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Pre-strain effect of on fracture performance of high-strength steel welds
안규백,박정은,Mituru Ohata,Fumiyoshi Minami 대한기계학회 2018 JOURNAL OF MECHANICAL SCIENCE AND TECHNOLOGY Vol.32 No.7
Fracture toughness of pre-strain effect was determined as a function of the temperature in structural steels of the 600 to 780 MPa class. Cyclic loading during earthquakes produces pre-strain in the component, which is enhanced at the region of strain concentration. During the Kobe Great Earthquake in 1995 in Japan, 10 to 15 % pre-strain was recorded at the beam-to-column connection. The relationship between critical CTOD and CGHAZ length was sampled by fatigue pre-crack for pre-strained HAZ, which is a significant decrease compared to that of the base metal. Furthermore, the effect of pre-strain is discussed in terms of the CTOD and Charpy impact energy of the local brittle zone.
유한요소법과 초음파 메카트로닉스 시스템에 의한 강도적 불균질 이음부의 노치위치에 따른 균열발생 한계 조건
안규백,방한서,풍전정남,An Gyu-Baek,Bang Han-Sur,Toyoda Masao 대한용접접합학회 2005 대한용접·접합학회지 Vol.23 No.6
It has been well hewn that ductile fracture of steels is accelerated by triaxial stresses. The characteristics of ductile crack initiation in steels are evaluated quantitatively using a two-parameters criterion based on equivalent plastic strain and stress triaxiality. The present study focuses on the effects of strength mismatch, which can elevate plastic constraint due to heterogeneous plastic straining, on the critical condition for ductile fracture initiation usinga two-parameter criterion. Fracture initiation testing has been conducted under static loading using notched round bar specimens which had different notch locations. This study provides the fundamental clarification of the effect of strength mismatching and effect of notch location on the critical condition to ductile crack initiation from notch root using fuite element method and ultrasonic-mechatronics system. The critical condition of ductile crack initiation from notch root of strength mismatched tensile specimens under static loading appeared to be almost the same as those of homogeneous tensile specimens with circumferential sharp notch specimen. Also, the effect of notch location in mismatched specimens was estimated using finite element(FE) analyses.
용접잔류응력의 영향에 의한 조선용 극후물재 취성균열 전파 경로
안규백,배홍열,박정웅,우완측 대한용접접합학회 2018 대한용접·접합학회지 Vol.36 No.6
Recently, the size of the ship required to explore and produce oil and natural gas in the arctic offshore region has greatly increased the demand for large vessels. In the shipbuilding industry, the container ship size has gradually increased for mass transportation and cost reduction in the shipping industry. Thus, thick and highstrength steel plates are used for the upper deck structure of container ships because of their large hatch openings. As the increase of plate thickness, the fracture toughness decreases in it. Therefore, it is important to evaluate of the fracture toughness in thick steel plate. In order to evaluate the brittle crack propagation arrest toughness, generally large scale test carried out with full thickness despite the difficult of the experiment. In this study, to clarify the brittle crack propagation arrest mechanism in welds and to establish the basic theory of the technique to avoid brittle fracture, the brittle crack propagation mechanism was investigated using small scale specimens.
동적하중 하에서의 강도적 불균질부를 갖는 용접이음재의 강도 및 파괴 특성
안규백,방한서,望月正人,大細充,豊田政男 대한용접접합학회 2003 대한용접·접합학회지 Vol.21 No.6
Welded joint generally has heterogeneity of strength, material, and fracture toughness and it is important to understand the characteristics of material strength and fracture of welded joint considering heterogeneous effect. Characteristics of strength and fracture of an undermatched joint under dynamic loading was studied by round-bar tension tests and thermal elastic-plastic analyses in this paper. The strength and fracture of the undermatched joints should be evaluated based on the effects of the strain rate and the temperature including temperature rise during the dynamic loading. The differences of fracture characteristics like such as ductile-to-brittle transition behavior are well precisely explained from the stress-strain distribution obtained by numerical analysis.
동적하중하에서의 용접이음부의 강도적특성에 대한 온도상승을 고려한 열탄소성 해석
안규백,망월정인,대전흉,방한서,농전정남 대한용접접합학회 2003 대한용접·접합학회지 Vol.21 No.3
It is important to understand the characteristics of material strength and fracture under the dynamic loading like as earthquakes to assure the integrity of welded structures. The characteristics of dynamic strength and fracture in structural steels and their welded joints should be evaluated based on the effects of the strain rate and the service temperature. It is difficult to predict or measure temperature rise history with the corresponding stress-strain behavior. In particular, material behaviors beyond the uniform elongation can not be precisely evaluated, though the behavior at large strain region after the maximum loading point is much important for the evaluation of fracture. In this paper, the coupling phenomena of temperature and stress-strain fields under the dynamic loading was simulated by using the finite element method. The modified rate-temperature parameter was defined by accounting for the effect of temperature rise under the dynamic deformation, and it was applied to the fully-coupled analysis between heat conduction and thermal elastic-plastic behavior. Temperature rise and stress-strain behavior including complicated phenomena were studies after the maximum loading point in structural steels and their undermatched joints and compared with the measured values.
동적하중하에서의 강도적 불균질재의 연성크랙 발생한계의 해석적 검토 - 강도적 불균질 및 동적부하의 영향에 의한 연성크랙 발생조건 (제 2 보) -
안규백,방한서,Mitsuru Ohata,Masahito Mochizuki,Masao Toyoda 대한용접접합학회 2003 대한용접·접합학회지 Vol.21 No.7
It has been well known that ductile fracture of steel is accelerated by triaxiality stresses. The characteristics of ductile crack initiation in steels are evaluate quantitatively using two-parameter criterion based on equivalent plastic strain and stress triaxiality. Recently, the characteristics of critical crack initiation of steels are quantitatively estimated using the two-parameter, that is, equivalent plastic strain and stress triaxiality, criterion. This study is paid to the fundamental clarification of the effect of geometrical heterogeneity and strength mismatching, which can elevate plastic constraint due to heterogeneous plastic straining, and loading rate on critical condition to initiate ductile crack using two-parameter. Then, the crack initiation testing were conducted under static and dynamic loading. To evaluate the stress/strain state in the specimens especially under dynamic loading, thermal elastic-plastic dynamic FE-analysis considering the temperature rise was used. The result showed that the critical global strain to initiate ductile fracture in specimens with strength mismatch under various loading rate cu be estimated based on the local criterion, that is two-parameter criterion obtained on homogeneous specimens under static tension, by mean of FE-analysis taken into account accurately both strength mismatch and dynamic loading effects on stress/strain behavior.