이차응력 지배 조건의 계단형 배관 시험편에 대한 탄소성 보정기법 적용 환경피로 평가 비교 연구

In Small Modular Reactors (SMRs), integral designs result in significant secondary stresses due to increased structural constraints under thermal loading. Conventional ASME Code-based fatigue evaluations, relying on elastic analysis and simplified plasticity correction factors, tend to overestimate fatigue damage under such conditions. Moreover, in cases where secondary stresses dominate, overestimation of fatigue damage becomes further amplified when environmental correction factors are applied, as prescribed in NRC Reg. Guide 1.207, Rev.1. This study investigates various plasticity correction methods, including simplified ASME approaches, Neuber method, Glinka method and elastic follow-up for their applicability to environmental fatigue evaluation of a thermally loaded stepped pipe specimen tested at Bettis. Finite element analysis under cyclic thermal loads was performed, and fatigue lives predicted using each method were compared against experimental crack initiation data. The results indicate that ASME Code-based procedures yield overly conservative predictions, while methods incorporating secondary stress redistribution provide fatigue life estimates that closely match experimental results. These findings support the adoption of advanced plasticity correction techniques to reduce conservatism in environmental fatigue evaluations for SMR structures.