• Journal of the Korea Concrete Institute
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• v.14 no.5
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• pp.689-697
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• 2002

The dynamic loads and load-point displacements of concrete three-point bend (TPB) specimens had been measured. The average crack velocities measured with strain gages were 0.16 ㎜/sec ∼ 66 m/sec. The fracture energy for crack extension was determined from the difference of the kinetic energy for the load-point velocity and the strain energy without permanent deformation from the measure external work. For all crack velocities, there were micro-cracking for 23 ㎜ crack extension, stable cracking for 61 ㎜ crack extension at the maximum strain energy, and then unstable cracking. The unstable crack extension was arrested at 80 ㎜ crack extension except the tests of 66 m/sec crack velocity. The tests less than 13 ㎜/sec crack velocity and faster than 1.9 m/sec showed static and dynamic fracture behaviors, respectively. In spite of much difference of the load and load-point displacement relations for the crack velocities, the crack velocities of dynamic tests did not affect on fracture energy rate during the stable crack extension due to the reciprocal action of kinetic force, crack extension and strain energy. During stable crack extension, the maximum fracture resistances of the dynamic tests was 147％ larger than that of the static tests.

• Journal of Korean Tunnelling and Underground Space Association
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• v.22 no.2
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• pp.145-154
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• 2020

Steel set is a structure that stabilize the NATM tunnel until the installation of shotcrete, and it is combined after the shotcrete is installed to improve stability. In this study, determination approach for the equivalent elastic modulus of shotcrete-lattice girder composite is newly suggested for tunneling simulation. Also, a method was presented to calibrate the equivalent elastic modulus through the comparison of the full 3D model and equivalent model. When the conventional equivalent elastic modulus is used for shotcrete-lattice girder composite, the flexural strength of equivalent model is 130% smaller than that of full 3D model. Equivalent elastic modulus is adjusted considering the error of flexural strength. It is found that the error of flexural strength obtained from adjusted equivalent model using adjusted equivalent elastic modulus is reduced less than 1%.

• International Journal of Highway Engineering
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• v.3 no.1 s.7
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• pp.185-197
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• 2001

This paper dealt with modification of effective crack length model (ECM) by adding Poisson's ratio term to evaluate fracture toughness of asphalt concrete which varies its material property by temperature. The original ECM model was developed for solid materials, such as cement concrete, and Poisson's ratio of materials was not considered. However, since asphalt concrete is sensitive to temperature variation and changes its Poisson's ratio by temperature, it should be taken into consideration to know exact fracture property under various temperatures. Four binders, including 3 polymer-modified asphalt (PMA) binders, were used to make a dense-grade asphalt mixture and 3-point bending test was peformed on notched beam at low temperatures, from -5oC to 35oC. Elastic modulus, flexural strength and fracture toughness were obtained from the test. The results showed that, since Poisson's ratio was considered, the more accurate test values could be obtained using modified ECM equation than original ECM. PMA mixture showed higher stiffness and fracture toughness than normal asphalt mixture under very low temperatures.

• Journal of the Korean Wood Science and Technology
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• v.22 no.2
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• pp.46-53
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• 1994

This study was conducted to investigate a feasibility of manufacturing wood fiber thermoplastic composites and to evaluate their mechanical properties. Wood fiber as a potential reinforcing filler was compounded with two thermoplastics (polypropylene and high density polyethylene) in high intensity thermokinetic plastic mixer aided with a wetting agent. It was found that wood fiber thermoplastic composites could be manufactured by injection molding process. The tensile and flexural strength of injection molded specimens were improved greatly with increasing wood fiber concentration. Tensile and flexural modulus increased proportionately with wood fiber concentration. Wood fiber provided reinforcement with thermoplastics in terms of strength and modulus. However, the percent elongation at break and energy to break were reduced with increasing wood fiber loadings. Impact strength also showed similar trend.

• Journal of agriculture & life science
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• v.45 no.6
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• pp.41-46
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• 2011

In this study, in addition to the green tea-wood fiber hybrid composite boards of previous researches, to make effective use of saw dust of domestic cypress tree with functionalities and application as interior materials, eco-friendly hybrid composite boards were manufactured from wood fiber, green tea and saw dust of cypress tree. We investigated the effect of the component ratio of saw dust and green tea on static bending strength performances. Static bending MOE (modulus of elasticity) was within 0.956~1.18GPa, and showed the highest value in wood fiber : green tea : saw dust = 50 : 40 : 10 of the component ratio, and had the lowest value in 50 : 30 : 20 of component ratio. These values were 2.0~3.1times lower than those of green tea-wood fiber hybrid composite boards reported in the previous researches. The bending MOR (modulus of rupture) showed 8.99~11.5MPa, the change of the bending MOR with component ratio of the factors was the same as that of bending MOE. These values had 1.9~3.5 times lower value than those of green tea-wood fiber hybrid composite boards, and showed the slightly lower values than the MOR of particle boards (PB) and medium density fiberboards (MDF) prescribed in Korean Industrial Standard. Therefore, it is considered that these hybrid composite boards need to improve strength performances by component ratio change, hybrid composite with other materials and adhesive change etc. in order to industrialize the hybrid composite boards.

• Journal of the Microelectronics and Packaging Society
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• v.25 no.1
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• pp.41-45
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• 2018

As the size of semiconductor chip shrinks, the electronic industry has been paying close attention to fan-out wafer level packaging (FO-WLP) as an emerging solution to accommodate high input and output density. FO-WLP also has several advantages, such as thin thickness and good thermal resistance, compared to conventional packaging technologies. However, one major challenge in current FO-WLP manufacturing process is to control wafer warpage, caused by the difference of coefficient of thermal expansion and Young's modulus among the materials. Wafer warpage induces misalignment of chips and interconnects, which eventually reduces product quality and reliability in high volume manufacturing. In order to control wafer warpage, it is necessary to understand the effect of material properties and design parameters, such as chip size, chip to mold ratio, and carrier thickness, during packaging processes. This paper focuses on the effects of thickness of chip and molding compound on 12" wafer warpage after PMC of EMC using finite element analysis. As a result, the largest warpage was observed at specific thickness ratio of chip and EMC.

• Proceedings of the Korea Concrete Institute Conference
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• 2008.04a
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• pp.1085-1088
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• 2008

Splice of reinforcement is inevitable in reinforced concrete structures and, generally, lap splices are used. Lap length for tension splice is determined from development length in tension. The development length is calculated from an experimental model which was based on data of tests on anchorage and splice. Longitudinal reinforcements in flexural members are deformed and, therefore, prying action happens in spliced reinforcements unlike anchored reinforcements. The prying action induces tensile stress in cover concrete and this tensile stress plays the same role to a circumferential tensile stress caused by bond. Because splitting failure is assumed to occur when the summation of tensile stresses caused by the prying action and the bond is equal to the tensile strength of the concrete, the prying action reduces the bond strength of spliced reinforcements. A theoretical model for the prying action is developed and effects of the prying action on the bond strength are assessed. The tensile stress by the prying action is proportional to tensile strength and modulus of elasticity of reinforcements. In addition, the tensile stress is inversely proportional to spacing of reinforcements. Consequently, longer splice length is required for spliced reinforcements with small spacing in flexible members.

• Journal of the Korean Professional Engineers Association
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• v.20 no.3
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• pp.5-14
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• 1987

연속체의 해석에 있어서, 특별한 경우를 제외하고는, 구조물의 개략적인 거동을 파악해야 될 경우가 종종 있다. 이러한 요구에 부응하기 위해서 강체요소법(Rigid Element Method)이라 불리우는 새로운 해석법이 개발되었다. 강체요소법은 원래 평정연구실에서 벽식프리캐스트 철근콘크리트 구조물의 탄소성해석을 하기 위해서 개발된 해석법에 착안하여, 내수벽과 같은 연속체에 적용함으로서 시작된 수치해석법이다. 그 후 저자들은 도통쉘, 구형쉘 혹은 이들이 조합된 쉘구조물에 적용할 수 있도록 개발 확장하였다. 강체요소법의 기본개념은 연속체의 분해된 각 요소를 강체(rigid body)라고 가정하고, 각 요소들은 요소의 강성으로 치환된 가상스프링으로 서로 연결되어 있다고 가정하여, 이 가상스프링의 거동을 평가함으로서 전체구조물의 거동을 파악하는 해석법이다. 이때 요소의 주변에 취해진 스프링은 해석을 단순화하기 위해서 축력, 면내전단력 및 면외전단력만을 전달한다고 가정하고, 요소의 강체변위(자유도)는 요소내의 임의의 한 점에서 취하며, 이 점에서의 강체변위(rigid displacements)는 요소의 주변에 취해진 스프링을 통하여 다른 요소로 전달된다. 상기와 같은 강체요소법의 개념을 연속체의 탄성 및 탄소성해석에 적용하면, 해석적 개념이 단순할 뿐만 아니라 구조물 전체의 자유도수를 대폭 줄여 컴퓨터 계산시간을 절약할 수 있는 잇점이 있고, 거시적인 모델(macroscopic modeling)과 미시적인 모델 (microscopic modeling)의 중간적인 성격을 가지기 때문에 구조물의 파괴상황에 대해서도 그 개략을 파악할 수 있다. 본 논문에서는 강체요소법을 보다 일반화된 해석법으로 개발, 확장하기 위해서 종전에 단층스프링시스템(single-layer spring system)으로 해석이 어려웠던 문제점들을 보완한 복층프링시스템(double-layer spring system)을 사용함으로서 휨, 비틀림의 효과를 파악할 수 있는 이론적 개념을 적용한 새로운 구요소, 원통요소 및 평면요소를 개발하고, 이러한 강체요소들의 적합매트릭스의 유도 및 해석저긴 방법을 정식화하였다. 또 휨, 비틀림 및 전단력의 효과를 고려한 사각형원통요소 및 능형원 통요소를 이용하여 원통쉘의 탄성 및 탄소성해석할 수 있는 프로그램을 개발하고, 이 프로그램으로 캔틸레버로된 연속형철근콘크리트 원통쉘의 탄성 및 탄소성해석에 적용하여 구조물의 거동에 관한 수치해석의 결과, 즉 내력의 분포, 균열의 진전, 파괴의 상황 및 변형의 상태 등을 파악해 보았다.

• Journal of the Korea institute for structural maintenance and inspection
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• v.18 no.1
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• pp.31-39
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• 2014

In order to design rigid zone area of frame structures, it is important to predict the direction change of internal stresses according to the bending moment acting on the joint region. In this study, it was examined whether current highway bridge design standards are useful to design different types of rigid joints having a various haunch shapes. In addition, stress distributions of inside of various rigid joints were inspected using the linear elastic finite element analysis. Based on the results of finite element analysis, the strut-tie models to design rigid joints are proposed. Suggested by this study, the strut-tie models have a same level of accuracy to a linear elastic finite element analysis. The proposed strut-tie models will be useful to design reinforcement details of rigid joints having a various haunch types.

• Journal of the Earthquake Engineering Society of Korea
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• v.11 no.5
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• pp.1-9
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• 2007

This study summarizes the results of a research project aimed at investigating the inelastic rotation capacity of beam-column connections of reinforced concrete moment frames. A total of 91 test specimens for beam-column joint connections were examined in detail, and 28 specimens were classified as special moment frame connections based on the design and detailing requirements in the ACI 318-02 Provisions. Then the acceptance criteria, originally defined for steel moment frame connections in the AISC-02 Seismic Provisions, were used to evaluate the joint connections of concrete moment frames. Twenty-seven out of 28 test specimens that satisfy the design requirements for special moment frame structures provide sufficient strength and are ductile up to a plastic rotation of 0.03 rad. without any major degradation in strength. Joint shear stress, column-to-beam flexural strength ratio, and transverse reinforcement ratio in a joint all play a key role in good performance of the connections.