• Computers and Concrete
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• 제25권3호
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• pp.225-244
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• 2020

The influence of boundary conditions on the bending and free vibration behavior of functionally graded sandwich plates resting on a two-parameter elastic foundation is examined using an original novel high order shear theory. The Hamilton's principle is used herein to derive the equations of motion. The number of unknowns and governing equations of the present theory is reduced, and hence makes it simple to use. This theory includes indeterminate integral variables and contains only four unknowns in which any shear correction factor not used, with even less than the conventional theory of first shear strain (FSDT). Unlike any other theory, the number of unknown functions involved in displacement field is only four, as against five, six or more in the case of other shear deformation theories. Galerkin's approach is utilized for FGM sandwich plates with six different boundary conditions. The accuracy of the proposed solution is checked by comparing it with other closed form solutions available in the literature.

• 한국전산구조공학회논문집
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• 제29권5호
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• pp.463-469
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• 2016

본 논문에서는 저차원 확률 함수를 사용하여 연속적인 선형 공극비 기울기를 갖는 기능경사재료 형태의 가상 시편을 제작하였다. 실제 물 시멘트 비가 다른 두 종류 시멘트 풀 시편의 X-선 CT 단면 이미지를 통해 3차원 공극 분포 모델을 제작하여 이를 기반으로 가상 시편을 제작하였다. 가상 시편이 두 실제 시편 사이에 위치하도록 공극 분포 특성을 저차원 확률 함수와 공극비 기울기로 구성하였다. 공극 분포의 기울기는 연속적인 형태의 수직 방향의 선형 공극비 기울기로 구현하며, 이를 위해 확률적 최적화의 목적 함수로 저차원 확률 함수와 공극비 기울기에 관한 함수를 사용하였다. 가상 시편 내 공극 분포 특성 확인을 위해 본 연구에서는 투기율 분석을 유한요소해석을 통해 수행하였다. 공극 분포 특성과 투기율 해석 결과를 비교하여 가상 시편이 갖는 공극 분포 특성이 투기율에 미치는 영향을 확인하고, 투기율의 실제 실험 결과와 해석 결과의 비교를 통해 해석 결과의 타당성을 입증하였다.

• Ocean Systems Engineering
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• 제3권1호
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• pp.9-34
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• 2013

Submarine pipelines encounter significant wave forces in shallow coastal waters due to the action of waves. In order to reduce such forces (also to protect the pipe against anchors and dropped objects) they are buried below the seabed. The wave force variation due to burial depends on the engineering characteristics of the sub soil like hydraulic conductivity and porosity, apart from the design environmental conditions. For a given wave condition, in certain type of soil, the wave force can reduce drastically with increased burial and in certain other type of soil, it may not. It is hence essential to understand how the wave forces vary in soils of different hydraulic conductivity. Based on physical model study, the wave forces on the buried pipeline model is assessed for a wide range of wave conditions, for different burial depths and for four types of cohesion-less soils, covering hydraulic conductivity in the range of 0.286 to 1.84 mm/s. It is found that for all the four soil types, the horizontal wave force reduces with increase in depth of burial, whereas the vertical force is high for half buried condition. Among the soils, well graded one is better for half buried case, since the least vertical force is experienced for this situation. It is found that uniformly graded and low hydraulic conductivity soil attracts the maximum vertical force for half buried case. A case study analysis is carried out and is reported. The results of this study are useful for submarine buried pipeline design.

• 한국진공학회:학술대회논문집
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• 한국진공학회 2014년도 제46회 동계 정기학술대회 초록집
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• pp.320-320
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• 2014

Amorphous silicon alloy (a-Si) solar cells and modules have been receiving a great deal of attention as a low-cost alternate energy source for large-scale terrestrial applications. Key to the achievement of high-efficiency solar cells using the multi-junction approach is the development of high quality, low band-gap materials which can capture the low-energy photons of the solar spectrum. Several cell designs have been reported in the past where grading or buffer layers have been incorporated at the junction interface to reduce carrier recombination near the junction. We have investigated profiling the composition of the a-SiGe alloy throughout the bulk of the intrinsic material so as to have a built-in electrical field in a substantial portion of the intrinsic material. As a result, the band gap mismatch between a-Si:H and $a-Si_{1-x}Ge_x:H$ creates a barrier for carrier transport. Previous reports have proposed a graded band gap structure in the absorber layer not only effectively increases the short wavelength absorption near the p/i interface, but also enhances the hole transport near the i-n interface. Here, we modulated the GeH4 flow rate to control the band gap to be graded from 1.75 eV (a-Si:H) to 1.55 eV ($a-Si_{1-x}Ge_x:H$). The band structure in the absorber layer thus became like a U-shape in which the lowest band gap was located in the middle of the i-layer. Incorporation of this structure in the middle and top cell of the triple-cell configuration is expected to increase the conversion efficiency further.

• 한국재난정보학회 논문집
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• 제16권4호
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• pp.832-841
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• 2020

연구목적: 본 연구에서는 고차전단변형이론을 적용한 등기하해석 방법을 이용하여 기능경사복합재 판의 휨에 의한 역학적 거동을 해석하고자 하였다. 연구방법: 기능경사복합재 판의 역학적 거동을 보다 더 정확하게 해석하기 위해서 전단보정계수를 도입할 필요가 없는 기하학적 비선형을 고려한 고차전단변형이론을 이용하여 휨을 받는 기능경사복합재 판의 평형방정식과 지배방정식을 도출하였으며, 등기하 해석방법에 의한 수정된 Newton-Raphson 반복법을 이용하여 방정식들을 풀었다. 연구결과: 판의 용적비, 길이-두께 비 및 경계조건은 기능경사복합재 판의 휨 거동에 상당한 영향을 미치는 것을 알 수 있었다. 결론: 제안된 등기하해석 방법은 휨을 받는 기능경사복합재 판의 역학적 거동을 해석하는데 있어 정확하고 효과적인 수치해석 방법임을 확인하였다.

• Steel and Composite Structures
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• 제38권3호
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• pp.337-353
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• 2021

Micro-electro-mechanical systems (MEMS) are widely employed in sensors, biomedical devices, optic sectors, and micro-accelerometers. New reinforcement materials such as carbon nanotubes as well as graphene platelets provide stiffer structures with controllable mechanical specifications by changing the graphene platelet features. This paper deals with buckling analyses of functionally graded graphene platelets micro plates with two piezoelectric layers subjected to external applied voltage. Governing equations are based on Kirchhoff plate theory assumptions beside the modified couple stress theory to incorporate the micro scale influences. A uniform temperature change and external electric field are regarded along the micro plate thickness. Moreover, an external in-plane mechanical load is uniformly distributed along the micro plate edges. The Hamilton's principle is employed to extract the governing equations. The material properties of each composite layer reinforced with graphene platelets of the considered micro plate are evaluated by the Halpin-Tsai micromechanical model. The governing equations are solved by the Navier's approach for the case of simply-supported boundary condition. The effects of the external applied voltage, the material length scale parameter, the thickness of the piezoelectric layers, the side, the length and the weight fraction of the graphene platelets as well as the graphene platelets distribution pattern on the critical buckling temperature change and on the critical buckling in-plane load are investigated. The outcomes illustrate the reduction of the thermal buckling strength independent of the graphene platelets distribution pattern while meanwhile the mechanical buckling strength is promoted. Furthermore, a negative voltage, -50 Volt, strengthens the micro plate stability against the thermal buckling occurrence about 9% while a positive voltage, 50 Volt, decreases the critical buckling load about 9% independent of the graphene platelet distribution pattern.

• Steel and Composite Structures
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• 제49권4호
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• pp.361-380
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• 2023

A size dependent bending behavior of piezoelectrical flexoelectric layered perforated functionally graded (FG) composite nanobeam rested on an elastic foundation is investigated analytically. The composite beam is composed of regularly cutout FG core and two piezoelectric face sheets. The material characteristics is graded through the core thickness by power law function. Regular squared cutout perforation pattern is considered and closed forms of the equivalent stiffness parameters are derived. The modified nonlocal strain gradient elasticity theory is employed to incorporate the microstructure as well as nonlocality effects into governing equations. The Winkler as well as the Pasternak elastic foundation models are employed to simulate the substrate medium. The Hamiltonian approach is adopted to derive the governing equilibrium equation including piezoelectric and flexoelectric effects. Analytical solution methodology is developed to derive closed forms for the size dependent electromechanical as well as mechanical bending profiles. The model is verified by comparing the obtained results with the available corresponding results in the literature. To demonstrate the applicability of the developed procedure, parametric studies are performed to explore influences of gradation index, elastic medium parameters, flexoelectric and piezoelectric parameters, geometrical and peroration parameters, and material parameters on the size dependent bending behavior of piezoelectrically layered PFG nanobeams. Results obtained revealed the significant effects both the flexoelectric and piezoelectric parameters on the bending behavior of the piezoelectric composite nanobeams. These parameters could be controlled to improve the size dependent electromechanical as well as mechanical behaviors. The obtained results and the developed procedure are helpful for design and manufacturing of MEMS and NEMS.

• Computers and Concrete
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• 제20권2호
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• pp.229-246
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• 2017

The dynamic bending response of single walled carbon nanotube reinforced composite (SWCNTRC) plates subjected to hygro-thermo-mechanical loading are investigated in this paper. The mechanical load is considered as wind pressure for dynamic bending responses of SWCNTRC plate. The dynamic version of the High Order shear deformation Theory (HSDT) for a composite plate with Matrix and SWCNTRC plate is first formulated. Distribution of fibers through the thickness of the SWCNTRC plate could be uniform or functionally graded (FG). The dynamic displacement response is predicted by using Nemarck integration method. The effective material properties of SWCNTRC are estimated by using micromechanics based modeling approach. The effect of different environmental condition, volume fraction of SWCNT, Width-to-thickness ratio, wind pressure, different SWCNTRC-FG plates, boundary condition, E1/E2 ratio, different temperature on dynamic displacement response is investigated. The dynamic displacement response is compared with the available literature and it shows good agreement.

• 한국항공운항학회지
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• 제15권2호
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• pp.9-17
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• 2007

A Green's function approach based on the laminate theory is adopted for solving the two-dimensional unsteady temperature field and the associated thermal stresses in an infinite plate made of functionally graded material (FGM). All material properties are assumed to depend only on the coordinate x (perpendicular to the surface). The unsteady heat conduction equation is formulated into an eigenvalue problem by making use of the eigenfunction expansion theory and the laminate theory. The eigenvalues and the corresponding eigenfunctions obtained by solving an eigenvalue problem for each layer constitute the Green's function solution for analyzing the two-dimensional unsteady temperature. The associated thermoelastic field is analyzed by making use of the thermal stress function. Numerical analysis for a FGM plate is carried out and effects of material properties on unsteady thermoelastic behaviors are discussed.

• 산업경영시스템학회지
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• 제33권4호
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• pp.114-121
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• 2010

Traditionally Nuclear Research and Development (R&D) result has been big influence on other industries and societies and it requires large scale investments and study period. So it is essential to apply Quality Assurance (QA) for systematic R&D management. This paper investigates QA System for U.S. Nuclear R&D and reviews QA elements. Based on this investigation, we applied QA requirements to Nuclear Hydrogen R&D project, and the scope of application be enlarged as R&D stage in progress. We also present QA system improvement way through consideration for Nuclear Hydrogen Project's QA application. As the need for QA in R&D is expected to increase in the future, it is necessary to prepare guidelines for R&D QA.