13th International Conference on Fracture June 16–21, 2013, Beijing, China -1- Advanced Assessment of Ductile Tearing in Nuclear Reactor Pressure Vessel Steel Using X-ray Tomography Michael Daly1*, Fabien Leonard1, John K Sharples2, Andrew H Sherry1 1 Dalton Nuclear Institute, The University of Manchester, Pariser Building - G Floor, Sackville Street, Manchester, M13 9PL, UK 2 AMEC Technical Services, Walton House, Birchwood Park, Warrington, Cheshire, WA3 6AT, UK * Corresponding author: michael.daly@postgrad.manchester.ac.uk Abstract Reactor pressure vessels (RPV) are manufactured from medium strength low allow ferritic steel specifically selected of its high toughness and weldability. The ability of the RPV to withstand crack propagation is crucial to maintaining the operational safety of the reactor plant. Current generations of RPV steels operate at sufficiently high temperatures to ensure that the material remains ductile during its service life. Furthermore, new materials are engineered to exhibit greater ductility and fracture toughness throughout their operating life. Therefore understanding and being able to predict the ductile fracture behaviour is critical for assuring the safety of RPV steels during operating conditions. This paper presents the results of an experimental programme aimed at using 3D X-ray tomography to quantify the volume fraction of ductile voids in tested pre-cracked specimens manufactured from A508 Class 3 RPV steel. The results indicate a high concentration of voids very close to the fracture surface and voids extending 3.6mm below the crack. The data and experimental methodology could be used to calibrate predictive mechanistically based models such as the Gurson-Tvergaard-Needlman (GTN). Keywords Ductile, Tearing, Steel, X-ray, Tomography 1. Introduction The mechanism of ductile fracture is characterised by the nucleation, growth and coalescence of voids at initiating particles. These particles are categorised as inclusions and second phase particles, and in ferritic steel are most often manganese sulphide (MnS) inclusions and metallic carbide particles (MnC). The voids form at these particles within the volume of high plastic strain and triaxial stresses ahead of a crack-tip or stress concentrator. Two nucleating processes have been observed [1]: voids formation by either decohesion of the interface between the matrix and the inclusion/particle, or by cracking of the inclusion/particle itself. Voids then grow under the influence of increasing plastic strain and high hydrostatic stress within the material. A crack will propagate once neighbouring voids coalesce and/or reach a critical size producing a macroscopic flaw. The coalescence of the voids can be considered as the final stage in the crack growth mechanism. The larger particles nucleate voids at lower stresses and strains [2] . Smaller particles will start contributing to void nucleation when the material is subjected to greater plastic deformation. The nucleation of these smaller voids at proximity to smaller particles, often between larger voids or
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