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Article Citation - WoS: 3On the Yielding of Two-Layer Composite Spherical Pressure Vessels(Gazi Univ, 2017) Akis, TolgaThe yielding of two-layer composite spherical pressure vessels under either internal or external pressure is investigated analytically in the framework of small deformations and von Mises yield criterion. It is shown for both pressure cases that depending on the material properties and sphere dimensions, different modes of plasticization may take place. Unlike the deformation behavior of a single layer spherical pressure vessel, yielding may commence at the inner layer or at the outer layer or simultaneously at both layers of the assembly.Article Citation - WoS: 20Citation - Scopus: 20The Stress Response of Partially Plastic Rotating Fgm Hollow Shafts: Analytical Treatment for Axially Constrained Ends(Taylor & Francis inc, 2006) Eraslan, Ahmet N.; Akis, Tolgaanalytical solutions to estimating the elastoplastic response of rotating functionally graded (FGM) hollow shafts with fixed ends are presented. The modulus of elasticity, as well as the uniaxial yield limit of the shaft material, are assumed to vary nonlinearly in the radial direction. The plastic model is based on Tresca's yield criterion, its associated flow rule, and ideal plastic material behaviour. Elastic, partially plastic, fully plastic, and residual stress states are investigated. It is shown that the elastoplastic stress response of a rotating FGM hollow shaft is affected significantly by the nonhomogeneity of the material. Unlike the case of a homogeneous hollow shaft, plastic deformation may commence at the inner surface, at the outer surface, or simultaneously at both surfaces. Accordingly, each case requires different mathematical treatment to arrive at its partially plastic solution. It is also shown that, by taking a numerical limit, the complete FGM solution presented herein converge to the solution of a homogeneous rotating shaft.Article Citation - WoS: 33Citation - Scopus: 35Exact Solution of Rotating Fgm Shaft Problem in the Elastoplastic State of Stress(Springer, 2007) Akis, Tolga; Eraslan, Ahmet N.Plane strain analytical solutions to estimate purely elastic, partially plastic and fully plastic deformation behavior of rotating functionally graded (FGM) hollow shafts are presented. The modulus of elasticity of the shaft material is assumed to vary nonlinearly in the radial direction. Tresca's yield criterion and its associated flow rule are used to formulate three different plastic regions for an ideal plastic material. By considerina different material compositions as well as a wide range of bore radii, it is demonstrated in this article that both the elastic and the elastoplastic responses of a rotating FGM hollow shaft are affected significantly by the material nonhomogeneity.Article Citation - WoS: 11Citation - Scopus: 13Automated Selection of Optimal Material for Pressurized Multi-Layer Composite Tubes Based on an Evolutionary Approach(Springer London Ltd, 2018) Azad, Saeid Kazemzadeh; Akis, TolgaDecision making on the configuration of material layers as well as thickness of each layer in composite assemblies has long been recognized as an optimization problem. Today, on the one hand, abundance of industrial alloys with different material properties and costs facilitates fabrication of more economical or light weight assemblies. On the other hand, in the design stage, availability of different alternative materials apparently increases the complexity of the design optimization problem and arises the need for efficient optimization techniques. In the present study, the well-known big bang-big crunch optimization algorithm is reformulated for optimum design of internally pressurized tightly fitted multi-layer composite tubes with axially constrained ends. An automated material selection and thickness optimization approach is employed for both weight and cost minimization of one-, two-, and three-layer tubes, and the obtained results are compared. The numerical results indicate the efficiency of the proposed approach in practical optimum design of multi-layer composite tubes under internal pressure and quantify the optimality of different composite assemblies compared to one-layer tubes.Article Citation - WoS: 50Citation - Scopus: 55Plane Strain Analytical Solutions for a Functionally Graded Elastic-Plastic Pressurized Tube(Elsevier Sci Ltd, 2006) Eraslan, Ahmet N.; Akis, TolgaPlane strain analytical solutions to functionally graded elastic and elastic-plastic pressurized tube problems are obtained in the framework of small deformation theory. The modulus of elasticity and the uniaxial yield limit of the tube material are assumed to vary radially according to two parametric parabolic forms. The analytical plastic model is based on Tresca's yield criterion, its associated flow rule and ideally plastic material behaviour. Elastic, partially plastic and fully plastic stress states are investigated. It is shown that the elastoplastic response of the functionally graded pressurized tube is affected significantly by the material nonhomogeneity. Different modes of plasticization may take place unlike the homogeneous case. It is also shown mathematically that the nonhomogeneous elastoplastic solution presented here reduces to that of a homogeneous one by appropriate choice of the material parameters. (c) 2006 Elsevier Ltd. All rights reserved.Letter Discussion of the Article "approximate Solution of Limit Angular Speed for Externally Loaded Rotating Solid Disk" by S. Bhowmick, D. Misra and Nk Saha [int J Mech Sci 2008;50:163-74](Pergamon-elsevier Science Ltd, 2008) Eraslan, Ahmet N.; Akis, Tolga[No Abstract Available]Letter Amendments on "application of Mises Yield Criterion To Rotating Solid Disk Problem" by N. Aleksandrova [int. J. Eng. Sci. 51 (2012) 333-337](Pergamon-elsevier Science Ltd, 2012) Eraslan, Ahmet N.; Akis, Tolga[No Abstract Available]Article Citation - WoS: 2Citation - Scopus: 2Structural Design Optimization of Multi-Layer Spherical Pressure Vessels: a Metaheuristic Approach(Springer, 2019) Akis, Tolga; Azad, Saeid KazemzadehThis study addresses the optimum design problem of multi-layer spherical pressure vessels based on von Mises yield criterion. In order to compute the structural responses under internal pressure, analytical solutions for one-, two-, and three-layer spherical pressure vessels are provided. A population-based metaheuristic algorithm is reformulated for optimum material selection as well as thickness optimization of multi-layer spherical pressure vessels. Furthermore, in order to enhance the computational efficiency of the optimization algorithm, upper bound strategy is also integrated with the algorithm for reducing the total number of structural response evaluations during the optimization iterations. The performance of the algorithm is investigated through weight and cost minimization of one-, two- and three-layer spherical pressure vessels and the results are presented in detail. The obtained numerical results, based on different internal pressures as well as vessel sizes, indicate the usefulness and efficiency of the employed methodology in optimum design of multi-layer spherical pressure vessels.Article Citation - WoS: 2Yielding of Radially Pressurized Functionally Graded Long Tubes Based on Von Mises Criterion(Gazi Univ, 2015) Akis, Tolga; Eren, OmurThe elastic behaviour of functionally graded long tubes with axially constrained ends subject to either internal or external pressure is investigated analytically. The modulus of elasticity and the yield limit of the tube material are assumed to vary radially in nonlinear forms. Making use of von Mises yield criterion, the effect of grading parameters on the yielding behavior of the tube is investigated and it is observed that for both pressure cases, the plastic flow may commence at the inner surface, at the outer surface or simultaneously at both surfaces depending on the functionally grading parameters.Article Citation - WoS: 9Citation - Scopus: 10Influence of Cement Replacement by Calcinated Kaolinitic and Montmorillonite Clays on the Properties of Mortars(Springer Heidelberg, 2023) Al-Noaimat, Yazeed A. A.; Akis, TolgaThis study aims to investigate the decomposition and pozzolanic reactivity of two different clays (kaolinitic and montmorillonite) from different origins and to determine their effects after calcination on the properties of cement mortars when used to replace Portland cement partially. Mineralogical and chemical compositions of the clay samples were determined using XRD (X-ray Diffractometer) and XRF (X-ray Fluorescence) tests, respectively. TG-DTA (Thermogravimetry-Differential Thermal Analyses) was used to determine the temperature profiles and the burning temperatures of the clays. The density and fineness of the burnt clays were also determined. In order to investigate the optimum material properties, different burning temperatures and replacement levels were considered. It was found that for all temperatures, the two burnt clays possess good pozzolanic activity. The highest compressive strength and lowest water absorption capacity were achieved when the clay determined as kaolinitic was burned at 700 & DEG;C and with 10% replacement level. While for the clay determined as montmorillonite, the optimum properties were obtained at 700 & DEG;C with a 20% substitution level. Kaolinite had better pozzolanic reactivity than montmorillonite, achieving higher strength performance with lower water absorption when partially replaced with cement. Moreover, it had compressive strength values even higher than plain cement.
