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YAPICI, Güney Güven

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Güney Güven

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Now showing 1 - 10 of 67
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    Optimization of the intermediate layer friction stir spot welding process
    (Springer Nature, 2019-09) Bajilane, Isam Jabbar Ibrahim; Yapıcı, Güney Güven; Mechanical Engineering; YAPICI, Güney Güven; Bajilane, Isam Jabbar Ibrahim
    Friction stir spot welding was performed for joining sheets of 2024 and 6061 aluminum alloys, which is otherwise difficult using conventional welding techniques. The presented approach utilizes an intermediate layer to avoid the keyhole problem. Design of experiment analysis was carried out to evaluate the influence of process parameters. The optimized set of parameters led to the fabrication of sound joints with strength properties exceeding twice the applicable standard requirements as discussed with the evidence of branched hook formations with extensive penetration. Tool rotational speed was determined to be the most significant parameter influencing the mechanical performance. The failure mode revealed itself as sheet tearing-nugget pull out in the joints produced under optimum conditions with various sized dimples apparent on the fracture surface.
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    On the high-temperature flow response of friction stir processed magnesium metal matrix composites
    (ASME, 2021-01-01) Ghobadlou, Alı Hosseınzadeh; Yapıcı, Güney Güven; Mechanical Engineering; YAPICI, Güney Güven; Ghobadlou, Alı Hosseınzadeh
    In the current work, multi-pass friction stir processing (FSP) was utilized to fabricate samples of fine-grained aluminum-zinc (AZ) magnesium alloy and its metal matrix composite (MMC). The microstructure and high-temperature tensile behavior of friction stir processed (FSPed) AZ31 and AZ31/SiC MMC at various strain rates in the range of 10(-2) to 10(-4) s(-1) were investigated, and the fracture mechanisms of each condition were analyzed. The results verified that MMC samples exhibited a remarkable enhancement in microhardness. The evolution of inclined basal texture was observed after processing for both FSPed and MMC samples. The ambient temperature stress-strain response revealed that the formability of AZ31 has improved after friction stir processing, whereas high-temperature flow curves were discernibly sensitive to strain rate. Equiaxed deep dimples were detected on the fracture surfaces of FSPed samples, but decreased strain rate led to an increase in the number of dimples as attributed to the recrystallization of new grains.
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    Effect of purity levels on the high-temperature deformation characteristics of severely deformed titanium
    (Springer International Publishing, 2017) Sajadifar, Seyed Vahid; Yapıcı, Güney Güven; Mechanical Engineering; YAPICI, Güney Güven; Sajadifar, Seyed Vahid
    In the present investigation, high-temperature compression tests were conducted at strain rates of 0.001 to 0.1 s−1 and at temperatures of 873 K to 1173 K (600 °C to 900 °C) in order to study the hot deformation characteristics and dynamic softening mechanisms of two different grades of commercial purity titanium after severe plastic deformation. It was observed that the effects of deformation rate and temperature are significant on obtained flow stress curves of both grades. Higher compressive strength exhibited by grade 2 titanium at relatively lower deformation temperatures was attributed to the grain boundary characteristics in relation with its lower processing temperature. However, severely deformed grade 4 titanium demonstrated higher compressive strength at relatively higher deformation temperatures (above 800 °C) due to suppressed grain growth via oxygen segregation limiting grain boundary motion. Constitutive equations were established to model the flow behavior, and the validity of the predictions was demonstrated with decent agreement accompanied by average error levels less than 5 pct for all the deformation conditions.
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    Low-cycle fatigue behavior of friction stir-welded copper joints
    (Springer, 2021-11) Ghobadlou, Ali Hosseinzadeh; Salahi, Salar; Radi, Amin; Sajadifar, Seyed Vahid; Yapıcı, Güney Güven; Mechanical Engineering; YAPICI, Güney Güven; Ghobadlou, Ali Hosseinzadeh; Salahi, Salar; Radi, Amin; Sajadifar, Seyed Vahid
    Investigation of the fatigue response of friction stir-welded (FSWed) joints is especially important in the design and manufacturing of components with exposure to cyclic loading. In this study, cyclic response of FSWed pure copper joints is investigated in the low-cycle fatigue regime. Microstructural characterizations revealed that FSW introduced a severely deformed microstructure in the nugget zone (NZ). Fatigue response was determined at a strain ratio of 0.1 by varying the total strain amplitude from 0.1 to 0.6%. Cyclic softening was observed for the low strain amplitude of 0.1%, whereas hardening was detected at higher strain amplitudes. The hysteresis loops demonstrated symmetricity along with noticeable linear behavior after the reversals. Typical fractures occurred in the heat affected zone (HAZ) rather than the NZ or the base metal due to grain coarsening of the HAZ. Improved cyclic properties of the NZ along with stable behavior up to 1000 cycles at a total strain amplitude of 0.3% were attributed to its fine and homogeneous microstructure. Moreover, fracture surface analysis demonstrated a ductile behavior represented by dimples in the sample strained at 0.1% in contrast with a brittle fracture surface of the sample fatigued at 0.5% strain amplitude.
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    ReviewPublicationOpen Access
    Graphene as a piezoresistive material in strain sensing applications
    (MDPI, 2022-01-12) Irani, F. S.; Shafaghi, A. H.; Tasdelen, M. C.; Delipinar, T.; Kaya, C. E.; Yapıcı, Güney Güven; Yapıcı, M. K.; Mechanical Engineering; YAPICI, Güney Güven
    High accuracy measurement of mechanical strain is critical and broadly practiced in several application areas including structural health monitoring, industrial process control, manufacturing, avionics and the automotive industry, to name a few. Strain sensors, otherwise known as strain gauges, are fueled by various nanomaterials, among which graphene has attracted great interest in recent years, due to its unique electro-mechanical characteristics. Graphene shows not only exceptional physical properties but also has remarkable mechanical properties, such as piezoresistivity, which makes it a perfect candidate for strain sensing applications. In the present review, we provide an in-depth overview of the latest studies focusing on graphene and its strain sensing mechanism along with various applications. We start by providing a description of the fundamental properties, synthesis techniques and characterization methods of graphene, and then build forward to the discussion of numerous types of graphene-based strain sensors with side-by-side tabular comparison in terms of figures-of-merit, including strain range and sensitivity, otherwise referred to as the gauge factor. We demonstrate the material synthesis, device fabrication and integration challenges for researchers to achieve both wide strain range and high sensitivity in graphene-based strain sensors. Last of all, several applications of graphene-based strain sensors for different purposes are described. All in all, the evolutionary process of graphene-based strain sensors in recent years, as well as the upcoming challenges and future directions for emerging studies are highlighted.
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    High temperature deformation behavior of 4340 steel: activation energy calculation and modeling of flow
    (Elsevier, 2013-12) Sajadifar, Seyed Vahid; Yapıcı, Güney Güven; Ketabchi, M.; Bemanizadeh, B.; Mechanical Engineering; YAPICI, Güney Güven; Sajadifar, Seyed Vahid
    The 4340 steel is extensively utilized in several industries including automotive and aerospace for manufacturing a large number of structural components. Due to the importance of thermo-mechanical processing in the production of steels, the dynamic recrystallization (DRX) characteristics of 4340 steel were investigated. Namely, hot compression tests on 4340 steel have been performed in a temperature range of 900–1200 °C and a strain rate range of 0. 01–1 s−1 and the strain of up to 0. 9. The resulting flow stress curves show the occurrence of dynamic recrystallization. The flow stress values decrease with the increase of deformation temperature and the decrease of strain rate. The microstructure of 4340 steel after deformation has been studied and it is suggested that the evolution of DRX grain structures can be accompanied by considerable migration of grain boundaries. The constitutive equations were developed to model the hot deformation behavior. Finally based on the classical stress-dislocation relations and the kinematics of the dynamic recrystallization; the flow stress constitutive equations for the dynamic recovery period and dynamic recrystallization period were derived for 4340 steel, respectively. The validity of the model was demonstrated by demonstrating the experimental data with the numerical results with reasonable agreement.
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    Effect of severe plastic deformation on the damping behavior of titanium
    (Elsevier, 2019-06-01) Sajadifar, Seyed Vahid; Atli, C.; Yapıcı, Güney Güven; Mechanical Engineering; YAPICI, Güney Güven; Sajadifar, Seyed Vahid
    In this work, influence of severe plastic deformation on the damping properties of commercial purity titanium is explored. Deformation induced grain refinement improved the strength at room temperature while this effect diminished at a high temperature of 600 degrees C. Ultrafine-grained titanium demonstrated higher damping capacity as rationalized with the enhanced contribution of increased defect density.
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    Effect of grain size on the very high cycle fatigue behavior and notch sensitivity of titanium
    (Elsevier, 2019-12) Sajadifar, Seyed Vahid; Wegener, T.; Yapıcı, Güney Güven; Niendorf, T.; Mechanical Engineering; YAPICI, Güney Güven; Sajadifar, Seyed Vahid
    The very high-cycle fatigue performances of coarse-grained and ultrafine-grained titanium samples with different geometries at ambient temperature and various stress amplitudes were investigated. Severe plastic deformation improves monotonic strength of titanium at the cost of a loss in ductility. Ultrafine-grained titanium demonstrates a superior fatigue performance compared to that of coarse-grained counterparts in the high-cycle fatigue regime, however, suffers notch sensitivity. Furthermore, in the very high-cycle fatigue regime stress-life curves merge unexpectedly. Microstructural inhomogeneity in the ultrafine-grained titanium is expected to be the reason. Analysis of fracture surfaces reveals that the formation of fatigue slip marks is evident on the fatigued samples of both microstructural states. Ultrafine-grained titanium is more prone to the intergranular fracture.
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    On the low-cycle fatigue behavior of a multi-phase high entropy alloy with enhanced plasticity
    (Elsevier, 2023-08) Radi, Amin; Sajadifar, S.; Seyedmohammadi, Seyedveghar; Krochmal, M.; Bolender, A.; Wegener, T.; Niendorf, T.; Yapıcı, Güney Güven; Mechanical Engineering; YAPICI, Güney Güven; Radi, Amin; Seyedmohammadi, Seyedveghar
    A multi-phase non-equiatomic FeCrNiMnCo high entropy alloy (HEA) was fabricated using vacuum induction melting. Thermo-mechanical treatments consisting of cold rolling and annealing at 750 °C and 850 °C were employed to improve the mechanical properties of the HEA in focus. Tensile experiments revealed that yield strength and ultimate tensile strength levels can be enhanced significantly after thermo-mechanical processing (TMP). At the same time, ductility remains at an adequate level. Strain-controlled low-cycle fatigue (LCF) experiments were carried out in order to assess the mechanical properties of this HEA under cyclic loading conditions. At the same strain amplitude, the stress levels of the processed samples were considerably higher than that of the as-received counterpart. Similarly, fatigue lives of the former could surpass the base condition at the strain amplitudes of 0.2% and 0.4%; however, at the higher strain amplitudes, cyclic softening was observed. Electron backscatter diffraction (EBSD) and X-ray diffraction (XRD) results revealed that phase transformation from face-centered cubic (FCC) to body-centered cubic (BCC/B2) took place at a higher occurrence with increasing strain amplitude (0.2% to 0.6%). Furthermore, transmission electron microscopy (TEM) studies confirm that upon tensile deformation additional plasticity mechanisms, i.e., deformation twinning and phase transformation, contribute to the overall mechanical behavior of the multi-phase HEA.
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    A comparative study on phenomenological and artificial neural network models for high temperature flow behavior prediction in Ti6Al4V alloy
    (Elsevier, 2022-12) Uz, M. M.; Hazar Yoruç, A. B.; Çokgünlü, Okan; Aydoğan, C. S.; Yapıcı, Güney Güven; Mechanical Engineering; YAPICI, Güney Güven; Çokgünlü, Okan
    Due to its critical use in lightweight components requiring elevated temperature operation, it is very important to determine and model the high temperature thermomechanical flow behavior of Ti6Al4V. In this study, uniaxial tensile tests were performed at quasi-static strain rates and at temperatures ranging from 500 °C to 800 °C. The ductile behavior provided at a temperature of 800 °C and at a strain rate of 0.001 s−1 can be preferred for forming operations due to the steady state flow behavior. However, stress peaks during deformation at the strain rates of 0.1 s−1 and 0.01 s−1 are indicative of an unsafe zone. For modeling the flow stress behavior, three models including the Artificial Neural Network, Modified Hensel-Spittel and Arrhenius are employed with varying prediction performance as shown by the correlation coefficient (R) and average absolute relative error (AARE) values. Accordingly, the Artificial Neural Network model is claimed to be a more suitable approach for capturing the mechanical behavior of Ti6Al4V within the forming temperature range utilized in this study.