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    A new method for determining limit strains of materials that show post-uniform elongation behavior
    (SAGE PUBLICATIONS LTD, 2014) Turkoz, Mevlut; Halkaci, Huseyin S.; Yigit, Osman; Dilmec, Murat; Ozturk, Fahrettin
    In this study, a new method is proposed to determine limit strains at the onset of localized necking for ductile materials that show post-uniform elongations. The new method is first applied for AA 5754-O using the ISO 12004-2 forming limit diagram determination standard. The method is also applied for ductile materials of 7114 steel, 304 stainless steel, and CuZn37 brass and finally for AA 2024-T4 having brittle fracture behavior. The results indicate that the new proposed method is quite successful, easy, and accurate for ductile materials that show post-uniform elongations.
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    A novel approach to enhance formability in Ti-6Al-4V alloy: Experimental investigations and microstructural analysis of pulsating tensile test
    (Elsevier, 2024) Korkmaz, Habip Gokay; Yapan, Yusuf Furkan; Toros, Serkan; Turkoz, Mevlut
    Ti-6Al-4V alloy, widely utilized in aerospace, medical industries, and specialized applications, boasts exceptional properties. However, its limited formability poses challenges in manufacturing processes. The pulsating loading method emerges as a promising solution to enhance formability in such materials. This study delves into the impact of stress relaxation and loading-unloading tests on the formability of Ti-6Al-4V alloy, conducting tensile tests on sheets of two different thicknesses: 0.5 mm and 2.65 mm. Investigating parameters such as pulse starting strain, relaxation time, and strain increment in stress relaxation experiments, as well as unloading ratio and strain increment in loading-unloading experiments, enabled a comprehensive comparison of the two pulsating loading methods across different sheet thicknesses. Results indicate a notable increase in material formability, up to approximately 20 % for the 2.65 mm thickness and up to 50 % for the 0.5 mm thickness compared to monotonic loading. Stress relaxation time emerged as the most influential parameter for both thicknesses. Additionally, XRD analysis was employed to elucidate the microstructural reasons behind the observed formability enhancement, while SEM imaging provided insights into the fracture surface morphology. This systematic approach sheds light on the microstructural mechanisms underlying the effect of pulsating loading on material behavior.
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    Detailed Investigation of Forming Limit Determination Standards for Aluminum Alloys
    (AMER SOC TESTING MATERIALS, 2013) Dilmec, Murat; Halkaci, H. Selcuk; Ozturk, Fahrettin; Turkoz, Mevlut
    In this study, experimental studies were conducted to evaluate the differences between the ASTM E2218-02 and ISO 12004-2 standards that are used for construction of the forming limit curve (FLC) and that made various assumptions, which create dissimilar FLCs for the same material. The comparison was made for two materials which have moderate brittle and ductile characteristics, AA2024-T4 and AA5754-O alloys, respectively. The effects of a specimen's geometry, lubrication condition, and determination methods of limit strains on FLCs were considered and compared. Because the same strain evaluation method should be used for the standards, so as to be able to investigate the effect of only standards, a simple method in the computer grid analysis system was used. To test the validity and the reliability of the method, limit strains on the same specimens were also determined with using a real-time measurement method for the ISO experiments, and the results reveal that the method is reliable. Failure mechanisms were inspected for further investigation. The Nakajima specimens formed with the two standards showed different failure mechanisms. Finally, conducting the case studies, it was concluded that ISO 12004-2 yields more reliable and reproducible results than the ASTM standard.
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    EFFECT OF SHEET THICKNESS ON THE ANISOTROPY AND THICKNESS DISTRIBUTION FOR AA2024-T4
    (INST ZA KOVINSKE MATERIALE I IN TEHNOLOGIE, 2013) Dilmec, Murat; Halkaci, Huseyin Selcuk; Ozturk, Fahrettin; Turkoz, Mevlut
    In this study, the effect of sheet thickness on the anisotropy and thickness distribution at room temperature (RT) was investigated for AA2024-T4 sheets. The anisotropy was determined using automated strain measurement with a grid analysis and profile-projector methods. The results indicate that the effects of the thicknesses of 0.8 mm, 1 mm, and 2 mm on the anisotropy were insignificant. In addition to the anisotropy measurement, the thickness variation of the specimens was also monitored. Besides the anisotropy values, no significant differences were observed between various thicknesses and directions.
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    Effect of sheet thickness on the anisotropy and thickness distribution for AA2024-T4
    (2013) Dilmec, Murat; Halkaci, Huseyin Selcuk; Ozturk, Fahrettin; Turkoz, Mevlut
    In this study, the effect of sheet thickness on the anisotropy and thickness distribution at room temperature (RT) was investigated for AA2024-T4 sheets. The anisotropy was determined using automated strain measurement with a grid analysis and profile-projector methods. The results indicate that the effects of the thicknesses of 0.8 mm, 1 mm, and 2 mm on the anisotropy were insignificant. In addition to the anisotropy measurement, the thickness variation of the specimens was also monitored. Besides the anisotropy values, no significant differences were observed between various thicknesses and directions.
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    Experimental and numerical investigation of the damage state of Ti-6Al-4V alloy sheet in the tensile test, hydraulic bulging, and hydroforming processes
    (Springer London Ltd, 2024) Yapan, Yusuf Furkan; Korkmaz, Habip Gokay; Toros, Serkan; Turkoz, Mevlut
    There has not been any damage prediction using Johnson-Cook's (JC) hardening and damage model in the hydraulic bulging (HB) and hydroforming (HF), which are the advanced manufacturing processes, of the Ti-6Al-4V (Ti64) alloy. In the presented study, the damage behavior of the Ti64 alloy sheet in the HB and HF processes was investigated both experimentally and numerically for the first time to address the existing research gap. In this context, firstly, tensile tests (TT) were carried out on samples with different stress triaxiality values at three different tensile speeds, and the fracture morphologies of the samples were examined to evaluate whether it was appropriate to use the JC hardening and damage model. Since the fracture surfaces generally exhibit a ductile fracture morphology and are affected by stress triaxiality and strain rate, it was determined that it would be appropriate to use the JC hardening model and damage criterion to predict the damage of the Ti64 alloy in finite element analysis (FEA). Then, JC model parameters were determined by fitting the stress-strain curve obtained from the FEA and experimental tensile tests. In the HB experiments, bulging height and thickness thinning were predicted by FEA with an accuracy of 97% and 96.85%, respectively. In the HF experiments, the experimental burst pressure, die inlet radius, and base radius were predicted correctly at a rate of 92.5%, 95.5%, and 97.8%, respectively. Also, the thickness of the sample showed good agreement with the FEA results. The fracture zones in each process exhibited good agreement with the experimental results. Thus, it has been demonstrated that the JC damage criterion can be successfully applied in FEA if the Ti64 titanium alloy is damaged in various processes.
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    Formability improvement in Ti-6Al-4 V sheet at room temperature by pulsating hydraulic bulging: experimental and numerical investigations
    (Springer London Ltd, 2023) Ozturk, Osman; Korkmaz, Habip G.; Atas, Gurkan; Aydin, Mevlut; Turkoz, Mevlut; Toros, Serkan; Dilmec, Murat
    Ti-6Al-4 V sheets possess limited formability at room temperature due to low ductility with almost no strain hardening. Pressure pulsation during hydroforming may bring significant improvement as an alternative to the widespread solution hot forming. However, much uncertainty exists on the deformation mechanism and effects of pulsating on difficult-to-form materials. In this study, the effect of pulsating pressure on the hydraulic bulge test was investigated to increase the limited formability of the Ti-6Al-4 V sheet at room temperature. Experimental results of thickness distribution and bulge height obtained from the bulge tests were compared with the finite element simulation results. The results show that the tests with pulsation allow a higher thickness reduction with a slightly more homogenous thickness distribution. Pulsation causes a delay in the material's failure resulting in a 15.4% increase in the dome height with a 17% increased burst pressure compared to monotonic loading. The underlying microstructural phenomena of increased formability were elaborated using dislocation estimations, fracture surface analysis, and hardness. Test results suggest that pulsation improves formability by 47% in terms of maximum elongation due to stress relaxation.