13th International Conference on Fracture June 16–21, 2013, Beijing, China -5- fracture surface. For the parent material, metallographic sections were taken to view the material in the axial-radial plane. The fractured sections were machined through the tested specimen halves in the region where plain strain fracture was expected to take place. The metallographic sections were progressively ground and polished to a mirror finish of 0.25 µm using diamond paste and etched using colloidal silica and 2% Nital. 2.4 X-ray Tomography Analysis The test samples for X-ray tomography imaging were machined below the fracture surface of three CT specimens using electrical discharge machining (EDM). The samples were approximately 0.5mm in diameter and 12mm in length and were extracted at regular intervals starting at the pre-cracked region but before the initiation of ductile tearing. The remaining specimens were extracted from below the ductile crack path and beyond the crack arrest point. The sections were extracted as close as possible to the region where plane strain was expected to take place with the greatest amount of ductile tearing damage. The surfaces of these small cylinders were lightly polished to remove any rust or scaling resulting from the EDM. The top 4mm very close to the fracture surface of these specimens were scanned at the Henry Moseley X-ray Imaging Facility at The University of Manchester using the Nikon Metrology 225/320 kV Custom Bay system equipped with a 225 kV static multi-metal anode source and a PerkinElmer 2000 × 2000 pixels 16-bit amorphous silicon flat panel detector. The scanning was performed with a molybdenum target using a voltage of 80 kV and a current of 130 µA. The data acquisition was carried out with an exposure time of 1000 ms with no filtration. The number of projections was set to 3,142 and the number of frames per projection was 1. The entire volume was reconstructed at full resolution with a voxel size of 2.0 µm along the x, y, and z directions. The data processing was performed with Avizo® Fire 7.0 software. An edge preserving smoothing filter was applied to the raw data to reduce image noise in each data set. Standard data processing was used to determine the void size distribution whereas a methodology similar to [11] was employed to determine the void to fracture surface distance and evolution of void volume fraction. 2.5 Quantification of ductile tearing damage Using the Avizo Fire data, the void volume fraction was estimated by measuring the voxel counts of metallic voxels against the count of porous voxels below the fracture surface. The VVF was calculated for each Regions Of Interest (ROI). A ROI of 100µm in height was utilised to divide the specimens into smaller cylinder regions which were comparable to the units used in Daly et al [8]. The VVF was calculated for the specimens originating below the pre-cracked surface as well as the region below the ductile tearing surface and beyond the crack arrest.
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