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Article Citation - WoS: 29Citation - Scopus: 37A Guide for Validation of Fe-Simulations in Bulk Metal Forming(Springer Heidelberg, 2005) Tekkaya, AE; Manufacturing EngineeringNumerical analysis of metal forming processes is an everyday practice in industry. Forming loads, material flow, forming defects such as underfills, laps, and even cracks, stresses in dies and punches, as well as product properties like new hardness distribution, dimensional accuracies, and residual stresses are predicted by numerical analysis and used for technology generation. Most of the numerical analysis is done by the finite element method made available for engineers and technicians by numerous powerful commercial software packages. These software packages act as black-boxes and usually hide the complicated numerical procedures and even their crucial parameters from the applier. Therefore, the question arises during industrial applications: how accurate is the simulation, and how can the results be assessed? The aim of this paper is to provide a guideline to assess the results of metal forming simulations. Although some ideas are valid for any metal forming process, bulk forming is the primary concern. The paper will address firstly the possible sources of error in a finite element analysis of bulk forming processes. Then, some useful elementary knowledge will be summarized. Various levels of validation such as result and ability validation and assessment will be discussed. Finally, interpretation of results will be treated. In this content also some suggestions will be given.Article Citation - WoS: 15Citation - Scopus: 21Comparison of Various Preforms for Hot Forging of Bearing Rings(Elsevier Science Sa, 2005) Arbak, M; Tekkaya, AE; Özhan, FProduction of bearing rings by hot forging is investigated in this study. The aim of the study is to determine a feasible preform at the first station of the forming process such that the tool wear is prolonged and tool fracture is prevented. For this purpose, it is assumed that the contact pressure at the interface between tools and workpiece is the predominant process parameter. The contact pressures are determined by precise thermo-mechanical coupled finite element analyses based on elastic-plastic material description. Material flow curves for various temperatures and strain-rates are determined in velocity controlled upsetting tests. Cooling experiments are used to determine the heat transfer coefficients. Accuracy of the numerical models has been verified by extensive numerical convergence studies and finally by comparing with experimental measurements. The analysed preforms are evaluated using a scheme of weight-factors for the various tool parts. Finally, a preform is suggested for which the weighted total tool pressure could be reduced by 15%. (c) 2005 Elsevier B.V. All rights reserved.Article Citation - Scopus: 1Assessment and Improvement of Elementary Force Computations for Cold Forward Rod Extrusion(Springer Heidelberg, 2005) Öcal, M; Egemen, N; Tekkaya, AE; Manufacturing EngineeringTwo commonly used analytical force computation methods for cold forward rod extrusion are evaluated by means of precise finite element computations. The upperbound model by Avitzur based on the spherical velocity field and the model by Siebel based on a quasi-upper-bound solution are considered. It has been found that the pure deformation forces obtained by summing the ideal force and shear force terms deviate between +25% and -20% from the finite element solutions. Larger deviations, however, occur for the Coulomb-friction term in the container. A new model based on an elasto-static analysis combined with numerical analysis is suggested to compute this term. This new model supplies also the accurate pressure distribution within the container.
