Master Degree / Yüksek Lisans Tezleri

Permanent URI for this collectionhttps://hdl.handle.net/11147/3008

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COAR Access Rights: search.filters.coarAccessRights.open accessSubject: search.filters.subject.Mechanical properties

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Now showing 1 - 4 of 4
  • Master Thesis
    The Constitutive and Damage Models of Additively Manufactured Ti6al4v Alloy
    (01. Izmir Institute of Technology, 2021) Hızlı, Burak; Güden, Mustafa
    Electron Beam Melting (EBM) is one of the metal additive manufacturing methods that enable the fabrication of Ti6Al4V alloy parts with intended shapes in where this alloy is of significant interest such as aerospace and biomedical industries due to its outstanding properties. In this study, the microstructural and mechanical properties of EBM-produced Ti64 were comprehensively investigated. Microstructural analysis was conducted on as-built specimens. Microstructural analysis showed that EBM-produced Ti64 possesses α+β duplex phase with directional microstructural alterations and high porosity fraction in the part volume. Mechanical properties were investigated under tension loadings at quasi-static rates (0.001-0.1 1/s) and compression loading at quasi-static and high strain rates (0.001-2154 1/s). Thereafter, Johnson-Cook (JC) strength and damage models were individually calibrated from the experimental results of tension and compression behaviors and experimental fracture strains in order to numerically predict the material flow behavior of EBM-produced Ti64 considering the strain, strain rate, and temperature effects in the case of various loadings combined with temperature changes. EBM-produced Ti64 exhibited proximate mechanical properties in terms of tension and compression behaviors, however extremely low ductile behavior under tension loadings resulting premature failure without necking. Eventual fracture of this material occurred via tearing of the scanned layers for tension loadings and shear crack following the shear band formation propagation on 45° to loading axis for compression loadings. Calibrated JC strength and damage models for EBM-produced Ti64 were able to predict flow behavior and fracture strains within strain rate range between 0.001 and 1000 1/s. However, the JC strength model could not predict the flow behavior at excessively high strain rates (2154 1/s) due to complex deformation mechanisms including adiabatic heating.
  • Master Thesis
    The Effect of Strain Rate on the Dynamic Mechanical Behaviour of Concrete
    (Izmir Institute of Technology, 2018) Uysal, Çetin Erkam; Taşdemirci, Alper; Güden, Mustafa
    The fast-growing population of mankind has brought out household needs and working structures that might be subjected to static and dynamic loads. Impact loads and repetitive dynamic loads can produce an overload on the structures in a very short period that causes relentless casualties and unfortunate property losses. The response of the concrete material on strain rate increase is critical. The dynamic characterization of concrete, lack of adequate and consistent study causes disagreement about strain rate sensitivity of concrete, so a consensus has not been reached. In this study, quasi-static (3.55x10-5, 3.23x10-4, and 2.97x10-3 s-1) and high strain rate (140-250 s-1) tests were conducted and the effect of strain rate on the mechanical behavior of concrete was investigated both experimental and numerical. A modified Split Hopkinson Pressure Bar test setup was used, by using an EPDM (Ethylene Propylene Diene Monomers) rubber pulse shaper, non-oscillatory results and nearly constant strain rate were reached, and premature failure was prevented. Modeling the test setup was conducted in Ls-Dyna and the Holmquist-Johnson Cook material model parameters were found. A good agreement between experimental and numerical results was reached. The strength enhancements of concrete material, while increasing strain rate was noticed. Using both experimental and numerical studies, the total strength increase is due to inertia effect and strain rate sensitivity effects were observed.
  • Master Thesis
    The Development of a New Testing Methodology in Dynamic Mechanical Chracterization of Concrete
    (Izmir Institute of Technology, 2018) Seven, Semih Berk; Güden, Mustafa; Taşdemirci, Alper; Taşdemirci, Alper; Güden, Mustafa
    Concrete is one of the most used material types in the world. Due to its structural complexity and insufficient testing techniques, the dynamic mechanical behavior of concrete has not yet been revealed sufficiently. This thesis aims to develop reliable and accurate mechanical characterization methodology for concrete using the combination of experimental and numerical methods together. The dynamic mechanical characterization of concrete at quasi-static and high strain rates was performed implementing unique techniques for both experimental and numerical studies. In quasi-static testing, universal compression test machine was used with strain gage mounted specimen for better strain measurements. In high strain rate tests, two modifications were implemented on the conventional Split Hopkinson Pressure Bar (SHPB) test apparatus. The first modification is the usage of pulse shaper to obtain nearly constant strain rate and dynamic stress equilibrium in the specimen. Second, piezo-electric quartz crystal force transducers were implemented on the specimen-bar interfaces to increase accuracy and sensitivity of the force measurement on the front and back forces of the specimen. Experimental results were validated constituting numerical study using finite element tool LS-DYNA. Concrete was modeled using Holmquist-Johnson-Cook (MAT_111) material model. HJC material model parameters were determined using experimental results coupling with the numerical analysis and the mechanical behavior of concrete was constituted. It was concluded that using pulse shaper and quartz crystals pretty useful when testing concrete and other brittle materials at high strain rates. Modification of new specimen geometries on numerical analysis showed better understandings of the effect of geometry on the dynamic stress equilibrium.
  • Master Thesis
    Development of Aluminum Honeycomb Cored Carbon Fiber Reinforced Polymer Composite Based Sandwich Structures
    (Izmir Institute of Technology, 2016) Okur, Mehmet Ziya; Tanoğlu, Metin
    Lightweight composite sandwich structures are composed of composite structures that are laminated between thin stiff facesheets bonded to a thicker lightweight core. These structures have high potatial to be used in civil engineering applications, marine, aerospace industry etc. applications due to their high strength to weight ratios and energy absorption capacity. In these structures, the bending loads are generally carried by the force couple formed by the face sheets while the shear loads are carried by the lightweight core materials. Main purpose of the core material is to provide a high moment of inertia. Therefore, under flexural loading, sandwich panels have higher specific mechanical properties relative to the monocoque structures. Also, the core resists transverse forces and stabilizes the laminates against global buckling and local buckling. The resulting structure provides increased buckling resistance and its rigidity. In this study, sandwich composite structures were developed with carbon fiber reinforced polymer composite facesheets and the cores made by Aluminum (Al) based honeycomb with various thicknesses. Carbon fiber/epoxy composite facesheets were fabricated with non-woven unidirectional (UD) fabrics (with 0o/90o orientation) and epoxy resin by vacuum infusion technique. Al honeycomb layers were sandwiched together with carbon/epoxy facesheets using a thermosetting adhesive. Mechanical tests were carried out to determine the mechanical behavior of face sheets, aluminum cores and the composite sandwich structures. Effect of core thickness on the mechanical properties of the sandwich structures was investigated.