A new simulation approach for crack initiation, propagation and arrest in hollow cylinders under thermal shock based on XFEM

Abstract

The core region of the reactor pressure vessel (RPV) can be considered as a hollow cylinder disregarding the geometrical details such as the nozzles. Under this consideration, a cylindrical mock-up with an axial crack can be used to investigate the behavior of initial defects under thermal shock (TS). In order to assess the crack initiation, propagation and arrest of an initial defect and to design mock-ups for TS experiments, a combined simulation approach of the Initiation-Growth-Arrest (IGA) algorithm and XFEM is proposed. The mock-up is designed taking into account the propagation of the initial defect and its arrest as well as several experimental criteria such as weight, size of the specimen, experimental feasibility, etc. The simulation strategy uses the stress intensity factors at every time increment of the TS event to estimate the possible crack propagation and brittle failure of the cylinder. The criterion for initiation of the crack propagation uses the temperature-dependent fracture toughness (KIc) of the material described by ASME fracture toughness model. For the crack arrest, the simulation approach uses the crack arrest fracture toughness of the material.After introducing the IGA algorithm and its implementation for XFEM, a 3D finite element model of a cylinder and material properties corresponding to an embrittled steel are introduced. Then, the IGA algorithm is combined with a closed-weight function formula (WFF) for axial crack in hollow cylinders, which is used as reference solution. The XFEM-initiation-growth-arrest (XFEM-IGA) and closed-weight function-initiation-growth-arrest (WFF-IGA) methods are applied on cylinders with different geometries to select the geometry of the mock-up designed for a thermal shock experiment. The results show that the crack stops for thick cylinders after several initiation-arrest cycles and that a reduction of the thickness provokes propagation of the crack until through-wall-crack happens. Beside the performance and application to an axial edged crack in a hollow cylinder, some limitations of the presented model are also discussed.