• Earthquakes and Structures
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• 제11권6호
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• pp.943-966
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• 2016

Recently, magnetorheological elastomer (MRE) material and its devices have been developed and attracted a good deal of attention for their potentials in vibration control. Among them, a highly adaptive base isolator based on MRE was designed, fabricated and tested for real-time adaptive control of base isolated structures against a suite of earthquakes. To perfectly take advantage of this new device, an accurate and robust model should be built to characterize its nonlinearity and hysteresis for its application in structural control. This paper first proposes a novel hysteresis model, in which a nonlinear hyperbolic sine function spring is used to portray the strain stiffening phenomenon and a Voigt component is incorporated in parallel to describe the solid-material behaviours. Then the fruit fly optimization algorithm (FFOA) is employed for model parameter identification using testing data of shear force, displacement and velocity obtained from different loading conditions. The relationships between model parameters and applied current are also explored to obtain a current-dependent generalized model for the control application. Based on the proposed model of MRE base isolator, a second-order sliding mode controller is designed and applied to the device to provide a real-time feedback control of smart structures. The performance of the proposed technique is evaluated in simulation through utilizing a three-storey benchmark building model under four benchmark earthquake excitations. The results verify the effectiveness of the proposed current-dependent model and corresponding controller for semi-active control of MRE base isolator incorporated smart structures.

• Steel and Composite Structures
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• 제34권6호
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• pp.891-907
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• 2020

Many studies reveal that during destructive earthquakes, most of the structures enter the inelastic phase. The amount of hysteretic energy in a structure is considered as an important criterion in structure design and an important indicator for the degree of its damage or vulnerability. The hysteretic energy value wasted after the structure yields is the most important component of the energy equation that affects the structures system damage thereof. Controlling this value of energy leads to controlling the structure behavior. Here, for the first time, the hysteretic behavior and energy dissipation capacity are assessed at presence of elliptical braced resisting frames (ELBRFs), through an experimental study and numerical analysis of FEM. The ELBRFs are of lateral load systems, when located in the middle bay of the frame and connected properly to the beams and columns, in addition to improving the structural behavior, do not have the problem of architectural space in the bracing systems. The energy dissipation capacity is assessed in four frames of small single-story single-bay ELBRFs at ½ scale with different accessories, and compared with SMRF and X-bracing systems. The frames are analyzed through a nonlinear FEM and a quasi-static cyclic loading. The performance features here consist of hysteresis behavior, plasticity factor, energy dissipation, resistance and stiffness variation, shear strength and Von-Mises stress distribution. The test results indicate that the good behavior of the elliptical bracing resisting frame improves strength, stiffness, ductility and dissipated energy capacity in a significant manner.

• Geomechanics and Engineering
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• 제22권4호
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• pp.361-374
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• 2020

This paper is concerned with the thermoelastic bending of FG beams resting on two-layer elastic foundations. One of these layers is Winkler springs with a variable modulus while the other is considered as a shear layer with a constant modulus. The beams are considered simply supported and subjected to thermo-mechanical loading. Temperature-dependent material properties are considered for the FG beams, which are assumed to be graded continuously across the panel thickness. The used theories contain undetermined integral terms which lead to a reduction of unknowns functions. Several micromechanical models are used to estimate the effective two-phase FG material properties as a function of the particles' volume fraction considering thermal effects. Analytical solutions for the thermo-mechanical bending analysis are obtained based on Navier's method that satisfies the boundary conditions. Finally, the numerical results are provided to reveal the effect of explicit micromechanical models, geometric parameters, temperature distribution and elastic foundation parameters on the thermoelastic response of FG beams.

• Steel and Composite Structures
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• 제35권3호
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• pp.439-447
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• 2020

This paper presents two sets of full three-dimensional thermoelastic finite element analyses of superimposed thermo-mechanically loaded Spar Wingskin Joints made with laminated Graphite Fiber Reinforced Plastic composites. The study emphasizes the influence of residual thermal stresses and material anisotropy on the inter-laminar delamination behavior of the joint structure. The delamination has been pre-embedded at the most likely location, i.e., in resin layer between the top and next ply of the fiber reinforced plastic laminated wingskin and near the spar overlap end. Multi-Point Constraint finite elements have been made use of at the vicinity of the delamination fronts. This helps in simulating the growth of the embedded delamination at both ends. The inter-laminar thermoelastic peel and shear stresses responsible for causing delamination damage due to a combined thermal and a static loading have been evaluated. Strain energy release rate components corresponding to the Mode I (opening), Mode II (sliding) and Mode III (tearing) of delamination are determined using the principle of Virtual Crack Closure Technique. These are seen to be different and non-self-similar at the two fronts of the embedded delamination. Residual stresses developed due to the thermoelastic anisotropy of the laminae are found to strongly influence the delamination onset and propagation characteristics, which have been reflected by the asymmetries in the nature of energy release rate plots and their significant variation along the delamination front.

• Steel and Composite Structures
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• 제36권5호
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• pp.507-519
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• 2020

Double skin composite walls are increasingly popular and have been applied to many safety-related facilities. They come from the concept of composite slabs. Conventional connectors such as shear studs and binding bars were used in previous studies to act as the internal mechanical connectors to lock the external steel faceplates to the concrete core. However, the restraint effects of these connectors were sometimes not strong enough. In this research, a recently proposed unique type of steel truss was employed along the wall height to enhance the composite action between the two materials. Concrete-filled tube columns were used as the boundary elements. Due to the existence of boundary columns, the restraints of steel faceplates to the concrete differ significantly for the walls with different widths. Therefore, there is a need to explore the effect of height-to-width ratio on the structural behavior of the wall. In the test program, three specimens were designed with the height of 3000 mm, the thickness of 150 mm, and different widths, to simulate the real walls in practice. Axial compression was applied by two actuators on the tested walls. The axial behavior of the walls was evaluated based on the analysis of test results. The influences of height-to-width ratio on structural performance were evaluated. Finally, discussion was made on code-based design.

• 한국구조물진단유지관리공학회 논문집
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• 제17권1호
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• pp.27-36
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• 2013

본 연구에서는 등대 구조물을 대상으로 한 면진기술 적용방안을 제시함으로써 지진에 대해 무방비상태에 놓여 있는 등대구조물의 지진안전도 확보방안을 제시하였다. 또한, 제안된 면진기술 적용방안을 활용하여 내진설계 이전에 지어진 등대를 대상으로 등대전체를 면진하였을 경우와 등대 렌즈만을 면진하였을 경우에 대하여 면진효과를 분석해 보았다. 해석결과, 등대 전체를 면진화하였을 경우 최대응답가속도와 층전단력 측면에서 충분한 면진효과를 얻은 반면, 등대 렌즈만을 면진화한 경우에는 렌즈부의 면진효과는 유효한 것으로 나타났으나 등대 자체의 안전이 확보되지 못한 상황에서는 등대의 파괴가 선행될 가능성이 있음에 주의할 필요가 있는 것으로 나타났다. 또한 등대전체와 렌즈를 동시에 면진화한 경우 비 면진구조 대비 유효한 면진효과는 나타내었지만, 최대응답가속도가 등대전체를 면진화한 경우보다 증가하는 경향을 나타내었으며, 렌즈의 면진화로 인한 층전단력의 감소가 미소함으로 이중면진의 적용으로 인한 실효성은 그 효과가 크지 않은 것으로 나타났다.

• 한국의류학회지
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• 제38권3호
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• pp.372-385
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• 2014

Lignin is an abundant natural polymer in the biosphere and second only to cellulose; however, it is under-utilized and considered a waste. In this study, lignin was fabricated into nanofibers via electrospinning. The critical parameters that affected the electrospinnability and morphology of the resulting fibers were examined with the aim to utilize lignin as a resource for a new textile material. Poly(vinyl alcohol) (PVA) was added as a carrier polymer to facilitate the fiber formation of lignin, and the electrospun fibers were deposited on polyester (PET) nonwoven substrate. Eleven lignin/PVA hybrid solutions with a different lignin to PVA mass ratio were prepared and then electrospun to find an optimum concentration. Lignin nano-fibers were electrospun under a variety of conditions such as various feed rates, needle gauges, electric voltage, and tip-to-collector distances in order to find an optimum spinning condition. We found that the optimum concentration for electrospinning was a 5wt% PVA precursor solution upon the addition of lignin with the mass ratio of PVA:lignin=1:5.6. The viscosity of the lignin/PVA hybrid solution was determined as an important parameter that affected the electrospinning process; in addition, the interrelation between the viscosity of hybrid solution and the electrospinnability was examined. The solution viscosity increased with lignin loading, but exhibited a shear thinning behavior beyond a certain concentration that resulted in needle clogging. A steep increase in viscosity was also noted when the electrospun system started to form fibers. Consequently, the viscosity range to produce bead-free lignin nanofibers was revealed. The energy dispersive X-ray analysis confirmed that lignin remained after being transformed into nanofibers. The results indicate the possibility of developing a new fiber material that utilizes biomass with resulting fibers that can be applied to various applications such as filtration to wound dressing.

• 비파괴검사학회지
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• 제19권2호
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• pp.110-117
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• 1999

크리프 손상을 단계적으로 받은 순수 구리 시편에 대하여 손상 진전에 따른 기공의 미시구조 변화를 관찰하고 초음파 속도와 주파수 의존 진폭 스펙트럼 및 감쇠를 측정하였다. 손상 진전에 따른 입계기공의 비등방적 성장으로 인하여 하중축에 대하여 세 방향으로 측정한 속도는 이방성 거동을 보였다. 최대 2%의 기공에 대하여 종파와 횡파는 각각 11%와 4%의 속도 감소를 보였으며, 기공량의 증가와 함께 비선형적인 감소 경향을 보였다. 진폭 스펙트럼은 기공량이 증가함에 따라 고주파수 성분이 감소하고 중심 주파수가 저주파수로 이동하였으며, 감쇠는 사용 주파수 범위에서 거의 선형적인 거동을 보였다. 기공량의 정량적 비파괴평가 파라미터로서 정규화 속도, 진폭 스펙트럼의 중심 주파수, 감쇠 기울기를 선정하였으며 기공량과의 상관관계를 제시하였다.

• 대한기계학회논문집
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• 제10권6호
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• pp.876-888
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• 1986

본 논문에서는 자유 표면과 강체부분이 많은 피어싱 공정의 해석에서 발생하 는 해의 수렴성 저하를 방지하기 위하여 비정상 상태의 가공경화 효과와 강체 연속처리 리방법을 프로그램 개발에 적용하고 프로그램 개발에 적용하고 프로그램 범용성을 높 이기 위해 초기 속도장 발생 프로그램을 개발하여 본 공정 해석에 적용하여 보려고 한 다. 또한 재료를 가공 경화를 고려한 강소성체(rigid-plastic material)로 가정해서 시편에 크랙이 발생하여 파괴될때까지 컴퓨터 시뮬레이션을 수행한 후 냉간 피어싱 공 정에 있어서 표면 크랙의 발생과 성장 그리고 파괴에 가장 많은 영향을 미치는 인자들 을 계산결과를 토대로 실험과 비교조사하고, 실험 시편의 지름과 높이 그리고 마찰상 태를 다르게 조합해서, 실험과 계산을 시하여 시편 모양에 따른 변형 모우드(mode)와 파괴 모우드의 차이를 규명해 보기로 한다.

• 한국강구조학회 논문집
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• 제26권4호
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• pp.349-359
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• 2014

강판콘크리트(SC)구조 벽-바닥접합부는 표면 강판에 의해 콘크리트가 나뉘게 되며 이 때문에 바닥 단부의 응력 전달기구가 RC구조의 것과 다를 것으로 예상된다. 따라서 본 연구에서는 리브가 부착된 SC구조 벽-바닥 실험체의 거동 및 내력평가를 수행하였다. 리브가 부착된 SC벽바닥 접합부의 접합성능을 확인하기위하여 7가지의 접합부 모형을 제작하고 단조 가력실험을 수행하여 구조내력 및 거동특성을 실험결과분석을 통해 고찰하였다. 제안식의 신뢰성을 확보하기위해 유한요소 해석을 수행하였다. 실험체 변수로는 가력지점, 마찰계수이며 이들의 영향에 대해 검토되었다. 실험체 파괴모드 분석을 통해 접합부 내력을 확인하였다. 제안식은 대부분 보수적인 값으로 확인되었다.