melih papila | Sabanci University (original) (raw)

Papers by melih papila

Research paper thumbnail of Optimization of clamped circular piezoelectric composite actuators

Sensors and Actuators A: Physical, 2008

This paper addresses the design of clamped circular piezoceramic composite unimorph and bimorph c... more This paper addresses the design of clamped circular piezoceramic composite unimorph and bimorph configurations, specifically the conflicting requirements of maximum volume displacement for a prescribed bandwidth. An optimization problem is formulated that implements analytical solutions for unimorph and bimorph configurations using laminated plate theory, including the use of oppositely polarized piezoceramic patches. A range of actuator geometric parameters are studied, and bounds for volume displacement and natural frequency of optimal designs are determined and presented via design curves. In the selected design space, Pareto optimization results for unimorph and bimorph configurations show that optimal volume displacement is related to the bandwidth by a universal power law such that the product of the square of the natural frequency and the displaced volume, a "gain-bandwidth" product, is a constant. Characteristic trends are also described that are independent of the actuator radius for the Pareto optimal piezoceramic patch thickness and radius versus normalized bandwidth. The results are relevant, for example, in the design of zero-net mass-flux or synthetic jet actuators used in flow control applications.

Research paper thumbnail of Dielectric behavior characterization of a fibrous-ZnO/PVDF nanocomposite

Polymer Composites, 2009

This study is focused on forming a fibrous-zinc oxide/ polyvinylidine fluoride (ZnO/PVDF) nanocom... more This study is focused on forming a fibrous-zinc oxide/ polyvinylidine fluoride (ZnO/PVDF) nanocomposite and characterizing its dielectric behavior. The nanocomposite is prepared in two steps. First, a network of nanoscale diameter ZnO fibers is produced by sintering electrospun PVA/Zinc Acetate fibers. Second, the ZnO fibrous nonwoven mat is sandwiched between two PVDF thermoplastic polymer films by hot-press casting. Scanning electron microscope images of the nanocomposite show that hot-press casting of the fibrous-ZnO network breaks the network up into short fibers. The in-plane distribution of the ZnO fillers (i.e., the short fibers) in the PVDF matrix appears to comply with that of the pristine ZnO fibers before hot-pressing, indicating that the fillers remain well-dispersed in the polymer matrix. To the authors' knowledge, the work reported herein is the first demonstration of the use of electrospinning to secure the dispersion and distribution of a network of inorganic fillers. Moreover, processing a fibrous-ZnO/PVDF flexible composite as described in this report would facilitate material handling and enable dielectric property measurement, in contrast to that on a fibrous mat of pure ZnO. Because of the high surface area of the short ZnO fibers and their polycrystalline structure, interfacial polarization is pronounced in the nanocomposite film. The dielectric constant is enhanced significantly-up to a factor of 10 at low frequencies compared to the dielectric constant of constituent materials (both bulk ZnO and PVDF), and up to a factor of two compared to a bulk-ZnO/PVDF composite. POLYM. COMPOS.

Research paper thumbnail of Response approximations - Noise, error repair, modeling errors

Research paper thumbnail of Error-based design space windowing

40th AIAA Aerospace Sciences Meeting & Exhibit, 2002

Windowing of design space is considered in order to reduce the bias errors due to low-order polyn... more Windowing of design space is considered in order to reduce the bias errors due to low-order polynomial response surfaces (RS). Standard design space windowing (DSW) uses a region of interest by setting a requirement on response level and checks it by a global RS predictions over the design space. This approach, however, is vulnerable since RS modeling errors may lead to the wrong region to zoom on. The approach is modified by introducing an eigenvalue error measure based on point-topoint mean squared error criterion. Two examples are presented to demonstrate the benefit of the error-based DSW.

Research paper thumbnail of Post-Buckling of Composite I-Sections. Part 2: Experimental Validation

Journal of Composite Materials, 2001

Page 1. http://jcm.sagepub.com/ Materials Journal of Composite http://jcm.sagepub. com/content/35... more Page 1. http://jcm.sagepub.com/ Materials Journal of Composite http://jcm.sagepub. com/content/35/9/797 The online version of this article can be found at: DOI: 10.1177/002199801772662497 2001 35: 797 Journal of Composite ...

Research paper thumbnail of Post-Buckling of Composite I-Sections. Part 2: Experimental Validation

Journal of Composite Materials, 2001

Page 1. http://jcm.sagepub.com/ Materials Journal of Composite http://jcm.sagepub. com/content/35... more Page 1. http://jcm.sagepub.com/ Materials Journal of Composite http://jcm.sagepub. com/content/35/9/797 The online version of this article can be found at: DOI: 10.1177/002199801772662497 2001 35: 797 Journal of Composite ...

Research paper thumbnail of Post-Buckling of Composite I-Sections. Part 1: Theory

Journal of Composite Materials, 2001

... mx L f g dy p N a K Page 7. tal strain energy due to bending of the plates to the total work ... more ... mx L f g dy p N a K Page 7. tal strain energy due to bending of the plates to the total work done by the edge forces on the plates. The buckling coefficient expression was used to create buck-ling coefficient vs. ... 780 MELIH PAPILA AND MEHMET A. AKGÜN ...

Research paper thumbnail of Post-Buckling of Composite I-Sections. Part 1: Theory

Journal of Composite Materials, 2001

... mx L f g dy p N a K Page 7. tal strain energy due to bending of the plates to the total work ... more ... mx L f g dy p N a K Page 7. tal strain energy due to bending of the plates to the total work done by the edge forces on the plates. The buckling coefficient expression was used to create buck-ling coefficient vs. ... 780 MELIH PAPILA AND MEHMET A. AKGÜN ...

Research paper thumbnail of Pointwise Bias Error Bounds and Min?Max Design for Response Surface Approximations

Aiaa Journal, 2005

Two approaches addressing response surface approximation errors due to model inadequacy (bias err... more Two approaches addressing response surface approximation errors due to model inadequacy (bias error) are presented, and a design of experiments minimizing the maximal bias error is proposed. Both approaches assume that the functional form of the true model is known and seek, at each point in design space, worst case bounds on the absolute error. The first approach is implemented prior to data generation. This data independent error bound can identify locations in the design space where the accuracy of the approximation fitted on a given design of experiments may be poor. The data independent error bound can easily be implemented in a search for a design of experiments that minimize the bias error bound as it requires very little computation.

Research paper thumbnail of Uncertainty and Wing Structural Weight Approximations

Research paper thumbnail of Equivalent” Electromechanical Coefficient for IPMC Actuator Design Based on Equivalent Bimorph Beam Theory

Experimental Mechanics, 2010

This paper addresses an “equivalent” electromechanical coupling coefficient that may be used in d... more This paper addresses an “equivalent” electromechanical coupling coefficient that may be used in designing Ionic Polymer Metal Composite (IPMC) actuators. The coefficient is not a material constant and derived from equivalent bimorph beam model. The collective effect of the membrane thickness and operating voltage on the coefficient is demonstrated by using a design of experiment (DOE) of three and five levels of the two factors, respectively. Experiments and linear finite element analyses with MD.NASTRAN at DOE points are performed. The tip displacement and the coupling coefficient are reported and their response surface (RS) approximations as function of the thickness and voltage are constructed. Experiments and RS predictions indicate that actuator thickness and applied voltage are two interacting major factors for maximum tip displacement. The equivalent coupling coefficient is primarily driven by the thickness of actuator moreover voltage appears to contribute as the thickness increases. The initial curvature of the strips before electrical excitation is also shown to be a factor for “equivalent” coupling coefficient, it is not, however sufficient to explain the variation in the experimental data. A correction factor approach is proposed and applied to the straight beam tip displacement RS that filters out experimental variation. A corrected RS enables including the pre-imposed initial curvature as design parameter along with the actuator thickness and the operating peak voltage when predicting the tip displacement and the equivalent coupling coefficient.

Research paper thumbnail of Engineering Chemistry of Electrospun Nanofibers and Interfaces in Nanocomposites for Superior Mechanical Properties

Acs Applied Materials & Interfaces, 2010

The novelty of this work is based on designing the chemistry of the electrospun nanofibers, so th... more The novelty of this work is based on designing the chemistry of the electrospun nanofibers, so that the resultant composites substantially benefit from cross-linking between the nanofibers and the polymer matrix. Specifically, the solution of in-house synthesized copolymers polystyrene-co-glycidyl methacrylate P(St-co-GMA) is electrospun to produce mats of surface reactive nanoto-submicron scale fibers that are accompanied later by spraying over the ethylenediamine (EDA) as a supplementary cross-linking agent for epoxy. The P(St-co-GMA)/EDA fiber mats are then embedded into an epoxy resin. Analysis of the three-point-bending mode of the composites reveals that the storage modulus of P(St-co-GMA)/EDA nanofiber-reinforced epoxy are about 10 and 2.5 times higher than that of neat and P(St-co-GMA) nanofiber-reinforced epoxy, respectively, even though the weight fraction of the nanofibers was as low as 2 wt %. The significant increase in the mechanical response is attributed to the inherently cross-linked fiber structure and the surface modification/chemistry of the electrospun fibers, that results in cross-linked polymer matrix-nanofiber interfacial bonding.

Research paper thumbnail of The effect of IPMC parameters in electromechanical coefficient based on equivalent beam theory

"Effective" electromechanical coupling coefficient for IPMC by equivalent bimorph beam model is s... more "Effective" electromechanical coupling coefficient for IPMC by equivalent bimorph beam model is studied. The collective effect of the membrane thickness and operating voltage is demonstrated by using a design of experiment of three and four levels of the two factors, respectively. Experiments and finite element analyses using MSC.NASTRAN are used to evaluate the tip displacement and the coupling coefficient for which approximations as function of the thickness and voltage are constructed. Initial curvature of the strips before electrical excitation is also shown to be a factor in "effective" coupling coefficient. A correction factor approach is proposed to include the effect of the preimposed curvature.

Research paper thumbnail of Effects of electrospinning parameters on polyacrylonitrile nanofiber diameter: An investigation by response surface methodology

Materials & Design, 2008

Effects of material and process parameters on the diameter of electrospun polyacrylonitrile fiber... more Effects of material and process parameters on the diameter of electrospun polyacrylonitrile fibers were experimentally investigated. Response surface methodology (RSM) was utilized to design the experiments at the settings of solution concentration, voltage and the collector distance. It also imparted the evaluation of the significance of each parameter on the resultant fiber diameter. The investigations were carried out in the two-variable process domains of several collector distances as applied voltage and the solution concentration were varied at a fixed polymer molecular weight. The mean diameter and coefficient of variation were modeled by polynomial response surfaces as functions of solution concentration and voltage at each collector distance. Effect of applied voltage in micron-scale fiber diameter was observed to be almost negligible when solution concentration and collector distance were high. However, all three factors were found statistically significant in the production of nano-scale fibers. The response surface predictions revealed the parameter interactions for the resultant fiber diameter, and showed that there is a negative correlation between the mean diameter and coefficient of variation for the fiber diameter. A sub-domain of the parameter space consisting of the solution concentration, applied voltage and collector distance, was suggested for the potential nano-scale fiber production. (O.S. Yö rdem), mpapila@sabanciuniv.edu (M. Papila), yusufm@sabanciuniv.edu (Y.Z. Menceloglu). 1 Tel.: +90 216 483 9000/2079; fax: +90 216 483 9550. 2 Tel.: +90 216 483 9535; fax: +90 216 483 9550. www.elsevier.com/locate/matdes Materials and Design xxx (2007) xxx-xxx Materials & Design ARTICLE IN PRESS Please cite this article in press as: Yö rdem OS et al., Effects of electrospinning parameters on polyacrylonitrile .

Research paper thumbnail of Poly(vinylidene fluoride)/zinc oxide smart composite material

This work aimed at fabrication and electromechanical characterization of a smart material system ... more This work aimed at fabrication and electromechanical characterization of a smart material system composed of electroactive polymer and ceramic materials. The idea of composite material system is on account of complementary characteristics of the polymer and ceramic for flexibility and piezoelectric activity. Our preliminary work included Polyvinylidene Fluoride (PVDF) as the flexible piezoelectric polymer, and Zinc Oxide (ZnO) as the piezoelectric ceramic brittle, but capable to respond strains without poling. Two alternative processes were investigated. The first process makes use of ZnO fibrous formation achieved by sintering PVA/zinc acetate precursor fibers via electrospinning. Highly brittle fibrous ZnO mat was dipped into a PVDF polymer solution and then pressed to form pellets. The second process employed commercial ZnO nanopowder material. The powder was mixed into a PVDF/acetone polymer solution, and the resultant paste was pressed to form pellets. The free standing composite pellets with electrodes on the top and bottom surfaces were then subjected to sinusoidal electric excitation and response was recorded using a fotonic sensor. An earlier work on electrospun PVDF fiber mats was also summarized here and the electromechanical characterization is reported.

Research paper thumbnail of Poly(vinylidene fluoride)/Zinc Oxide Smart Composite Material

This work aimed at fabrication and electromechanical characterization of a smart material system ... more This work aimed at fabrication and electromechanical characterization of a smart material system composed of electroactive polymer and ceramic materials. The idea of composite material system is on account of complementary characteristics of the polymer and ceramic for flexibility and piezoelectric activity. Our preliminary work included Polyvinylidene Fluoride (PVDF) as the flexible piezoelectric polymer, and Zinc Oxide (ZnO) as the piezoelectric ceramic brittle, but capable to respond strains without poling. Two alternative processes were investigated. The first process makes use of ZnO fibrous formation achieved by sintering PVA/zinc acetate precursor fibers via electrospinning. Highly brittle fibrous ZnO mat was dipped into a PVDF polymer solution and then pressed to form pellets. The second process employed commercial ZnO nanopowder material. The powder was mixed into a PVDF/acetone polymer solution, and the resultant paste was pressed to form pellets. The free standing composite pellets with electrodes on the top and bottom surfaces were then subjected to sinusoidal electric excitation and response was recorded using a fotonic sensor. An earlier work on electrospun PVDF fiber mats was also summarized here and the electromechanical characterization is reported.

Research paper thumbnail of Generalized pointwise bias error bounds for response surface approximations

International Journal for Numerical Methods in Engineering, 2006

This paper proposes a generalized pointwise bias error bounds estimation method for polynomial-ba... more This paper proposes a generalized pointwise bias error bounds estimation method for polynomial-based response surface approximations when bias errors are substantial. A relaxation parameter is introduced to account for inconsistencies between the data and the assumed true model. The method is demonstrated with a polynomial example where the model is a quadratic polynomial while the true function is assumed to be cubic polynomial. The effect of relaxation parameter is studied. It is demonstrated that when bias errors dominate, the bias error bounds characterize the actual error field better than the standard error. The bias error bound estimates also help to identify regions in the design space where the accuracy of the response surface approximations is inadequate. It is demonstrated that this information can be utilized for adaptive sampling in order to improve accuracy in such regions. Copyright © 2005 John Wiley & Sons, Ltd.

Research paper thumbnail of Response Surfaces for Optimal Weight of Cracked Composite Panels: Noise and Accuracy

Two levels of fidelity are used for minimum weight design of a composite bladestiffened panel sub... more Two levels of fidelity are used for minimum weight design of a composite bladestiffened panel subject to crack propagation constraints. The low fidelity approach makes use of an equivalent strain constraint calculated by a closed form solution for the stress intensity factor. The high fidelity approach uses the stress intensity factor directly as the constraint and computes it from the stress distribution around the crack. A number of panels were optimized by both approaches for different values of applied load, crack length, and blade height, and response surface approximations for optimal weight as function of these configuration variables were constructed. Computational cost, noise and accuracy for the results are compared.

Research paper thumbnail of Optimal Weight of Cracked Composite Panels By An Equivalent Strain Constraint

An equivalent strain constraint can be used instead of a crack propagation constraint based on th... more An equivalent strain constraint can be used instead of a crack propagation constraint based on the value o f the stress intensity factor, when the latter constraint is not available in a structural optimization package. The approach is demonstrated at two levels of fidelity for minimum weight design of a composite blade-stiffened panel with a crack. The low-fidelity approach utilizes a closed form solution of stress intensity factor while the high-fidelity approach computes it from the stress distribution around the crack. Results are compared with a direct approach that implements stress intensity factor as a constraint in the structural optimizations. The equivalent strain was computationally efficient, but did not converge to the optimum design when started from a very different initial design. Using the low-fidelity direct approach to obtain initial design for optimization based on the highfidelity model reduced computational cost for both direct optimization and the equivalent-strain approach, and it eliminated the problem of convergence to the wrong design for the latter approach.

Research paper thumbnail of Modeling and optimization of a side-implanted piezoresistive shear stress sensor

This paper presents the modeling and design optimization of a micromachined floating element piez... more This paper presents the modeling and design optimization of a micromachined floating element piezoresistive shear stress sensor for the time-resolved, direct measurement of fluctuating wall shear stress in a turbulent flow. The sensor structure integrates side-implanted diffused resistors into the silicon tethers for piezoresistive detection. A theoretical nonlinear mechanical model is combined with a piezoresistive model to determine the electromechanical sensitivity. Lumped element modeling (LEM) is used to estimate the resonant frequency. Finite element modeling is employed to verify the mechanical models and LEM results. Two dominant noise sources, 1/f noise and thermal noise, were considered to determine the noise floor. These models were then leveraged to obtain optimal sensor designs for several sets of specifications. The cost function is the minimum detectable shear stress that is formulated in terms of sensitivity and noise floor. This cost function is subjected to the constraints of geometry, linearity, bandwidth, power and resistance. The results indicate the possibility of designs possessing dynamic ranges of greater than 85dB.

Research paper thumbnail of Optimization of clamped circular piezoelectric composite actuators

Sensors and Actuators A: Physical, 2008

This paper addresses the design of clamped circular piezoceramic composite unimorph and bimorph c... more This paper addresses the design of clamped circular piezoceramic composite unimorph and bimorph configurations, specifically the conflicting requirements of maximum volume displacement for a prescribed bandwidth. An optimization problem is formulated that implements analytical solutions for unimorph and bimorph configurations using laminated plate theory, including the use of oppositely polarized piezoceramic patches. A range of actuator geometric parameters are studied, and bounds for volume displacement and natural frequency of optimal designs are determined and presented via design curves. In the selected design space, Pareto optimization results for unimorph and bimorph configurations show that optimal volume displacement is related to the bandwidth by a universal power law such that the product of the square of the natural frequency and the displaced volume, a "gain-bandwidth" product, is a constant. Characteristic trends are also described that are independent of the actuator radius for the Pareto optimal piezoceramic patch thickness and radius versus normalized bandwidth. The results are relevant, for example, in the design of zero-net mass-flux or synthetic jet actuators used in flow control applications.

Research paper thumbnail of Dielectric behavior characterization of a fibrous-ZnO/PVDF nanocomposite

Polymer Composites, 2009

This study is focused on forming a fibrous-zinc oxide/ polyvinylidine fluoride (ZnO/PVDF) nanocom... more This study is focused on forming a fibrous-zinc oxide/ polyvinylidine fluoride (ZnO/PVDF) nanocomposite and characterizing its dielectric behavior. The nanocomposite is prepared in two steps. First, a network of nanoscale diameter ZnO fibers is produced by sintering electrospun PVA/Zinc Acetate fibers. Second, the ZnO fibrous nonwoven mat is sandwiched between two PVDF thermoplastic polymer films by hot-press casting. Scanning electron microscope images of the nanocomposite show that hot-press casting of the fibrous-ZnO network breaks the network up into short fibers. The in-plane distribution of the ZnO fillers (i.e., the short fibers) in the PVDF matrix appears to comply with that of the pristine ZnO fibers before hot-pressing, indicating that the fillers remain well-dispersed in the polymer matrix. To the authors' knowledge, the work reported herein is the first demonstration of the use of electrospinning to secure the dispersion and distribution of a network of inorganic fillers. Moreover, processing a fibrous-ZnO/PVDF flexible composite as described in this report would facilitate material handling and enable dielectric property measurement, in contrast to that on a fibrous mat of pure ZnO. Because of the high surface area of the short ZnO fibers and their polycrystalline structure, interfacial polarization is pronounced in the nanocomposite film. The dielectric constant is enhanced significantly-up to a factor of 10 at low frequencies compared to the dielectric constant of constituent materials (both bulk ZnO and PVDF), and up to a factor of two compared to a bulk-ZnO/PVDF composite. POLYM. COMPOS.

Research paper thumbnail of Response approximations - Noise, error repair, modeling errors

Research paper thumbnail of Error-based design space windowing

40th AIAA Aerospace Sciences Meeting & Exhibit, 2002

Windowing of design space is considered in order to reduce the bias errors due to low-order polyn... more Windowing of design space is considered in order to reduce the bias errors due to low-order polynomial response surfaces (RS). Standard design space windowing (DSW) uses a region of interest by setting a requirement on response level and checks it by a global RS predictions over the design space. This approach, however, is vulnerable since RS modeling errors may lead to the wrong region to zoom on. The approach is modified by introducing an eigenvalue error measure based on point-topoint mean squared error criterion. Two examples are presented to demonstrate the benefit of the error-based DSW.

Research paper thumbnail of Post-Buckling of Composite I-Sections. Part 2: Experimental Validation

Journal of Composite Materials, 2001

Page 1. http://jcm.sagepub.com/ Materials Journal of Composite http://jcm.sagepub. com/content/35... more Page 1. http://jcm.sagepub.com/ Materials Journal of Composite http://jcm.sagepub. com/content/35/9/797 The online version of this article can be found at: DOI: 10.1177/002199801772662497 2001 35: 797 Journal of Composite ...

Research paper thumbnail of Post-Buckling of Composite I-Sections. Part 2: Experimental Validation

Journal of Composite Materials, 2001

Page 1. http://jcm.sagepub.com/ Materials Journal of Composite http://jcm.sagepub. com/content/35... more Page 1. http://jcm.sagepub.com/ Materials Journal of Composite http://jcm.sagepub. com/content/35/9/797 The online version of this article can be found at: DOI: 10.1177/002199801772662497 2001 35: 797 Journal of Composite ...

Research paper thumbnail of Post-Buckling of Composite I-Sections. Part 1: Theory

Journal of Composite Materials, 2001

... mx L f g dy p N a K Page 7. tal strain energy due to bending of the plates to the total work ... more ... mx L f g dy p N a K Page 7. tal strain energy due to bending of the plates to the total work done by the edge forces on the plates. The buckling coefficient expression was used to create buck-ling coefficient vs. ... 780 MELIH PAPILA AND MEHMET A. AKGÜN ...

Research paper thumbnail of Post-Buckling of Composite I-Sections. Part 1: Theory

Journal of Composite Materials, 2001

... mx L f g dy p N a K Page 7. tal strain energy due to bending of the plates to the total work ... more ... mx L f g dy p N a K Page 7. tal strain energy due to bending of the plates to the total work done by the edge forces on the plates. The buckling coefficient expression was used to create buck-ling coefficient vs. ... 780 MELIH PAPILA AND MEHMET A. AKGÜN ...

Research paper thumbnail of Pointwise Bias Error Bounds and Min?Max Design for Response Surface Approximations

Aiaa Journal, 2005

Two approaches addressing response surface approximation errors due to model inadequacy (bias err... more Two approaches addressing response surface approximation errors due to model inadequacy (bias error) are presented, and a design of experiments minimizing the maximal bias error is proposed. Both approaches assume that the functional form of the true model is known and seek, at each point in design space, worst case bounds on the absolute error. The first approach is implemented prior to data generation. This data independent error bound can identify locations in the design space where the accuracy of the approximation fitted on a given design of experiments may be poor. The data independent error bound can easily be implemented in a search for a design of experiments that minimize the bias error bound as it requires very little computation.

Research paper thumbnail of Uncertainty and Wing Structural Weight Approximations

Research paper thumbnail of Equivalent” Electromechanical Coefficient for IPMC Actuator Design Based on Equivalent Bimorph Beam Theory

Experimental Mechanics, 2010

This paper addresses an “equivalent” electromechanical coupling coefficient that may be used in d... more This paper addresses an “equivalent” electromechanical coupling coefficient that may be used in designing Ionic Polymer Metal Composite (IPMC) actuators. The coefficient is not a material constant and derived from equivalent bimorph beam model. The collective effect of the membrane thickness and operating voltage on the coefficient is demonstrated by using a design of experiment (DOE) of three and five levels of the two factors, respectively. Experiments and linear finite element analyses with MD.NASTRAN at DOE points are performed. The tip displacement and the coupling coefficient are reported and their response surface (RS) approximations as function of the thickness and voltage are constructed. Experiments and RS predictions indicate that actuator thickness and applied voltage are two interacting major factors for maximum tip displacement. The equivalent coupling coefficient is primarily driven by the thickness of actuator moreover voltage appears to contribute as the thickness increases. The initial curvature of the strips before electrical excitation is also shown to be a factor for “equivalent” coupling coefficient, it is not, however sufficient to explain the variation in the experimental data. A correction factor approach is proposed and applied to the straight beam tip displacement RS that filters out experimental variation. A corrected RS enables including the pre-imposed initial curvature as design parameter along with the actuator thickness and the operating peak voltage when predicting the tip displacement and the equivalent coupling coefficient.

Research paper thumbnail of Engineering Chemistry of Electrospun Nanofibers and Interfaces in Nanocomposites for Superior Mechanical Properties

Acs Applied Materials & Interfaces, 2010

The novelty of this work is based on designing the chemistry of the electrospun nanofibers, so th... more The novelty of this work is based on designing the chemistry of the electrospun nanofibers, so that the resultant composites substantially benefit from cross-linking between the nanofibers and the polymer matrix. Specifically, the solution of in-house synthesized copolymers polystyrene-co-glycidyl methacrylate P(St-co-GMA) is electrospun to produce mats of surface reactive nanoto-submicron scale fibers that are accompanied later by spraying over the ethylenediamine (EDA) as a supplementary cross-linking agent for epoxy. The P(St-co-GMA)/EDA fiber mats are then embedded into an epoxy resin. Analysis of the three-point-bending mode of the composites reveals that the storage modulus of P(St-co-GMA)/EDA nanofiber-reinforced epoxy are about 10 and 2.5 times higher than that of neat and P(St-co-GMA) nanofiber-reinforced epoxy, respectively, even though the weight fraction of the nanofibers was as low as 2 wt %. The significant increase in the mechanical response is attributed to the inherently cross-linked fiber structure and the surface modification/chemistry of the electrospun fibers, that results in cross-linked polymer matrix-nanofiber interfacial bonding.

Research paper thumbnail of The effect of IPMC parameters in electromechanical coefficient based on equivalent beam theory

"Effective" electromechanical coupling coefficient for IPMC by equivalent bimorph beam model is s... more "Effective" electromechanical coupling coefficient for IPMC by equivalent bimorph beam model is studied. The collective effect of the membrane thickness and operating voltage is demonstrated by using a design of experiment of three and four levels of the two factors, respectively. Experiments and finite element analyses using MSC.NASTRAN are used to evaluate the tip displacement and the coupling coefficient for which approximations as function of the thickness and voltage are constructed. Initial curvature of the strips before electrical excitation is also shown to be a factor in "effective" coupling coefficient. A correction factor approach is proposed to include the effect of the preimposed curvature.

Research paper thumbnail of Effects of electrospinning parameters on polyacrylonitrile nanofiber diameter: An investigation by response surface methodology

Materials & Design, 2008

Effects of material and process parameters on the diameter of electrospun polyacrylonitrile fiber... more Effects of material and process parameters on the diameter of electrospun polyacrylonitrile fibers were experimentally investigated. Response surface methodology (RSM) was utilized to design the experiments at the settings of solution concentration, voltage and the collector distance. It also imparted the evaluation of the significance of each parameter on the resultant fiber diameter. The investigations were carried out in the two-variable process domains of several collector distances as applied voltage and the solution concentration were varied at a fixed polymer molecular weight. The mean diameter and coefficient of variation were modeled by polynomial response surfaces as functions of solution concentration and voltage at each collector distance. Effect of applied voltage in micron-scale fiber diameter was observed to be almost negligible when solution concentration and collector distance were high. However, all three factors were found statistically significant in the production of nano-scale fibers. The response surface predictions revealed the parameter interactions for the resultant fiber diameter, and showed that there is a negative correlation between the mean diameter and coefficient of variation for the fiber diameter. A sub-domain of the parameter space consisting of the solution concentration, applied voltage and collector distance, was suggested for the potential nano-scale fiber production. (O.S. Yö rdem), mpapila@sabanciuniv.edu (M. Papila), yusufm@sabanciuniv.edu (Y.Z. Menceloglu). 1 Tel.: +90 216 483 9000/2079; fax: +90 216 483 9550. 2 Tel.: +90 216 483 9535; fax: +90 216 483 9550. www.elsevier.com/locate/matdes Materials and Design xxx (2007) xxx-xxx Materials & Design ARTICLE IN PRESS Please cite this article in press as: Yö rdem OS et al., Effects of electrospinning parameters on polyacrylonitrile .

Research paper thumbnail of Poly(vinylidene fluoride)/zinc oxide smart composite material

This work aimed at fabrication and electromechanical characterization of a smart material system ... more This work aimed at fabrication and electromechanical characterization of a smart material system composed of electroactive polymer and ceramic materials. The idea of composite material system is on account of complementary characteristics of the polymer and ceramic for flexibility and piezoelectric activity. Our preliminary work included Polyvinylidene Fluoride (PVDF) as the flexible piezoelectric polymer, and Zinc Oxide (ZnO) as the piezoelectric ceramic brittle, but capable to respond strains without poling. Two alternative processes were investigated. The first process makes use of ZnO fibrous formation achieved by sintering PVA/zinc acetate precursor fibers via electrospinning. Highly brittle fibrous ZnO mat was dipped into a PVDF polymer solution and then pressed to form pellets. The second process employed commercial ZnO nanopowder material. The powder was mixed into a PVDF/acetone polymer solution, and the resultant paste was pressed to form pellets. The free standing composite pellets with electrodes on the top and bottom surfaces were then subjected to sinusoidal electric excitation and response was recorded using a fotonic sensor. An earlier work on electrospun PVDF fiber mats was also summarized here and the electromechanical characterization is reported.

Research paper thumbnail of Poly(vinylidene fluoride)/Zinc Oxide Smart Composite Material

This work aimed at fabrication and electromechanical characterization of a smart material system ... more This work aimed at fabrication and electromechanical characterization of a smart material system composed of electroactive polymer and ceramic materials. The idea of composite material system is on account of complementary characteristics of the polymer and ceramic for flexibility and piezoelectric activity. Our preliminary work included Polyvinylidene Fluoride (PVDF) as the flexible piezoelectric polymer, and Zinc Oxide (ZnO) as the piezoelectric ceramic brittle, but capable to respond strains without poling. Two alternative processes were investigated. The first process makes use of ZnO fibrous formation achieved by sintering PVA/zinc acetate precursor fibers via electrospinning. Highly brittle fibrous ZnO mat was dipped into a PVDF polymer solution and then pressed to form pellets. The second process employed commercial ZnO nanopowder material. The powder was mixed into a PVDF/acetone polymer solution, and the resultant paste was pressed to form pellets. The free standing composite pellets with electrodes on the top and bottom surfaces were then subjected to sinusoidal electric excitation and response was recorded using a fotonic sensor. An earlier work on electrospun PVDF fiber mats was also summarized here and the electromechanical characterization is reported.

Research paper thumbnail of Generalized pointwise bias error bounds for response surface approximations

International Journal for Numerical Methods in Engineering, 2006

This paper proposes a generalized pointwise bias error bounds estimation method for polynomial-ba... more This paper proposes a generalized pointwise bias error bounds estimation method for polynomial-based response surface approximations when bias errors are substantial. A relaxation parameter is introduced to account for inconsistencies between the data and the assumed true model. The method is demonstrated with a polynomial example where the model is a quadratic polynomial while the true function is assumed to be cubic polynomial. The effect of relaxation parameter is studied. It is demonstrated that when bias errors dominate, the bias error bounds characterize the actual error field better than the standard error. The bias error bound estimates also help to identify regions in the design space where the accuracy of the response surface approximations is inadequate. It is demonstrated that this information can be utilized for adaptive sampling in order to improve accuracy in such regions. Copyright © 2005 John Wiley & Sons, Ltd.

Research paper thumbnail of Response Surfaces for Optimal Weight of Cracked Composite Panels: Noise and Accuracy

Two levels of fidelity are used for minimum weight design of a composite bladestiffened panel sub... more Two levels of fidelity are used for minimum weight design of a composite bladestiffened panel subject to crack propagation constraints. The low fidelity approach makes use of an equivalent strain constraint calculated by a closed form solution for the stress intensity factor. The high fidelity approach uses the stress intensity factor directly as the constraint and computes it from the stress distribution around the crack. A number of panels were optimized by both approaches for different values of applied load, crack length, and blade height, and response surface approximations for optimal weight as function of these configuration variables were constructed. Computational cost, noise and accuracy for the results are compared.

Research paper thumbnail of Optimal Weight of Cracked Composite Panels By An Equivalent Strain Constraint

An equivalent strain constraint can be used instead of a crack propagation constraint based on th... more An equivalent strain constraint can be used instead of a crack propagation constraint based on the value o f the stress intensity factor, when the latter constraint is not available in a structural optimization package. The approach is demonstrated at two levels of fidelity for minimum weight design of a composite blade-stiffened panel with a crack. The low-fidelity approach utilizes a closed form solution of stress intensity factor while the high-fidelity approach computes it from the stress distribution around the crack. Results are compared with a direct approach that implements stress intensity factor as a constraint in the structural optimizations. The equivalent strain was computationally efficient, but did not converge to the optimum design when started from a very different initial design. Using the low-fidelity direct approach to obtain initial design for optimization based on the highfidelity model reduced computational cost for both direct optimization and the equivalent-strain approach, and it eliminated the problem of convergence to the wrong design for the latter approach.

Research paper thumbnail of Modeling and optimization of a side-implanted piezoresistive shear stress sensor

This paper presents the modeling and design optimization of a micromachined floating element piez... more This paper presents the modeling and design optimization of a micromachined floating element piezoresistive shear stress sensor for the time-resolved, direct measurement of fluctuating wall shear stress in a turbulent flow. The sensor structure integrates side-implanted diffused resistors into the silicon tethers for piezoresistive detection. A theoretical nonlinear mechanical model is combined with a piezoresistive model to determine the electromechanical sensitivity. Lumped element modeling (LEM) is used to estimate the resonant frequency. Finite element modeling is employed to verify the mechanical models and LEM results. Two dominant noise sources, 1/f noise and thermal noise, were considered to determine the noise floor. These models were then leveraged to obtain optimal sensor designs for several sets of specifications. The cost function is the minimum detectable shear stress that is formulated in terms of sensitivity and noise floor. This cost function is subjected to the constraints of geometry, linearity, bandwidth, power and resistance. The results indicate the possibility of designs possessing dynamic ranges of greater than 85dB.