• 한국전자파학회논문지
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• 제12권1호
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• pp.18-26
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• 2001

본 논문에서는 압전체를 전기기계 압전파동방정식과 경계조건을 이용하여 고유치 제로 정식화하고, 3차원 유한요소법을 적용하여 초고주파 대역에서 동작하는 압전박막공진기의 공진모드 및 공진특성을 공진기의 입력 임피던스를 통하여 해석하였다. 이를 통하여 1차원 해석에서는 불가능하였던 공진기의 전극형상과 상, 하부 전극의 비대칭 구조에 따른 공진특성과 스퓨리어스특성을 추출하였다. 본 논문에서 제안한 방법으로 계산된 공진주파수를 Mason 등기모델 해석결과 및 실제 제작한 ZnO 압전박막공진기의 공진 특성과 비교한 결과 정확하게 일치함을 확인하였다. 또한 두께진동모드로 동작하기 위한 최적의 길이와 두께의 비가 20 : 1이고 최소한의 길이와 두께의 비가 5:1 이상임을 알 수 있었다.

• Advances in nano research
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• 제8권1호
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• pp.59-68
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• 2020

In the present study, buckling analysis of sandwich composite (carbon nanotube reinforced composite and fiber reinforced composite) Euler-Bernoulli beam in two configurations (core and layers material), three laminates (combination of different angles) and two models (relative thickness of core according to peripheral layers) using differential quadrature method (DQM) is studied. Also, the effects of porosity coefficient and different types of porosity distribution on critical buckling load are discussed. Using sandwich beam, it shows a considerable enhancement in the critical buckling load when compared to ordinary composite. Actually, resistance against buckling in sandwich beam is between two to four times more. It is also showed the critical buckling loads of laminate 1 and 3 are significantly larger than the results of laminate 2. When Configuration 2 is used, the critical buckling load rises about 3 percent in laminate 1 and 3 compared to the results of configuration 1. The amount of enhancement for laminate 3 is about 17 percent. It is also demonstrated that the influence of the core height (thickness) in the case of lower carbon volume fractions is ignorable. Even though, when volume fraction of fiber increases, differences grow smoothly. It should be noticed the amount of decline has inverse relationship with the beam aspect ratio. Among three porosity patterns investigated, beam with the distribution of porosity Type 2 (downward parabolic) has the maximum critical buckling load. At the end, the first three modes of buckling will be demonstrated to investigate the effect of spring constants.

• 대한금속재료학회지
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• 제50권10호
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• pp.775-783
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• 2012

The effect of thermal annealing on the in-situ growth characteristics of intermetallics (IMCs) and the mechanical strength of Cu pillar/Sn-3.5Ag microbumps are systematically investigated. The $Cu_6Sn_5$ phase formed at the Cu/solder interface right after bonding and grew with increased annealing time, while the $Cu_3Sn$ phase formed at the $Cu/Cu_6Sn_5$ interface and grew with increased annealing time. IMC growth followed a linear relationship with the square root of the annealing time due to a diffusion-controlled mechanism. The shear strength measured by the die shear test monotonically increased with annealing time. It then changed the slope with further annealing, which correlated with the change in fracture modes from ductile to brittle at a critical transition time. This is ascribed not only to the increasing thickness of brittle IMCs but also to the decreasing thickness of the solder, as there exists a critical annealing time for a fracture mode transition in our thin solder-capped Cu pillar microbump structures.

• Structural Engineering and Mechanics
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• 제72권1호
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• pp.43-60
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• 2019

This paper presents a finite element model which can simulate the axial compression behavior of steel reinforced recycled concrete (SRRC) filled square steel tube columns using the ABAQUS software. The analytical model was established by selecting the reasonable nonlinear analysis theory and the constitutive relationship of material in the columns. The nonlinear analysis of failure modes, deformation characteristics, stress nephogram, and load-strain curves of columns under axial loads was performed in detail. Meanwhile, the influences of recycled coarse aggregate (RCA) replacement percentage, profile steel ratio, width thickness ratio of square steel tube, RAC strength and slenderness ratio on the axial compression behavior of columns were also analyzed carefully. It shows that the results of finite element analysis are in good agreement with the experimental results, which verifies the validity of the analytical model. The axial bearing capacity of columns decreased with the increase of RCA replacement percentage. While the increase of wall thickness of square steel tube, profile steel ratio and RAC strength were all beneficial to improve the bearing capacity of columns. Additionally, the parameter analysis of finite element analysis on the columns was also carried out by using the above numerical model. In general, the SRRC filled square steel tube columns have high bearing capacity and good deformation ability. On the basis of the above analysis, a modified formula based on the American ANSI/AISC 360-10 was proposed to calculate the nominal axial bearing capacity of the columns under axial loads. The research conclusions can provide some references for the engineering application of this kind of columns.

• Steel and Composite Structures
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• 제38권1호
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• pp.103-120
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• 2021

This research examines the eccentric compression performance of composite columns composed of recycled aggregate concrete (RAC)-filled square steel tube and profile steel. A total of 17 specimens on the composite columns with different recycled coarse aggregate (RCA) replacement percentage, RAC strength, width to thickness ratio of square steel tube, profile steel ratio, eccentricity and slenderness ratio were subjected to eccentric compression tests. The failure process and characteristic of specimens under eccentric compression loading were observed in detail. The load-lateral deflection curves, load-train curves and strain distribution on the cross section of the composite columns were also obtained and described on the basis of test data. Results corroborate that the failure characteristics and modes of the specimens with different design parameters were basically similar under eccentric compression loads. The compression side of square steel tube yields first, followed by the compression side of profile steel. Finally, the RAC in the columns was crushed and the apparent local bulging of square steel tube was also observed, which meant that the composite column was damaged and failed. The composite columns under eccentric compression loading suffered from typical bending failure. Moreover, the eccentric bearing capacity and deformation of the specimens decreased as the RCA replacement percentage and width to thickness ratio of square steel tube increased, respectively. Slenderness ratio and eccentricity had a significantly adverse effect on the eccentric compression performance of composite columns. But overall, the composite columns generally had high-bearing capacity and good deformation. Meanwhile, the mechanism of the composite columns under eccentric compression loads was also analysed in detail, and the calculation formulas on the eccentric compression capacity of composite columns were proposed via the limit equilibrium analysis method. The calculation results of the eccentric compression capacity of columns are consistent with the test results, which verify the validity of the formulas, and the conclusions can serve as references for the engineering application of this kind of composite columns.

• Structural Engineering and Mechanics
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• 제81권6호
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• pp.715-728
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• 2022

A new type of buckling-restrained braces (BRBs) with a longitudinally profiled steel plate working as the core (LPBRB) is proposed and experimentally investigated. Different from conventional BRBs with a constant thickness core, both stiffness and strength of the longitudinally profiled steel core along its longitudinal direction can change through itself variable thickness, thus the construction of LPBRB saves material and reduces the processing cost. Four full-scale component tests were conducted under quasi-static cyclic loading to evaluate the seismic performance of LPBRB. Three stiffening methods were used to improve the fatigue performance of LPBRBs, which were bolt-assembled T-shaped stiffening ribs, partly-welded stiffening ribs and stiffening segment without rib. The experimental results showed LPBRB specimens displayed stable hysteretic behavior and satisfactory seismic property. There was no instability or rupture until the axial ductility ratio achieved 11.0. Failure modes included the out-of-plane buckling of the stiffening part outside the restraining member and core plate fatigue fracture around the longitudinally profiled segment. The effect of the stiffening methods on the fatigue performance is discussed. The critical buckling load of longitudinally profiled segment is derived using Euler theory. The local bulging behavior of the outer steel tube is analyzed with an equivalent beam model. The design recommendations for LPBRB are presented finally.

• Steel and Composite Structures
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• 제45권5호
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• pp.729-747
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• 2022

The critical buckling loads and buckling modes of bi-directional functionally graded porous unified higher order shear plate with elastic foundation are investigated. A mathematical model based on neutral axis rather than midplane is developed in comprehensive way for the first time in this article. The material constituents form ceramic and metal are graded through thickness and axial direction by the power function distribution. The voids and cavities inside the material are proposed by three different porosity models through the thickness of plate. The constitutive parameters and force resultants are evaluated relative to the neutral axis. Unified higher order shear plate theories are used to satisfy the zero-shear strain/stress at the top and bottom surfaces. The governing equilibrium equations of bi-directional functionally graded porous unified plate (BDFGPUP) are derived by Hamilton's principle. The equilibrium equations in the form of coupled variable coefficients partial differential equations is solved by using numerical differential integral quadrature method (DIQM). The validation of the present model is presented and compared with previous works for bucking. Deviation in buckling loads for both mid-plane and neutral plane are developed and discussed. The numerical results prove that the shear functions, distribution indices, boundary conditions, elastic foundation and porosity type have significant influence on buckling stability of BDFGPUP. The current mathematical model may be used in design and analysis of BDFGPU used in nuclear, mechanical, aerospace, and naval application.

• 한국지반공학회논문집
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• 제24권3호
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• pp.13-23
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• 2008

시간영역에서 수행되는 비선형 지반응답해석에서 지반의 미소변형률 감쇠는 Rayleigh 감쇠공식을 이용하여 점성감쇠로서 모사된다. 실제 지반의 미소변형률 감쇠는 주파수의 영향을 받지 않는 반면 시간영역해석에서의 점성감쇠는 주파수의 영향을 크게 받으며 이의 영향정도는 Rayleigh 감쇠공식에 따라서 결정된다. 본 연구에서는 국내 지반에 대한 비선형 지반응답해석시 감쇠공식의 영향을 평가하고자 일련의 해석을 수행하였다. 해석결과 점성감쇠공식은 계산된 응답에 매우 큰 영향을 미치는 것으로 나타났다. 널리 사용되는 Simplified Rayleigh 공식은 심도 30m 이상의 지반에서 수치적으로 발생하는 인공감쇠로 인하여 고주파수에서의 에너지 소산을 과대예측하는 것으로 나타난 반면, Full Rayleigh 공식을 사용하며 적절하게 최적주파수를 선정한 경우, 인공감쇠는 크게 감소하는 것으로 나타났다. 나아가 해석결과를 등가선형해석과 비교한 결과 20m 미만의 얕은 심도 지반에서도 등가선형 해석은 최대가속도를 과대예측 할 수 있는 것으로 나타났다.

• Composites Research
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• 제20권5호
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• pp.34-42
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• 2007

본 논문에서는 탄소 복합재와 알루미늄으로 구성된 이종재료 단일겹침 접착 체결부에서, 파손하중에 영향을 미치는 주요인자들의 효과를 실험적으로 연구하였다. 실험을 위해 접착압력 4가지(2, 3, 4, 6기압), 겹침길이 6가지(15, 20, 25, 30, 35, 40 mm), 모재 두께 2가지(1.58, 3.01 mm)에 대한 시편 총 66개를 제작하였다. 실험 결과 접착제 FM73에 대해 제작사에서 제시한 접착압력은 약 3기압이었지만 본 연구에서 사용한 이종재료 접착의 경우, 최소 4기압 이상의 접착압력이 필요함을 확인하였다. 겹침길이를 증가시킬 경우 파손하중이 증가하지만 접착부의 폭과 길이의 비가 1을 넘어갈 경우 접착강도 즉 단위 접착면적당의 파손하중의 증가는 크지 않았다. 모재의 두께도 접착부 파손하중 및 강도에 큰 영향을 미쳤으며 모재의 두께가 약 2배로 증가할 때 접착강도는 $12{\sim}32%$까지 증가하였다. 접착부의 파손은 대부분 복합재 모재의 층간분리 형태로 발생하였으며, 접착압력이 높아질수록, 접착길이가 길어질수록 층간분리가 발생하는 위치가 적층판 내부로 깊게 확대되는 경향이 있다.

• Steel and Composite Structures
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• 제47권6호
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• pp.709-728
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• 2023

Experimental and analytical studies were conducted to clarify the influencing mechanisms of the longitudinal reinforcement on performance of axially loaded Reinforced Concrete-Filled Steel Tube (R-CFST) short columns. The longitudinal reinforcement ratio was set as parameter, and 10 R-CFST specimens with five different ratios and three Concrete-Filled Steel Tube (CFST) specimens for comparison were prepared and tested. Based on the test results, the failure modes, load transfer responses, peak load, stiffness, yield to strength ratio, ductility, fracture toughness, composite efficiency and stress state of steel tube were theoretically analyzed. To further examine, analytical investigations were then performed, material model for concrete core was proposed and verified against the test, and thereafter 36 model specimens with four different wall-thickness of steel tube, coupling with nine reinforcement ratios, were simulated. Finally, considering the experimental and analytical results, the prediction equations for ultimate load bearing capacity of R-CFSTs were modified from the equations of CFSTs given in codes, and a new equation which embeds the effect of reinforcement was proposed, and equations were validated against experimental data. The results indicate that longitudinal reinforcement significantly impacts the behavior of R-CFST as steel tube does; the proposed analytical model is effective and reasonable; proper ratios of longitudinal reinforcement enable the R-CFSTs obtain better balance between the performance and the construction cost, and the range for the proper ratios is recommended between 1.0% and 3.0%, regardless of wall-thickness of steel tube; the proposed equation is recommended for more accurate and stable prediction of the strength of R-CFSTs.