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Application of strain gradient elasticity theory for buckling analysis of protein microtubules
Bekir Akgöz,Ömer Civalek 한국물리학회 2011 Current Applied Physics Vol.11 No.5
In this paper, size effect of microtubules (MTs) is studied via modified strain gradient elasticity theory for buckling. MTs are modeled by Bernoulli―Euler beam theory. By using the variational principle, the governing equations for buckling and related boundary conditions are obtained in conjunctions with the strain gradient elasticity. The size effect for buckling analysis of MTs is investigated and results are presented in graph form. The results obtained by strain gradient elasticity theory are discussed through the numerical simulations. The results based on the modified couple stress theory, nonlocal elasticity theory and classical elasticity theories have been also presented for comparison purposes.
Laser cutting of Kevlar laminates: First and second law analysis
Bekir S. Yilbas,Ahmet Z. Sahin,C. Chatwin,Tahir Ayar 대한기계학회 2011 JOURNAL OF MECHANICAL SCIENCE AND TECHNOLOGY Vol.25 No.4
Kevlar laminates are difficult to machine using conventional machining methods because of their thermal and mechanical properties. Laser cutting offers advantages over conventional machining methods, such as precision of operation, non-frictional processing, and operational cost. This provides the motivation for the present study, which reports on laser cutting of Kevlar laminates of different thicknesses. The first and second law efficiencies of the cutting process are formulated and predicted in line with the experimental parameters. The laser cut surfaces are examined using optical and electron scanning microscopes. It is demonstrated that the first and second law efficiencies improve at high laser cutting speeds and low laser output power levels. For these conditions parallel sided kerfs with no sideways burning are produced.
Bekir Akgöz 국제구조공학회 2019 Steel and Composite Structures, An International J Vol.33 No.1
In the present study, microstructure-dependent static stability analysis of inhomogeneous tapered micro-columns is performed. It is considered that the micro column is made of functionally graded materials and has a variable cross-section. The material and geometrical properties of micro column vary continuously throughout the axial direction. Euler-Bernoulli beam and modified couple stress theories are used to model the nonhomogeneous micro column with variable cross section. Rayleigh-Ritz solution method is implemented to obtain the critical buckling loads for various parameters. A detailed parametric study is performed to examine the influences of taper ratio, material gradation, length scale parameter, and boundary conditions. The validity of the present results is demonstrated by comparing them with some related results available in the literature. It can be emphasized that the size-dependency on the critical buckling loads is more prominent for bigger length scale parameter-to-thickness ratio and changes in the material gradation and taper ratio affect significantly the values of critical buckling loads.
Bekir Akgöz,Ömer Civalek 국제구조공학회 2011 Steel and Composite Structures, An International J Vol.11 No.5
In the present manuscript, geometrically nonlinear free vibration analysis of thin laminated plates resting on non-linear elastic foundations is investigated. Winkler-Pasternak type foundation model is used. Governing equations of motions are obtained using the von Karman type nonlinear theory. The method of discrete singular convolution is used to obtain the discretised equations of motion of plates. The effects of plate geometry, boundary conditions, material properties and foundation parameters on nonlinear vibration behavior of plates are presented.
Buckling analysis of linearly tapered micro-columns based on strain gradient elasticity
Bekir Akgöz,Ö mer Civalek 국제구조공학회 2013 Structural Engineering and Mechanics, An Int'l Jou Vol.48 No.2
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.
Bekir Sami Yilbas,Syed Sohail Akhtar,Omer Keles,Kurtulus Boran 대한기계학회 2015 JOURNAL OF MECHANICAL SCIENCE AND TECHNOLOGY Vol.29 No.8
Laser cutting of a triangular geometry into aluminum 2024 alloy is carried out. Thermal stress field in the cutting section is predictedusing the finite element code ABAQUS. Surface temperature predictions are validated through the thermocouple data. Morphologicalchanges in the cut section are examined incorporating optical and electron scanning microscopes. The effects of the size of the triangulargeometry on thermal stress field are also examined. It is found that surface temperature predictions agree well with thermocouple data. von Mises stress remains high in the region close to the corners of the triangular geometry, which is more pronounced for the small sizetriangle. This behavior is associated with the occurrence of the high cooling rates in this region. Laser cut edges are free from large scalesideways burning and large size burr attachments. However, some locally scattered dross attachments are observed at the kerf exit.
Laser short-pulse heating of metallic surface: Consideration of Seebeck effect
Bekir Sami Yilbas 한국물리학회 2009 Current Applied Physics Vol.9 No.3
In the present study, laser short-pulse heating is formulated using an electron kinetic theory approach. Temperature predictions are compared with that obtained from two-equation model. Seebeck effect is considered during the heating process. The predicted Seebeck coefficients are compared with the results based on the early formulation. Electron excess energy loss due to Seebeck effect is compared with electron mean energy. It is found that Seebeck coefficient decays sharply in the surface region due to sharp decay of electron temperature in this region. Seebeck coefficient obtained from the present study is in agreement with the predictions based on the early formulation, provided electron temperature is used in the previous formulation. However, Seebeck coefficient differs significantly once the lattice site temperature is used in the previous formulation. Electron excess energy loss due to Seebeck effect is considerably less than electron mean energy, i.e. the ratio is in the order of 10-5. In the present study, laser short-pulse heating is formulated using an electron kinetic theory approach. Temperature predictions are compared with that obtained from two-equation model. Seebeck effect is considered during the heating process. The predicted Seebeck coefficients are compared with the results based on the early formulation. Electron excess energy loss due to Seebeck effect is compared with electron mean energy. It is found that Seebeck coefficient decays sharply in the surface region due to sharp decay of electron temperature in this region. Seebeck coefficient obtained from the present study is in agreement with the predictions based on the early formulation, provided electron temperature is used in the previous formulation. However, Seebeck coefficient differs significantly once the lattice site temperature is used in the previous formulation. Electron excess energy loss due to Seebeck effect is considerably less than electron mean energy, i.e. the ratio is in the order of 10-5.