Bekir Akgöz

Mechanical Engineering, Civil Engineering

PhD. Candidate
24.86

Publications

  • Bekir Akgöz, Ömer Civalek
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    ABSTRACT: A new non-classical sinusoidal plate model is developed on the basis of modified strain gradient theory. This model takes into account the effects of shear deformation without any shear correction factors and also can capture the size effects due to additional material length scale parameters. The governing equations and corresponding boundary conditions for bending, buckling, and free vibration analysis of the microplate are derived by implementing Hamilton’s principle. Analytical solutions based on the Fourier series solution are presented for simply supported square microplates. A detailed parametric study is performed to demonstrate the influences of thickness-to-length scale parameter ratio, length-to-thickness ratio, and shear deformation on deflection, critical buckling load, and fundamental frequencies of microplates. It is observed that the effect of shear deformation becomes more significant for smaller values of length-to-thickness ratio.
    Acta Mechanica 07/2015; 226(7). DOI:10.1007/s00707-015-1308-4 · 1.47 Impact Factor
  • Bekir Akgöz, Ömer Civalek
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    ABSTRACT: A new size-dependent beam model is introduced on the basis of hyperbolic shear deformation beam and modified strain gradient theory. The governing differential equations and corresponding boundary conditions are obtained with the aid of minimum total potential energy principle. The static bending and buckling behaviors of simply supported microbeams embedded in an elastic medium are investigated. The interactions between the microbeam and elastic medium are simulated by Winkler foundation model. Navier solution procedure is employed to obtain analytical solutions for deflections under sinusoidal load and critical buckling loads. The effects of material length scale parameter, length-to-thickness ratio, shear correction factors and Winkler modulus on the bending and buckling responses of microbeams are discussed in detail. The results are comparatively presented with the results of other beam theories. It is observed that the new results predicted by the present model and the results evaluated by sinusoidal shear deformation beam model are in good agreement.
    International Journal of Mechanical Sciences 05/2015; 12. DOI:10.1016/j.ijmecsci.2015.05.003 · 2.06 Impact Factor
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    ABSTRACT: Modal analysis of tapered piles embedded in elastic foundations is investigated. The pile is modeled via Bernoulli-Euler beam theory and discrete singular convolution is used for modeling. Some parametric results have been presented for tapered pile in elastic foundation.
  • Bekir Akgöz, Ömer Civalek
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    ABSTRACT: Nonlocal elasticity theory is presented for the free vibration and bending analysis of nano scaled graphene sheets having sector shape. An eight-node curvilinear domain is used for transformation of the governing equation of motion of sector graphene from physical region to computational region in conjunctions with the Kirchhoff plate theory. The discrete singular convolution (DSC) method is employed for numerical solution of resulting nonlocal governing differential equation and related boundary conditions. Then, the effects of nonlocal parameter, mode numbers, sector angle and radius ratio on the static and vibration results of nano-scaled sector shaped graphene sheets are discussed.
    Mechanics of Advanced Materials and Structures 01/2015; DOI:10.1080/15376494.2014.984089 · 0.66 Impact Factor
  • International Conference on Advances in Composite Materials and Structures; 01/2015
  • Bekir Akgöz, Ömer Civalek
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    ABSTRACT: In this paper, mechanical responses of isolated microtubules are investigated. Microtubules can be defined as bio-composite structures that are a component of the cytoskeleton in eukaryotic cells and play important roles in cellular processes. They have superior mechanical properties such as high rigidity and flexibility. In order to model the microtubules such as a hollow beam, a trigonometric shear deformation beam model is employed on the basis of modified strain gradient theory. The governing equations and related boundary conditions are derived by implementing Hamilton’s principle. A detailed parametric study is performed to investigate the influences of shear deformation, material length scale parameter-to-outer radius ratio, aspect ratio and shear modulus ratio on mechanical responses of microtubules. It is observed that microstructure-dependent behavior is more considerable when material length scale parameters are closer to the outer diameter of microtubules. Also, it can be stated that effects of shear deformation become more significant for smaller shear modulus and aspect ratios.
    Composite Structures 12/2014; 118:9–18. DOI:10.1016/j.compstruct.2014.07.029 · 3.32 Impact Factor
  • Bekir Akgöz, Ömer Civalek
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    ABSTRACT: Thermo-mechanical size-dependent buckling analysis of embedded functionally graded (FG) microbeams is performed based on sinusoidal shear deformation beam and modified couple stress theories. It is assumed that material properties vary smoothly and continuously throughout the thickness. Winkler elastic foundation model is used to simulate the interaction between FG microbeam and elastic medium. The governing equations and corresponding boundary conditions are obtained with the aid of minimum total potential energy principle. The buckling characteristics of simply supported embedded FG microbeams in thermal environment are investigated. The obtained results are compared with the results of simple beam theory with no shear deformation effects and classical theory. Influences of thickness-to-material length scale parameter ratio, material property gradient index, slenderness ratio, temperature change and Winkler parameter on critical buckling loads of embedded FG microbeams are discussed in detail.
    International Journal of Engineering Science 12/2014; 85:90–104. DOI:10.1016/j.ijengsci.2014.08.011 · 2.29 Impact Factor
  • Hakan Ersoy, Bekir Akgöz, Ömer Civalek
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    ABSTRACT: Static analyses of laminated conical and cylindrical shell are presented. The governing equation is derived using Love’s first approximation thin shell theory for conical geometry. Then, the method of Discrete Singular Convolution (DSC) is applied to the solution of the resulting governing equation and boundary conditions for bending. Numerical results for stress and deflections of laminated conical and cylindrical shells are presented for different geometric and material parameters. The numerical results show that the present method is quite easy to implement, accurate and efficient for the problems considered.
    KSCE Journal of Civil Engineering 06/2014; 18(5):1455-1463. DOI:10.1007/s12205-014-0314-8 · 0.51 Impact Factor
  • Bekir Akgöz, Ömer Civalek
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    ABSTRACT: A shear deformation beam model and new shear correction factors are presented for nonhomogeneous microbeams. The governing equations and corresponding boundary conditions in bending and buckling are obtained by implementing minimum total potential energy principle. Bending and buckling problems of a simply supported functionally graded microbeam are analytically solved by Navier solution procedure. Several comparative results are given for different material property gradient index, thickness-to-material length scale parameter ratio (or vice versa), slenderness ratio and shear correction factors. It is observed that size effect and shear deformation are more significant for lower values of thickness-to-material length scale parameter and slenderness ratios, respectively.
    Composite Structures 06/2014; 112. DOI:10.1016/j.compstruct.2014.02.022 · 3.32 Impact Factor
  • Bekir Akgöz, Ömer Civalek
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    ABSTRACT: In this paper, a new microstructure-dependent sinusoidal beam model for buckling of microbeams is presented using modified strain gradient theory. This microbeam model can take into consideration microstructural and shear deformation effects. The equilibrium equations and corresponding boundary conditions in buckling are derived with the minimum total potential energy principle. Buckling problem of a simply supported microbeam subjected to an axial compressive force is analytically solved by Navier solution procedure. Influences of thickness-to-length scale parameter and slenderness ratios on buckling behavior are discussed in detail. It is observed that the size dependency becomes more important when the thickness of the microbeam is closer to material length scale parameter. Also, it can be said that the effects of shear deformation are more considerable for short and thick beams with lower slenderness ratios.
    International Journal of Mechanical Sciences 04/2014; 81. DOI:10.1016/j.ijmecsci.2014.02.013 · 2.06 Impact Factor
  • Bekir Akgöz, Ömer Civalek
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    ABSTRACT: The longitudinal free vibration problem of a micro-scaled bar is formulated using the strain gradient elasticity theory. The equation of motion together with initial conditions, classical and non-classical corresponding boundary conditions for a micro-scaled elastic bar is derived via Hamilton's principle. The resulting higher-order equation is solved for clamped-clamped and clamped-free boundary conditions. Effects of the additional length scale parameters on the frequencies are investigated. It is observed that size effect is more significant when the ratio of the microbar diameter to the additional length scale parameter is small. It is also observed that the difference between natural frequencies predicted by current and classical models becomes more prominent for both lower values of slenderness ratio of the microbar and for higher modes.
    Journal of Vibration and Control 03/2014; 20(4):606-616. DOI:10.1177/1077546312463752 · 4.36 Impact Factor
  • Bekir Akgöz, Ömer Civalek
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    ABSTRACT: Longitudinal free vibration analysis of axially functionally graded microbars is investigated on the basis of strain gradient elasticity theory. Functionally graded materials can be defined as nonhomogeneous composites which are obtained by combining of two different materials in order to obtain a new desired material. In this study, material properties of microbars are assumed to be smoothly varied along the axial direction. Rayleigh-Ritz solution technique is utilized to obtain an approximate solution to the free longitudinal vibration problem of strain gradient microbars for clamped-clamped and clamped-free boundary conditions. A parametric study is carried out to show the influences of additional material length scale parameters, material ratio, slenderness ratio and ratio of Young's modulus on natural frequencies of axially functionally graded microbars.
    Composites Part B Engineering 12/2013; 55:263-268. DOI:10.1016/j.compositesb.2013.06.035 · 2.98 Impact Factor
  • Bekir Akgöz, Ömer Civalek
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    ABSTRACT: The buckling problem of linearly tapered micro-columns is investigated on the basis of modified strain gradient elasticity theory. Bernoulli-Euler beam theory is used to model the non-uniform micro column. Rayleigh-Ritz solution method is utilized to obtain the critical buckling loads of the tapered cantilever micro-columns for different taper ratios. Some comparative results for the cases of rectangular and circular cross-sections are presented in graphical and tabular form to show the differences between the results obtained by modified strain gradient elasticity theory and those achieved by modified couple stress and classical theories. From the results, it is observed that the differences between critical buckling loads achieved by classical and those predicted by non-classical theories are considerable for smaller values of the ratio of the micro-column thickness (or diameter) at its bottom end to the additional material length scale parameters and the differences also increase due to increasing of the taper ratio.
    Structural Engineering & Mechanics 10/2013; 48(2):195-205. DOI:10.12989/sem.2013.48.2.195 · 0.80 Impact Factor
  • Bekir Akgöz, Ömer Civalek
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    ABSTRACT: The buckling behavior of size-dependent microbeams made of functionally graded materials (FGMs) for different boundary conditions is investigated on the basis of Bernoulli–Euler beam and modified strain gradient theory. The higher-order governing differential equation for buckling with all possible classical and non-classical boundary conditions is obtained by a variational statement. The effects of the power of the material property variation function, boundary conditions, slenderness ratio, ratio of additional material length scale parameters for two constituents, beam thickness-to-additional material length scale parameter ratio on the buckling response of FGM microbeams are investigated. Some comparative results are presented in tabular and graphical form in order to show the differences between the results obtained by the present model and those predicted by modified couple stress and classical continuum models.
    Acta Mechanica 09/2013; 224(9). DOI:10.1007/s00707-013-0883-5 · 1.47 Impact Factor
  • Ömer Civalek, Bekir Akgöz
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    ABSTRACT: Free vibration analysis of micro-scaled annular sector and sector shaped graphene located on an elastic matrix are studied via nonlocal elasticity theory. An eight-node curvilinear element is used for transformation of the governing equation of motion of annular sector graphene from physical region to computational region in conjunctions with the thin plate theory. Elastic matrix is modeled via two-parameters which are Winkler–Pasternak elastic foundations. The discrete singular convolution (DSC) method is employed for numerical solution of resulting nonlocal governing differential equations and related boundary conditions. Then, the effects of nonlocal parameter, mode numbers, sector angles and foundation parameters on the frequency response of micron-scaled annular sector and sector graphene are discussed.
    Computational Materials Science 09/2013; 77:295–303. DOI:10.1016/j.commatsci.2013.04.055 · 2.13 Impact Factor
  • Bekir Akgöz, Ömer Civalek
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    ABSTRACT: A new size-dependent higher-order shear deformation beam model is developed based on modified strain gradient theory. The model captures both the microstructural and shear deformation effects without the need for any shear correction factors. The governing equations and boundary conditions are derived by using Hamilton's principle. The static bending and free vibration behavior of simply supported microbeams are investigated. Analytical solutions including Poisson effect for deflections under point and uniform loads and for first three natural frequencies are obtained by Navier solution. The results are compared with other beam theories and other classical and non-classical models. A detailed parametric study is carried out to show the influences of thickness-to-material length scale parameter ratio, slenderness ratio and shear deformation on deflections and natural frequencies of microbeams. It is observed that effect of shear deformation becomes more significant for both smaller slenderness ratios and higher modes.
    International Journal of Engineering Science 09/2013; 70:1-14. DOI:10.1016/j.ijengsci.2013.04.004 · 2.29 Impact Factor
  • Bekir Akgöz, Ömer Civalek
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    ABSTRACT: Analytical solutions for bending, buckling, and vibration of micro-sized plates on elastic medium using the modified couple stress theory are presented. The governing equations for bending, buckling and vibration are obtained via Hamilton’s principles in conjunctions with the modified couple stress and Kirchhoff plate theories. The surrounding elastic medium is modeled as the Winkler elastic foundation. Navier’s method is being employed and analytical solutions for the bending, buckling and free vibration problems are obtained. Influences of the elastic medium and the length scale parameter on the bending, buckling, and vibration properties are discussed.
    Meccanica 05/2013; 48(4). DOI:10.1007/s11012-012-9639-x · 1.82 Impact Factor
  • Bekir Akgöz, Ömer Civalek
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    ABSTRACT: In the present study, vibration response of non-homogenous and non-uniform microbeams is investigated in conjunction with Bernoulli–Euler beam and modified couple stress theory. The boundary conditions of the microbeam are considered as fixed at one end and free at the other end. It is taken into consideration that material properties and cross section of the microbeam vary continuously along the longitudinal direction. Rayleigh–Ritz solution method is utilized to obtain an approximate solution to the free transverse vibration problem. A detailed study is carried out to show the effects of material properties and taper ratios on natural frequencies of axially functionally graded tapered microbeams. In order to demonstrate the validity and accuracy of the current analysis, some of present results are compared with previous results in the literature and an excellent agreement is observed between them.
    Composite Structures 04/2013; 98:314–322. DOI:10.1016/j.compstruct.2012.11.020 · 3.32 Impact Factor
  • Bekir Akgöz, Ömer Civalek
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    ABSTRACT: Free vibration of single-layered graphene sheet (SLGS) resting on an elastic matrix as Pasternak foundation model is investigated by using the modified couple stress theory. Governing equation of motion for SLGS is obtained via thin plate theory in conjunction with Hamilton’s principle. All edges simply supported boundary condition is considered. Analytical solution of the resulting equation is obtained via Fourier series approach. Effects of the material length scale parameter and elastic matrix parameters on vibration frequencies of SLGS are investigated. The influence of the mode numbers on frequencies for two-different matrix parameters and aspect ratio of graphene sheet are also studied. Numerical results reveal that the frequency values increase significantly with the increase of the material length scale parameter. It has been shown that scale effects are quite significant on frequencies especially when length and width of the SLGS is smaller and in higher modes of vibration and need to be included in the mechanical modeling of SLGS.
    Materials and Design 12/2012; 42:164–171. DOI:10.1016/j.matdes.2012.06.002 · 3.50 Impact Factor
  • Murat Gürses, Bekir Akgöz, Ömer Civalek
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    ABSTRACT: In the present study, free vibration analysis of nano-sized annular sector plate is analyzed using the nonlocal continuum theory. The method of discrete singular convolution (DSC) is used for numerical computations. Firstly, equation of motion of thin plates is formulated via nonlocal elasticity. Then, irregular physical domain is transformed into a rectangular domain by using geometric coordinate transformation. The DSC procedures are then applied to discretization of the transformed set of governing equations and related boundary conditions. The effects of nonlocal parameter, mode numbers, sector angle and radius ratio on the vibration frequencies are investigated in detail. It is seen that the size effects are significant in vibration analysis of nano-scaled annular sector plates and need to be included in the mechanical model.
    Applied Mathematics and Computation 11/2012; 219(6):3226–3240. DOI:10.1016/j.amc.2012.09.062 · 1.60 Impact Factor

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