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Papers by Vahit Mermertas

Research paper thumbnail of Optimal design of manipulator with four-bar mechanism

Mechanism and Machine Theory, 2004

In this paper, optimal kinematic design of planar manipulator with four-bar mechanism is presente... more In this paper, optimal kinematic design of planar manipulator with four-bar mechanism is presented. For the parallel manipulator, there is only one location defined between force and motion where local mobility index equals one. In this work, optimum link measurements of the manipulator that maximize the local mobility index depending on the input link location have been found and design charts for the optimum manipulator design have been obtained. As a result of this, it is shown that based on the determined link measurements, performance of the manipulator can be maximum not only for a certain position, but also for a position interval, as well. Additionally, better conditions have been obtained in terms of force-motion relationship compared to the parallel manipulator also at the positions where local mobility index is different than one.

Research paper thumbnail of A study on annular plates with radial through cracks by means of sector type element

Journal of Sound and Vibration, 2007

In this study, the natural frequencies of annular plates with periodic radial through cracks are ... more In this study, the natural frequencies of annular plates with periodic radial through cracks are investigated by means of finite element method. The position of the crack in the plate surface was selected as parallel to radial direction. Both the cracks emanating from the inner and outer boundary of the plate is non-propagating and open. The crack in the element was modeled by an additional flexibility matrix, the terms of which were calculated using fracture mechanics. For this purpose, an isoparametric sector type element with radial through crack of four nodes and three degrees of freedom at each node is considered. In the event of the selection of appropriate half sector angle, taking into account the closeness of the geometries of the sector and trapezoidal type element, the flexibility matrix of the sector type element with crack are derived by means of the flexibility matrix of the trapezoidal type element. The natural frequencies of annular plates are found for different length and number of cracks. The theoretical results are obtained with different boundary conditions. The obtained results of improved elements are compared with experimental results in the literature. It is observed that the increment of the length and number of cracks has various effects on the natural frequencies and mode shapes of the annular plate.

Research paper thumbnail of Longitudinal Vibrations of Rods Coupled by a Double Spring-Mass System

Journal of Sound and Vibration, 1997

Research paper thumbnail of Stability of variably thick orthotropic annular plates

International Journal of Mechanical Sciences, 1994

Al~tract-The static and dynamic stability of variably thick orthotropic annular plates subjected ... more Al~tract-The static and dynamic stability of variably thick orthotropic annular plates subjected to in-plane periodic forces with respect to time is studied. It is assumed that the periodic radial forces with the same period act along both the inner and outer edges of plate. The finite element method is used with a sector element based on the Mindlin plate theory. The wave propagation technique of cyclic symmetry is used in the formulation. The choice of the element enables the analyst to solve the axi-symmctric and asymmetric dynamic stability problem at the same time. Variations in the plate thickness in which it increases or decreases in the radial direction according to the equations h = h,~,(r/ro) +~ or h = h,~,(r/r~)-~, respectively are considered. The instability regions for different boundary conditions are determined by Bolotin's method. It is found that the critical buckling load, buckling mode and instability regions are changed by variations in the polar orthotropic material properties.

Research paper thumbnail of Natural frequencies of annular plates with circumferential cracks by means of sector element

Engineering Fracture Mechanics, 2008

In this study, the natural frequencies of annular plates with circumferential cracks are investig... more In this study, the natural frequencies of annular plates with circumferential cracks are investigated by using finite element method. The cracks were non-propagating and open. In the event of the selection of appropriate half sector angle, taking into account the closeness of the geometries of the sector and trapezoidal element, the flexibility matrix of the sector element with crack was obtained by the flexibility matrix of the trapezoidal element. Also, because of small half sector angles, the circumferential cracks in the elements were assumed to be straight cracks. The results of this study with improved elements are compared with the theoretical results in the literature. It is observed that the location and the number of cracks have various effects on the natural frequencies related to the vibration modes.

Research paper thumbnail of Dynamic stability of radially stiffened annular plates

Computers & Structures, 1991

... stiffeners on the annular plates can be lost when the plate is subjected to a parametric exit... more ... stiffeners on the annular plates can be lost when the plate is subjected to a parametric exitation. ... that the instability region is moved towards a lower frequency as a whole and the instability region is ... RC Duffield and N. Willems, Parametric resonance of stiffened rectangular plates ...

Research paper thumbnail of Dynamic interaction between the vehicle and simply supported curved bridge deck

Computer Methods in Applied Mechanics and Engineering, 1998

ABSTRACT Vehicle—guideway interaction for a vehicle on simply-supported curved bridge decks is st... more ABSTRACT Vehicle—guideway interaction for a vehicle on simply-supported curved bridge decks is studied with the effects of vehicle speed and guideway parameters taken into account. In the dynamic model, the vehicle is considered to have four-degrees of freedom, with lumped masses, linear springs and dampers. Although the material and geometric linearities are preserved in the formulation, the consideration of interaction between the vehicle and the bridge results in a nonlinear structural response. The finite element method is used for solving the governing differential equation of the vehicle—bridge system. In this paper, multi-predictor—corrector procedure in connection with the Newmark method, is employed for the solution of the resulting matrix ordinary differential equations. The deflection at the middle of the beam is determined for different vehicle speed and different radius of curvatures of the curved deck.

Research paper thumbnail of Optimal design of manipulator with four-bar mechanism

Mechanism and Machine Theory, 2004

In this paper, optimal kinematic design of planar manipulator with four-bar mechanism is presente... more In this paper, optimal kinematic design of planar manipulator with four-bar mechanism is presented. For the parallel manipulator, there is only one location defined between force and motion where local mobility index equals one. In this work, optimum link measurements of the manipulator that maximize the local mobility index depending on the input link location have been found and design charts for the optimum manipulator design have been obtained. As a result of this, it is shown that based on the determined link measurements, performance of the manipulator can be maximum not only for a certain position, but also for a position interval, as well. Additionally, better conditions have been obtained in terms of force-motion relationship compared to the parallel manipulator also at the positions where local mobility index is different than one.

Research paper thumbnail of A study on annular plates with radial through cracks by means of sector type element

Journal of Sound and Vibration, 2007

In this study, the natural frequencies of annular plates with periodic radial through cracks are ... more In this study, the natural frequencies of annular plates with periodic radial through cracks are investigated by means of finite element method. The position of the crack in the plate surface was selected as parallel to radial direction. Both the cracks emanating from the inner and outer boundary of the plate is non-propagating and open. The crack in the element was modeled by an additional flexibility matrix, the terms of which were calculated using fracture mechanics. For this purpose, an isoparametric sector type element with radial through crack of four nodes and three degrees of freedom at each node is considered. In the event of the selection of appropriate half sector angle, taking into account the closeness of the geometries of the sector and trapezoidal type element, the flexibility matrix of the sector type element with crack are derived by means of the flexibility matrix of the trapezoidal type element. The natural frequencies of annular plates are found for different length and number of cracks. The theoretical results are obtained with different boundary conditions. The obtained results of improved elements are compared with experimental results in the literature. It is observed that the increment of the length and number of cracks has various effects on the natural frequencies and mode shapes of the annular plate.

Research paper thumbnail of Longitudinal Vibrations of Rods Coupled by a Double Spring-Mass System

Journal of Sound and Vibration, 1997

Research paper thumbnail of Stability of variably thick orthotropic annular plates

International Journal of Mechanical Sciences, 1994

Al~tract-The static and dynamic stability of variably thick orthotropic annular plates subjected ... more Al~tract-The static and dynamic stability of variably thick orthotropic annular plates subjected to in-plane periodic forces with respect to time is studied. It is assumed that the periodic radial forces with the same period act along both the inner and outer edges of plate. The finite element method is used with a sector element based on the Mindlin plate theory. The wave propagation technique of cyclic symmetry is used in the formulation. The choice of the element enables the analyst to solve the axi-symmctric and asymmetric dynamic stability problem at the same time. Variations in the plate thickness in which it increases or decreases in the radial direction according to the equations h = h,~,(r/ro) +~ or h = h,~,(r/r~)-~, respectively are considered. The instability regions for different boundary conditions are determined by Bolotin's method. It is found that the critical buckling load, buckling mode and instability regions are changed by variations in the polar orthotropic material properties.

Research paper thumbnail of Natural frequencies of annular plates with circumferential cracks by means of sector element

Engineering Fracture Mechanics, 2008

In this study, the natural frequencies of annular plates with circumferential cracks are investig... more In this study, the natural frequencies of annular plates with circumferential cracks are investigated by using finite element method. The cracks were non-propagating and open. In the event of the selection of appropriate half sector angle, taking into account the closeness of the geometries of the sector and trapezoidal element, the flexibility matrix of the sector element with crack was obtained by the flexibility matrix of the trapezoidal element. Also, because of small half sector angles, the circumferential cracks in the elements were assumed to be straight cracks. The results of this study with improved elements are compared with the theoretical results in the literature. It is observed that the location and the number of cracks have various effects on the natural frequencies related to the vibration modes.

Research paper thumbnail of Dynamic stability of radially stiffened annular plates

Computers & Structures, 1991

... stiffeners on the annular plates can be lost when the plate is subjected to a parametric exit... more ... stiffeners on the annular plates can be lost when the plate is subjected to a parametric exitation. ... that the instability region is moved towards a lower frequency as a whole and the instability region is ... RC Duffield and N. Willems, Parametric resonance of stiffened rectangular plates ...

Research paper thumbnail of Dynamic interaction between the vehicle and simply supported curved bridge deck

Computer Methods in Applied Mechanics and Engineering, 1998

ABSTRACT Vehicle—guideway interaction for a vehicle on simply-supported curved bridge decks is st... more ABSTRACT Vehicle—guideway interaction for a vehicle on simply-supported curved bridge decks is studied with the effects of vehicle speed and guideway parameters taken into account. In the dynamic model, the vehicle is considered to have four-degrees of freedom, with lumped masses, linear springs and dampers. Although the material and geometric linearities are preserved in the formulation, the consideration of interaction between the vehicle and the bridge results in a nonlinear structural response. The finite element method is used for solving the governing differential equation of the vehicle—bridge system. In this paper, multi-predictor—corrector procedure in connection with the Newmark method, is employed for the solution of the resulting matrix ordinary differential equations. The deflection at the middle of the beam is determined for different vehicle speed and different radius of curvatures of the curved deck.