해양 구조용강 F690의 신뢰성 향상에 관한 연구 = A Study on Improvement of Reliability of Offshore Structural Steel F690

다국어 초록 (Multilingual Abstract)

[First Topic: Evaluation of Small Cracks]
The stress intensity factor （Κ） is essential for analyzing fracture and fatigue crack growth in metals and ceramics. While the threshold stress intensity factor （ΔΚRth） increases with crack length, it becomes constant beyond a certain length. This study utilized evaluation equations proposed by Tange et al. and Ando et al. to assess ΔΚRth（s） and the fatigue limit （Δσwc） for small cracks in F690 steel. Despite their different conceptual origins, the results from both equations were nearly identical, proving their utility in evaluating crack length dependence across all steel grades.

[Second Topic: Rendering Cracks Harmless via Needle Peening]
Semi-circular cracks （depth 0.4–1.6 mm） were machined into DNV F690 steel, followed by needle peening （NP） and four-point bending fatigue tests. The fatigue limits estimated by Ando’s equation aligned well with the experimental results. NP-induced compressive residual stress significantly enhanced the fatigue limit of both smooth and cracked specimens. Notably, cracks up to 0.8 mm in depth were rendered harmless, maintaining over 95% of the smooth specimen's fatigue limit. The maximum ＂harmless＂ crack depth for this NP treatment was estimated to be 1.14 mm.

[Third Topic: Reliability and Maintenance Strategies]
High-strength steels like F690 are susceptible to rapid microcrack growth, making it vital to render such cracks harmless for the safety of offshore structures. This study evaluated the maximum harmless crack depth （αhml） under various conditions of aspect ratio （As）, ΔΚth, and residual stress. Key findings include: ① αhml increases as the compressive residual stress distribution becomes larger and deeper. ② Larger values of As and ΔΚth（l） also lead to an increased αhml. ③ With a safety factor （N） of 2.0, long-term reliability can be secured using needle peening. ④ Standard non-destructive inspection （NDI） may struggle to detect cracks that significantly reduce fatigue limits, necessitating more sensitive NDI techniques. ⑤ The study reviewed the relationship between αhml, detectable crack depth （σNDI）, and critical crack depth （σcrN） to establish effective maintenance protocols.

목차 (Table of Contents)

• 제1장 서론 1
• 1.1 연구 배경 및 필요성 2
• 1.2 연구 목적 및 내용 4
• 참고문헌 5
• 제2장 미소 균열의 하한계응력확대계수 및 피로한도 비교 11
• 2.1 서언 12
• 2.2 평가 방법 및 재료 14
• 2.3 평가 결과 22
• 2.4 고찰 36
• 2.5 결언 39
• 참고문헌 40
• 제3장 니들 피닝으로 균열 무해화 평가 44
• 3.1 서언 45
• 3.2 재료 및 실험 방법 47
• 3.2.1 재료 47
• 3.2.2 피닝 48
• 3.2.3 잔류 응력분포 53
• 3.2.4 피로 실험 55
• 3.2.5 균열 시험편의 피로한도 (Δσwc)와 하한계응력확대계수(ΔΚth) 평가 56
• 3.2.6 파괴역학에 의한 무해화 가능 균열 치수 평가 59
• 3.3 결과 61
• 3.3.1 하한계 응력확대계수의 결정 61
• 3.3.2 피로한도 결정 63
• 3.3.3 균열재의 피로한도 비교 65
• 3.3.4 미소 균열의 하한계 응력확대계수 72
• 3.3.5 무해화 가능 균열 크기 74
• 3.4 고찰 78
• 3.4.1 미소 균열의 특성 78
• 3.4.2 물리적 타당성 확인 78
• 3.4.3 니들 피닝과 무해화 79
• 3.4.4 한계와 적용 가이드 라인 79
• 3.5 결언 80
• 참고문헌 82
• 제4장 표면균열 무해화 기술에 의한 신뢰성 향상 89
• 4.1 서언 90
• 4.2 재료 및 실험 방법 92
• 4.2.1 재료 및 모델 시험편 92
• 4.2.2 잔류 응력분포 94
• 4.2.3 평가 방법 96
• 4.3 결과 101
• 4.3.1 ΔΚ의 균열 길이 의존성 101
• 4.3.2 무해화 가능 균열 크기 (αhml) 109
• 4.3.3 재료 건전성 평가 모식도 113
• 4.4 고찰 116
• 4.4.1 비파괴검사(NDI) 한계와 무해화 기술의 결합 116
• 4.4.2 설계 단계별 신뢰성 평가 116
• 4.4.3 해양구조물 유지보수의 가이드라인 117
• 4.5 결언 118
• 참고문헌 120
• 제5장 결론 129
• 발표 논문 132
• 감사의 글 135