13th International Conference on Fracture June 16–21, 2013, Beijing, China -10- The effect of the plastic strain field produced from the propagating crack may provide sufficient strains at inclusions and particles to nucleate and grow relatively large voids deep below the fracture surface. A correlation of the experimental work with a finite element analysis is required to attain an estimate of the gradient of strains ahead and below the crack during propagation. Finally, the tomographic images suffered from a blurring effect as a result of the X-ray beam hardening and X-ray spot being too large in comparison with the void sizes that were being imaged. Consequently, a substantial number of voids were only partially resolved and quantified since the pixel colours of these voids matched other metallic areas of the specimens and needed to be culled to obtain a reproducible and systematic quantifying tool. Further work will aim to use a synchrotron facility that will reduce such imaging artefacts and will ultimately increase the overall void volume fraction within the specimens. 5. Conclusions This paper has described preliminary work undertaken to characterise the ductile fracture properties and fracture mechanism in A508 Class 3 steel using X-ray tomography analysis. The main conclusions from the work are as follows: • The mechanical and fracture toughness properties have been quantified in the hoop direction. The average yield stress is 446 MPa and the initiation toughness defined by the 0.2 mm blunting line is ~ 475 kJ/m2. • The ductile fracture mechanism was identified to occur by the decohesion of the matrix from inclusions and second phase particles. • The ductile damage was successfully imaged and quantified in 3D using X-ray laboratory sources to image voids of approximately 10µm in diameter and larger below the crack surface of compact test specimens. • A relatively high VVF was quantified for specimens extracted at a 2mm crack extension but high concentrations of voids were also observed at 350µm and intermittently until 3600µm for specimens extracted along the length of the crack path. The results differ from experiments carried out with aluminium alloys where the VVF reached the background level at 300µm below the fracture surface. • As a result of the limitations of the X-ray machine and software, a proportion of the voids were not quantified which has produced a VVF lower than is actually present in the material. • The distribution of the voids deep below the fracture surface is attributed to microstructural banding and larger inclusions requiring lower strains. 6. Acknowledgements The authors are grateful to AMEC in Risley for their support in the use of their material testing and
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