• Steel and Composite Structures
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• 제45권3호
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• pp.349-367
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• 2022

In this research, an approach combining a semi-analytical method and an analytical method is presented to investigate the static and dynamic post-buckling behavior of the sandwich functionally graded (FG) porous cylindrical shells exposed to external pressure. The sandwich cylindrical shell considered is composed of a viscoelastic core and two FG porous (FGP) face layers. The viscoelastic core is made of Kelvin-Voigt-type material. The material properties of the FG porous face layer are considered continuous through each face thickness according to a porosity coefficient and a volume fraction index. Two types of sandwich FG porous viscoelastic cylindrical shells named Type A and Type B are considered in the research. Type A shell has the porosity evenly distributed across the thickness direction, and Type B has the porosity unevenly distributes across the thickness direction. The FG face layers are considered in two cases: outside metal surface, inside ceramic surface (OMS-ICS), and inside metal surface, outside ceramic surface (IMS-OCS). According to Donnell shell theory, von-Karman equation, and Galerkin's method, a discretized nonlinear governing equation is derived for analyzing the behavior of the shells. The explicit expressions for static and dynamic critical buckling loading are thus developed. To study the dynamic buckling of the shells, the governing equation is examined via a numerical approach implementing the fourth-order Runge-Kutta method. With a procedure presented by Budiansky-Roth, the critical load for dynamic post-buckling is obtained. The effects of various parameters, such as material and geometrical parameters, on the post-buckling behaviors are investigated.

• Steel and Composite Structures
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• 제44권5호
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• pp.651-676
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• 2022

Most of the experimental, theoretical, and numerical studies on the stability of functionally graded composites are deterministic, while there are full of complex interactions of variables with an inherently probabilistic nature, this paper presents a non-intrusive framework to investigate the stochastic nonlinear buckling behaviors of porous functionally graded cylindrical shells exposed to inevitable source-uncertainties. Euler-Lagrange equations are theoretically derived based on the three variable refined shear deformation theory. Closed-form solutions for the shell buckling loads are achieved by solving the deterministic eigenvalue problems. The analytical results are verified with numerical results obtained from finite element analyses that are conducted in the commercial software ABAQUS. The non-intrusive framework is completed by integrating the Monte Carlo simulation with the verified closed-form solutions. The convergence studies are performed to determine the effective pseudorandom draws of the simulation. The accuracy and efficiency of the framework are verified with statistical results that are obtained from the first and second-order perturbation techniques. Eleven cases of individual and compound uncertainties are investigated. Sensitivity analyses are conducted to figure out the five cases that have profound perturbative effects on the shell buckling loads. Complete probability distributions of the first three critical buckling loads are completely presented for each profound uncertainty case. The effects of the shell thickness, volume fraction index, and stochasticity degree on the shell buckling load under compound uncertainties are studied. There is a high probability that the shell has non-unique buckling modes in stochastic environments, which should be known for reliable analysis and design of engineering structures.

• Nuclear Engineering and Technology
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• 제55권10호
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• pp.3617-3627
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• 2023

This study presents a method to solidify spent ion-exchange resin (IER) in a metakaolin-based geopolymer and shows results of mechanical strength, immersion, leaching, irradiation, and thermal cycling tests for waste acceptance criteria (WAC) to repository. The geopolymer waste form with 20 wt% of simulated spent IER met the WAC in South Korea (ROK), and the leaching tests of various sized-waste forms up to 15.0 × 30.0 cm and waste loadings up to 20 wt% for 1-5 d and 1-90 d achieved a leachability index, L_i > 6. In a leaching test for 5 d, the cumulative fraction leached (CFL) for Cs, which leached the most, was linearly correlated with the square root of leaching time for all waste forms, and L_i increased as the size of the waste form increased. The CFL was also correlated with elapsed time in the 90 d leaching test. The correlations among CFL, time, and volume-to-surface area ratio of waste forms used to estimate the L_i of Cs of a 200-L sized geopolymer with 15 wt% IER showed the L_i values as 14.73 (5 d) and 17.71 (90 d), respectively, indicating that the large-sized geopolymer waste form met the WAC.

• Structural Engineering and Mechanics
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• 제49권5호
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• pp.569-595
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• 2014

In this paper, a novel semi-energy finite strip method (FSM) is developed based on the concept of first order shear deformation theory (FSDT) in order to attempt the post-buckling solution for thin and relatively thick functionally graded (FG) plates under uniform end-shortening. In order to study the effects of through-the-thickness shear stresses on the post-buckling behavior of FG plates, two previously developed finite strip methods, i.e., semi-energy FSM based on the concept of classical laminated plate theory (CLPT) and a CLPT full-energy FSM, are also implemented. Moreover, the effects of aspect ratio on initial post-buckling stiffness of FG rectangular plates are studied. It has been shown that the variation of the ratio of initial post-buckling stiffness to pre-buckling stiffness ($S^*/S$) with respect to aspects ratios is quite independent of volume fractions of constituents in thin FG plates. It has also been seen that the universal curve representing the variation of ($S^*/S$) with aspect ratio of a FG plate demonstrate a saw shape curve. Moreover, it is revealed that for the thin FG plates in contrast to relatively thick plates, the variations of non-dimensional load versus end-shortening is independent of ceramic-metal volume fraction index. This means that the post-buckling behavior of thin FG plates and the thin pure isotropic plates is similar. The results are discussed in detail and compared with those obtained from finite element method (FEM) of analysis. The study of the results may have a great influence in design of FG plates encountering post-buckling behavior.

• Structural Engineering and Mechanics
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• 제75권2호
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• pp.193-209
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• 2020

This paper presents a new hyperbolic shear deformation plate theory including the stretching effect for free vibration of the simply supported functionally graded plates in thermal environments. The theory accounts for parabolic distribution of the transverse shear strains and satisfies the zero traction boundary conditions on the surfaces of the plate without using shear correction factors. This theory has only five unknowns, which is even less than the other shear and normal deformation theories. The present one has a new displacement field which introduces undetermined integral variables. Material properties are assumed to be temperature-dependent, and graded in the thickness direction according to a simple power law distribution in terms of the volume power laws of the constituents. The equation of motion of the vibrated plate obtained via the classical Hamilton's principle and solved using Navier's steps. The accuracy of the proposed solution is checked by comparing the present results with those available in existing literature. The effects of the temperature field, volume fraction index of functionally graded material, side-to-thickness ratio on free vibration responses of the functionally graded plates are investigated. It can be concluded that the present theory is not only accurate but also simple in predicting the natural frequencies of functionally graded plates with stretching effect in thermal environments.

• 지능정보연구
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• 제28권4호
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• pp.81-94
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• 2022

최근 고액의 실물자산이나 채권을 분할하여 여러 투자자가 공동으로 투자하는 이른바 조각투자가 인기를 얻고 있다. 2016년 설립된 뮤직카우는 음원 유통에 따른 저작권료 참여 청구권을 조각투자할 수 있는 서비스를 세계 최초로 시작하였다. 본 연구에서는 딥러닝 알고리즘 중 하나인 LSTM 모델을 사용하여 뮤직카우에서 거래되는 저작권료 참여 청구권의 가격을 예측하는 연구를 진행하였다. 청구권의 이전 가격과 거래량, 저작권료와 같은 청구권과 관련된 변수 외에도, 음악저작권료 참여 청구권 시장 상황을 나타내는 종합 지표와 경제 상황을 반영하는 환율, 국고채 금리, 한국종합주가지수도 변수로 사용하였다. 연구 결과 상대적으로 거래량이 낮은 조각투자의 사례에서도 LSTM 모델이 거래가격을 잘 예측하는 것을 확인할 수 있었다.

• Structural Engineering and Mechanics
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• 제85권1호
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• pp.105-118
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• 2023

The present work is an attempt to develop a simple and accurate finite element formulation for the assessment of thermal shock/thermally induced vibrations in pretwisted and tapered functionally graded material thin (FGM) blades obtained from Voigt and local representative volume elements (LRVE) homogenization models, based on neutral surface approach. The neutral surface of the FGM blade does not coincide with its mid-surface. A finite element model (FEM) is developed using first-order shear deformation theory (FSDT) and the FGM turbine blade is modelled according to the shallow shell theory. The top and the bottom layers of the FGM blade are made of pure ceramic and pure metal, respectively and temperature-dependent material properties are functionally graded in the thickness direction, the position of the neutral surface also depends on the temperature. The material properties are estimated according to two different homogenization models viz., Voigt or LRVE. The top layer of the FGM blade is subjected to high temperature and the bottom surface is either thermally insulated or kept at room temperature. The solution of the nonlinear profile of the temperature in the thickness direction is obtained from the Fourier law of heat conduction in the unsteady state. The results obtained from the present FEM are compared with the benchmark examples. Next, the effect of angle of twist, intensity of thermal shock, variable chord and span and volume fraction index on the transient response due to thermal shock obtained from the two homogenization models viz., Voigt and LRVE scheme is investigated. It is shown that there can be a significant difference in the transient response calculated by the two homogenization models for a particular set of material and geometric parameters.

• 콘크리트학회지
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• 제11권2호
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• pp.209-220
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• 1999

터널의 숏크리트 라이닝이나 포장 콘크리트 보강용으로 요접철망(wire mesh)을 대신해서 강섬유가 사용되고 있다. 본 연구에서는 강섬유 보강으로 인한 인성평가 대상 구조물을 slab panel 구조($60{\times}60{\times}10$cm)로 하고, 강섬유 혼입률은 콘크리트 용적의 0.5% ~ 2%로 다양화하였다. 이 때 사용한 강섬유는 Dramix ZC type으로 직경은 0.8mm, 길이는 60mm이다. 강섬유 효과의 상대평가를 위한 용접철망(wire mesh)보강은 상면, 하면, 상하면 보강으로 하였다. 이들 실험 결과를 각국의 인성 평가 방법으로 비교 검토한 결과 슬래브(slab) 시험체 적용을 위한 EFNARC의 방법은 25mm의 처짐까지 측정하는 것이 너무 큰 것으로 평가되었고, 보의 휨인성 평가법을 적용하여 검토한 결과에서는 Johnston(II)방법에 의한 $I_{5.5}$ 가장 적절하였으며, JCI-SF4방법에서 지간의 1/150까지 측정하는 것은 너무 작았다. 또한 강섬유로 용접철망(wire mesh)을 효과적으로 대치할 수 있음을 알 수 있었고, 인성효과에 유용한 강섬유 혼입량은 0.5% ~ 1%범위에 있는 것으로 나타났다.

• 대한토목학회논문집
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• 제12권4호
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• pp.95-106
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• 1992

강섬유의 형태가 콘크리트의 휨거동에 미치는 영향을 알아 보기 위하여 5종의 강섬유를 사용한 실험을 실시하였다. 강섬유 혼입율은 1~2%로 변화를 주었고 전단 스팬비와 철근비는 고정하였다. 형태가 다른 강섬유의 보강 효과를 비교 분석하기 위한 방법으로 섬유 보강 효과 지수를 채택하였고, 인성 효과를 분석하기 위하여 유효 인성 지수를 제안하였으며, 이들 지수들은 강섬유 보강 효과를 비교 분석하는데 유용한 방법으로 평가되었다. 실험 결과에 의하면 강섬유 혼입율은 양단이 hook 형태인 강섬유의 경우에는 1% 이내가 효과적이며, dog bone 형태인 강섬유는 1~2%, 기타 강섬유는 2% 정도가 적절하였다. 강섬유의 혼입으로 인한 휨강도 및 파괴인성의 증진 효과는 무근 콘크리트보의 경우가 철근 콘크리트보의 경우보다 크게 나타났다. 또한 강섬유의 형태에 의한 휨강도와 인성의 증진 효과는 양단이 hook나 dog bone(paddled)형태의 강섬유가 섬유 혼입율이 상대적으로 낮음에도 straight나 crimped 및 wavy 형태보다 우수한 것으로 판명되었다.

• Steel and Composite Structures
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• 제38권5호
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• pp.477-496
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• 2021

The main objective of this paper is to study vibration of sandwich open cylindrical panel with damaged core and FG face sheets based on three-dimensional theory of elasticity. The structures are made of a damaged isotropic core and two external face sheets. These skins are strengthened at the nanoscale level by randomly oriented Carbon nanotubes (CNTs) and are reinforced at the microscale stage by oriented straight fibers. These reinforcing phases are included in a polymer matrix and a three-phase approach based on the Eshelby-Mori-Tanaka scheme and on the Halpin-Tsai approach, which is developed to compute the overall mechanical properties of the composite material. Three complicated equations of motion for the panel under consideration are semi-analytically solved by using 2-D differential quadrature method. Several parametric analyses are carried out to investigate the mechanical behavior of these multi-layered structures depending on the damage features, through-the-thickness distribution and boundary conditions. It is seen that for the large amount of power-law index "P", increasing this parameter does not have significant effect on the non-dimensional natural frequency parameters of the FG sandwich curved panel. Results indicate that by increasing the value of isotropic damage parameter "D" up to the unity (fully damaged core) the frequency would tend to become zero. One can dictate the fiber variation profile through the radial direction of the sandwich panel via the amount of "P", "b" and "c" parameters. It should be noticed that with increase of volume fraction of fibers, the frequency parameter of the panels does not increase necessarily, so by considering suitable amounts of power-law index "P" and the parameters "b" and "c", one can get dynamic characteristics similar or better than the isotropic limit case for laminated FG curved panels.