Earticle

연구목적: 기시공되어 현재 운행 중인 2-Arch 터널에 대하여 피난연결통로 신설의 필요성이 대두되고 있다. 이에 본 연구는 중앙벽체 굴착에 따른 터널 및 벽체 거동 분석과 안정성 분석에 목적을 두고 있다. 연구방법: 2-Arch 터널과 피난연결통로의 이론적 배경을 설명하고, 3차원의 유한요소 수치해석을 통하 여 터널 및 벽체의 거동을 집중적으로 분석하였다. 다양한 지반 조건을 변수로 하여 암반 등급에 따른 매개변수 해석을 수행하였으며, 중앙벽체의 변위와 응력을 집중적으로 분석하였다. 연구결과: 중앙벽 체 굴착 시 개구부 중앙에서 가장 큰 침하량을 보였으며, 응력은 첫 번째 굴착 시 가장 크게 발생하였다. 또한 개구부 양 상단에서 응력집중 현상이 발생함을 알 수 있었으며, 허용응력 개념을 고려한 안정성 검 토 결과 허용응력을 초과하는 것으로 나타났다. 결론: 터널의 천단변위는 허용기준치 내로 안정성을 확 보하였지만, 발생응력은 허용기준치를 초과하는 것으로 나타나 시공 시 적절한 보강공법을 적용하여 갑작스런 응력 해방에 대한 방지가 필요할 것으로 사료된다.

Purpose: There is a need to construct an evacuation passageway for the 2-Arch tunnel, which has been constructed and is in operation. Therefore, it aims to analyze tunnel and center wall behaviour and stability due to excavation of the center wall. Method: We describe the theoretical background of 2-Arch tunnel and evacuation passageway, and focused on analyzing the behaviour of tunnel and wall using 3-dimensional finite element analysis. Parametric analysis according to rock rating was performed with various ground conditions, and the displacement and stress of the center wall were intensively analyzed. Result: With the center wall excavation, the largest amount of settlement was shown in the center of the opening, and the stress was greatest during the first excavation. In addition, it was shown that stress concentration occurred at the top of both openings, and stability reviews considering the concept of allowable stress showed that it exceeded the allowable stress. Conclusion: Although the displacement of the tunnel has secured stability within the allowable standard, the generated stress is found to exceed the allowable standard, so it is necessary to prevent sudden stress release by applying appropriate reinforcement methods during construction.

During the initial design of military vehicles, one of the most fundamental tasks is to meet the durability requirements with the minimum weight. The prototype of a Korean light tactical vehicle is a new car with a bulletproof glass and body to secure survivability, but with a greater total weight due to these structures. Thus, multidirectional actions have been taken to reduce the weights of non-bulletproof structures and to satisfy the total weight constraint. This paper suggests a design optimization process for weight reduction of the trimmed body and suspension arms of the new tactical vehicle. Inertia relief analysis and normal mode analysis of each part are conducted to determine the allowable stress and the natural frequency. Using the optimization analysis method (SOL200) in the MSC/Nastran software, the optimum thicknesses of 32 design variables are computed to extract the minimum weight within the strength and vibration constraints. Finally, durability analysis of the optimum model of each component is performed to evaluate whether or not it satisfies the target life. <Key Words> Tactical Vehicle, Allowable Stress, Natural Frequency, Design Optimization, Durability

Military vehicles are subject to variable extreme loads that consequently lead to fractures of chassis components. In the early design stage, it is important to evaluate their strengths and service load conditions not only for preventing these failures but also for reducing cost and weight. Thus, it is required to determine the allowable stresses of main parts under static load cases while no data on road load is available. This study now describes the analytic method applied to obtain the strength criterion of each subsystem using stress analysis and durability load history for a heavy-duty military truck. Selecting the maximum load history applied to the component through static durability analysis was thus necessary. A sine-wave dynamic stress history was constructed to reach the damage similar to the main load history. The allowable stress corresponding to a target life of each part was established using the optimization algorithm at MSC/Fatigue software. This process is very useful because the durability performances of chassis components can be investigated from static stress analysis based on the effective exciting load and comparison of allowable stress and maximum stress in the layout stage.

The aim of this study was to investigate the processes for evaluating the allowable bending stress. The confidence levels and the size-adjustment in standards were reviewed with experimental data. The results show that, (1) KS F 2152 was more strict than others overseas standards due to the higher confidence level. The 5% NTL of bending strengths by a method in KS F 2152 were lower than the overseas standards and more specimens were required for evaluating the structural properties according to KS F 2152. (2) Due to the absence of size-adjustment method in domestic standards, the specified size and the exponential parameters on the size-adjustment equation were reviewed by size factors. The specified size (width: 286 mm, length: 6096 mm), and the exponential parameters (w: 0.29, l: 0.14) will be suitable for developing the allowable bending stress in domestic standard. (3) The size adjusted allowable bending stresses of No. 2 grade Korean pine were lower than the allowable stresses tabulated in KBC even though less strict method (75% confidence level) to calculate 5% value was used. The allowable stresses tabulated in KBC are needed to be reviewed by continuous experimental data.

The ceramic coatings are excellent materials of cutting tools and sliding parts. To evaluate the wear characteristics of very thin ceramic coated layer, it is very important to investigate its wear process microscopically. An effective method for investigating the wear of a thin layer is the observation of wear process in microscopic detail using in-situ system. In this study, using the SEM Tribosystem as in-situ system, the microscopic wear mode of TiN coatings was investigated in repeated sliding. As results, four modes were observed for TiN coatings: Ploughing, powder formation, flake formation and coating delimitation. The observation of the microscopic wear by in-situ system can clarify the allowable stress of TiN coating.

본 연구에서는 국산 침엽수재의 육안 등급구분과 허용응력설정에 관한 국내의 학술논문을 정리하였다. 국립산림과학원 고시(KFRI 1995-27, KFRI 2000-39, KFRI 2007-3, KFRI 2009-1)를 활용하여 국산 낙엽송재의 육안 등급 구분한 연구를 비교한 결과 등급구분 비율이 연구자마다 상이했다. 보다 신뢰할 수 있는 등급구분 분류를 위해서 공증된 목재이용 기관에서 숙련된 연구자에 의한 등급구분을 시행하여야 할 것으로 사료된다. 허용응력설정에 관한 연구를 고찰한 결과, 연구자 마다 허용응력산출 방법이 ASTM D 245, KS F 2152, JAS 1990으로 모두 상이했다. 이는 침엽수 구조용재(KS F 3020)에서 기준허용응력을 제시하고 있지만 허용응력을 산출하는 명확한 방법이 제시되어 있지 않기 때문인 것으로 나타났다. 따라서 공식적인 허용응력 결정방법이 제정되어야 할 것으로 사료된다.

The goal of this study was to review the visual grading and allowable stress determination methodologies for the domestic softwood. Previous studies used different grading (KFRI 1995-27, KFRI 2000-39, KFRI 2007-3, KFRI 2009-1) and allowable stress determination methodologies (ASTM D 245, KS F 2152, JAS 1990). The results of the visual grading were different by each researcher. Compared to the $1^{st}$ grade proportion from the previous studies using the previous specification on visual grading (KFRI 1995-2007), a higher $1^{st}$ grade proportion was found from the studies using the current specification (KFRI 2009). Compared to the allowable stress values from the small clear sample, the higher allowable stress values from the structural size were found. The results indicated that the strength reduction factor used in small clear sample was too conservative for the different grades. To obtain consistent results for the grade, it is required to have experts in visual grading and authorized organizations. An official standard methodology for the allowable stress value determination needs to be defined for the reliable stress value.