13th International Conference on Fracture June 16–21, 2013, Beijing, China -9- for the 2mm specimens. The VVF remains relatively high up to 800µm below the fracture surface. • Secondary peaks are observed well below the fracture surface especially for the specimens extracted below extensive ductile tearing. The peaks are often representative of clusters of voids or very large voids at depths ranging from 1mm to 3.6mm. • Pre-cracking of the fracture toughness specimens produces very little observable ductile tearing damage. • At 2mm of ductile tearing, the ductile damage is immediately quantifiable very close to the fracture surface. For the other specimens, voids only become visible after 250µm below the fracture surface and peaks at a ratio of 7.60 × 10-3. • The data captured within the volume of material at proximity to the crack initiation and blunting (0.5mm) produce some of the highest VVF values with a maximum of 7.60 × 10-3 at 350µm below the crack surface. • Voids have been imaged and quantified beyond the crack tips indicating ductile tearing damage ahead of the crack path. 4. Discussion Taylor et al [7] quantified the void volume fraction below the fracture surface in failed CT specimens of AL2024-T351 aluminium alloy using optical and X-ray tomography. A critical void volume fraction ƒf of approximately 1.0×10 -2 was calculated for the aluminium alloy which compares favourably with the results obtained for the RPV ferritic steel of ƒf = 9.75×10 -3. It is worth noting that the peaks within the void volume fraction data may disproportionately increase the average volume fraction well below the fracture surface for the three test specimens. But as the material should be considered as a continuum with an even distribution of initial voids and initiating particles, the average void volume fraction over a large number of tests should be representative of the material’s bulk and fracture characteristics. On the other hand, the ductile damage extends further beyond the crack surface in the ferritic steel than in the aluminium alloy. The aluminium alloy exhibits a sharp reduction in the VVF which reaches ƒ = 0 at 300µm below the fracture surface. The ferritic steel exhibits ductile tearing damage up to 3.6 mm below the fracture surface identified by large voids and clusters of voids. The extent of the ductile damage was equally observed by previous work from Daly et al [8] using optical imaging analysis. The substantial extent of the ductile damage was further observed in an equivalent HY100 ferritic steel. The work from Everett et al [2] identified voids below the fracture surface of fractured notched tensile specimens using a synchrotron source with equivalent resolutions. The distribution of voids deep below the fracture surface was attributed to microstructural banding and larger MnS inclusions. These larger MnS inclusions preferentially promoted the nucleation of voids at relatively low strains.
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