Browsing by Author "Tekkaya, A. Erman"
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Article Citation Count: 38Application of Continuum Damage Mechanics in discontinuous crack formation: Forward extrusion chevron(Wiley-v C H verlag Gmbh, 2008) Soyarslan, Celal; Tekkaya, A. Erman; Tekkaya, Ahmet Eeman; Department of Mechatronics Engineering; Manufacturing EngineeringMaterializing Continuum Damage Mechanics (CDM), numerical modeling of discrete internal cracks, namely central bursts, in direct forward extrusion process is presented. Accordingly, in a thermodynamically consistent setting, a local Lemaitre variant damage model with quasi-unilateral evolution is coupled with hyperelastic-plasticity. The formulations are constructed in the principal axes where simultaneous local integration schemes are efficiently developed. To this end, the framework is implemented as ABAQUS/VUMAT subroutine to be used in an explicit FE solution scheme, and utilized in direct forward extrusion simulations for bearing steel, 100Cr6. Discontinuous cracks are obtained with the element deletion procedure, where the elements reaching the critical damage value are removed from the mesh. The periodicity of the cracks shows well accordance with the experimental facts. The investigations reveal that, application of the quasi-unilateral conditions together with the crack closure parameter has an indispensable effect on the damage accumulation zones by determining their internal or superficial character. (C) 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.Article Citation Count: 7Generalized transient temperature behavior in induction heated workpieces(Elsevier Science Sa, 2009) Tekkaya, Ahmet Eeman; Dursunkaya, Zafer; Tekkaya, A. Erman; Manufacturing EngineeringElectromagnetic-thermal coupled numerical analysis of induction heating is time intensive if three-dimensional models are used. In this study, by processing the results of finite element computations, a dimensional analysis is carried out to predict the transient temperature rise time at the surface of an induction heated workpiece. A unique set of curves is found that describes the transient dimensionless temperature at a point on the surface as a function of the electromagnetic skin depth alone. The effectiveness of the approach is verified by numerical calculations and comparing the dimensionless temperature curves for different heating cases and materials. Effect of temperature dependent thermal properties is also investigated. The consistency of numerical results is also verified by comparison with experiments. (C) 2009 Elsevier B.V. All rights reserved.Master Thesis Mikroşekillendirme analizi: Mikromekanik ve sayısal yönleri(2007) Demirci, Emrah; Demirci, Emrah; Tekkaya, Ahmet Eeman; Tekkaya, A. Erman; Soyarslan, Celal; Department of Mechatronics Engineering; Department of Mechatronics Engineering; Department of Mechatronics Engineering; Manufacturing EngineeringArtan elektronik ve mikromekanik parcaların piyasa hacmi donanımların daha yogun islevsellik kazanmalarına ve kuculmelerine yol acmaktadır. Talepleri karsılamak icin mikro-parcaların uretim yontemlerinin tasarımı hem uygulamalı ve hem de analitik olarak arastırılmalıdır. Bu tez mikro-¸sekillendirmenin analitik olarak arastırılmasına katkıda bulunmak icin hazırlanmıstır. Mikro-¸sekillendirme yontemlerinin sonlu elemanlar yontemi ile simulasyonu icin basitlestirilmis bir modelleme yaklasımı onerilmistir. Yeni modelleme yaklasımı boyut etkileri diye adlandırılan geleneksel sekillendirme ve mikro-¸sekillendirme arasındaki farkları acıklamayı hedefler. Azalan is parcası ebat olcegi nedeniyle olusan boyut etkileri malzemenin homojen olmayan tanecik yapısı ile acıklanmıstır. Sonucta sonlu elemanlar yontemi simulasyonu icin onerilen yaklasım, malzemeyi anisotropik mekanik ozelliklere sahip bireysel tanecikler olarak modellemektir. Taneciklerin yonsel tepkileri Hill'in anizotropik malzeme modeli ile temsil edilmistir. Uc mumkun malzeme modeli (izotropik, tek tanecik, ¸coklu tanecik) ile 2 ve 3 boyutlu yaygın sekillendirme yontemlerinin sim¨ulasyonları yapılmı¸stır ve literat¨ur ile kar¸sıla¸stırılmı¸stır. Dahası, konsept bir mikro-¸sekillendirme presi i¸ceren bir deney duzenegi tasarlanmı¸stır. Anahtar sozcukler : Mikro-¸sekillendirme, Geleneksel ¸sekillendirme, Boyut etkileri, Hill'in anizotropik malzeme modeli, Sonlu elemanlar y¨ontemi.Conference Object Citation Count: 12Modeling flexforming (fluid cell forming) process with finite element method(Trans Tech Publications Ltd, 2007) Tekkaya, Ahmet Eeman; Polat, Naki; Koeksal, Arif; Tekkaya, A. Erman; Manufacturing EngineeringIn this paper, the flexforming process is modeled by finite element method in order to investigate the operation window of the problem. Various models are established using explicit approach for the forming operation and implicit approach for the unloading one. In all analyses the rubber diaphragm has been modeled revealing that the modeling of this diaphragm is essential. Usine the material Aluminum 2024 T3 alclad sheet alloy, three basic experiments are conducted: Bending of a straight flange specimen, bending of a contoured flange specimen and bulging of a circular specimen. By these experiments tile effects of blank thickness, die bend radius, flange length and orientation of the rolling direction of the part have been investigated. Experimental results are compared with finite element results to verify the computational models.Article Citation Count: 42Numerical simulation of various cross sectional workpieces using conventional deep drawing and hydroforming technologies(Elsevier Sci Ltd, 2008) Tekkaya, Ahmet Eeman; Tekkaya, A. Erman; Manufacturing EngineeringThis study focuses on the determination of optimum sheet metal forming process and process parameters for various cross sectional workpieces by comparing the numerical results of high-pressure sheet metal forming, hydro-mechanical deep drawing (DD) and conventional DD simulations. Within the range of each cross section, depth, characteristic dimensions ratio and fillet radius have been altered systematically. Steel of types St14 and DC04 have been used as the specimen material in the numerical analyses and the experimental verification throughout the study. All numerical simulations have been carried out by using a dynamic-explicit commercial finite element code and an elasto-plastic material model. During the analyses each workpiece was simulated by the three competing processes. The results of analyses, such as sheet thickness distribution, necking, forming of radii etc., are used for assessing the success of each forming process alternative. The analyses revealed that depending on the workpiece geometry and dimensional properties certain processes are preferable for obtaining more satisfactory products. Working windows for each process have been established based on the analyzed parameters of the circular, elliptic, rectangular and square cross sectional product geometries. This data is expected to be useful for selecting the appropriate production process for a given workpiece geometry and understand the limits of each sheet metal forming processes. (c) 2007 Elsevier Ltd. All rights reserved.Master Thesis Profilli ovalamanın analizi(2009) Tekkaya, Ahmet Eeman; Tekkaya, A. Erman; Manufacturing EngineeringTornalama ile üretilen rulman bileziklerinin ovalama ile yapılması amaçlanmaktadır. Soğuk metal şekillendirme prosesi olan profilli ovalama ile imal edilen rulmanın ömrünün daha uzun olacağı tahmin edilmektedir. Soğuk metal şekillendirme proseslerinin genel bir özelliği olarak malzeme sertliği, dinamik yüklemelere dayanımı, yüzey kalitesi daha iyi olacaktır. Bu özellikler sayesinde ömrünün uzayacağı düşünülmektedir. Soğuk şekillendirmenin nihai ürüne uygulanmaya çalışılması, basitleştirilmiş simülasyon modelleri ve üç boyutlu profilli ovalamanın simülasyonu ile yapılan deneylerin sonuçlarının karşılaştırılası çalışmalarını içermektedir. Ovalama işlemi ile profildeki kapak yuvası gibi ayrıntıların doldurulması çok zordur. Bundan dolayı profile uygun bir ön şekil verilmesi amaçlanmıştır. Tornalama ile yapılacak olan bu ön şekil, sonlu eleman analizleriyle bilgisayar ortamında denemeler yaparak, tespit edilecektir. Çalışma sonucunda soğuk ovalama ile nihai profil şeklinin verme işlemi özgün bir üretim prosesi olacaktır.Article Citation Count: 60A semi-empirical approach for residual stresses in electric discharge machining (EDM)(Elsevier Sci Ltd, 2006) Erden, Abdulkadir; Tekkaya, A. Erman; Tekkaya, Ahmet Eeman; Department of Mechatronics Engineering; Manufacturing EngineeringHigh residual stresses are developed on the surfaces of electric discharge machined parts. In this study, layer removal method is used to measure the residual stress profile as a function of depth beneath the surface caused by die sinking type EDM. Cracking and its consequences on residual stresses are also studied on samples machined at long pulse durations. A modified empirical equation is developed for scaling residual stresses in machined surfaces with respect to operating conditions. In this model, a unit amplitude shape function representing change in curvature with respect to removal depth is proposed. The proposed form is found to be a special form of a Gauss Distribution. It is the sum of two Gaussian peaks, with the same amplitude and pulse width but opposite center location. The form can be represented by three constant coefficients. These coefficients depend on the released energy by a power function. (C) 2005 Elsevier Ltd. All rights reserved.