Investigation of thermal-loads at high primary-loads : Finite Element Analysis of Thermally Induced Secondary Shock Loads and Their Sequential Dependence on Structural Integrity with Elastoplastic Materials

Sammanfattning

This thesis investigates the influence of thermal secondary stresses on fracture assessments at high 𝐿𝑟-values within the R6 FAD framework, following the methodology outlined in the SSM handbook (SSM 2018:18). This methodology, as implemented in the ISAAC tool, allows for a reduction of the secondary safety-factor when considering both primary and secondary-loads. Previous studies have indicated that this assumption may not always be conservative, particularly for load sequences involving mechanical loading followed by thermal loading (𝑀 → 𝑇). Finite Element Analysis (FEA) was performed in ABAQUS on cracked pipe geometries subjected to combined mechanical and thermal loading. Both axial and circumferential internal elliptic surface cracks in 304 stainless steel (304SS) were investigated using temperature-dependent elastic-plastic material modelling. The crack driving force was evaluated through the 𝐽-integral and compared with predictions from the R6-method and the structural integrity assessment software ISAAC. Two loading sequences were considered, mechanical loading followed by thermal loading (𝑀 → 𝑇) and thermal loading followed by mechanical loading (𝑇 → 𝑀). The results demonstrate that the loading sequence significantly influences the crack driving force. In particular, the 𝑀 → 𝑇 sequence showed that secondary stresses can still contribute to the crack driving force at high 𝐿𝑟-values, while (𝑇 → 𝑀) relaxes the thermal influence. The study shows that thermal secondary stresses can still contribute to fracture driving force near plastic collapse and that the current down-weighting of secondary safety-factor procedures in ISAAC may not always provide expected safety-factors for thermal transients i.e., shock loads. The findings contribute to improved understanding of primary and secondary stress interaction in NLFM and provide insights relevant to structural integrity assessments in nuclear applications.