• Advances in nano research
• /
• v.6 no.2
• /
• pp.113-133
• /
• 2018

In this work, free vibration characteristics of functionally graded piezoelectric (FGP) nanobeams based on third order parabolic shear deformation beam theory are studied by presenting a Navier type solution as the first attempt. Electro-mechanical properties of FGP nanobeam are supposed to change continuously throughout the thickness based on power-law model. To capture the small size effects, Eringen's nonlocal elasticity theory is adopted. Using Hamilton's principle, the nonlocal governing equations for third order shear deformable piezoelectric FG nanobeams are obtained and they are solved applying analytical solution. By presenting some numerical results, it is demonstrated that the suggested model presents accurate frequency results of the FGP nanobeams. The influences of several parameters including, external electric voltage, power-law exponent, nonlocal parameter and mode number on the natural frequencies of the size-dependent FGP nanobeams is discussed in detail.

• Computers and Concrete
• /
• v.28 no.5
• /
• pp.521-532
• /
• 2021

Present paper focuses on the modeling of size effect on the compressive strength of normal concrete with the application of Discrete Element Method (DEM). Test specimens with different size and shape were cast and uniaxial compressive strength test was performed on each sample. Five different concrete mixes were used, all belonging to a different normal strength concrete class (C20/25, C30/37, C35/45, C45/55, and C50/60). The numerical simulations were carried out by using the PFC 5 software, which applies rigid spheres and contacts between them to model the material. DEM modeling of size effect could be advantageous because the development of micro-cracks in the material can be observed and the failure mode can be visualized. The series of experiments were repeated with the model after calibration. The relationship of the parallel bond strength of the contacts and the laboratory compressive strength test was analyzed by aiming to determine a relation between the compressive strength and the bond strength of different sized models. An equation was derived based on Bazant's size effect law to estimate the parallel bond strength of differently sized specimens. The parameters of the equation were optimized based on measurement data using nonlinear least-squares method with SSE (sum of squared errors) objective function. The laboratory test results showed a good agreement with the literature data (compressive strength is decreasing with the increase of the size of the specimen regardless of the shape). The derived estimation models showed strong correlation with the measurement data. The results indicated that the size effect is stronger on concretes with lower strength class due to the higher level of inhomogeneity of the material. It was observed that size effect is more significant on cube specimens than on cylinder samples, which can be caused by the side ratios of the specimens and the size of the purely compressed zone. A limit value for the minimum size of DE model for cubes and cylinder was determined, above which the size effect on compressive strength can be neglected within the investigated size range. The relationship of model size (particle number) and computational time was analyzed and a method to decrease the computational time (number of iterations) of material genesis is proposed.

• 한국전산유체공학회:학술대회논문집
• /
• 2000.05a
• /
• pp.33-38
• /
• 2000

A Numerical study of laminar vortex-shedding past a circular cylinder has been performed widely by many researchers. Many factors, such as numerical technique and domain size, number and shape of grid, affected predicting vortex shedding and Strouhal number. In the present study, the effect of convection scheme, time discretization methods and grid dependence were investigated. The present paper presents the finite volume solution of unsteady flow past circular cylinder at Re=200, 400. The Strouhal number was predicted using UDS, CDS, Hybrid, Power-law, LUDS, QUICK scheme for convection term, implicit and crank-nicolson methods for time discretization. The grid dependence was investigated using H-type mesh and O-type mesh. It also studied that the effect of mesh size of the nearest adjacent grid of circular cylinder. The effect of convection scheme is greater than the effect of time discretization on predicting Strouhal. It has been found that the predicted Strouhal number changed with mesh size and shape.

• Journal of the Korea Concrete Institute
• /
• v.13 no.2
• /
• pp.153-160
• /
• 2001

In this study, the size effect on axial compressive strength for concrete members was experimentally investigated. Experiment of mode I failure, which is one of the two representative compressive failure modes, was carried out by using double cantilever beam specimens. By varying the eccentricity of applied loads with respect to the axis on each cantilever and the initial crack length, the size effect of axial compressive strength of concrete was investigated, and new parameters for the modified size effect law (MSEL) were suggested using least square method (LSM). The test results show that size effect appears for axial compressive strength of cracked specimens. For the eccentricity of loads, the influence of tensile and compressive stress at the crack tip are significant and so that the size effect is present. In other words, if the influence of tensile stress at the crack tip grows up, the size effect of concrete increases. And the effect of initial crack length on axial compressive strength is present, however, the differences with crack length are not apparent because the size of fracture process zone (FPZ) of all specimens in the high-strength concrete is similar regardless of differences of specimen slenderness.

• Journal of Korea Trade
• /
• v.26 no.1
• /
• pp.99-112
• /
• 2022

Purpose - This paper empirically investigates via a spatial lag model from the perspective of space economy to find the influencing factors of South Korea's OFDI along with 60 countries. Design/methodology - In the study of regional economic phenomena, we must first test the corresponding spatial correlation, and on this basis, complete the construction of the spatial model. For the target research object, after testing the spatial correlation, if there is spatial correlation, a spatial measurement model is needed. This paper uses the global Moran's I index for calculation. Based on the characteristics and research needs of the research object, this paper selects the spatial lag model to verify the existence of the spatial effect and factors affecting OFDI. Findings - Our results show that export scale, infrastructure, technology level, political stability, resource endowment, market size, distance and labor cost have a certain impact on Korea's OFDI, but at present the distance and market size factors are the most important influencing factors for South Korea's OFDI, The technical level and political stability have little effect on South Korea's OFDI, and are not main factors determining South Korea's OFDI. Originality/value - Through spatial measurement verification, it was found that the spatial effect has a significant impact on OFDI, along with more than 60 countries. On this basis, relevant suggestions are put forward, which have strong practical significance for South Korea's OFDI to achieve healthy and sustainable development.

• KSCE Journal of Civil and Environmental Engineering Research
• /
• v.7 no.2
• /
• pp.29-36
• /
• 1987

In most of the structural members with initial cracks, the strength tends to decrease as the member size increases. This phenomenon is known as size effect. Among the structural materials of glass, metal or concrete, etc., concrete represents the size effect even without initial crack. According to the previous size effect law, the concrete member of very large size can resist little stress. Actually, however, even the large size member can resist some stress if there is no initial notch. This means that the fracture mechanism of very small or very large size member follows strength criterion, but the medium size member follows non-linear fracture mechanics (NLFM). In this study, the empirical models which are derived based on nonlinear fracture mechanics are proposed according to the regression analysis with the existing test data of large size specimens for uni-axial compression test, splitting tensile test and shear test of reinforced concrete beams.

• Structural Engineering and Mechanics
• /
• v.62 no.1
• /
• pp.97-106
• /
• 2017

This paper discusses an experimental programme that was carried out to study the effects of specimen size-shape and type of moulds on the compressive strength of concrete. For this purpose, cube specimens with 150 mm dimensions, cylinder specimens with $150{\times}300mm$ dimensions, and prism specimens with $150{\times}150{\times}375mm$ dimensions were prepared. The experimental programme was carried out with several concrete compositions belonging to strength classes C20/25, C25/30, C30/37, C40/50 and C60/75. Furthermore, the test results were curve-fitted using the least squares method to obtain the new parameters for the modified size effect law.

• Advances in nano research
• /
• v.6 no.2
• /
• pp.93-112
• /
• 2018

An analytical solution of the buckling governing equations of functionally graded piezoelectric (FGP) nanobeams obtained by using a developed third-order shear deformation theory is presented. Electro-mechanical properties of FGP nanobeam are supposed to change continuously in the thickness direction based on power-law model. To capture the small size effects, Eringen's nonlocal elasticity theory is adopted. Employing Hamilton's principle, the nonlocal governing equations of a FG nanobeams made of piezoelectric materials are obtained and they are solved using Navier-type analytical solution. Results are provided to show the effect of different external electric voltage, power-law index, nonlocal parameter and slenderness ratio on the buckling loads of the size-dependent FGP nanobeams. The accuracy of the present model is verified by comparing it with nonlocal Timoshenko FG beams. So, this study makes the first attempt for analyzing buckling behavior of higher order shear deformable FGP nanobeams.

• Advances in nano research
• /
• v.7 no.1
• /
• pp.63-75
• /
• 2019

This article represents a quasi-3D theory for the buckling investigation of magneto-electro-elastic functionally graded (MEE-FG) nanoplates. All the effects of shear deformation and thickness stretching are considered within the presented theory. Magneto-electro-elastic material properties are considered to be graded in thickness direction employing power-law distribution. Eringen's nonlocal elasticity theory is exploited to describe the size dependency of such nanoplates. Using Hamilton's principle, the nonlocal governing equations based on quasi-3D plate theory are obtained for the buckling analysis of MEE-FG nanoplates including size effect and they are solved applying analytical solution. It is found that magnetic potential, electric voltage, boundary conditions, nonlocal parameter, power-law index and plate geometrical parameters have significant effects on critical buckling loads of MEE-FG nanoscale plates.

• Proceedings of the Korea Concrete Institute Conference
• /
• 2003.11a
• /
• pp.285-288
• /
• 2003

The compressive strength of concrete is used as the most fundamental and important material property when reinforced concrete structures are designed. It has been problem to use this value, however, because the control specimen sizes and shapes are different from every country. In this study, the effect of specimen shapes and sizes on compressive strength of concrete members was experimentally investigated based on fracture mechanics. Experiments for the mode I failure was earned out by using cylinder, cube, and prism specimens. The test results are curve fitted using least square method(LSM) to obtain the new parameters for the modified size effect law(MSEL). The analysis results show that the effect of specimen sizes and shapes on ultimate strength is apparent. The results also show stronger size effect in member when the casting direction is perpendicular to loading direction