• Wind and Structures
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• v.29 no.6
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• pp.457-469
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• 2019

In this paper, a numerical solution is presented for supersonic flutter analysis of cantilever non-symmetric functionally graded (FG) sandwich plates. The plate is considered to be composed of two different functionally graded face sheets and an isotropic homogeneous core made of ceramic. Based on the first order shear deformation theory (FSDT) and linear piston theory, the set of governing equations and boundary conditions are derived. Dimensionless form of the governing equations and boundary conditions are derived and solved numerically using generalized differential quadrature method (GDQM) and critical velocity and flutter frequencies are calculated. For various values of the yaw angle, effect of different parameters like aspect ratio, thickness of the plate, power law indices and thickness of the core on the flutter boundaries are investigated. Numerical examples show that wings and tail fins with larger length and shorter width are more stable in supersonic flights. It is concluded for FG sandwich plates made of Al-Al₂O₃ that increase in volume fraction of ceramic (Al₂O₃) increases aeroelastic stability of the plate. Presented study confirms that improvement of aeroelastic behavior and weight of wings and tail fins of aircrafts are not consistent items. It is shown that value of the critical yaw angle depends on aspect ratio of the plate and other parameters including thickness and variation of properties have no considerable effect on it. Results of this paper can be used in design and analysis of wing and tail fin of supersonic airplanes.

• Computers and Concrete
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• v.26 no.1
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• pp.75-94
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• 2020

The aim of this research is to investigate free vibration of a novel five layer Timoshenko microbeam which consists of a transversely flexible porous core made of Al-foam, two graphen platelets (GPL) nanocomposite reinforced layers to enhance the mechanical behavior of the structure as well as two piezo-magneto-electric face sheets layers. This microbeam is subjected to a thermal load and resting on Pasternak's foundation. To accomplish the analysis, constitutive equations of each layer are derived by means of nonlocal strain gradient theory (NSGT) to capture size dependent effects. Then, the Hamilton's principle is employed to obtain the equations of motion for five layer Timoshenko microbeam. They are subsequently solved analytically by applying Navier's method so that discretized governing equations are determined in form of dynamic matrix giving the possibility to gain the natural frequencies of the Timoshenko microbeam. Eventually, after a validation study, the numerical results are presented to study and discuss the influences of various parameters such as nonlocal parameter, strain gradient parameter, aspect ratio, porosity, various volume fraction and distributions of graphene platelets, temperature change and elastic foundation coefficients on natural frequencies of the sandwich microbeam.

• Korean Journal of Materials Research
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• v.28 no.5
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• pp.286-294
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• 2018

Hot-press forming(HPF) steel can be applied successfully to auto parts because of its superior mechanical properties. However, its resistances to aqueous corrosion and the subsequent hydrogen embrittlement(HE) decrease significantly when the steel is exposed to corrosive environments. Considering that the resistances are greatly dependent on the properties of coating materials formed on the steel surface, the characteristics of the corrosion and hydrogen diffusion behaviors regarding the types of coating material should be clearly understood. Electrochemical polarization and impedance measurements reveal a higher corrosion potential and polarization resistance and a lower corrosion current of the Al-coating compared with Zn-coating. Furthermore, it was expected that the diffusion kinetics of the hydrogen atoms would be much slower in the Al-coating, and this would be due mainly to the much lower diffusion coefficient of hydrogen in the Al-coating with a face-centered cubic structure. The superior surface inhibiting effect of the Al-coating, however, is degraded by the formation of local cracks in the coated layer under severe stress conditions, and therefore further study will be necessary to gain a clearer understanding of the effect of cracks formed on the coated layer on the subsequent corrosion and hydrogen diffusion behaviors.

• Journal of Sasang Constitutional Medicine
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• v.21 no.1
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• pp.165-185
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• 2009

1. Objectives This study was designed to research the relationship between Sasang Constitution and Framingham coronary risk score(FRS), 2. Methods 666 people, over 40 years old, were participated in the community-based cohort in Wonju, South Korea from July 2nd to August 30th in 2006, Sasang Constitutional Diagnosis was carried out using PSSC(Phonetic System of Sasang Constitution), face and tongue photos and a checkup list, Framingham coronary risk score(FRS) was calculated using previously studied sheets adopted from KCDC(Korea Center for Disease Control and Prevention). 3. Results The average FRS of men was 7,06 and that of female was 8,19 which was significantly higher than that of men. There were significant differences between Taeeumin and other constitutions in women, whereas no differences among male constitutions. Analyzing adjusted ORs for CHD risk intervals according to Sasang constitution, they were 0.96 in Taeeumin and 1.70 in Soyangin to Soeumin. Also 0.55 in Taeeumin and 3.39 in Soyangin to Soeumin in men, 1.96 in Taeeumin and 1.87 in Soyangin to Soeumin in women. Women had highet FRS than men and female Taeeumin group have highet FRS points and CRP than other constitutions. 4. Conclusions Regarding the positive relationship between FRS, APG and CRP, it is thought to be useful to understand APG and CRP results at the same time when calculating FRS. When it comes to prevention and screening of CHD, female Taeeumin should be more careful and female Soyangin also should have a consideration of heart disease.

• Journal of the Korea Concrete Institute
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• v.17 no.6 s.90
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• pp.903-910
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• 2005

In recent years, fiber reinforced polymer(FRP) plates have shown a great promise as an alternative to steel plates for reinforced concrete beam rehabilitation. Reinforced concrete beams strengthened with externally bonded FRP sheets to the tension face can exhibit ultimate flexural strengths several times greater than their original strength if their bond strength is enough. Debonding failure, however, may occur before the strengthened beam can achieve its enhanced flexural strength. The purpose of this paper is to investigate the debonding failure strength of FRP-strengthened reinforced concrete beams. An analytical procedure for calculating debonding load between concrete and strengthening FRP is presented. Based on the local bond stress-slip relationship in the previous studies, uniform bond stress is assumed on the effective bond length. The analytical expressions are developed from linear elastic theory and statistical analyses of experimantal results reported in the literature. The proposed method is verified by comparisons with experimental results reported in the previous researches.

• Steel and Composite Structures
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• v.39 no.5
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• pp.615-630
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• 2021

The main purpose of the present work was to study the dynamic instability of a three-layered, thick composite sandwich beam with the functionally graded (FG) flexible core subjected to an axial compressive follower force. Flutter instability of a sandwich cantilever beam was analyzed using the high-order theory of sandwich beams, for the first time. The governing equations in general for sandwich beams with an FG core were extracted and could be used for all types of sandwich beams with any types of face sheets and cores. A polynomial function is considered for the vertical distribution of the displacement field in the core layer along the thickness, based on the results of the first Frosting's higher order model. The governing partial differential equations and the equations of boundary conditions of the dynamic system are derived using Hamilton's principle. By applying the boundary conditions and numerical solution methods of squares quadrature, the beam flutter phenomenon is studied. In addition, the effects of different geometrical and material parameters on the flutter threshold were investigated. The results showed that the responses of the dynamic instability of the system were influenced by the follower force, the coefficients of FGs and the geometrical parameters like the core thickness. Comparison of the present results with the published results in the literature for the special case confirmed the accuracy of the proposed theory. The results showed that the follower force of the flutter phenomenon threshold for long beams tends to the corresponding results in the Timoshenko beam.

• International Journal of Computer Science & Network Security
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• v.21 no.5
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• pp.243-255
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• 2021

This study aimed to identify the effectiveness of the use of distance-evaluation tools and methods among students with learning difficulties from the teachers' point of view, to achieve this goal. A scale was built, and the psychometric characteristics were validated. It consisted, in its final form, of 17 items distributed on four axes, in addition to three open questions. It was applied to a random sample of (149) teachers of students with learning difficulties in Makkah Region. The results showed that teachers' keenness to encourage students with learning difficulties, so that they would not feel frustrated with the distance learning process. It was also evident that teachers did not use achievement portfolios in the evaluation process. In connection with the appropriate evaluation methods, the majority indicated the use of work sheets and visual evaluation methods that rely on audio and visual skills, such as presenting videos, pictures, audio and games, and applying short objective tests. Among the proposals to improve evaluation methods and tools: Individual evaluation, attention to individual treatment, obligating personal attendance of students to school, splitting the required tasks, and not increasing the skills required to be mastered. As for the obstacles that teachers face: Lack of time, difficulty in communicating with students with distance learning difficulties and problems related to the Internet such as interruption, weakness, or lack of availability.

• Journal of the Korean Wood Science and Technology
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• v.32 no.3
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• pp.42-51
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• 2004

Wood veneer-bamboo zephyr composite board (WBCB) was manufactured to evaluate the properties of bamboo as alternative raw materials for the manufacture of composite panels. Bamboo zephyr was prepared using Moso bamboo (Phyllostachys pubesens Mazel. et Z), Giant timber bamboo (Phyllostachys bambusoides Sieb. et Zucc), and Hachiku bamboo (Phyllostachys nigra var. henosos Stapf). The effect of age and species of bamboo on zephyr production was investigated in terms of the pass number of bamboo split through the rollers, and the width increasing rate of bamboo split. Five-ply WBCBs were produced with Keruing veneers as face and back layers, leading to three layers of bamboo zephyr sheets as core layer. Each layer was placed so that its grain direction was at right angles to that of the adjacent layer and the layers were bonded together with phenol-formaldehyde (PF) resin.The pass number of bamboo split was increased with an increase of the thickness of culm wall. At the same thickness, Moso bamboo showed no effect of the age of bamboo on the pass number. The pass number of split of Giant timber bamboo was lower than that of Moso bamboo. No significant effect of bamboo species and age on the width of zephyr produced was observed. The width of zephyr obtained could be expressed as a function of diameter multiplied by thickness of culm wall. The physical and mechanical properties of WBCB manufactured in all given conditions did not show any significant differences, and they were above the requirement of Korean Standard (KS).

• Structural Engineering and Mechanics
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• v.85 no.5
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• pp.593-605
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• 2023

This work presents a comparison between analytical and finite element analysis for bending of porous sandwich functionally graded material (FGM) plates. The plate is rectangular and simply supported under static sinusoidal loading. Material properties of FGM are assumed to vary continuously across the face sheets thickness according to a power-law function in terms of the volume fractions of the constituents while the core is homogeneous. Four types of porosity are considered. A refined higher-order shear with normal deformation theory is used. The number of unknowns in this theory is five, as against six or more in other shear and normal deformation theories. This theory assumes the nonlinear variation of transverse shear stresses and satisfies its nullity in the top and bottom surfaces of the plate without the use of a shear correction factor. The governing equations of equilibrium are derived from the virtual work principle. The Navier approach is used to solve equilibrium equations. The constitutive law of the porous FGM sandwich plate is implemented for a 3D finite element through a subroutine in FORTRAN (UMAT) in Abaqus software. Results show good agreement between the finite element model and the analytical method for some results, but the analytical method keeps giving symmetric results even with the thickness stretching effect and load applied to the top surface of the sandwich.

• Steel and Composite Structures
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• v.46 no.1
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• pp.1-13
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• 2023

Elastic bending of imperfect functionally graded sandwich plates (FGSPs) laying on the Winkler-Pasternak foundation and subjected to sinusoidal loads is analyzed. The analyses have been established using the quasi-3D sinusoidal shear deformation model. In this theory, the number of unknowns is condensed to only five unknowns using integral-undefined terms without requiring any correction shear factor. Moreover, the current constituent material properties of the middle layer is considered homogeneous and isotropic. But those of the top and bottom face sheets of the graded porous sandwich plate (FGSP) are supposed to vary regularly and continuously in the direction of thickness according to the trigonometric volume fraction's model. The corresponding equilibrium equations of FGSPs with simply supported edges are derived via the static version of the Hamilton's principle. The differential equations of the system are resolved via Navier's method for various schemes of FGSPs. The current study examine the impact of the material index, porosity, side-to-thickness ratio, aspect ratio, and the Winkler-Pasternak foundation on the displacements, axial and shear stresses of the sandwich structure.