Yazar "Elkoca, Oktay" seçeneğine göre listele

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    Determination of Modified Mohr-Coulomb Damage Model Parameters for DH780 Steel in Finite Element Analysis
    (Düzce Üniversitesi, 2023) Civek, Tolgahan; Şen, Nuri; Elkoca, Oktay
    In sheet metal forming processes, tearing problems might be occasionally encountered due to many reasons such as incorrect forming parameters. The trial and error methods that are used to solve such problems, on many occasions, are time-consuming and inefficient in terms of finding the correct forming parameters or die design for the forming process. The finite element analysis method, on the other hand, can be used as a tool that is both time and cost-saving. However, in order to effectively exploit the use of finite element analysis in sheet metal forming operations, the material that is used to be formed needs to be well characterized in terms of its hardening behaviour and failure criteria. In this study, a TRIP-aided DP steel (DH780) has been tensile tested in three different deformation conditions (uniaxial, plane stress and shear) and the parameters of its hardening model (Hollomon) and failure criteria (Modified Mohr-Coulomb) have been determined. According to the simulation results, obtained hardening parameters are able to describe the flow behaviour of the steel and the used failure criterion is able to predict the experimental failure correctly in each deformation condition.
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    DP600 Çeliğinin Farklı Süre ve Sıcaklıklarda Temperlenmesinin Mukavemet ve Delik Genişletme Oranına Etkisi
    (Düzce Üniversitesi, 2023) Albayrakoğlu, Süreyya; Şen, Nuri; Civek, Tolgahan; Elkoca, Oktay
    Gelişmiş Yüksek Mukavemetli Çelikler, şekillendirilen ürünlerin mukavemetine büyük katkı sağlayarak araçların toplam ağırlığının azaltılmasına önemli bir katkı sağlamaktadır. Bu çeliklerin mikroyapılarında bulunan çeşitli fazların birbirleri arasındaki yüksek mukavemet farklılıkları, şekillendirme işlemleri sırasında kenar yırtılma problemlerinin sıklıkla görülmesine yol açmaktadır. DP çelikleri, mikro yapılarında sertlik dereceleri birbirlerinden oldukça farklı olan ferrit ve martenzit fazlarından oluşan bir çelik olması sebebiyle kenar yırtılma problemlerinin sıklıkla görüldüğü bir AHSS çelik türüdür. Bu çalışmada, malzemenin mukavemetini önemli bir ölçüde korurken fazlar arasındaki mukavemet farkını azaltmak için DP600 çeliğine ılık sıcaklıklarda (75 °C, 375 °C) 300s, 900s, 1800s ve 3600s süreyle temperleme işlemi uygulanmıştır. Uygulanan temperleme işleminin DP600 çeliğinin mukavemetine ve delik genişleme oranına etkisini gözlemlemek amacıyla çekme testleri ve delik genişletme testleri yapılmıştır. DP600 çeliğinin 375 °C'de 900 saniye tavlanmasıyla delik genişleme oranının yaklaşık %55,12 oranında arttığı ve mukavemet azalmasının ise sadece %10,60 civarında olduğu saptanmıştır.
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    Mechanical properties of ultra-high-strength steels at elevated temperatures
    (Taylor & Francis Ltd, 2019) Şen, Nuri; Durucan, Gökhan; Elkoca, Oktay; Uygur, İlyas
    This paper presents an experimental study on the mechanical properties of ultra-high-strength steels at elevated temperatures. Tensile tests were carried out at 300-600 degrees C on Docol 1200M and Docol 1400M steel samples. The results indicate that as the temperature increases Young's Modulus, yield strength (YS) and ultimate tensile strength (UTS) display a decrease. YS/UTS ratios at 300 degrees C are lower than those at room temperature, they make peaks at 400 and 500 degrees C for Docol 1400M and Docol 1200M, respectively, and then decrease again beyond those temperatures. While total elongation continuously increases, uniform elongation slightly decreases with increasing temperature. Present carbides in tempered matrix continue to grow and new carbides are observed at the grain boundaries. Considering all roll forming parameters, 300 degrees C seems the most convenient temperature for warm forming. In this sense, the warm roll forming has a potential for forming complex-shaped parts by reconciling strength with formability.
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    A STUDY ON THE SURFACE ROUGHNESS OF GALVANNEALED LOW CARBON Al-KILLED AND Ti-Nb STABILIZED INTERSTITIAL FREE STEELS
    (Polska Akad Nauk, Polish Acad Sciences, Inst Metall & Mater Sci Pas, 2021) Sen Elkoca, Candan; Ekmekci, Bulent; Elkoca, Oktay
    In this study, the surface roughness of galvannealed low carbon Al-killed and Ti-Nb stabilized interstitial free steels was investigated using the industrial galvannealing process parameters. The iron content of the coatings was also analysed to establish a relationship with the surface roughness and coating composition. The surface roughness displayed an exponential behaviour with increasing of annealing time at each annealing temperature in both steel coatings, which was in an increasing order in the galvannealed low carbon Al-killed steel coating, whereas it was a reverse order in the galvannealed Ti-Nb stabilized interstitial free steel coating. The craters were observed on the galvannealed coatings resulting in high surface roughness. Increasing the iron content of the coatings leads to a reduction in the surface roughness with delta(1k) phase.