• 한국정밀공학회:학술대회논문집
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• 한국정밀공학회 1997년도 추계학술대회 논문집
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• pp.163-166
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• 1997

Aluminium foam material is a highly porous material having complicated cellular structure defined by randomly distributed air pores in metallic matrix. this structure gives the aluminium a set of properties which cannot be achieved by any of conventional treatments. The properties of aluminium foam material significantly depend on its porosity, so that a desired profile of properties can be tailored by changing the foam density. Melting method is the one of foaming processes, which the production has long been considered difficult to realize becaues of such problems as the low foamability of molten metal, the varying size of. cellular structures, solidification shrinkage and so on. These problems, however, have gradually been solved by researchers and some manufacturers are now producing foamed aluminum by their own methods. Most of all, the parameters of solving problem in electric furnace were stirring temperature, stirring velocity, foaming temper:iture, and so on. But it has not considered about those in induction heating, foaming velocity and foaming temperature in semi-solid state yet. Therefore, this paper presents the effects on these parameter to control cell size, quantity and distribution.

• Structural Engineering and Mechanics
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• 제44권1호
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• pp.73-84
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• 2012

This paper strongly addresses to the problem of the mechanical systems in which parameters are uncertain and bounded. Interval calculation is used to find sharp bounds of the structural parameters for infilled frame system modeled with finite element method. Infill walls are generally treated as non-structural elements considerably to improve the lateral stiffness, strength and ductility of the structure together with the frame elements. Because of their complex nature, they are often neglected in the analytical model of building structures. However, in seismic design, ignoring the effect of infill wall in a numerical model does not accurately simulate the physical behavior. In this context, there are still some uncertainties in mechanical and also geometrical properties in the analysis and design procedure of infill walls. Structural uncertainties can be studied with a finite element formulation to determine sharp bounds of the structural parameters such as wall thickness and Young's modulus. In order to accomplish this sharp solution as much as possible, interval finite element approach can be considered, too. The structural parameters can be considered as interval variables by using the interval number, thus the structural stiffness matrix may be divided into the product of two parts which correspond to the interval values and the deterministic value.

• Steel and Composite Structures
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• 제7권3호
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• pp.219-240
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• 2007

An analytical-numerical procedure has been presented in this paper to take into account the nonlinear effects of concrete cracking and time-dependent effects of creep and shrinkage in the concrete portion of the continuous composite beams under service load. The procedure is analytical at the element level and numerical at the structural level. The cracked span length beam element consisting of uncracked zone in middle and cracked zones near the ends has been proposed to reduce the computational effort. The progressive nature of cracking of concrete has been taken into account by division of the time into a number of time intervals. Closed form expressions for stiffness matrix, load vector, crack lengths and mid-span deflection of the beam element have been presented in order to reduce the computational effort and bookkeeping. The procedure has been validated by comparison with the experimental and analytical results reported elsewhere and with FEM. The procedure can be readily extended for the analysis of composite building frames where saving in computational effort would be very considerable.

• 센서학회지
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• 제29권4호
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• pp.237-242
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• 2020

This paper deals with relative position estimation using a Kalman filter (KF) based on inertial sensors that have been widely used in various biomechanics-related outdoor applications. In previous studies, the relative position is determined using relative orientation and predetermined segment-to-joint (S2J) vectors, which are assumed to be constant. However, because body segments are influenced by soft tissue artifacts (STAs), including the deformation and sliding of the skin over the underlying bone structures, they are not constant, resulting in significant errors during relative position estimation. In this study, relative position estimation was performed using a KF, where the S2J vectors were adopted as time-varying states. The joint constraint and the variations of the S2J vectors were used to develop a measurement model of the proposed KF. Accordingly, the covariance matrix corresponding to the variations of the S2J vectors continuously changed within the ranges of the STA-causing flexion angles. The experimental results of the knee flexion tests showed that the proposed KF decreased the estimation errors in the longitudinal and lateral directions by 8.86 and 17.89 mm, respectively, compared with a conventional approach based on the application of constant S2J vectors.

• 한국소음진동공학회:학술대회논문집
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• 한국소음진동공학회 1995년도 추계학술대회논문집; 한국종합전시장, 24 Nov. 1995
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• pp.208-216
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• 1995

대형선박, 항공기, 초고층 건축물 등은 얇은 박판 형태의 보로 이상화하여 구조 및 진동해석을 수행할 수 있다. 이러한 형태로 이상화한 구조물은 비틀림 강도면에서 매우 취약함을 보이고, 굽힘-비틀림 진동은 단면형상에 따라 연성도가 심화된다. 상하 굽힘 진동은 탄성거동 영역에서 도심과 전단중심이 일치하는 대칭 진동(Symmetric vibration) 현상을 보인다. 그러나, 수평 굽힘 진동은 도심과 전단중심의 차이가 커질수록 즉, 연성도가 높아질수록 비틀림 진동과 복합되어 복잡한 비대칭 진동(Antisymmetric vibration) 현상을 나타낸다. 본 논문에서는 연성효과에 의한 수평 굽힘 진동과 비틀림 진동 현상에 대한 연구를 수행하였고, 진동계산을 위해서 전달행렬법(Transfer Matrix Method)을 사용하였다. 수치계산은 첫번째로, 도심과 전단중심의 차이가 매우 작아 연성도를 무시할 수 있을 정도의 구조물에 대해서 일반적인 수평 굽힘 진동 현상과 비틀림 진동 현상을 연구하였다. 두번째로, 연성도가 매우 심할 경우에 굽힘-비틀림 연성 진동 현상을 Timoshenko 보의 이론과 Vlasov 보의이로네 따라 각각 계산을 수행하였다. 마지막으로, 첫번째와 두번째 구조를 결합한 경우에 대해서 굽힘-비틀림 연성 진동 현상을 연구하였다. 이 경우에 두 구조물의 결합부에서 비틀림 강성과 Warping 강성의 심한 변화로 인한 불연속 경계면이 발생하게 되고 이때의 진동해석을 위해서 보 이론에 기초를 두고 상당히 높은 정확도를 제공하는 Haslum[2] 등과 Pedersen[3]이 제시한 이론을 이용하였다.

• Journal of the Korean Wood Science and Technology
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• 제42권5호
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• pp.605-614
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• 2014

This study investigated microstructural changes of cured urea-formaldehyde (UF) resins mixed with aqueous rubber latex emulsion after intentional acid etching. Transmission electron microscopy (TEM) was used in order to better understand a hydrolytic degradation process of cured UF resins responsible for the formaldehyde emission from wood-based composite panels. A liquid UF resin with a formaldehyde to urea (F/U) molar ratio 1.0 was mixed with a rubber latex emulsion at three different mixing mass ratios (UF resin to latex = 30:70, 50:50, and 70:30). The rate of curing of the liquid modified UF resins decreased with an increase of the rubber latex proportion as determined by differential scanning calorimetry (DSC) measurement. Ultrathin sections of modified and cured UF resin films were exposed to hydrochloric acid etching in order to mimic a certain hydrolytic degradation. TEM observation showed spherical particles and various cavities in the cured UF resins after the acid etching, indicating that the acid etching had hydrolytically degraded some part of the cured UF resin by acid hydrolysis, also showing spherical particles of cured UF resin dispersed in the latex matrix. These results suggested that spherical structures of cured UF resin might play an important role in hindering the hydrolysis degradation of cured UF resin.

• Journal of the Korean Wood Science and Technology
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• 제42권5호
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• pp.499-509
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• 2014

Wood filler is a porous and anisotropic material having different size, shape, and aspect ratio. The use of wood fillers such as wood particle, wood flour, and wood pulp in wood plastic composites (WPCs) are growing rapidly because these wood fillers give improved strength and stiffness to WPCs. However, the wood fillers have originally poor compatibility with plastic matrix affecting the mechanical properties of WPCs. Therefore, to improve compatibility between wood and plastic, numbers of physical and chemical treatments were investigated. While the various treatments led to improved performances in WPC industries using petroleum-based plastics, full biodegradation is still issues due to increased environmental concerns. Hence, bio-based plastics such as polylactide and polyhydroxybutyrate having biodegradable characteristics are being applied to WPCs, but relatively expensive prices of existing bio-based plastics prevent further uses. As conventional processing methods, extrusion, injection, and compression moldings have been used in WPC industries, but to apply WPCs to engineered or structural places, new processing methods should be developed. As one system, co-extrusion technique was introduced to WPCs and the co-extruded WPCs having core-shell structures make the extended applications of WPCs possible.

• Carbon letters
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• 제9권4호
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• pp.298-302
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• 2008

Carbon blacks could be used as the filler for the electromagnetic interference (EMI) shielding. The poly vinyl alcohol (PVA) and polyvinylidene fluoride (PVDF) were used as the matrix for the carbon black fillers. Porous carbon blacks were prepared by $CO_2$ activation. The activation was performed by treating the carbon blacks in $CO_2$ to different degrees of burnoff. During the activation, the enlargement of pore diameters, and development of microporous and mesoporous structures were introduced in the carbon blacks, resulting in an increase of extremely large specific surface areas. The porosity of carbon blacks was an increasing function of the degree of burn-off. The surface area increased from $80\;m^2/g$ to $1142\;m^2/g$ and the total pore volume increased from $0.14073\;cc{\cdot}g^{-1}$ to $0.9343\;cc{\cdot}g^{-1}$. Also, the C=O functional group characterized by aldehydes, ketones, carboxylic acids and esters was enhanced during the activation process. The EMI shielding effectiveness (SE) of raw N330 carbon blacks filled with PVA was about 1 dB and those of the activated carbon blacks increased to the values between 6 and 9 dB. The EMI SE of raw N330 carbon blacks filled with PVDF was about 7 dB and the EMI SE increased to the range from 11 to 15 dB by the activation.

• Smart Structures and Systems
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• 제3권4호
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• pp.455-474
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• 2007

A flexibility-based distributed computing strategy (DCS) for structural health monitoring (SHM) has recently been proposed which is suitable for implementation on a network of densely distributed smart sensors. This approach uses a hierarchical strategy in which adjacent smart sensors are grouped together to form sensor communities. A flexibility-based damage detection method is employed to evaluate the condition of the local elements within the communities by utilizing only locally measured information. The damage detection results in these communities are then communicated with the surrounding communities and sent back to a central station. Structural health monitoring can be done without relying on central data acquisition and processing. The main purpose of this paper is to experimentally verify this flexibility-based DCS approach using wired sensors; such verification is essential prior to implementation on a smart sensor platform. The damage locating vector method that forms foundation of the DCS approach is briefly reviewed, followed by an overview of the DCS approach. This flexibility-based approach is then experimentally verified employing a 5.6 m long three-dimensional truss structure. To simulate damage in the structure, the original truss members are replaced by ones with a reduced cross section. Both single and multiple damage scenarios are studied. Experimental results show that the DCS approach can successfully detect the damage at local elements using only locally measured information.

• Wind and Structures
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• 제24권4호
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• pp.351-365
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• 2017

The minimization method is widely used to predict the dynamic characteristics of a system. Generally, data recorded by experiment (for example displacement) tends to contain noise, and the error in the properties of the system is proportional to the noise level (NL). In addition, the accuracy of the results depends on various factors such as the signal character, filtering method or cut off frequency. In particular, coupled terms in multimode systems show larger differences compared to the true value when measured in an environment with a high NL. The iterative least square (ILS) method was proposed to reduce these errors that occur under a high NL, and has been verified in previous research. However, the ILS method might be sensitive to the signal processing, including the determination of cutoff frequency. This paper focused on improving the accuracy of the ILS method, and proposed the modified ILS (MILS) method, which differs from the ILS method by the addition of a new calculation process based on correlation coefficients for each degree of freedom. Comparing the results of these systems with those of a numerical simulation revealed that both ILS and the proposed MILS method provided good prediction of the dynamic properties of the system under investigation (in this case, the damping ratio and damped frequency). Moreover, the proposed MILS method provided even better prediction results for the coupling terms of stiffness and damping coefficient matrix.