Master Degree / Yüksek Lisans Tezleri
Permanent URI for this collectionhttps://hdl.handle.net/11147/3008
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Master Thesis The Investigation of the Dynamic Compression Characteristics of a Layered Glass System(01. Izmir Institute of Technology, 2023) Ağırdıcı, Burak; Taşdemirci, AlperLayered glass structures are one of the most common material types used in air, land, and sea vehicles. Since these structures are exposed to external impact loads, it is important to determine their dynamic mechanical behavior. In this study, dynamic compression characteristics of the layered glass system were investigated numerically using the LS-DYNA finite element program. The Johnson Holmquist Ceramics material model was used for the glass layer, the Ogden Rubber material model, which is used in material models with high elastic structural behavior was used for the polyvinyl butyral (PVB) interlayer, and the SAMP-1 material model was used for the polycarbonate interlayer. Numerical studies were carried out to investigate the stress wave propagation, the amount of energy released, and the deceleration rate of the penetration velocity. Split Hopkinson Pressure Bar setup was used to numerically load the layered glass systems at high strain rates for a reliably easy controlled wave generation. The layered glass structure consisting of two interlayer types with different thicknesses was loaded in the SHPB system, and the effect of the interlayer material type and thickness on the stress wave propagation was investigated. Then, the projectile impact test was modeled at different impact velocities for a square plate of PVB-layered glass structure. The thickness of the PVB interlayer was kept constant, while the thickness and location of the glass layer varied. From the results, the slowing rate of the projectile, the amount of erosion energy, and the energy balance were determined.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, MustafaThe 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, MustafaConcrete 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.