• Composites Research
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• v.28 no.5
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• pp.260-264
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• 2015

In this paper, molecular dynamics (MD) simulation was carried to predict thermo-mechanical behaviors for carbon nanotube (CNT) reinforced epoxy composites and to analyze the trends. Total of six models having the volume fractions of CNT from 0 to 25% in epoxy were constructed. To predict thermal behaviors, temperature was increased constantly from 300 to 600 K, and the glass transition temperature ($T_g$) and coefficient of thermal expansion (CTE) analyzed using the relationship between temperature and specific volume. The elastic moduli that represented to the mechanical behaviors were also predicted by constant strain. Additionally, the effects of functionalization of CNT on mechanical behaviors of composite were analyzed. Models were constructed to represent CNTs functionalized by nitrogen doping and COOH groops, and interfacial behaviors and elastic moduli were analyzed. Results showed that the agglomerations of CNTs in epoxy cause by perturbations of thermo-mechanical behaviors, and the functionalization of CNTs improved the interfacial response as well as mechanical properties.

• Journal of the Computational Structural Engineering Institute of Korea
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• v.19 no.3 s.73
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• pp.259-269
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• 2006

In this paper, the stress intensity factor, energy release rate and crack opening displacement are computed using the finite element method for axisymmetric viscoelastic cylinders with the penny-shaped and circumferential cracks. The triangular elements with quarter point nodes are used to describe the stress singularity around the crack edge. The analytical solutions are also derived by using the elastic-viscoelastic correspondence principle and compared with the numerical results to show the validity and accuracy of the presented method. Viscoelastic materials are assumed to behave elastically in dilatation and like a three-parameter standard linear solid.

• Composites Research
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• v.30 no.6
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• pp.371-378
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• 2017

In this study, hygroelastic behavior of thermosetting epoxy is predicted by molecular dynamics simulations. Since consistent exposures to humid environments lead to macroscopic degradation of polymer composite, computational simulation study of the hygroscopically aged epoxy cell is essential for long-time durability. Therefore, we modeled amorphous epoxy molecular unit cell structures at a crosslinking ratio of 30, 90% and with the moisture weight fraction of 0, 4 wt% respectively. Diglycidyl ether of bisphenol F (EPON862) and triethylenetetramine (TETA) are chosen as resin and curing agent respectively. Incorporating equilibrium and non-equilibrium ensemble simulation with a classical interatomic potential, various hygroelastic properties including diffusion coefficient of water, coefficient of moisture expansion (CME), stress-strain curve and elastic modulus are predicted. To establish the structural property relationship of pure epoxy, free volume and internal non-bond potential energy of epoxy are examined.

• Journal of Ocean Engineering and Technology
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• v.14 no.2
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• pp.65-69
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• 2000

This study is to investigate the characteristics of fatigue crack propagation in rolled super duplex stainless steel that was changed austenite-ferrite volume fraction by heat treatment. It was used two kinds of specimen the rolling and the transverse directions δ-phase fraction affected sound velocity hardness and Young's modulus. Characteristics of fatigue crack propagation was affected by anisotropy and (δ+γ) phase volume fractions.

• Computational Structural Engineering
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• v.10 no.2
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• pp.225-232
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• 1997

Material properties of a new composite composed of components with known material properties are usually investigated through experiments. Elastic modulus and Poisson's ratio are measured at various volume fractions of mixed components and utilized as the base information on an analytical model for predicting the mechanical behaviors of a structure constructed by the composite. Elastic material properties of a composite at various volume fractions are numerically estimated by minimizing the error between the static displacements computed from a model for the composite and those computed from a model of homogeneous and isotropic material. A finite element model for a composite is proposed to distribute different types of material components easily into the model depending on the volume fraction. Then, the material properties of a composite filled with solid mircospheres are predicted numerically through a sample study and the estimated results are compared with experimental results and some theoretical equations.

• 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%.

• Journal of Biomedical Engineering Research
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• v.37 no.2
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• pp.90-103
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• 2016

This research was aimed to analyze load sharing ratios between cortical shell and trabecular bone of a degraded lumbar vertebra with aging, and also evaluate elastic moduli assigned into an FE model, using finite element method. For the better analysis of trabecular bone, effective elastic moduli, that is, nominal elastic moduli divided by the volumetric porosities was used. The elastic moduli of the cortical shell suitable for the trabecular bone were obtained from the equations on the basis of idealized stress-strain relations, including areal porosities. To minimize numerical errors, p-element was used. Using eight parameters that refer to some published papers, the geometry of L3 with a removed posterior part. After the constant compressive displacement was applied, the load sharing ratios were obtained by using both every elastic strain energy and every vertical force between two bones in each 8-volume. As results, 1) according to an increase in age from 20-year to 80-year, load sharing ratios of trabecular bone decreased from 55% to 49%; 2) the maximal ratios of each bone were occurred in the mid-plane of centrums and the endplate of cortical shells, respectively; 3) effective elastic moduli assigned into a porous centrum/cortex were found to be adequate; 4) for load sharing ratios, the difference of two methods showed that the total ratios were almost same within less than 1% but the partial ratios at every depth were more or less different each other.

• Journal of the Korean Ceramic Society
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• v.37 no.12
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• pp.1198-1203
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• 2000

AlN와 Si을 이용하여 체적 탄성파 over-moded 공진기를 형성하였다. 높은 c-축 배향성을 갖는 AlN 압전박막은 sputtering에 의해 저온에서 증착하였다. AlN 박막의 c-축 배향성은 기판과 타겥의 거리가 가까울수록, 증착 압력이 낮을수록 (002) 면으로의 성장이 촉진되었다. Si 기판을 이용한 over-moded 공진기로부터 TFR의 임피던스를 산출한 결과 공진영역의 면적에 가장 의존하였다. Al/AlN/Al로 이루어진 TFR의 입력 임피던스는 공진 영역이 크기가 200㎛×200㎛인 경우 가장 50Ω에 근접하였다. Over-moded 공진 특성은 Si 기판의 낮은 Q로 인해 mode 수 294인 2.60976 GHz에서 0.109%의 유효 전기기계결합계수(K/sub eff/²)와 0.3의 K/sub eff/²·Q값을 보였다.

• Transactions of the Korean Society of Mechanical Engineers A
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• v.39 no.2
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• pp.129-135
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• 2015

In this experimental study, the thermal stability values of micrometer-sized silica particle-reinforced epoxy composite specimens were evaluated by measuring their thermal expansion coefficients and Young's moduli. For all specimens used in this study (from the baseline specimen to that containing 70 wt% silica filler), the thermal expansion coefficients and Young's moduli were gradually reduced down to 25% and increased up to 51%, respectively. The results of the experiment were compared with those of certain empirical models. The experimental results of the measurement of thermal expansion coefficients corresponded well with those of Kerner's model, which considers the bulk and shear moduli of the matrix and silica filler. However, the results of the measurement of Young's moduli using the empirical Mori-Tanaka model were observed to match better with those of the experiment. The comparison of the results of the experiment with those of the empirical models demonstrated that a reliable model for measuring the thermal expansion coefficients and Young's moduli of composite specimens needs to consider certain property variations in the composites in addition to volume fraction changes in the filler and matrix.

• Journal of the Korean Society for Aeronautical & Space Sciences
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• v.36 no.11
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• pp.1049-1055
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• 2008

Modern multidisciplinary computational fluid dynamics often incorporates moving boundaries, as would be required in the applications such as design optimization, aeroelasticity, or forced boundary motion. It is challenging to develop robust, efficient grid deformation algorithms when large displacement of the moving boundaries is required. In this paper, a volume grid deformation code is developed based on the finite macro-element and the transfinite Interpolation, and then interfaces to a structured multi-block Navier-Stokes in-house code. As demonstrated by an airfoil with pitching motion, the hysteresis loops of lift, drag and moment coefficients of the developed method are shown to be in good agreement with those of experimental data.