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    Assessment of Thermal Performance of the Functionally Graded Materials in Annular Fins
    (MAIK NAUKA/INTERPERIODICA/SPRINGER, 2016) Hassanzadeh, R.; Pekel, H.
    Heat transfer enhancement in the annular fins with uniform profile is investigated in the present study. Conventional fin materials are replaced with the functionally graded materials in order to increase the rate of heat transfer and corresponding fin efficiency. Here, the fin material properties, such as conductivity, are assumed to be graded along the annular fin radius, R, as a linear function. The governing equation of the annular fin is computed with an approximate analytical method using the mean value theorem. It is revealed that the in-homogeneity index, beta, of the fin material plays an important role on the thermal characteristics of the annular fins. Alternatively, heat dissipation between the fin surface and surrounding fluid in the annular fins increases with increasing the inhomogeneity indices, beta. Finally, it is stated that application of the functionally graded material in the annular fins enhances the fin efficiency in comparison to the annular fins with homogeneous material. It is hoped that the results obtained from this study arouse interest among thermal designers and heat exchanger industries.
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    Determination of natural frequencies of non-uniform aluminum beams coated with functionally graded materialModalanalyse von ungleichmäßigen Aluminiumträgern, die mit funktional abgestuftem Material beschichtet sind
    (Wiley-V C H Verlag Gmbh, 2024) Pekel, H.; Erdurcan, E. F.
    The present study comprises a numerical analysis used to find the dimensionless natural frequencies of non-uniform aluminum beams coated with functionally graded material. The beams have variable width, and their variation is described by exponential and linear functions. While the coating material properties vary with a polynomial function, the lamination theory is used to calculate the overall properties of the functionally graded material coating. The beam is modeled as a modified Timoshenko beam and the gradual transition of the coating material properties as 25 layers of homogeneous isotropic material. In order to find the natural frequencies of the beam, finite element analysis was used, and the numerical results were processed with MATLAB, which were in good agreement with literature values. A parametric study is performed to study the effects of slenderness (L/H), coating thickness (h/H), skewness rate (S) and taper ratio (w2/w1) on the dimensionless natural frequencies. The study showed that tapering and skewing had a limited effect on the natural frequencies in general, however there exists a critical slenderness for every taper ratio and skewness rate where shape variation has a significant effect on the natural frequencies and should be considered. In der vorliegenden Studie wird eine numerische Analyse durchgefuhrt, um die dimensionslosen Eigenfrequenzen von ungleichmassigen Aluminiumtragern, die mit funktional abgestuftem Material beschichtet sind, zu ermitteln. Die Trager haben eine variable Breite und ihre Variation wird durch Exponential- und Linearfunktionen beschrieben. Weil die Eigenschaften des Beschichtungswerkstoffs mit einer Polynomfunktion variieren, wird die Laminierungstheorie verwendet, um die Gesamteigenschaften der funktional abgestuften Material-Beschichtung zu berechnen. Der Trager wird als modifizierter Timoshenko-Balken modelliert, und der allmahliche ubergang der Beschichtungsmaterialeigenschaften wird als 25 Schichten aus homogenem isotropem Material modelliert. Um die Eigenfrequenzen des Tragers zu ermitteln, wurde die Finite-Elemente-Analyse verwendet. Die von MATLAB gefundenen numerischen Ergebnisse stimmen mit Literaturwerten uberein. Eine Parameterstudie wurde durchgefuhrt, um die Auswirkungen von Schlankheit (L/H), Beschichtungsdicke (h/H), Schieferate (S) und Verjungungsverhaltnis (w2/w1) auf die dimensionslosen Eigenfrequenzen zu untersuchen. Die Studie zeigte, dass Verjungung und Schragstellung im Allgemeinen einen begrenzten Einfluss auf die Eigenfrequenzen haben, jedoch gibt es eine kritische Schlankheit fur jedes Verjungungsverhaltnis und jede Schieferate, bei der Formvariationen einen grossen Einfluss auf die Eigenfrequenzen haben und berucksichtigt werden sollten. The current paper elucidates the factors which should be considered when designing a metal structural element with non-uniform shape coated with functionally graded material. As a result, the effects of both material and shape parameters on the dimensionless natural frequencies are presented individually. image
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    Evaluation of tensile behaviour of 5754 aluminium-magnesium alloy at cold and warm temperatures
    (TAYLOR & FRANCIS LTD, 2009) Ozturk, F.; Toros, S.; Pekel, H.
    Cost effective lightweight vehicles have recently become a key target for automotive manufactures due to increasing concerns about minimising environmental impact and maximising fuel economy without sacrificing the vehicle performance and comfort. Aluminium-magnesium alloys are the most important candidate materials due to their excellent high strength to weight ratio, good formability, good corrosion resistance and recycling potential. Although their deformations leave undesirable traces on the surface of the final product and their formability are not very favorable at room temperatures, the formability can be improved by changing the forming temperature to either cryogenic or warm. In this study, uniaxial tensile deformation behaviour of 5754 aluminium-magnesium alloy sheet was extensively studied at cold (-60 to 0 degrees C) and warm (room to 250 degrees C) forming temperatures and a strain rate range of 0.0016-0.042 s(-1). Results indicate that the formability of this material at cold and warm temperatures is better than at room temperature. Stretching lines were eliminated at both cold and warm temperatures especially above 175 degrees C and 0.0016 s(-1) strain rate. The most suitable forming conditions were obtained at 250 degrees C and 0.0083 s(-1) or 175 degrees C and 0.0016 s(-1).
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    Evaluation of tensile properties of 5754-O type aluminum- magnesium alloy at cold and warm temperature
    (Hanrimwon Publishing Co., 2008) Ozturk, F.; Toros, S.; Pekel, H.
    In this research, uniaxial tensile deformation behavior of 5754-O type aluminum-magnesium (Al- Mg) alloy sheets which are used widely in diverse applications ranging from automotive bodies to food processing due to their excellent high strength to weight ratio, corrosion resistance, and weldability were investigated at temperature range between -60 to 250oC and at the range of strain rates 0.0016-0.04 s-1. Mechanical properties such as yield strength (YS), ultimate tensile strength (UTS), strain hardening coefficient (n), strain rate sensitivity (m), total elongation (TE) and reduction of area (RA) were determined. In addition, the elimination of Luder's band (stretcher marks or line) elimination was investigated. The ductility of the alloy was calculated by two different methods. As a result, the ductility was found to increase with increasing temperature and to decrease with increasing strain rate. Luder's band elimination was observed with increasing temperatures and strain rates. The test results at cold temperatures indicated that an increase in total elongation and strain hardening coefficient were observed. Optimum formability condition was obtained at high temperature and low strain rate.