소구경 튜브의 인장물성 평가를 위한 최적 비표준 시편 형상 및 시험 방법에 관한 연구 = A Study of Optimized Non-Standard Specimen Geometry and Test Method for Evaluating the Tensile Properties of Small-Diameter Tubes

다국어 초록 (Multilingual Abstract)

A Study of Optimized Non-Standard Specimen Geometry and Test Method for Evaluating the Tensile Properties of Small-Diameter Tubes WonJoon Na Advisor : Prof. Jin Weon Kim, Ph.D. Department of Nuclear Engineering, Graduate School of Chosun University To perform a structural integrity assessment involving an elastic-plastic analysis, it is essential to determine the tensile properties of the constituent materials. The ASTM E8/E8M standard recommends tensile testing using a full-section specimen for small-diameter tubular materials, such as steam generator tubes and nuclear fuel cladding, which are used in nuclear power plants. However, unstable behaviors such as fracturing outside the gauge section or slipping may occur when a full-section specimen is used, which makes it difficult to obtain reliable and consistent tensile properties of materials. Various non-standard specimens have therefore been proposed to overcome these limitations, but their geometries have not yet been standardized. In this study, tensile tests were performed using on two types of non-standard specimens with different geometries, as well as a standard full-section specimen, to suitable specimen geometries and test methods for evaluating the tensile properties of small-diameter tubes. The non-standard specimens were the double strip dogbone (DSD) specimen and the reduced-section specimen with a decreased outer diameter in the gauge section. Three types of seamless tube made from SA-213 TP316 stainless steel with different geometries were used as test materials. All tests were conducted at room temperature at a quasi-static rate, with strain measured using both an extensometer and digital image correlation (DIC). The results showed that the yield strength and uniform elongation of the DSD specimens were comparable to those of the standard full-section specimen. However, the tensile strength and total elongation differed from those of the full-section specimen, depending on the thickness and width of the gauge section. In particular, the total elongation was consistently smaller than that of the full-section specimen. The reduced-section specimens exhibited stable fracture behavior within the gauge section. Furthermore, when the cross-sectional area of the specimens was appropriately reduced, their tensile properties were comparable to those of the full-section specimens. It was found that an optimal reduction in cross-sectional area of less than 5.0 % was required. Consequently, while the DSD specimens provide reasonable evaluations of yield and tensile strength, as well as uniform elongation, they consistently underestimate total elongation. In contrast, reduced-section specimens reliably provide tensile properties such as yield strength, tensile strength, and uniform and total elongation, overcoming the limitations of standard full-section specimens. Therefore, the reduced-section specimen is considered a valid alternative geometry for evaluating the tensile properties of small-diameter tubes.

목차 (Table of Contents)

• 제 1 장 서론 1
• 제 2 장 시험 재료 및 시편 3
• 제 1 절 시험 재료 3
• 제 2 절 시편 4
• 1. Full-section 시편 4
• 2. Double Strip Dogbone 시편 6
• 3. Reduced-section 시편 10
• 제 3 장 시험 조건 및 절차 12
• 제 1 절 시험 조건 12
• 제 2 절 시험 방법 및 절차 14
• 1. 시험 장비 14
• 2. 시험 절차 및 데이터 취득 14
• 제 3 절 데이터 처리 방법 15
• 1. 변형률 측정 15
• 가. 신율계를 이용한 변형률 측정 15
• 나. DIC를 이용한 변형률 측정 16
• 2. 인장 물성 데이터 17
• 가. 공칭 응력-변형률 곡선 17
• 나. 인장 물성치 18
• 다. 진 응력-변형률 곡선 18
• 제 4 장 시험 결과 및 토의 20
• 제 1 절 표준 시편 시험 20
• 1. 표준 시편의 변형률 측정 결과 신뢰성 20
• 2. 인장시험 결과 22
• 가. 표준 시편 손상 거동 22
• 나. 표준 시편 공칭 응력-변형률 및 진 응력-변형률 곡선 23
• 다. 표준 시편 인장물성 비교 24
• 제 2 절 DSD 시편 25
• 1. DSD 시편의 변형률 측정 결과 신뢰성 25
• 2. 인장시험 결과 28
• 가. DSD 시편 손상 거동 28
• 나. DSD 시편 공칭 응력-변형률 및 진 응력-변형률 곡선 30
• 다. DSD 시편 인장물성 비교 37
• 3. DSD 시편의 적용성 검토 38
• 제 3 절 Reduced-section 시편 42
• 1. Reduced-section 시편의 변형률 측정 결과 신뢰성 42
• 2. 인장시험 결과 44
• 가. Reduced-section 시편 손상 거동 44
• 나. Reduced-section 시편 공칭 응력-변형률 및 진 응력-변형률 곡선 46
• 다. Reduced-section 시편 인장물성 비교 53
• 3. Reduced-section 시편의 적용성 검토 55
• 가. 표준 시편과 비교를 통한 결과 55
• 나. 단면적 감소율에 따른 영향 및 적절한 형상 제안 55
• 제 5 장 결론 58
• 참고문헌 59