced by (limited-angle) CT [13]. The sample geometry shown in figure3 (a) was used. A hole with a radius of 5 mm was punched out from a sheet of DP steel and an elongated crack was machined up to one edge. The loading was achieved perpendicular to the crack, via a two-screw displacement-controlled wedging device that controls the specimen notch crack mouth opening displacement (CMOD) similar to the one used in Ref. [13,14]. To avoid the sample buckling and out-of-plane motion, an anti-buckling device was used. The entire rig was mounted in a dedicated plate that was removed from the SRCL rotation stage between loading steps. The loading was applied via stepwise increases in the CMOD, one turn of the screw corresponding to 1 mm of CMOD. 3 scans were performed before any loading in order to map and image the initial state of the cut-edge. After each loading step, a scan of the ROI containing the crack tip was carried out. -3-
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