It is important for safety case justification of the continued use of nuclear power plant that any changes in the ferritic RPV steels' fracture toughness with temperature, irradiation and geometry can be accounted for, particularly with regards to plant life extension. It has been demonstrated that local approach methods have the potential to provide such estimates by assessing the likelihood of cleavage rupture.Here a micro-structurally informed, post-processing, local approach methodology with a new rupture criterion is presented. This has been applied to a base and weld material pair, made available under the EU FP7 PERFORM 60 Programme. This material has been selected as tensile and fracture data are available along with results of recently performed analyses to characterise and quantify the size distribution of fracture initiators for the weld material (predominantly found to be alumino-silicates) which is used as an input to the model.A series of finite element analyses have been performed for two-dimensional three-point bend specimens over a range of temperatures, constraint and irradiation states. Application of the local approach model to these results has then been favourably compared to the experimentally determined toughness. This has been achieved for a range of conditions when using the experimentally determined initiator particle distribution and maintaining the same calibration terms throughout for each material.
Within the EU 7th framework programme, STYLE, a number of large-scale tests have been performed. One of these tests, Mock-Up 2 (MU-2), was performed on a through wall crack located at a repair weld adjacent to a multi-pass narrow-gap weld. The aim of MU-2 was to investigate ductile crack growth under conditions with significant levels of residual stress. As part of the materials testing programme, low-constraint fracture specimens (three-point bend specimens with a/t=0.1) were extracted from the weld to test the weld materials fracture toughness. An overview of these tests is provided here. However, these low constraint tests demonstrated somewhat unusual fatigue crack growth on inserting the crack, leading to the crack depth being shorter in the centre of the specimens to the outside. Subsequently, although it has not been possible to use these specimens to determine the materials J-R curve, it does provide a features test for ductile modelling with the Gurson-Tvergaard-Needleman (GTN) local approach model for ductile crack growth. This paper provides an overview of the modelling associated to understand these observations, including an estimate of the retained residual stress, fatigue growth estimates and subsequent ductile modelling. An overview of the calibration of the GTN model is also provided using the weld material’s tensile tests, high constraint compact-tension tests and MU-2.
This paper presents work that has been carried out to extract leakage rates from finite element models. The benefit of using a more sophisticated leak rate model in combination with finite elements is the increased accuracy in leakage rate calculation for more complex crack shapes and loading conditions. The methodology involves extracting the crack opening displacement (COD) from the Extended Finite Element Method (XFEM) and standard finite elements, then outputting the data to an Excel spreadsheet. A polynomial is then fitted to the data and this is inputted into a MATHCAD code which calculates leakage rate using the Ordinary Differential Equation (ODE) model developed in a previous paper. Test cases were considered which were based on a thick plate with a central crack under various loading conditions. These included primary membrane loading, through-wall bending, and a residual-stress field. The through wall bending and residual stress fields were induced from a temperature field imposed on the plate. A difference of 20% in leak rate is observed between results from the DAFTCAT (Reference 1) code and the ODE method for the residual stress field considered. The paper concludes with recommendations for improved guidance in R6 Section III.11 (Reference 2) for cases when complex crack shapes and loading are present.
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