• 한국재료학회지
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• 제17권3호
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• pp.179-183
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• 2007

A plate heat exchanger (PHE) normally uses vacuum brazing technology for connecting plates and fins. However, the reliability of high temperature brazing, especially with nickel-based filler metals containing boron the formation of brittle intermetallic compounds (IMCs) in brazed joints is of major concern. since they considerably degrade the mechanical properties. This research was examined the vacuum brazing of commercially SUS304 stainless steel with BNi-2 (Ni-Cr-B-Si) filler metal, and discussed to determine the influence of brazing temperatures on the microstructure and mechanical strength of brazed joints. In the metallographic analysis it is observed that considerable large area of Cr-B intermetallic compound phases at the brazing layer and the brazing tensile strength is related to removal of this brittle phase greatly. The mechanical properties of brazing layer could be stabilized through increasing the brazing temperature over $100^{\circ}C$ more than melting temperature of filler metals, and diffusing enough the brittle intermetallic compound formed in the brazing layer to the base metal.

• 대한토목학회논문집
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• 제28권5A호
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• pp.623-629
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• 2008

일반적으로 각국의 강구조물 시공현장에서 사용되고 있는 고장력볼트의 인장강도는 1,000 MPa급이 주종을 이루고 있으나, 고강도강과 극후판의 개발 및 보급과 강교량 건설기술의 발전에 따른 교량 지간의 장대화로 인하여 강도가 큰 새로운 볼트 개발이 요구되어 왔다. 따라서, 본 연구에서는 새롭게 개발된 F13T급 고장력볼트를 이용하여 마찰계수에 따른 마찰연결부 피로실험을 수행하여 피로강도를 평가하였다. 결과적으로 피로실험결과 마찰연결부의 피로강도 B등급을 만족하였다. 또한, 마찰연결부의 피로파단에 대해서 분석하였다.

• 대한조선학회논문집
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• 제36권2호
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• pp.94-104
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• 1999

고장력강을 주로 사용하는 선체에서 좌굴은 중요한 설계기준이 된다. 판부재는 주로 면내강성을 갖는다. 만약 2차좌굴로 인하여 선체판의 면내강성이 저하한다면 선박전체의 종강도는 크게 저하한다. 그러므로, 판부재의 좌굴후거동과 같은 기하학적 비선형거동을 정확히 규명하여야 하는 것은 구조물 전체적으로 매우 필요하다. 이상과 같은 관점에서 본 논문에서는 에너지법을 이용하여 압축하중을 받는 단순지지판의 기하학적 비선형거동을 규명하였다. 에너지 법을 바탕으로 선체판의 탄성대변형해석을 수행하였고 분기점형좌굴과 극한점형좌굴에 대하여 규명하였다.

• 대한조선학회논문집
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• 제31권1호
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• pp.121-132
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• 1994

본 연구에서는 선저보강판부재의 좌초시 손상거동특성을 규명하기 위하여 시리즈 실험을 수행하였다. 즉, 강체웨지가 선저보강판을 연직방향에서 준정적으로 진입할 때 여러가지 파라메타 즉 1) 판의 종횡비, 2)웨지각도, 3)판두께, 4)웨지형상 및 5)종 횡보강재의 변화에 따른 시리즈 실험을 수행하고, 좌초시 선저보강판의 좌초하중-진입량 관계와 흡수에너지-진입량 관계 등에 관한 파라메타들의 영향을 분석하였다.

• 지질공학
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• 제30권1호
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• pp.43-57
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• 2020

본 연구는 기존의 강지보재보다 우수한 고강도 격자지보재를 개발하여 그 성능을 평가하기 위함이다. 이를 위해 수치해석을 이용한 지보재의 구조적 특성을 분석하였고, 시작품에 대한 최대 굽힘하중시험과 용접이음부 인장강도시험을 실시하여 그 성능을 평가하였다. 구조해석 결과에 따른 최적의 상하부부재와 플레이트의 규격은 50 mm × 31.8 mm × 25.4 mm로 시공성과 경제적이 우수한 것으로 나타났다. 굽힘 하중시험으로부터 확정된 규격 55 mm × 30 mm × 20 mm와 85 mm × 30 mm × 20 mm의 고강도 격자지보재는 기준값과 H형강 100과 125의 목표값 모두 만족하는 것으로 나타났다. 이론 처짐량과 실제 처짐량의 비를 검토한 결과, 이 연구에서 개발된 고강도 격자지보재는 독일연방 철도국에서 제시하고 있는 격자지보재 처짐량의 평가기준 5 이하로 나타났다. 마지막으로 용접이음부에 대한 인장시험 결과는 고강도 격자지보재의 주봉-보조봉-플레이트가 접하는 용접이음부 인장강도가 목표값 이상으로 나타나 용접이음부에 대한 안정성은 충분한 것으로 나타났다.

• Structural Engineering and Mechanics
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• 제30권4호
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• pp.403-426
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• 2008

Externally bonding fiber reinforced polymer (FRP) sheets with an epoxy resin is an effective technique for strengthening and repairing reinforced concrete (RC) beams under flexural loads. Their resistance to electro-chemical corrosion, high strength-to-weight ratio, larger creep strain, fatigue resistance, and nonmagnetic and nonmetallic properties make carbon fiber reinforced polymer (CFRP) composites a viable alternative to bonding of steel plates in repair and rehabilitation of RC structures. The objective of this investigation is to study the effectiveness of CFRP sheets on ductility and flexural strength of reinforced high strength concrete (HSC) beams. This objective is achieved by conducting the following tasks: (1) flexural four-point testing of reinforced HSC beams strengthened with different amounts of cross-ply of CFRP sheets with different amount of tensile reinforcement up to failure; (2) calculating the effect of different layouts of CFRP sheets on the flexural strength; (3) Evaluating the failure modes; (4) developing an analytical procedure based on compatibility of deformations and equilibrium of forces to calculate the flexural strength of reinforced HSC beams strengthened with CFRP composites; and (5) comparing the analytical calculations with experimental results.

• Nuclear Engineering and Technology
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• 제46권6호
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• pp.857-862
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• 2014

Mechanical alloying under various gas atmospheres such as Ar, an Ar-$H_2$ mixture, and He gases were carried out, and its effects on the powder properties, microstructure and mechanical properties of ODS ferritic steels were investigated. Hot isostatic pressing and hot rolling processes were employed to consolidate the ODS steel plates. While the mechanical alloyed powder in He had a high oxygen concentration, a milling in Ar showed fine particle diameters with comparably low oxygen concentration. The microstructural observation revealed that low oxygen concentration contributed to the formation of fine grains and homogeneous oxide particle distribution by the Y-Ti-O complex oxides. A milling in Ar was sufficient to lower the oxygen concentration, and this led a high tensile strength and fracture elongation at a high temperature. It is concluded that the mechanical alloying atmosphere affects oxygen concentration as well as powder particle properties. This leads to a homogeneous grain and oxide particle distribution with excellent creep strength at high temperature.

• 대한기계학회논문집A
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• 제26권3호
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• pp.576-585
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• 2002

A three-dimensional Lagrangian explicit time-integration finite element code for analyzing the dynamic impact phenomena was developed. It uses four node tetrahedral elements. In order to consider the effects of strain rate hardening, strain hardening and thermal softening, which are frequently observed in high-velocity deformation phenomena, Johnson-Cook model is used as constitutive model. For more accurate and robust contact force computation, the defense node contact algorithm was adopted and implemented. In order to evaluate the performance of the newly developed three-dimensional hydrocode NET3D, numerical simulations of the oblique impact of mild steel plate by mild steel sphere were carried out. Ballistic limit about various oblique angle between 0 degree and 80 degree was estimated through a series of simulations with different initial velocities of sphere. Element eroding by equivalent plastic strain was applied to mild steel spheres and targets. Ballistic limits and fracture characteristics obtained from simulation were compared with experimental results conducted by Finnegan et al. From numerical studies, the following conclusions were reached. (1) Simulations could successfully reproduce the key features observed in experiment such as tensile failure termed "disking"at normal impacts and outwards bending of partially formed plus segments termed "hinge-mode"at oblique impacts. (2) Simulation results fur 60 degrees oblique impact at 0.70 km/s and 0.91 km/s were compared with experimental results and Eulerian hydrocode CTH simulation results. The Lagrangian code NET3D is superior to Eulerian code CTH in the computational accuracy. Agreement with the experimentally obtained final deformed cross-sections of the projectile is excellent. (3) Agreement with the experimental ballistic limit data, particularly at the high-obliquity impacts, is reasonably good. (4) The simulation result is not very sensitive to eroding condition but slightly influenced by friction coefficient.

• Steel and Composite Structures
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• 제47권2호
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• pp.269-287
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• 2023

The WGJ420 fire-resistant weathering (FRW) steel is developed and manufactured with standard yield strength of 420 MPa at room temperature, which is expected to significantly enhance the performance of steel structures with excellent fire and corrosion resistances, strong seismic capacity, high strength and ductility, good resilience and robustness. In this paper, the mechanical properties of FRW steel plates and buckling behavior of columns are investigated through tests at elevated temperatures. The stress-strain curves, mechanical properties of FRW steel such as modulus of elasticity, proof strength, tensile strength, as well as corresponding reduction factors are obtained and discussed. The recommended constitutive model based on the Ramberg-Osgood relationship, as well as the relevant formulas for mechanical properties are proposed, which provide fundamental mechanical parameters and references. A total of 12 FRW steel welded I-section columns with different slenderness ratios and buckling load ratios are tested under standard fire to understand the global buckling behavior in-depth. The influences of boundary conditions on the buckling failure modes as well as the critical temperatures are also investigated. In addition, the temperature distributions at different sections/locations of the columns are obtained. It is found that the buckling deformation curve can be divided into four stages: initial expansion stage, stable stage, compression stage and failure stage. The fire test results concluded that the residual buckling capacities of FRW steel columns are substantially higher than the conventional steel columns at elevated temperatures. Furthermore, the numerical results show good agreement with the fire test results in terms of the critical temperature and maximum axial elongation. Finally, the critical temperatures between the numerical results and various code/standard curves (GB 51249, Eurocode 3, AS 4100, BS 5950 and AISC) are compared and verified both in the buckling resistance domain and in the temperature domain. It is demonstrated that the FRW steel columns have sufficient safety redundancy for fire resistance when they are designed according to current codes or standards.

• Journal of Welding and Joining
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• 제34권1호
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• pp.41-46
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• 2016

With the recent increase in size of ships and offshore structures, there are more demand for thicker plates. As the thickness increases, it is known that fatigue life of the structures decrease. To improve the fatigue life, post weld treatments techniques, such as toe grinding, TIG dressing and hammer peening, are typically employed. However, these techniques require additional construction time and production cost. Therefore, it is of crucial interest steels with longer fatigue crack growth life compared to conventional steels. This study investigates fatigue crack growth rate (FCGR) behaviours of conventional EH36 steel and Ferrite-Bainite dual phase EH36 steel (F-B steel). F-B steel is known to have improved fatigue performance associated with the existence of two different phases. Ferrite-Bainite dual phase microstructures are obtained by special thermo mechanical control process (TMCP). FCGR behaviours are investigated by a series of constant stress-controlled FCGR tests. Considering all test conditions (ambient, low temperature, high stress ratio), it is shown that FCGR of F-B steel is slower than that of conventional EH36 steel. From the tensile tests and impact tests, F-B steel exhibits higher values of strength and impact energy leading to slower FCGR.