• Journal of the Korean Society for Aeronautical & Space Sciences
• /
• v.45 no.2
• /
• pp.92-98
• /
• 2017

Porosity in polymer matrix composites generated during pyrolysis process affect the thermomechanical behavior of the composites. In this paper, multislice finite element models for the porous composite materials are developed, and poroelastic and failure analysis for these models are performed. In order to investigate the three-dimensional effects, finite element meshes are modeled considering different porosity(up to 0.5) and the number of slices (up to five). As a result, effective Young's moduli and poroelastic parameters exhibit the maximum differences of 74.0% and 442.1% with respect to porosity respectively, and 98.7% and 37.2% with respect to the number of slices. First and last failure strengths are decreased 88.2% and 90.0% with respect to porosity respectively, and 53.8% and 171.8% with respect to the number of slices.

• Journal of the Korean Society for Railway
• /
• v.18 no.1
• /
• pp.53-62
• /
• 2015

LWDT was developed for use as an alternative technique to measure the stiffness of trackbed soils. In this study, numerical and theoretical analyses of LWDT's acting mechanism were performed. The effectiveness of the adapted elastic formula used for calculation of the dynamic modulus, Evd, was investigated theoretically and also numerically by running ABAQUS analysis. The minimum thickness of the upper layer is proposed based on the analysis. Correction factors for the formula of elastic modulus are also proposed in this study. In the future, following field test results and laboratory mechanical tests such as the resonant column test, a guideline for the use of LWDT as a standard test protocol in track construction sites, as a measuring tool for the degree of compaction and/or stiffness and dynamic modulus, will be proposed based on this analysis.

• Journal of Korean Society of Steel Construction
• /
• v.13 no.2
• /
• pp.133-141
• /
• 2001

A tubular member holding an axially through-gusset connection is often used to transmit axial compression in a steel truss structures. The elastic buckling loads of the member is affected by the stiffness ratio($\beta$) and the length ratio(G) because of two elements with different properties. In current code, however, the strength is evaluated with an effective length factor k=0.9 without considering the above effect. Therefore this study analyzed a theoretical mechanism based on the elasticity theory and performed a finite element analysis to investigate the influence parameters on the elastic buckling strength of axially loaded member.

• Korean Journal of Optics and Photonics
• /
• v.25 no.1
• /
• pp.1-7
• /
• 2014

Multiple scattering effects in cloud are important error sources of the Mie scattering Lidar inversion method, which should be measured to correct the Lidar equation in single wavelength Mie Lidar. We have calculated the multiple scattering effects in liquid water clouds by using a Monte Carlo method, and we have applied these multiple scattering effects in measuring water cloud effective size and LWC (Liquid Water Content). When cloud effective size is less than $2.5{\mu}m$, we can easily extract cloud effective size and LWC by using two wavelength Lidar such as extinction coefficients measured at 355nm and 1064nm. For a larger size cloud, we can find that saturated degree of linear polarization is strongly correlated with cloud effective size, LWC, and extinction coefficients. From these correlations we know that we can measure LWC and cloud effective size if we use single wavelength Rotational Raman Lidar and Mie scattering polarization Lidar.

• Composites Research
• /
• v.17 no.1
• /
• pp.29-37
• /
• 2004

Honeycomb sandwich composite(HSC) structures have been widely used in aircraft and military industry owing to their light weight and high stiffness. Mechanical properties of honeycomb core materials are needed for accurate analysis of the sandwich composites. In this study. theoretical formula for effective elastic modulus and Poisson's ratio of honeycomb core materials was established using an energy method considering the bending, axial and shear deformations of honeycomb core walls. Finite-element analysis results obtained by using commercial FEA code, ABAQUS 6.3 were comparable to the theoretical ones. In addition, we performed tensile test of HSC plates and analyzed deformation behaviors and interlaminar stresses through its FEA simulation. An increased shear stress along the interface between surface and honeycomb core layers was shown to be the main reason for interfacial delamination in HSC plate under tensile loading.

• Journal of the Korean Society for Nondestructive Testing
• /
• v.32 no.1
• /
• pp.34-40
• /
• 2012

In this study, effects of bonding agent, e.g. epoxy, on the behavior of fundamental guided wave modes propagated in FRP plate bonded on a concrete, are investigated. Global matrix model of multilayered FRP-epoxy-concrete system was constructed to obtain the velocity and attenuation dispersion curves of the fundamental A0 and S0 modes. Two variables, thickness and elastic modulus of epoxy layer, were considered in the dispersion analysis. It was found that both the thickness and the elastic modulus of epoxy layer greatly affect the phase velocity and attenuation of S0 mode while those are negligible for A0 mode. Based on the results, it was concluded that S0 mode is more effective than A0 mode for bonding condition assessment for FRP plate bonded concrete.

• Journal of the Korea institute for structural maintenance and inspection
• /
• v.16 no.1
• /
• pp.131-139
• /
• 2012

The lateral reinforcements of concrete such as hoops and spiral bars are known to confine concrete to compensate the strength loss due to fire by reducing explosive spalling and improving the capacity of ductility. In this context, a study was conducted to investigate the residual mechanical properties of confined and unconfined concrete($f_{ck}$=60MPa) after a single thermal cycle at 300, 600, $800^{\circ}C$. The main parameters required to establish the stress-strain relationship are the peak stress, the elastic modulus, and the strain at peak stress. The knowledge of the residual mechanical properties of concrete is necessary whenever the thermally damaged structure is required to bear a significant share of the loads, even after a severe thermal accident. Based on the results obtained in this study, the residual stress of confined concrete under thermal damage is higher according to the level of confinement and the larger strain made it to have better ductility. The decreasing ratio of elastic modulus from the relationship of stress and strain was also smaller than that of unconfined concrete.

• The Journal of the Acoustical Society of Korea
• /
• v.11 no.4
• /
• pp.22-30
• /
• 1992

The propagation of coherent time-harmonic elastic SH waves in a medium with random distribution of cylindrical inclusions is studied for characterizing the dynamic elastic modulus and the attenuation property of fiber-reinforced composite materials. A multiple scattering theory using the single scattering coefficients in conjunction with the Lax's quasicrystalline approximation is derived and from which the dispersion relation for such medium is obtained. The pair-correlation functions between the cylinders which are needed to formulate the multiple scattering interaction between the cylinders are obtained by Monte Carlo simulation method.From the numerically calculated complex wavenumbers, the propagation speed of the average wave, the coherent attenuation coefficient and the effective shear modulus are presented as functions of frequency and area density.

• Transactions of the Korean Society of Mechanical Engineers
• /
• v.15 no.2
• /
• pp.611-617
• /
• 1991

Effective stiffness of short fiber composite with a three-dimensional random orientation of fibers is derived theoretically and compared with available experimental data. The laminate analogy and transformed laminate analogy are used for modulus prediction of 2-D and 3-D random composites, respectively. The effective stiffness of random oriented fiber composite can be expressed in terms of longitudinal and transverse stiffnesses of unidirectional composites. The result of transformed laminate analogy is more accurate than other approaches such as, Christensen-Waals equational and Lavengood-Goettler equation, etc. Also the effective properties of random oriented fiber composite can be expressed in terms of fiber and matrix properties such as elastic modulus, shear modulus and Poisson's ratio.

• Proceeding of EDISON Challenge
• /
• 2015.03a
• /
• pp.197-200
• /
• 2015

푸아송비(poisson's ratio)는 종방향 변형률에 대한 횡방향 변형률의 비로, 우리 주위 대부분의 재료들은 양의 푸아송비를 지닌다. 그러나 재료가 특정한 격자구조를 이루도록 설계할 경우 구성물질이 양의 푸아송비를 가지더라도 거시적으로는 음의 푸아송비를 구현할 수 있으며, 이러한 극한물성물질(metamaterial)을 오그제틱 물질(auxetic material)이라고 부른다. 이전까지 오그제틱 물질을 구현하기 위한 많은 메커니즘들이 개발되고 역학적, 수치적으로 해석되어 왔다. 이 논문에서는 가장 대표적인 오그제틱 구조인 리엔트런트(re-entrant) 오그제틱 구조를 빔 구조물로 모델링하여 유한요소해석을 수행하고 주요 설계 변수인 리엔트런트 각에 따라서 푸아송비와 유효 탄성계수가 어떻게 변화하는지 확인하였다.