큰 진폭의 반복하중 조건에서 원자력 구조재료의 변형 및 손상 거동 평가 연구 = A Study on the Evaluation of Deformation and Failure Behaviors of Nuclear Structural Materials under Large-Amplitude Cyclic Loads

다국어 초록 (Multilingual Abstract)

A Study on the Evaluation of Deformation and Failure Behaviors of Nuclear Structural Materials under Large-Amplitude Cyclic Loads Sang Eon Kim Advisor : Prof. Jin Weon Kim, Ph.D. Department of Nuclear Engineering, Graduate School of Chosun University Nuclear structures, systems, and components （SSCs） are generally designed to maintain structural integrity under design basis earthquake （DBE） conditions. However, recent studies and post-Fukushima safety evaluations have indicated that significant plastic deformation may occur in SSCs under beyond design basis earthquake （BDBE） conditions, particularly at geometrical discontinuities where strain localization is concentrated. Under such extreme seismic loading, structural materials can experience very low cycle fatigue （VLCF）, characterized by large plastic deformation and failure within a limited number of loading cycles. Although the deformation and failure behaviors associated with conventional low cycle fatigue （LCF） have been extensively investigated, the deformation and failure characteristics of nuclear structural materials in the VLCF regime, particularly under load-controlled cyclic loading conditions involving ratcheting deformation, are not yet fully understood. Therefore, a systematic investigation of the deformation behavior, fatigue life, and failure characteristics of nuclear structural materials under both displacement- and load-controlled cyclic loading conditions is required for reliable structural integrity assessments of nuclear SSCs subjected to BDBE conditions. The objective of this study is to investigate the deformation behavior, failure mode, and fatigue life characteristics of nuclear structural materials subjected to large-amplitude cyclic loads corresponding to BDBE conditions. For this purpose, a series of cyclic tests, including VLCF and LCF regimes, were conducted on notched specimens of SA508 Gr.1a low-alloy steel （LAS） and SA312 TP316 stainless steel （SS） under both displacement- and load-controlled mode cyclic loads at room temperature. The effects of notch geometry, loading amplitude, load ratio, material type, and loading mode on deformation, fatigue life, and failure characteristics under large-amplitude cyclic loads were systematically investigated based on the test results. Under displacement-controlled mode cyclic loads, both materials exhibited typical strain-controlled fatigue behavior dominated by cyclic plastic deformation. SA312 TP316 SS showed pronounced cyclic hardening behavior, whereas SA508 Gr.1a LAS exhibited relatively limited cyclic hardening characteristics. The cyclic hardening behavior of the stainless steel became more significant with decreasing notch radius due to localized strain concentration. The fatigue lives of both materials in the VLCF regime generally followed the conventional Coffin-Manson relationship when the number of cycles to failure exceeded approximately 10 cycles. In addition, the dominant failure mechanism under displacement-controlled conditions was identified as surface crack initiation and propagation, which is similar to conventional LCF behavior. These results indicate that the fatigue behavior under displacement-controlled VLCF conditions can still be reasonably characterized using conventional strain-life approaches within a certain fatigue life range. In load-controlled mode, however, the deformation and failure behaviors were significantly influenced by the load amplitude, notch radius, and material type, since the cyclic loads caused ratcheting deformation. The accumulation of plastic strain increased with both increasing load amplitude and increasing notch radius at the same load level. SA508 Gr.1a LAS, which exhibited lower cyclic hardening ability, showed significantly larger strain accumulation rate than SA312 TP316 SS, which exhibited higher cyclic hardening ability. The relationship between load amplitude and fatigue life exhibited non-linear in the VLCF regime. Accelerated reduction in fatigue life was particularly evident above a certain level of load amplitude, due to the accumulation of ratcheting deformation and the exhaustion of ductility. At the same load level, SA508 Gr.1a LAS exhibited a shorter fatigue life than SA312 TP316 SS in the VLCF regime; however, the opposite was true in the LCF regime. The dominant failure modes depended on the loading conditions and the type of material. In the LCF regime, ratchet-fatigue cracking was the dominant failure mode, regardless of the type of material. For SA508 Gr.1a LAS, ductile fracture induced by ratcheting deformation was dominant over most of the VLCF regime, particularly at higher load amplitudes. By contrast, SA312 TP316 SS exhibited a transition in failure mode depending on fatigue life: ductile fracture dominated in the extremely short-life regime, whereas ratchet-fatigue cracking became dominant with increasing fatigue life. These indicate that the failure behaviors observed in the VLCF regime cannot be fully explained by those observed in a conventional LCF regime in load-controlled mode. The experimental results demonstrate that the deformation and failure behaviors of nuclear structural materials under large-amplitude cyclic loads depend heavily on the loading mode and the cyclic hardening characteristics of the materials. Therefore, structural integrity assessments for nuclear SSCs subjected to BDBE conditions require an integrated consideration of both displacement- and load-controlled modes cyclic loading behaviors, based on material-specific deformation and failure characteristics. The findings of this study provide valuable experimental insights into the deformation and failure characteristics of nuclear structural materials under displacement- and load-controlled cyclic loading and contribute to the improvement of structural integrity assessment methodologies for nuclear SSCs under extreme seismic loading conditions.

목차 (Table of Contents)

• 제1장 서론 1
• 제2장 선행 연구 조사 5
• 제1절 지진하중 하의 피로 손상 거동 5
• 제2절 초저주기 피로 손상 연구 10
• 1. 변형 거동 10
• 2. 피로 손상 메커니즘 16
• 3. 피로수명 평가 20
• 제3절 반복하중 형태에 따른 손상 거동 24
• 1. 변위제어모드 반복하중 24
• 2. 하중제어모드 반복하중 28
• 제4절 기존 연구의 한계 33
• 제3장 실험 방법 및 절차 36
• 제1절 재료 및 시편 36
• 1. 재료 36
• 2. 시편 38
• 제2절 시험 조건 및 절차 39
• 1. 변위제어모드 반복하중 조건 시험 39
• 가. 시험 조건 39
• 나. 시험 장치 및 절차 43
• 2. 하중제어모드 반복하중 조건 시험 44
• 가. 시험 조건 44
• 나. 시험 장치 및 절차 50
• 제3절 시편 형상별 응력 및 변형률 상태 52
• 1. 유한요소해석 모델 52
• 2. 노치 시편의 응력과 변형률 분포 54
• 제4장 변위제어모드 반복하중 하의 손상시험 57
• 제1절 서론 57
• 제2절 변형률 진폭 결정 57
• 1. 유한요소해석 모델 58
• 2. 변형률 진폭 정의 63
• 제3절 변위제어모드 반복하중 하의 변형 거동 67
• 1. 하중-변위 곡선 67
• 2. 반복 변형 거동 70
• 제4절 변위제어모드 반복하중 하의 손상 거동 74
• 1. 손상 사이클 74
• 2. 손상 모드 79
• 제5장 하중제어모드 반복하중 하의 손상시험 83
• 제1절 서론 83
• 제2절 하중 수준 별 응력과 변형률 분포 83
• 1. 유한요소해석 모델 83
• 2. 응력 및 변형률 분포 84
• 제3절 단조하중 하의 손상시험 88
• 제4절 하중제어모드 반복하중 하의 변형 거동 90
• 1. 정규화 하중-변위 곡선 90
• 2. 반복 변형 거동 95
• 가. 하중 진폭 영향 95
• 나. 재료 종류 영향 101
• 다. 노치 반경 영향 104
• 라. 라체팅 변형 거동 109
• 제5절 하중제어모드 반복하중 하의 손상 거동 113
• 1. 손상 사이클 113
• 가. 하중 진폭 영향 113
• 나. 노치 반경 영향 116
• 2. 손상 모드 116
• 제6절 하중 비에 따른 영향 122
• 1. 정규화 하중-변위 곡선 122
• 2. 반복 변형 거동 125
• 3. 손상 거동 139
• 가. 손상 사이클 139
• 나. 손상 모드 143
• 제6장 결론 147
• 참고문헌 150