THE EFFECTS OF MODEL THICKNESS ON THE VARIATION OF LOCALISED STRESS FIELDS IN FOUR-POINT BENDING TESTS: A CPFE STUDY

Hydrogen embrittlement is one of the principal mechanisms leading to the degradation of engineering materials used in the core of nuclear reactors. This mechanism plays a major role in deciding when to replace the Zr-2.5Nb pressure tubes used in the core ofCANDUnuclear reactors. The presence of localised stress fields in CANDU pressure tubes affects the diffusion of hydrogen atoms toward cracks, scratches, and other imperfections. These high-stress regions, in which the hydrogen accumulates, serve as the nucleation sites for hydrides. In this study, an in-house crystal plasticity finite element model was used to study the effects of model thicknesses on the distribution of stresses. This was done to provide a road map for comparing the results obtained from electron backscatter diffraction measurements (EBSD) to those calculated by CPFE modelling. It is shown, that within the ROI, the variation of von Mises stresses changes significantly with model thickness, while the hoop and hydrostatic stresses are only slightly affected. In all cases, the hydrostatic stresses were more pronounced at the midpoint of the sample. Additionally, it was found that to minimise the impact of the boundary conditions on the stresses at the free surface a thickness of 500 mu m was found to be optimal.