Phd Degree / Doktora
Permanent URI for this collectionhttps://hdl.handle.net/11147/2869
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Doctoral Thesis Advanced Material Characterization and Modeling the Foreign Body Impact Damage Initiation and Progression of a Laminated Carbon Composite(01. Izmir Institute of Technology, 2023) Bayhan, Mesut; Taşdemirci, Alper; Güden, MustafaThe coupon level composite sample tests and the accompanying numerical models were carried out to predict the response of woven carbon fiber composite structures against impact. The numerical models of the coupon-level tests were implemented in LSDYNA software using the MAT_162 and MAT_58 composite material models. The results obtained by both quasi-static and dynamic tests were used to determine their constants. In addition to the tests that were used for the determination and calibration of the material model parameters, separate tests and their models were performed for the validation, including punch shear tests and low-velocity impact tests. It could be said that the material models examined were considered comprehensive and precise as the experimental results were well predicted by the numerical models. Also, the rate sensitivity of the woven carbon composite in the in-plane and thickness directions was investigated experimentally and numerically. In the tests, the DIC method was employed in the determination of the displacement and strain of the specimen. Based on the results obtained, it was concluded that the in-plane tensile properties are rate insensitive. Besides, the simulations of the component level tests, such as bird strike and drone impact, were established to investigate the damage initiation and propagation within the composite. It was found that the drone impact results in more severe damage compared to the bird impact. It is worth noting that the development of such precise composite material models to simulate dynamic loadings will definitely shorten the time between the beginning of designing and the component testing.Doctoral Thesis The Development of Constitutive Equations of Polycarbonate and Modeling the Impact Behavior(01. Izmir Institute of Technology, 2023) Sarıkaya, Mustafa Kemal; Güden, Mustafa; Taşdemirci, AlperThe Johnson and Cook (JC) flow stress and damage parameters of a polycarbonate were determined by the mechanical tests and numerical simulations. The experimental tests included quasi-static and high strain rate tension and compression, quasi-static notched-specimen tension, quasi-static indentation (QSI), low velocity impact (LVI) and projectile impact (PI). The flow stress equation determined from the experimental average true stress-true strain curve well agreed with the effective stress-strain obtained from the quasi-static numerical tension test. The numerical QSI force-displacement curve based on the experimental average true stress-true strain equation was further shown to be very similar to that of the experiment. The LVI and PI test simulations were then continued with the experimental average true stress-true strain equation using five different flow stress-strain rate relations: JC, Huh and Kang, Allen-Rule and Jones, Cowper-Symonds and the nonlinear rate approach. No strain rate sensitivity in the LVI tests was ascribed to low strain rate dependency of the flow stress at intermediate strain rates and large strains. On the other side, all the stress-strain rate relations investigated nearly predicted the experimental damage types in the PI tests, except the Cowper-Symonds relation which predicted the fracture of the polycarbonate plate at 140 m s-1. The absorbed energy at 160 m s-1 test was determined 1.6 times that of the QSI test, proving an increased energy absorption of the tested polycarbonate at the investigated impact velocities. The verified parameters were finally used to model the damages formed on a canopy against bird strike.Doctoral Thesis Geometric Properties of Coupler-Curve Equation of Planar Slider-Crank and Four-Bar Linkages(01. Izmir Institute of Technology, 2022) Eraz, Talha; Kiper, GökhanThis thesis study focuses on coupler-curve of planar slider-crank and four-bar mechanisms. The geometric properties of the coupler-curve equations are investigated. The coupler-curve equations of both slider-crank and four bar mechanisms are shown to consist of quadratic and linear components. The quadratic components that appear in the coupler-curve equations are circles which determine the area the coupler-curve may be located. The path generation problem of the slider-crank mechanism is another aspect of this thesis. A limited solution to the path generation problem is introduced and tested numerically. A method that is a combination of the discovered geometric properties of the coupler-curve and numerical approximation methods is introduced. The solution approach works for the task of fitting a coupler-curve on a cluster of points and five precision points problem.Doctoral Thesis Compliant Control of Robotic Co-Workers in Surgical Applications(01. Izmir Institute of Technology, 2023) Ayit, Orhan; Dede, Mehmet İsmet CanIn recent years, robots have taken place in surgical operations due to their advantages over humans, such as power, endurance, dexterity, and accuracy. Because of the lack of abilities, such as decision-making, adaptability, and creativity, human surgeons supervise the robots. The robots share the operation places with humans, called co-worker robots. Robots have the power to harm their environment; therefore, robots can generate dangerous situations for surgeons and patients. To deal with the issues, this dissertation aims to design active compliant control algorithms such as impedance control, admittance control, and hybrid position/force control to achieve safe interaction forces in surgical operations by considering the performance. The surgical co-worker robot’s type, actuation system, robot dynamics, and environment dynamics are important factors for designing the active compliant controller. Besides these, stability and robustness for safety, and agility and human effort for performance are considered for designing the controller. This dissertation takes into account three interaction scenarios encountered in surgical operations. In these scenarios, it is expected from the co-worker robot that it adapts to the sudden change in its environment dynamics. For instance, safe interaction is desired when the robot interacts with the stiff and soft tissues. To handle the issue, a switching control methodology is presented where the predefined control parameters are switched according to their environments. The methodology is implemented in a novel co-worker robot named NeuRoboScope, designed to assist the endoscopic pituitary gland surgery with the support of The Scientific and Technological Research Council of Turkey (TUBITAK). Moreover, active compliant control algorithms require a motion control algorithm as a low-level controller. In this dissertation, the computed torque method and independent joint controllers with gravity compensation are used as motion control algorithms. The computed torque method requires the dynamic model of the robot. Due to that, the dissertation proposes a simplified dynamic model with a correction coefficient for computational efficiency. ARM Cortex M4 processor runs the computed torque method with the proposed dynamic modeling method at 500 Hz. Also, this dissertation presents an independent joint controller which uses the simplified gravity matrix as a feedforward term for compensating the NeuRoboScope’s gravitational effect. The experimental results of both controllers are discussed in this dissertation.Doctoral Thesis Numerical Investigation of Gas Transport Through Micro/Nano-scale Porous Media at Slip Flow Regime(Izmir Institute of Technology, 2021) Sabet, Safa; Barışık, MuratGas flow in micro/nano-scale porous systems is observed in many applications and technologies. Gas dynamics at such small scales differ from conventional fluid dynamics estimations due to rarefaction effects. In the literature, the Knudsen number (Kn) for the characterization of rarefaction effects on permeability is calculated based on a characteristic flow height estimated from the pore size, while the geometric parameters such as pore shape and pore-throat ratios are mostly ignored. Therefore, an accurate characterization of rarefaction effects could not be ascertained. For the first time in literature, a general characterization of gas transport through systems at different porosity and pore throat size values and at different rarefaction levels was obtained using a modified Kn definition. The characteristic height required for an accurate Kn of a porous system is defined using the "equivalent diameter" calculated from the corresponding permeabilities. Pore-level calculations were performed in a wide range of systems while the observed permeability variation by porous parameters was successfully described by an extended volume-averaged model developed as a combination of the Darcy, Kozeny-Carman, and Klinkenberg models. The characterization systematic and volume-averaged model was applied for various cases of (i) two-dimensional porous, (ii) two-dimensional multi- porous, and (iii) three-dimensional complex porous system. For all these systems, the permeability values could be estimated in terms of the geometric parameters of the porous structures and rarefaction levels. In addition, the rarefaction effects on heat convection in metal foams were studied through Darcy to Forchheimer flow regimes using the Kelvin Cell structure. A 60% increase in permeability and a substantial decrease in inertial effects developed due to rarefaction, while Nusselt numbers were found mostly related to Reynolds number. Further, the influence of variation in gas thermophysical properties coupled with rarefaction as a function of increasing gas temperature for high heat flux applications was described. A 40% decrease in hydraulic conductivity for a temperature increase from 300K to 400K is observed, independent from the Kn number.Doctoral Thesis Examination of Fatigue Behaviour of Carbon Fiber Reinforced Polymer Composites(Izmir Institute of Technology, 2021) Güneş, Mehmet Deniz; Tanoğlu, Metin; Tanoğlu, MetinThis PhD thesis aims to examine the fatigue behavior of sandwich panels fabricated from adhesively bonded aluminum honeycomb core and carbon fiber reinforced polymer composite face sheets. Initially, sandwich panels were manufactured with three different amounts of adhesive in their interface. Static flexural behavior was characterized with three-point bending tests. Load-displacement curves and static flexural failure modes were obtained and utilized to compare the static flexural behavior of fabricated sandwich. Fatigue behavior of sandwich panels were characterized with the three-point bending fatigue tests. Stiffness degradation curves were used to identify the failure cycles of sandwich panels. Fatigue failure modes and S-N curves were obtained to find out the effect of amount of adhesive on fatigue behavior of sandwich panels. The other study within this thesis was made to investigate the effect of core thickness on the fatigue behavior of the sandwich panels based on aluminum honeycomb core and carbon fiber reinforced polymer composite face sheets. Sandwich panels were fabricated by using three different aluminum honeycomb core thickness. Static flexural tests were carried out to determine the static flexural behavior of developed sandwich panels. Load-displacement curves and failure modes were obtained from flexural tests. In addition to this, core shear tests were performed to investigate the core shear strength of the honeycomb cores with different core thickness. Effect of core thickness on fatigue behavior of sandwich panels were characterized with fatigue failure modes and S-N curves. Stiffness degradation method was used to determine the fatigue failure cycles of the sandwich panels.Doctoral Thesis Molecular Dynamics Studies on Interface Heat Transfer Control Using Electric Field(Izmir Institute of Technology, 2021) Yenigün, Onur; Barışık, MuratThermal management is considered as a bottleneck for the development of next generation micro/nano-electronics with high heat dissipation rates. When component sizes decrease to nanoscales, increase in surface to volume ratio leads the interfacial thermal resistance (ITR) to dominate the heat transfer behavior. The current study focuses on characterizing ITR at molecular level and exploring smart thermal management concepts for nano-scale systems. In sequence, the effect of solid thickness on ITR was investigated such that the altered phonon spectrum inside the solid domain creating the size dependency on thermal conductivity was also found to create a size dependency in ITR. Next, an active and local manipulation of heat transfer between water and various solids by an applied uniform and/or non-uniform electric field was examined. When the water molecules underwent electric field induced orientation polarization and liquid dielectrophoresis (LDEP), a substantial increase in heat transfer was developed due to the decrease in ITR and increment in thermal conductivity. Finally, an interface-localized heat transfer control technique was proposed, where interdigitated electrodes (IDEs) were embedded into the heat dissipating surface. IDEs created an electric field gradient exclusively near the electrode surface which resulted in LDEP forces on the water dipoles at near surface region enhancing solid/liquid interface energy and almost eliminating the ITR. We developed semi-empirical and theoretical relations to describe ITR variation by the electric field, which will be important for thermal management of current and future technologies.Doctoral Thesis Development and Characterization of Innovative Fiber Reinforced Prepregs and Their Composites Containing Functional Fillers(Izmir Institute of Technology, 2021) Uz, Yusuf Can; Tanoğlu, MetinThis Ph.D. thesis aims to prepare laboratory-scale carbon fiber reinforced prepregs and improve the performance of their composites by incorporating functionalized single-wall carbon nanotubes (SWCNTs). The effect of nano-scale functional fillers on the characterization of prepregs and their composites was investigated to develop innovative materials for primary structures. To affect dispersion characteristics, SWCNTs were functionalized by oxidizing their surface with the carboxyl (-COOH) group using acid treatment. The modified resin system containing 0.05, 0.1, and 0.2 wt. % F-SWCNTs were developed with novel multi-step dispersion techniques. FTIR spectroscopy was performed to identify new bonding groups formed after the covalent functionalization. Unidirectional carbon fiber reinforced prepregs with/without F-SWCNTs were prepared using a drum-type winding technique by utilizing the solvent-dip (solution impregnation) process. The effect of F‐SWCNTs on the curing process and kinetic parameters of the carbon fiber/epoxy-based prepregs were investigated using non‐isothermal DSC. The activation energy of the curing reaction was calculated by the isoconversional methods. Also, the new numerical approach called GMN was developed to determine the activation energy of the thermosetting materials. For the fabrication of prepreg-based composite laminates, the vacuum bag-only (VBO) method was performed. The fiber volume fractions of the CFRP samples changed between 55.3% and 50.16%. The mechanical and thermomechanical properties of prepreg-based CFRP composites with/without F-SWCNTs were investigated. The optimum mechanical properties of F-SWCNTs filled CFRP composite was achieved at 0.05 wt.% of F-SWCNTs. However, mechanical properties were decreased due to the addition of higher content of F-SWCNTs, in comparison with neat CFRP.Doctoral Thesis Multiphysics Modeling of Surface Charge and Pressure-Driven Electrokinetic Flow in Micro/Nano Scale Porous Media(Izmir Institute of Technology, 2021) Şen, Tümcan; Barışık, MuratAccurate characterization of fluid transport in micro/nano confinements is essential for numerous applications from industrial, agricultural, and medical sciences. In these applications, electrokinetic interactions dominate the fluid behavior, which causes conventional fluid dynamics to become incomplete. Specifically, near-wall hydrodynamics and liquid/solid coupling at the interface varies by electrokinetic effects. Therefore, the current study focuses on characterization of the fluid transport at various porous systems and ionic conditions. The Poisson-Nernst-Planck (PNP) equations were numerically solved coupled with the Navier-Stokes (NS) equations. Charge regulation (CR) boundary condition is employed to calculate the charging behavior of the surfaces. First, the surface charging of nano-scale systems was analyzed by considering the electric double layer (EDL) overlap and inlet/outlet effects. While EDL overlap decreased the surface charge, inlet/outlet effects presented an opposite behavior. Then, transport is characterized by calculating the hydraulic conductivity from Darcy's law under electrokinetic and boundary slip effects. The results showed that electrokinetic effects decrease the hydraulic conductivity with increasing concentrations and decreasing confinement sizes. At slipping condition with a constant slip length applied, velocity slip developing on surface showed strong dependence on porosity and ionic conditions. For low porosities and high concentrations almost no-slip conditions were observed even at high slip lengths. Results showed that the transport in micro/nano-scale porous systems is dominated by electrokinetic interactions depending on porous system parameters and ionic conditions.Doctoral Thesis Performance Improvement of Composite Materials Used as Hydrogen Storage Tanks by Microstructural Modifications(Izmir Institute of Technology, 2020) Ay, Zeynep; Tanoğlu, MetinThe goal of this Ph.D. thesis is to improve the performance of the cylindrical composites manufactured by filament winding method by using the toughened matrix resin with nano-sized filler (noncovalently functionalized with ethoxylated alcohol chemical-vapor-deposition-grown SWCNTs). The effect of SWCNT concentration on the mechanical performance of these composites was investigated and discussed. One of the main focus of this thesis is to examine the effect of nano-sized filler type and filler concentration on the performance of the epoxy-based composites. For this purpose, epoxy-based nanocomposites with different nano-sized filler types (SWCNT, TEGO, and HNT) at varying concentrations were developed by a calendaring (3-roll-mill) method. A series of mechanical tests were performed for reference composite and developed nanocomposites. The scanning electron microscopy (SEM) was used to reveal the morphology and toughening mechanisms by examining the fractured surface of nanocomposites. The rheological behaviors and contact angle measurements with glass fiber of the selected filler (SWCNT) incorporated epoxy suspensions were investigated to determine the suitability of suspensions for the filament winding process. The reference and SWCNT modified glass fiber (GF)-based cylindrical fiber-reinforced polymeric composites (CFRPCs) with an inner diameter of 60 and 275 mm were manufactured by the filament winding method. The split-disk and three-point bending tests were performed for GF-based CFRPCs. The double cantilever beam (DCB) test was also carried out for the reference and SWCNT modified GF-based CFRPCs to investigate the effect of SWCNT existence on the interlaminar fracture toughness of CFRPCs. The fractured surfaces after the DCB test were analyzed under the SEM to comprehend the toughening mechanisms, and micro-and nano-sized filler morphologies. Consequently, it was revealed that blending and hence toughening the epoxy resin with SWCNT improves the interlaminar properties of the GF-based composites.