Browsing by Author "Toparli, M"

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    Mechanical properties of magnesia and magnesia-chromite refractory materials
    Köksal, NS; Toparli, M; Altun, IA
    In this study, modulus of rupture, elastic modulus and fracture toughness values were investigated at elevated temperature for magnesia and magnesia-chromite content of chromite between 6% and 20%. Modulus of rupture and elastic modulus values were calculated by three point bending test at temperatures 400, 600, 800 and 1400 degreesC. Modulus of rupture and Young's modulus values decreased when temperature and content of chromite increased. Modulus of rupture and Young's modulus values at room temperature were compared to the values at 400, 600, 800 and 1400 degreesC. Young's modulus values decreased to about 40% and modulus of rupture values decreased to about 25% of the values at room temperature. The samples were notched as a/h = 0.3 by cutting tool of 0.2 mm thickness. Fracture toughness values were calculated at each temperature. Fracture toughness value at room temperature was compared to the values at 400, 600, 800 and 1400 degreesC. It was seen that fracture toughness values had decreased by the increase at temperature.
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    FINITE-ELEMENT MODEL OF THE INDENTATION FOR YBCO-BASED SUPERCONDUCTOR THIN FILMS
    Culha, O; Turkmen, I; Toparli, M; Celik, E
    Superconducting films with poor mechanical properties are useless even if they possess good transport and flux-pinning properties. Since additive particles as pinning centers are important changes in a microstructure, their effect on the micromechanical properties such as Young's modulus and hardness have to be investigated with respect to the additional-particle type and quantity, using experimental and numerical methods. In this study, films were dip-coated onto (001) SrTiO3 (STO) single-crystal substrates with metalorganic deposition using the trifluoroacetate (TFA-MOD) technique. The phase analysis and the microstructure of the superconducting thin films were determined with an X-ray diffractometer (XRD) and a scanning electron microscope (SEM). The mechanical-property variations of the pure YBCO and the YBCO thin films with Mn (reacting as BaMnO3) were experimentally obtained with nanoindentation techniques. Thus, the BaMnO3 nanoparticle effects on the structural and mechanical properties of the films were observed. According to the nanoindentation results, the Young's modulus and indentation hardness of the films decreased from 88.54 GPa to 76.47 GPa and from 12.51 GPa to 3.88 GPa, respectively, depending on the additive particles. In addition, the finite-element modeling (FEM) of the indentation was applied to estimate the failure stress/stress distribution relation at the contact region between the indenter and the surface of a YBCO-based thin film, obtaining the same force/penetration depth curve as with the indentation experiment. According to these main aims of FEM, the mesh-design effect, material properties and the boundary condition of the axisymmetric model were chosen and optimized to obtain the mechanical results of the instrumented indentation.
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    Calculation of temperature and thermal stress in refractory materials quenched in water using finite element methods
    Köksal, NS; Toparli, M
    In this study, temperature and thermal stress distribution calculated using a finite element method for magnesia and magnesia-chromite content of chromite between 6% and 15%. The thermal and mechanical properties of the materials were measured using standard laboratory procedure. The samples were dried at 110 degreesC and then transferred into electric furnace at 400, 600 and NOT and held there for 30 min. Samples were then moved into water and left there for 3 minutes. A quenching simulation was made for three different temperatures (400, 600 and 800 degreesC) using ANSYS 6.0 package. The finite element study was carried out using two-dimensional models. The refractory material was assumed to be homogenous and isotropic. Time dependence of temperature and thermal stress were calculated in surface and center of the plate specimen subjected to the thermal shock by quenching in water.
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    Hardness and yield strength of dentin from simulated nano-indentation tests
    Toparli, M; Koksal, NS
    The finite element method (FEM) is applied for studying the hardness (H) and yield strength (Y) of dentin subjected to a nano-indentation process. The nanoindentation experiments were simulated with the ABAQUS finite element software package. This test, performed with a spherical indenter, was simulated by axisymmetric finite element analysis. The load versus displacement was calculated during loading-unloading sequence for different elastic modulus (E) and yield strength. Hardness and maximum principal compressive and tensile stresses were plotted for different elastic modulus depending on yield strength. The dentin was assumed to be isotropic, homogenous and elasto-ptastic. The theoretical results outlined in this study were compared with the experimental works reported in the literature and then hardness and yield strength of dentin was estimated. (c) 2004 Elsevier Ireland Ltd. All rights reserved.
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    Characterization and determination of FexB layers' mechanical properties
    Culha, O; Toparli, M; Sahin, S; Aksoy, T
    The aim of this study is to obtain microstructural characteristics and investigate the mechanical properties such as hardness, Young's modulus and fracture toughness of the boride layer depending on process time and temperature. The produced double layers (FeB and Fe2B) were extensively analyzed with respect to X-ray, diffraction (XRD) and scanning electron microscopy (SEM). The XRD pattern of the boride layers, which were formed on SAE 1020 and 1040 quality steel at 900 degrees C for 2 h, 4 h and 6 h, include only FeB phase on surface with (111), (2 10), (10 1) and (111) planes. SEM cross-sectional investigations show that double-phase boride layer existence from surface to inside of substrate. The structural compositions of layers consist of boron rich phase (FeB) and iron rich phase (Fe2B), respectively. Surface roughness value of samples is important parameter for micro-indentation test with Dynamic Ultra Micro Hardness Tester. After diffusion controlled boriding process, material surface roughness can be high for micro-indentation test. So, surface polishing process is applied for decreasing roughness of FeB layers. Surface roughness values of 2 h, 4 h and 6 b borided SAE 1020 and SAE 1040 quality steel were decreased from 0.9 mu m to 0.05 mu m by polishing process. Mechanical properties of layers were examined by Shimadzu Dynamic Ultra-microhardness test machine for estimating Young's modulus due to load-unload sensing analysis and in addition to mechanical investigation hardness-depth curves of the layer were obtained for estimating indentation depth and load dependency of mechanical properties. Load depended elastic modulus (12S-624 GPa) and hardness (17-33 GPa) were obtained at 80 mN, 160 mN, 320 mN and 640 mN applied peak loads depending on boriding process time. Fracture toughness proper-ties of FeB surface layers were calculated by Vickers Fracture Toughness method for 1 N applied peak load with measuring crack length after loading stage was finished. (c) 2007 Elsevier B.V. All rights reserved.
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    Residual thermal stress analysis in cylindrical steel bars using finite element method and artificial neural networks
    Toparli, M; Sahin, S; Ozkaya, E; Sasaki, S
    In this study, it was proposed that the residual stresses within steel bars after quenching in water from 600 degreesC could be calculated by using the finite element method (FEM) and an artificial neural network (ANN) algorithm. Three modelled cylindrical specimens of AISI 1020 steel were heated and then quenched in water. Using FEM, temperature distribution with time and thermal residual stress values in the samples were calculated after cooling. The analysis was extended to elastic-plastic deformation during the quenching of steel cylinders of various diameters. The calculated temperature and thermal residual stress values were used in training a multi-layer, feed forward, back propagation ANN algorithm. The results obtained via the ANN algorithm method have been compared with the FEM results. Comparison showed good agreement. (C) 2002 Elsevier Science Ltd. All rights reserved.
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    Synthesis and characterization of Ag doped TiO2 heterojunction films and their photocatalytic performances
    Demirci, S; Dikici, T; Yurddaskal, M; Gultekin, S; Toparli, M; Celik, E
    In this study, undoped and silver (Ag) doped titanium dioxide (TiO2) films were successfully synthesized by sol-gel spin coating technique on the Si substrates. Photocatalytic activities of the TiO2 films with different Ag content were investigated for the degradation of methylene blue (MB) under UV light irradiation. The crystal phase structure, surface morphology, chemical and optical properties of Ag-doped TiO2 films were characterized using an X-ray diffractometer (XRD), scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS), UV-vis spectrophotometer, and FTIR spectrophotometer. The results showed that the Ag-doped TiO2 films calcined at 500 degrees C had the crystalline anatase phases and the surface morphologies with some cracks. Ag substitution into TiO2 matrix enhanced the photocatalytic activity of TiO2 films under UV light irradiation as compared to the undoped TiO2 film. Furthermore, the results indicated that the 0.7% Ag doped TiO2 film exhibited a superior photocatalytic activity than that of undoped and other Ag-doped TiO2 films. This study demonstrated the potential of an application of Ag doped films to efficiently treat dissolved organic contaminants in water. (C) 2016 Elsevier B.V. All rights reserved.
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    CHARACTERIZATION AND DETERMINATION OF MECHANICAL PROPERTIES OF YBCO SUPERCONDUCTING THIN FILMS WITH MANGANESE USING THE TFA-MOD METHOD
    Culha, O; Birlik, I; Toparli, M; Celik, E; Engel, S; Holzapfel, B
    The aim of this study is to determine the microstructure, superconducting and mechanical properties of YBa2Cu3O6 56 (YBCO) and YBCO thin films with a manganese (Mn) addition. All the YBCO superconducting films (undoped and Mn-doped) were dip-coated onto (001) SrTiO3 (STO) single-crystal substrates with a metalorganic deposition using the trifluoroacetate (TFA-MOD) technique. The phase analysis, microstructure, surface morphologies and critical temperature (T-c) of the superconducting thin films were determined with an X-ray diffractometer (XRD), a scanning electron microscope (SEM), an atomic force microscope (AFM) and an inductive T-c measurement system. Since the main issue of this study is to determine the mechanical-property variations of the superconducting thin films with/without a Mn addition, the adhesion strength of these films on a STO substrate was tested with a Shimadzu scratch tester. Depending on the Mn addition, the critical forces of pure films increase from 56.23 mN, 58.63 mN and 60.11 mN for pure YBCO, YBCO with 0.05 g and 0.10 g of Mn. Furthermore, Young's modulus and the hardness of the undoped and Mn-doped YBCO thin films were measured with a CSM Berkovich nanoindenter using the load-unload sensing analysis under a 0.3 mN applied load.
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    Fabrication and characterization of nanostructured anatase TiO2 films prepared by electrochemical anodization and their photocatalytic properties
    Yurddaskal, M; Dikici, T; Yildirim, S; Yurddaskal, M; Toparli, M; Celik, E
    In this study, nanostructured anatase titanium dioxide (TiO2) films were fabricated by electrochemical anodization of titanium first, and then annealed at 500 degrees C for 2 h. Effect of electrolyte concentration, anodization time and electrolyte temperature on the surface morphology of the resulting TiO2 thin films were investigated. The phase structures, surface morphology and chemical composition were analyzed using X-ray diffraction (XRD), scanning electron microscope (SEM), atomic force microscopy (AFM) and X-ray photoelectron spectroscopy (XPS). The photocatalytic activity tests of the samples were evaluated by the degradation of aqueous methylene blue (MB) solutions under UV light illumination for different periods of time. The results showed that the structure of nanostructured TiO2 films depended strongly on the anodization parameters. It was found that there were micro-scale pores (<10 mu m) and nano-scale pores (diameter in the range from 40 to 70 nm) on the anodized titanium surfaces. This study indicated that structures, surface morphology, and surface area of the nanostructured anatase TiO2 films played an important role on their photocatalytic performance. The results clearly proved that nanostructured anatase TiO2 film prepared with optimum process parameters resulted in enhancement of the photocatalytic activity. (C) 2015 Elsevier B.V. All rights reserved.
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    Heat Treatment Effects on Mechanical Properties of Atmospheric Plasma Sprayed FexB Coatings on Al Substrate
    Culha, O; Sahin, S; Ozdemir, I; Toparli, M
    In this work, we draw attention to determination of heat-treatment effects on mechanical properties of atmospheric plasma sprayed (APS) FexB coatings on aluminum substrates by micro-indentation technique. With this regard, iron boride powders of Fe-18.8B-0.2C-0.5Si-0.8Al (wt.%) were deposited onto Al substrates by APS in order to improve the mechanical properties of Al surface. As-sprayed coatings are composed of mainly FexB and iron matrix supersaturated with boron owing to the rapid solidification of molten droplets flattened on a substrate. It was observed that APS coatings exhibited characteristic wavy layered structure having porosity, inclusions, and semi-melted particles. The postfurnace treatment of APS coatings was carried out at temperatures ranged from 450 to 550 degrees C in an argon atmosphere. The post-treatment applied for APS deposits led to increase in hardness of 40% without showing cracks. Furthermore, micro-mechanical properties of FexB coatings were examined by Shimadzu Dynamic Ultra-Microhardness Tester for estimating Young's modulus and hardness due to load-unload sensing analysis by applying different loads such as 160, 320, and 640 mN to determine load and indentation depth dependency of APS FexB on Al substrate for each samples, in details.
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    A calculation of stress distribution in metal-porcelain crowns by using three-dimensional finite element method
    Aykul, H; Toparli, M; Dalkiz, M
    The objective of this study was to calculate stress distribution in a maxillary second premolar tooth which occurred by the mastication force. The tooth model was crowned with Au-Pd alloy, Ni-Cr alloy and porcelain. A load of 450 N, at an angle of 45degrees to the longitudinal axis was applied on the occlusal margin of the crown tooth. The tooth was assumed isotropic, homogenous, elastic and unsymmetrical. This numerical study was carried out using three-dimensional finite element models and calculation programs were prepared by the authors using FORTRAN 77. The distribution of compressive, tensile and shear stress were plotted for the dentine, dentine-metal and metal-porcelain interfaces. The highest stress values were observed when Ni-Cr alloy and porcelain was used.
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    Modelled and measured residual stresses in a bimaterial joint
    Sahin, S; Toparli, M; Ozdemir, I; Sasaki, S
    A finite element technique has been used to predict residual and thermal stresses due to welding. For this purpose, a steel-brass material couple was chosen and thin plates of the materials were hard brazed. The finite element study was carried out using two-dimensional models. After the temperature distributions as a result of welding were calculated, thermal and residual stress values obtained. Thermo-elasto-plastic formulations using a von-Mises yield criterion with linear isotropic-hardening were employed. For this deformation, the initial stress method was used and the kinematical Bauschinger effect was considered. The authors prepared all calculation programs using FORTRAN 77. To obtain residual stresses that occur during the welding, the hole-drilling strain-gage method was chosen and conducted in accordance with the ASTM Standard E 837-99. The agreement between the calculated results and the experimental data shows that the finite element analysis method is reliable. (C) 2002 Elsevier Science B.V. All rights reserved.