Master Degree / Yüksek Lisans Tezleri
Permanent URI for this collectionhttps://hdl.handle.net/11147/3008
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Master Thesis Determination of the Equivalent Stress-Strain Curves of Ductile Metals Through Image Analysis(01. Izmir Institute of Technology, 2024) Çakmak, Mehmet; Güden, Mustafa; Taşdemirci, AlperThis thesis presents a methodology for determining the equivalent stress-strain and the failure strain-stress triaxiality curves of ductile metallic materials using the advanced computing and image analysis methods. The determined curves were then used to calculate the parameters of the Johnson and Cook (JC) flow stress and damage models. A code was developed in Python to perform the numerical calculations and image analysis using the Python's libraries and image analysis tools. The main entries to the code were the experimental force-displacement curves at different strain rates, the experimental failure strain-stress triaxiality curve at a reference quasi-static strain rate, the experimental failure strain-strain rate curve at a constant stress triaxiality and the video images of the deforming test specimens. The correctness and reliability of the developed code in predicting the equivalent stress-strain curves and the parameters of the JC flow stress and damage models were clearly demonstrated for the selected 316L and AISI 4340 alloys. The code could also be easily adopted to other well-now constitutive equations commonly used in the finite element software. Finally, the results of present study contribute to the field of mechanical engineering by providing a robust tool for the materials characterization essential for designing and optimizing engineering components subjected to complex states of stresses.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 Development of Forming Simulation Methodology of a Plate Type Heat Exchanger(01. Izmir Institute of Technology, 2023) Şimşek, İbrahim; Taşdemirci, AlperIn this study, the production process of plate type heat exchangers was developed as a simulation methodology. Within the scope of the study, first, the parameters in the production process were determined. Then, mechanical characterization studies were planned with the AISI 316L stainless steel material used during production and the alternative AISI 304 stainless steel material, and the tests were completed with the support of the relevant stakeholders. The tests were determined according to the requirements of the simulation methodology. In this context, uniaxial tensile test, biaxial hydraulic bulge test and Split Hopkinson tensile tests were performed to obtain the necessary inputs for the mechanical characterization of the material and creating the material model. The material models established with the information obtained from the tests were validated with the modeling of the test setups in the numerical environment. The simulation methodology was developed in the LS-DYNA environment in the light of the process parameters obtained from the production and the data obtained from the mechanical characterization tests. The simulation model created with the developed methodology was verified because of comparison with the sample produced from AISI 316L stainless steel material taken from production. After the verified model was obtained, a simulation model was created with AISI 304 stainless steel. In addition, for the model formed with AISI 316L stainless steel, process parameters optimization study was carried out, and preliminary work activities related to reducing production times were carried out in numerical environment. After these modeling activities, the knowledge of the license plate was increased. In addition, effective plastic stress during the process, springback effect, residual stress values after springback, effective plastic strain, thickness distribution and thickness reduction values were obtained for the plate. By using the forming limit diagram of AISI 316L stainless steel, information about the final formability behavior was obtained.Master Thesis The Investigation of Energy Absorption Characteristics of Tpu Tpms Structures Subjected To Impact Loading(01. Izmir Institute of Technology, 2023) Bakıcı, Çetin; Taşdemirci, AlperIn this thesis, the energy absorption capability of a schwarz based TPMS structure both experimentally and numerically was invetigated. In the product, TPU material and FDM printer was used. Instead of the regular schwarz primitive cell structure, which has been frequently examined in the literature, the sandwich structure design was prepared with the geometry selected from the region between two cells was used and its advantages were compared. In the selection of the TPMS structure, both its high energy absorption capability per unit weight and its geometry suitable for mass production in the future was important. A hyperelastic material TPU and a printer suitable for its production were selected to show deformation behaviour of the structure against multiple loading. After material characterization with TPU specimens, the determined printer parameters were kept constant, and single and multiple cell structures were produced. Static and dynamic tests were performed, and single and multiple-cell structures were modeled and validated in the LS-DYNA finite element package program. It was observed that as the strain rate increases, the structures densification point also decreased and the first peak force and the energy absorption per unit weight (SAE) increase. In addition, it was observed that the deformation behaviour of single and multiple-cell structures were rate dependent. It has been observed that the structure with 9 cells absorbs 20% more energy than the structure with unit cell, which is 9 times higher than the unit cell structure due to the interaction of cells. The developed structure was numerically exposed to blast loads following Nato Stanag 4569 standart. In this standart, from the defined of the injury criteria,on the lower and upper tibia joint should experienced force values lower than 2.6 kN and 5.4 kN respectively. From the numerial simulations, it was found that the structure was able to mitigate the blast load transmitted to the during the accaptable limits.Master Thesis The Investigation of Static and Dynamic Compressive Deformation Behavior of a Paper Based Sandwich Material(01. Izmir Institute of Technology, 2022) İmrağ, Berkay Türkcan; Taşdemirci, Alper; Taşdemirci, AlperIn this study, dynamic and quasi-static compression behavior of paper-based honeycomb sandwich structures were investigated. It is known that the mechanical properties of paper-based honeycomb structures change with changing strain rate values. For this reason, dynamic and quasi-static loading conditions should be considered separately when investigating the compressive behavior of the structure. In the material characterization studies, a series of tests were conducted to examine mechanical properties of the paper layer material and sandwich structure. Using data from mechanical tests, numerical models were established in the finite element tool LS-DYNA. Outputs of numerical models were validated with mechanical test outputs. After the validation study, the effects that influence the dynamic compressive behavior of the paper-based honeycomb sandwich structure and their contribution percentages were investigated using the opportunities provided by the FE tool. The results showed a 150.48 % difference between the dynamic and quasi-static compressive behavior of the structure. The numerical results obtained from explicit and implicit solvers also showed good correlation with the experimental results. In addition, the micro-mechanical modeling approach in numerical models made it possible to investigate the effects such as strain rate sensitivity of the paper layer material, entrapped air inside the core cells, and micro-inertia individually. The contribution percentages of the effects were calculated by comparing the numerical and experimental results.Master Thesis On the Selection of Material Model for the 3d Printed Plastics(01. Izmir Institute of Technology, 2021) Yorulmazlar, Berika; Taşdemirci, AlperIn this study, the behavior of suitable material models which fulfil the need of representation of static and dynamic constitutive behavior ABS plastic produced with Fused Deposition Modeling (FDM) method was investigated. The accuracy of material model strongly depends on the accurate determination of its constants. These constants were obtained by conducting quasi-static and high strain rate experimental studies. The high strain rate tests of FDM built ABS samples were performed using split-Hopkinson pressure bar (SHPB) and split-Hopkinson tension bar (SHTB) and gas gun set-ups. Numerical models were conducted by using the commercial explicit finite element code LS-DYNA 971. Raw data obtained from experiments at low and high strain rates, were reduced and defined in material models. ΜΑΤ_24, ΜΑΤ_81, ΜΑΤ—187 material models were considered in numerical models to investigate the constitutive behavior of the FDM b^ilt ABS material. Good correlation was observed between the numerical and experimental data with the use of selected material models. Then, Generalized Incremental Stress-State dependent damage Model (GISSMO) was selected to characterize the failure behavior of the FDM built ABS. Parameters and curves that defines the state necking and failure occurs at, were found by using optimisation tool, LS-OPT. After observing successful match between the numerical and experimental forcedisplacement curves, GISSMO parameters were defined in SHTB and gas gun numerical models. The results showed good correlation for also the gas gun and SHTB tests in terms of failure behavior, eventually. These imply that GISSMO has the potential to predict necking and localization of deformation of the 3D-printed ABS plastics for different load cases.