Nuclear Engineering and Design 386 (2022) 111582 Available online 4 December 2021 0029-5493/© 2021 The Author(s). Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). A new simulation approach for crack initiation, propagation and arrest in hollow cylinders under thermal shock based on XFEM Diego F. Mora * , Markus Niffenegger Paul Scherrer Institut, Nuclear Energy and Safety Department, Structural Integrity Group, Forschungsstrasse 111, CH-5232 Villigen PSI, Switzerland A R T I C L E INFO Keywords: Pressurized thermal shock Stress intensity factor Crack initiation-growth-arrest algorithm Hollow cylinder Extended finite element method (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 (K Ic ) 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 com- bined 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 cyl- inder, some limitations of the presented model are also discussed. 1. Introduction The process of periodically renewable license for operating nuclear power plants (NPP) requires concerning integrity assessment of the reactor pressure vessel (RPV), especially for planned long-term opera- tion (LTO), where the embrittlement on the RPV steel due to cumulative neutron exposure is an issue. Since the RPV is a very important safety barrier for a large amount highly radioactive inventory, its integrity requires assessment for normal operation as well as for the accident case. The current integrity assessment of RPVs is often based on the analysis of a postulated initial defect and its associated stress intensity factor (SIF), calculated by the linear elastic fracture mechanics theory (EricksonKirk et al., 2006; Thamaraiselvi and Vishnuvardhan, 2020). The integrity assessment of these structures requires a crack initiation analysis. A crack propagation-arrest analysis is only needed if the crack is proven to initiate. Pressurized thermal shock (PTS) counts as one of the most relevant loading condition concerning the RPV safety. The propagation of initial defects in RPV steel has been evaluated in previous large scale experiments in thermal shock (TS) and PTS such as in Refs (Bass et al., 2001; Bass et al., 2000). It was observed that the initiation of the crack can be followed by stable or unstable propagation depending on different factors: the temperature of the crack tip, the constraint of the crack, the ligament size, etc. Experiments on real size samples to examine the fracture behavior under PTS loading condition are extremely expensive and one possible way to circumvent this problem is to perform experiments on mock-ups. In the past, several experiments were done on cylinders under PTS, i.e. those in References (Merkert, 2002; Schüle, 2002; Sievers and Shulz, 1996; Stumpfrock et al., 2003). A specially heat treated material was used to get a brittleness similar to a * Corresponding author. E-mail address: [email protected] (D.F. Mora). Contents lists available at ScienceDirect Nuclear Engineering and Design journal homepage: www.elsevier.com/locate/nucengdes https://doi.org/10.1016/j.nucengdes.2021.111582 Received 30 July 2021; Received in revised form 16 November 2021; Accepted 20 November 2021
A new simulation approach for crack initiation, propagation and arrest in hollow cylinders under thermal shock based on XFEM
May 30, 2023
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