Ph.D. Theses

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Energy absorption of composite structures under axial loading : design, manufacturing and analysis
(Thesis (Ph.D.) - Bogazici University. Institute for Graduate Studies in Science and Engineering, 2023., 2023) Engül, Mehmet Can.; Ersoy, Nuri.
Geometry has a significant influence on Specific Energy Absorption (SEA) of composite structures. However, the variation of crush induced failure mechanisms with the change in geometry and the way they affect energy absorption capability has not yet been fully clarified. Moving from simple to complex structures, this dissertation aims to conduct a systematic study investigating the influence of geometry on SEA and to design a novel crash box geometry which has a better performance than those currently in use. For that purpose, an experimental investigation is conducted first for the crushing process of the flat, semi-circle, and sinusoidal structures with different dimensions made of both unidirectional and woven fabric prepregs. The results are compared, and variation of the observed failure mechanisms are discussed. The sinusoidal geometry having the highest SEA is identified to utilize in the design of a novel crash box geometry. Design process based on trial and error through manufacturing and testing, on the other hand, is time consuming and expensive so that researchers are currently interested in Finite Element (FE) Model predictions. However, due to the complexity of the process, FE models applied to estimate the SEA of composites require high computational time. This dissertation proposes a novel numerical approach applied for the crushing process of composite structures in which computational cost is decreased by reducing the number of interfaces between plies and modifying the relevant properties. The novel approach is first applied to composite flat plates and later validated by modelling sinusoidal geometries. The same approach, which can estimate the SEA of the structures in more than 50% less time than conventional methods, is then utilized in the design process of a crash box geometry. As a result of various modelling attempts, a novel geometry, which is called "Sun-like Structure" is manufactured and tested. Results show that the SEA of the final geometry is 2.3 times and 16% higher than the flat plates and the single sinusoidal structures, respectively. In the final part of the thesis work, tapering is applied to edges of the crash box structure so that the initial peak load is decreased to prevent an excessive deceleration causing a fatal damage to the passengers during a crush event.
Optimization and application of stirling engine for waste heat recovery from a heavy-duty truck engine
(Thesis (Ph.D.) - Bogazici University. Institute for Graduate Studies in Science and Engineering, 2023., 2023) Güven, Metin.; Anlaş, Günay.
In this thesis, the use of the Stirling engine for recovery of waste heat from a heavy-duty truck diesel engine is studied in detail: First, the maximum theoretical non-dimensional shaft work for three types of Stirling engines is calculated, and out from the three types are compared by optimizing selected parameters in the shaft work equation using a first-order analysis. The results show that the Beta-type Stirling engine is more effective than the Alpha and Gamma types with higher power density. Results are then used for a preliminary design of the waste heat recovery (WHR) system from the exhaust gases of a commercial truck. Calculations carried out with exhaust gas temperatures measured on-road tests show that the WHR system can provide 1% of internal combustion engine (ICE) power output, corresponding to 3 kW, offering about 0.4% reduction in fuel consumption. Second, a dynamic analysis is performed on the Beta-type engine using a second-order analysis, considering size constraints. Four Beta-type Stirling engines that use the exhaust gas recirculation (EGR) cooler or the tailpipe as a heat source are simulated. The dynamic analysis shows that the Stirling engine WHR system does not generate a reasonable power output because of the low heat transfer rate from the truck’s exhaust gas. Therefore, it does not appear feasible for use in commercial vehicle applications.
Investigation of twinning nucleation and propagation in magnesium by molecular dynamics
(Thesis (Ph.D.) - Bogazici University. Institute for Graduate Studies in Science and Engineering, 2023., 2023) Kapan, Enver.; Aydıner, C. Can.; Alkan, Sertan.
Modeling deformation twin nucleation in magnesium has proven to be a challenging task. In particular, the absence of a heterogeneous twin nucleation model which provides accurate energetic descriptions for twin-related structures indicates a need to more deeply understand twin energetics. To address this problem, in a two-part study, molecular dynamics simulations are performed to follow the energetic evolution of {1012} tension twin embryos nucleating from an asymmetrically-tilted grain boundary. The line, surface and volumetric terms associated with twin nucleation are identified. A micromechanical model is proposed where the stress field around the twin nucleus is estimated using the Eshelby formalism, and the contributions of the various twin related structures to the total energy of the twin are evaluated. In the second part of the study, a geometric model is proposed to complement the micromechanical model such that the geometric variables associated with twin related structures are predicted from a few mesoscopic parameters. The results of the geometric model are substituted into the micromechanical model to obtain potential energy surfaces of a wide range of twin configurations. Classical nucleation theory is implemented to predict the nucleation energy barrier for {1012} twinning in Mg. The reduction in the grain boundary energy arising from the change in character of the prior grain boundary is found to be able to offset the energy costs of creating the other interfaces. The defect structures bounding the stacking faults that form inside the twin are also found to possibly have significant energetic contributions. These results suggest that both of these effects could be critical considerations when predicting twin nucleation sites in magnesium.
Crack initiation and growth in shape memory alloys
(Thesis (Ph.D.) - Bogazici University. Institute for Graduate Studies in Science and Engineering, 2023., 2023) Mutlu, Fatma.; Anlaş, Günay.
The aim of this thesis is to study the crack initiation and growth in shape memory alloys (SMAs), with emphasis given to martensitic transformation in front of the crack tip and non- transforming phase interactions along the crack path. In the first part, the effect of phase transformation on the fracture of polycrystalline NiTi SMAs is examined to introduce the complexities encountered in transforming materials. Fracture experiments are conducted at two different loading rates, and crack initiation and growth are monitored: In all samples, the crack starts and grows at a straight angle from the notch. Later, the J-Integral is evaluated experimentally and numerically. At a low loading rate the driving mechanism for resistance to fracture is phase transformation, but at a higher loading rate heat dissipation becomes more prominent due to higher magnitude of latent heat release. In the second part, crack initiation and growth in single-crystal CoNiAl SMAs are investigated using cyclic loading experiments. In crack growth tests, cracks initiate and grow at an angle similar to the martensite band angle of tensile specimens. Crack initiation angles of superelastic CoNiAl samples are calculated analytically and numerically, and the results are found to be in good agreement with the experiments. Stress intensity factors (SIFs) and crack growth rates (da/dN) are calculated for growing cracks, the behavior of which are investigated by examining the test samples with an optical microscope: When non-transforming secondary phase regions are encountered, cracks slow down/stop and SIFs increase/decrease. Overall, it is found that the non-transforming phase acts as a barrier against crack growth and improves the fracture resistance of the SMA. The results of the study underline the importance of including the effects of phase transformation and non- transforming microstructural barriers for a better understanding of the fracture behavior of SMAs.
Mesoscale investigation of constitutive behaviour of HCP metals based on full-field in situ strain measurement
(Thesis (M.S.) - Bogazici University. Institute for Graduate Studies in Science and Engineering, 2022., 2023) Şafak, Nıma.; Aydıner, C. Can.
Hexagonal close-packed (HCP) metals have received extensive attention from research and industrial communities for their exceptional properties. Among these HCP metals, Magnesium is the lightest and the most intriguing one. Its deformation incorporates significant twinning that depends heavily on microstructure and loading conditions. For this study, a common Mg alloy (AZ31) is considered with multiple crystallographic textures to aid the fundamental understanding of its deformation. This is predominantly an experimental effort with a novel in-situ digital image correlation (DIC) technique that provides deformation quantification with granular resolution over macroscopic fields. Further, samples are subjected to cyclic to study spatial aspects of strain accommodation via twinning, detwinning and slip plasticity. Shear band formation and progression due to the bursts of twins is quantitatively detailed and it is demonstrated that unlike twinning, detwinning is a nearly homogeneous phenomenon. Further tension after detwinning forces the material to plasticity. Another load reversal to compression pushes the material back into twinning regime but with reduced shear band compactness. It is also shown that the Lüders-like bands form with lower strain content in extruded samples compared to rolled samples that also show extreme plastic anisotropy. An open source spatially- resolved crystal plasticity solver (DAMASK) is used to test the fidelity level of basic constitutive models when confronted with twin coordination and detwinning. Two sets of representative volume elements (RVEs) are produced for both textures. In both cases, the simulated plastic deformation occurs earlier with considerable hardening at the twin plateau deviating from the near perfectly-plastic experimental results. The model is found to be incapable of predicting the strain compactness and intense anisotropy of the rolled sample shear bands. It is also not geared to model detwinning, highlighting the need for more advanced models if the goal is to attain high fidelity in micro-scale fields.
Development of a novel variable stiffness device based on magneto-rheological elastomers for soft robots
(Thesis (Ph.D.) - Bogazici University. Institute for Graduate Studies in Science and Engineering, 2023., 2023) Atakuru, Taylan.; Samur, Evren.; Aydıner, C. Can.
One of the biggest challenges in soft robotics is the variability and controllability of stiffness. Compliance is required for soft robots to enable dexterity and secure interactions with the environment, whereas rigidity is required to transmit forces when necessary. Stiffness variation of soft robots has been achieved through stiffening methods such as antagonistic arrangement of active elements, jamming by vacuum, and viscosity change under magnetic field. The methods can be compared in terms of speed of stiffening and destiffening, modes of stiffening, and stiffness variation. Magnetorheological elastomers (MREs) are effective in response time and suitable for different stiffening modes, such as bending, tension, and compression. However, stiffness variation data can only reach high values if a very high magnetic field is applied. Jamming-based methods appeal due to fabrication, low cost, and stiffness variation. However, the speed of this technology is not particularly remarkable. In addition, it requires an external membrane, creating design complications for system integration. No research that utilizes both methods simultaneously is found in the soft robotics literature. In this thesis, a hybrid method is proposed that combines a jamming-based approach with a viscosity-based one for stiffening of soft robots. The proposed method is innovative because stiffness variation is boosted by exploiting the advantages of magnetic jamming of MREs. In order to prove the proposed method, a number of steps was taken. First, the bending behavior of MREs is analytically, numerically, and experimentally investigated to analyze the effect of volume fraction of magnetic particles on stiffness variation. Second, a multi-layer jamming structure consisting of MRE layers and two flexible Neodymium-Iron-Boron (NdFeB) magnets is developed to investigate the unique mechanics of magnetic jamming of MRE sheets exploring stiffness change both due to jamming and variable viscoelasticity. Third, a fiber jamming structure consisting of MRE fibers and a flexible NdFeB magnet is developed and integrated into a soft robot, the STIFF-FLOP manipulator. Stiffening tests are performed on the manipulator to prove the concept of magnetic jamming of MRE fibers. Results show that stiffness gain in bending and compression is achieved with the proposed method. Finally, a possible implementation of electronically-controlled magnetic jamming and stiffening is demonstrated on the manipulator which is embedded with electro- permanent magnets. The findings of this thesis show that the proposed hybrid stiffening method combining jamming with viscoelasticity modification is a promising approach to achieve variable and controllable stiffness in soft robots.
Experimental analysis and modeling of porous NiTi shape memory alloys
(Thesis (Ph.D.) - Bogazici University. Institute for Graduate Studies in Science and Engineering, 2022., 2022) Özerim, Gülcan.; Anlaş, Günay.; Moumni, Ziad.
Porosity brings new features to NiTi SMAs, and raises its potential for biomedical applications. Although different techniques are provided in the literature for manufacturing porous NiTi samples, the subject is still open to further investigation to achieve superior shape memory characteristics. Based on this, the aim of the thesis is to analyze and model the mechanical behavior of porous NiTi SMAs. First, NiTi compacts were produced using spark plasma sintering. After sintering, because the samples did not show the expected pseudoelastic behavior, they were systematically subjected to heat treatment. The transformation behavior and the phase composition were analyzed using DSC and XRD. These characterization gave an insight to the micro-structure after heat treatment. Then, instrumented micro-indentation was carried out to measure the hardness that was altered by aging. Selected samples that were tested under uniaxial compression showed an enhancement in the pseudoelasticity of the SPSed NiTi that was heat-treated. In the modeling part, a macro-scale phenomenological model is proposed for the mechanical behavior of the porous NiTi by using poromechanics. The model considers the porous medium as a skeleton that consists of a solid matrix and connected porous space. The porosity is included as an internal state variable. Both the pseudoelastic and plastic deformations were considered. The phenomenological model was implemented into Abaqus through a UMAT, and validated using experimental results available in the literature, as well as the numerical results obtained from the unit cell (UC) technique used in this study. The model proposed in this thesis represents the mechanical behavior of porous SMAs with reasonable accuracy with a significant reduction in numerical cost when compared to the UC approach. The model can be especially useful in possible biomedical applications.
Turbulent combustion modelling of fuels using LES
(Thesis (Ph.D.) - Bogazici University. Institute for Graduate Studies in Science and Engineering, 2022., 2022) Güryuva, Serdar.; Bedir, Hasan.
Combustion simulations with high fidelity turbulence models and detailed chemistry may suffer from high computational power requirements due to the combined cost of time-scale dissipation and small integration steps. Such a limitation can be avoided by employing a hybrid reaction mechanism reduction method called local self-similarity tabulation (LS2T). LS2T directly solves several dominant species reactions and incorporates the effects of other species on dominant ones by data retrieval from pre-calculated tables. This study is based on the application of the LS2T method to high fidelity 3D combustion simulations with different fuels and combustion physics. The test cases that are selected for demonstration purposes are Sandia Flame-D, the premixed methane combustion, and Sandia Spray-A, the non-premixed n-dodecane combustion. The combustion simulations use large eddy simulation (LES) as turbulence solver and transported probability density function (TPDF) for species transport, to increase the accuracy of the simulation and avoid the use of any additional reaction model. This study is the first demonstrator of LS2T approach application to 3D combustion problem with Sandia Flame-D simulation and it is also the very first Spray-A simulation that is executed using LES and TPDF in a 3D resolved domain. The report consists of wide literature research regarding the LES combustion analyses of both test cases and chemistry reduction techniques applied, a detailed theory behind all the physics solution methods used, the computational methods implemented for the study, results, and discussions of the 0D and 3D simulations. The results show that by the use of the LS2T method, it is possible to have high accuracy and generate results similar to the detailed chemistry while maintaining an acceptable computational effort.
Adaptive boundary control using backstepping for 1D variable length string-mass system under disturbances
(Thesis (Ph.D.) - Bogazici University. Institute for Graduate Studies in Science and Engineering, 2022., 2022) Szczesiak, Mateusz.; Anlaş, Günay.; Yılmaz, Çetin.
In this thesis, an adaptive boundary control using delayed control methodology for a 1D wave equation is examined. The outlined problem is applied in the control of an ideal string- mass system with constant or time-varying length. The dynamics of the system, which constitutes the basis for the control problem, is first derived using the extended Hamilton‘s Principle. The resulting wave PDE is then transformed into two decoupled hyperbolic equations using the method of characteristics. The solution of the characteristic equation allows one to project the input signal at one boundary onto the dynamics describing the other boundary. Here, the input appears with an explicit delay. If the domain is characterized by a moving boundary, i.e., the length of the string is non -constant, the delay is time-varying. The problem then becomes that of control of a linear ODE with an input delay. Afterward, the transport PDE representation is used to re-express the delay in terms of a PDE‘s boundary value re sulting in an ODE- PDE cascade system. The backstepping transformation then gives the control law and transforms the system into the target system characterized by fa vorable control properties. The only feedback required for the control is the boundary measurements. Thereafter, Lyapunov‘s theory is used in the stability analysis. Any unknown in-domain or boundary disturbances, as well as uncertain boundary parame ters, are handled using the adaptive control strategies. The dynamics of the string-mass system and the performance of the derived controllers are illustrated using numerical simulations. This is followed by a case study where the deployment and control of an underwater sensor in the presence of the water waves are simulated.
Nonlinear viscoelastic material modeling using nested linkage mechanisms
(Thesis (Ph.D.) - Bogazici University. Institute for Graduate Studies in Science and Engineering, 2022., 2022) Özcan, Mustafa Umut.; Yılmaz, Çetin.; Sönmez, Fazıl Önder.
In this study, basic linear lumped elements such as springs and dashpots are used in nested linkage mechanisms in order to simulate and predict the mechanical behaviour of nonlinear viscoelastic materials. The proposed mechanism model containing two nested linkages can show initial softening followed by hardening response under quasi-static loading, which is commonly displayed by hyperelastic materials. Hence, material nonlinearity is simulated by geometric nonlinearity of the linkage mechanism. The mechanism also displays relaxation, hysteresis, and dynamic stiffness responses of viscoelastic materials with the help of dashpot elements. By tuning the geometric parameters of the mechanism, and the stiffness and damping parameters in the system, desired viscoelastic response can be obtained. Most of the previous experimental studies in the literature considered just two of different possible test scenarios. Comparisons with the experimental results in the literature show that the nested linkage mechanism with linear springs and dashpots can successfully simulate the material response in the tests for different double combinations of quasi-static loading, ramp-and-hold loading, hysteresis, and dynamic stiffness tests. When the experimental studies in the literature are investigated, it is seen that studies investigating three different test scenarios are rare. In this thesis, these four testing scenarios are considered in the same study for model validation for the first time. These four tests are conducted on three rubber samples with different stiffness and damping characteristics. It is shown that the nested linkage mechanism model can accurately mimic the material behaviour in these four different tests with a single set of values for the design parameters. In order to evaluate the prediction capability of the nested linkage mechanism model, optimization is conducted using only two test scenarios and the responses in the other two test scenarios are validated. To further assess the prediction capability of the model, parameter values are obtained for a sample and the responses of a sample from the same material with a different size is estimated for the four test scenarios. Finally, considering the hardening behaviour of the samples, the number of parameters in the model is reduced from 8 to 5 and it is shown that the reduced model also gives quite satisfactory results.
Mechanical behavior of low density polymeric foams under multiple loading and unloading
(Thesis (Ph.D.)-Bogazici University. Institute for Graduate Studies in Science and Engineering, 2008., 2008.) Öztürk, Umud Esat.; Anlaş, Günay.
In this thesis, mechanical behavior and energy absorption characteristics of low density polymeric foams under multiple loading and unloading are investigated for uniaxial and hydrostatic compression, uniaxial tension, simple shear, and cylinder and block indentation. Constitutive models and energy absorption diagrams available in literature for uniaxial compressive loading are reviewed. A new phenomenological constitutive model for accurate calculation of load, deformation, and absorbed energy is proposed for multiple loading and unloading. Results of the available and the new models are compared to those of experiments for expanded polystyrene (EPS) and polyethylene (PE) foams. A design procedure for multiple compressive loading and unloading is presented. A drop test rig for measuring uniaxial compressive behavior of foams at high loading speed and a hydrostatic compression test setup to study the mechanical behavior of foams under multiple hydrostatic loading and unloading are built. Tools to be used with Zwick Z020 universal tensile testing machine are prepared for uniaxial tension, simple shear, and cylinder and block indentation tests. Stressstrain results are presented for EPS and PE foam specimens. Finite element simulations of EPS and PE foam specimens under multiple loading and unloading for uniaxial and hydrostatic compression, uniaxial tension, simple shear, and cylinder and block indentation are performed using Abaqus finite element package for volumetric and isotropic hardening. The results of finite element simulations are compared to those of experiments.
Numerical investigation of high Knudsen number flow in rectangular enclosures
(Thesis (Ph.D.)-Bogazici University. Institute for Graduate Studies in Science and Engineering, 2006., 2006.) Orhan, Mehmet.; Ecder, Ali.; Tezel, Akın.
Nowadays, enlightening unknown aspects of rarefied gas flow is one of the critical issues of fluid dynamic research to ensure correct and proper operations of manyMicro- Electro-Mechanical-Systems (MEMS). Thermally driven motion of rarefied gases is gaining in importance to develop Knudsen compressors having better performance or to improve single crystal growth processes. Therefore, accurate prediction of the physics lying behind the thermal creep in the transition regime as well as slip flow regime is one of the main motivations of this study. The other emphasis is possible flow instability of the rarefied gases in enclosures. For this purpose, an asymptotic approximation has been performed in the first part of the study to find analytical solutions. In the second one, linear disturbance theory of hydrodynamic stability has been applied to the problem to determine bounds of instabilities. Analytical solutions of two-dimensional stability analysis have been introduced. Critical states have been identified for different models and for varying Knudsen numbers. More generally, eigen-spectrum of the perturbation equations has been identified in three-dimensions. At the last part, by applications of an artificial viscosity scheme, a computer program has been constructed to solve Burnett and also Navier-Stokes equations. Mechanisms of the thermal creep flow have also been verified by inspecting stress tensors of Burnett equations. Most importantly, the insufficiency and the failure of Navier-Stokes equations for the creeping flows have been proved. Moreover, it has been shown that Burnett equations can correctly model such creeping flows.
LPV modeling and robust control of yaw and roll modes of road vehicles
(Thesis (Ph.D.)-Bogazici University. Institute for Graduate Studies in Science and Engineering, 2007., 2007.) Başlamışlı, Selahattin Çağlar.; Köse, İbrahim Emre.; Anlaş, Günay.
In this thesis, the usefulness of linear parameter varying (LPV) modeling of vehicle dynamics is investigated for controller synthesis to take nonlinear tire behavior into account. The H infinity control framework is used to design combined active steering and differential controllers to improve vehicle handling during maneuvers involving large driver commanded steering angles. Two approaches are undertaken to reduce the size of the parameter set to minimize solver time during the controller synthesis step. The first approach is built on modeling tire stiffnesses as parametric uncertainties. This leads to a linear fractional transformation (LFT) model of the combined vehicle body and tire subsystems and to the design of a static state feedback controller intended to be robust against large variations in parameters. In the second approach, a rational fit is proposed for the nonlinear tire model used, and original parametric vehicle models are derived by integrating the fitting model into the equations of motion. This leads to the design of gainscheduled LPV controllers where scheduling is based on lateral and longitudinal tire slips. At small driver commanded steering angles, both controllers achieve decoupling of sideslip and yaw rate modes. However, at large driver commanded steering angles, the steering response of the first controller is observed to be unstable at the physical limit of the vehicle due to the shortcomings of the parametric uncertainty model in predicting tire behavior at large lateral slip. Meanwhile, the second controller achieves decoupling of all vehicle modes for the whole range of driver commanded steering angles up to and at the physical limit of the vehicle, revealing the importance of incorporating the tire friction circle concept into the controller synthesis.
Dynamic analysis of diesel engine crankshaft system using finite elements and multibody system simulation programs
(Thesis (Ph.D.)-Bogazici University. Institute for Graduate Studies in Science and Engineering, 2008., 2008.) Yılmaz, Yasin.; Anlaş, Günay.
In this thesis, dynamic analysis of in-line six cylinder diesel engine crankshaft system is carried out using analytical and numerical methods. The dynamic analysis of the crankshaft system consists of calculation of forces, displacements and stresses over a complete engine cycle (two revolutions of the crankshaft) under steady state (constant speed) conditions with a model of the whole cranktrain. Crankshaft system consists of crankshaft, engine block, pistons, piston pins, connecting rods, flywheel, torsional vibration damper, bearings and mounts that support the engine block. The loading on the system comes from the cylinder gas pressure and inertia of crankshaft system components. In the analytical part of the study, first, the forces acting on the crankshaft system are determined. Then, main bearing loads are calculated using a statically determinate system approach for each crank throw. Finally, torsional vibration and stress analyses of the crankshaft system are performed. In the numerical analysis of the crankshaft system, Msc. Nastran and Msc. Adams programs are used. The dynamic stress distribution in the crankshaft is evaluated using a flexible crankshaft model that is obtained through finite elements and Component Mode Synthesis (CMS) technique. To study the effect of oil holes on crankshaft dynamic stresses, crankshaft models with and without oil holes are used. The effect of TV damper on crankshaft stresses is investigated. Bearings are modeled using hydrodynamic bearing models of ADAMS. Coupled axial, bending and torsional vibrations of the crankshaft system are considered. Effect of each part of the crankshaft system on crankshaft dynamic stress and vibration characteristics are investigated. A separate chapter is devoted to effects of counterweight mass and position on main bearing load and crankshaft bending stresses. In the analysis, rigid, beam and 3D solid (flexible) crankshaft models are used. Main bearing load results for rigid, beam and 3D solid models are compared and beam model is used in counterweight configuration analyses. Twelve-counterweight configurations with a zero degree counterweight angle and eight-counterweight configurations with thirty degree counterweight angle, each for 0%, 50% and 100% counterweight balancing rates, are considered. It is found that maximum main bearing load and web bending stress increase with increasing balancing rate, and average main bearing load increases with decreasing balancing rate. Both configurations show the same trend. For this specific engine, the load from gas pressure rather than inertia forces is the parameter with the most important influence on design of the crankshaft. Results of bearing loads and web bending stresses are tabulated.
Characterization of NiMnGa magnetic shape memory alloys
(Thesis (Ph.D.)-Bogazici University. Institute for Graduate Studies in Science and Engineering, 2005., 2005.) Pirge, Gürsev.; Altıntaş, Sabri.
Magnetic shape memory (MSM) alloys are a new class of actuator materials withhigh actuation frequency, energy density and strain and they can be used in themanufacturing of actuators, smart structures, sensors and transducers. NiMnGa alloys experience a reversible martensitic transformation, which is a temperature-dependent phasetransformation from a highly symmetric crystallographic structure (austenite) to lowsymmetry (martensite). These materials are ferromagnetic. Ferromagnetism is aphenomenon by which a material can exhibit a spontaneous magnetization, and is one of the strongest forms of magnetization. Ferromagnetic metal alloys whose constituents arenot ferromagnetic in their pure forms are called Heusler alloys, named after Friedrich Heusler. Applying a strong magnetic field to some of the Heusler alloys may inducereorientation of martensite variants with high magneto-crystalline anisotropy energy, which leads to a net shape change of the material. In this study, the effect of alloycomposition, cutting direction and heat treatment on the microstructure, local composition,and thermal and dilatometric properties of NiMnGa alloys were investigated.Characterization tests involved various crystals, with and without post-crystal growth heat treatment, by chemical analysis, differential scanning calorimetry (DSC), dilatometry,optical microscopy, scanning electron microscopy and radiography. Metallographic studies showed that as solidified, off-stoichiometric alloys had three distinct microstructural features-a Heusler phase, a Mn rich phase and a eutectic or eutectoid region. Various heattreatment procedures were applied to successfully remove the last phase and produce MSM effect. Heat treatment was also essential for the production of a distinct martensitetransformation in DSC and dilatometry traces and a martensitic transformation to occurover a narrow temperature range. Bulk and microanalysis showed that there are significant concentration variations in the boules grown by the Bridgman method, that lead to changesin phase transformation behavior which were observed by DSC. The presence of composition variations in the boules is a major issue because of its effect on the martensitetransformation temperature. For boules with composition variations, both transformed anduntransformed regions will exist over some temperature range, degrading the performanceof any actuator made from them. Clearly, further effort on the improvement of the crystalgrowth technique is needed to remove the composition gradient and variations and toobtain a fine dendritic structure, which would be much easier to homogenize. For thecurrent growth conditions, coarse cellular structures have been obtained which showsignificant solute segregation. An increase in the thermal gradient during the directionalsolidification process resulted in a finer cellular structure.
Core-shell PVA / gelatin nanofibrous scaffolds using multinozzle aqueous electrospinning
(Thesis (Ph.D.) - Bogazici University. Institute for Graduate Studies in Science and Engineering, 2019., 2019.) Şengör, Mustafa.; Bedir, Hasan.; Altıntaş, Sabri.
Biological scaffolds have been used in the reconstruction of the damaged tissues. They have similar morphology and structure to the host tissues. However, they can be produced using materials that can be harmful to humans and the environment. In this context, core-shell nano ber based sca olds, whose mechanical strengths are provided by PVA(poly vinyl alcohol) and recognition sites are provided by gelatin, were fabricated in a non-woven manner using multiple nozzles of electrospinning technique. Instead of widely used toxic, acidic or salt-based ionic solvents, deionized \water" was used as the only solvent for both polymers. Firstly, nano bers were produced from 8 % (w / w) gelatin and 8%(w / w) PVA solutions individually. Limits were determined for parameters such as voltage, feed rate, temperature and polymer concentrations. Although pure gelatin nano bers have diameters of less than 50 nm, they have beaded structure and have lower mechanical strengths. Smooth bers were obtained from 8% PVA. Fibers with PVA: gelatin core shell morphology were then produced at di erent feed rate ratios (FRR). Based on the ber diameter, the optimal FRR with a 15 kV voltage magnitude and 15 cm electrode distance was found to be 1: 1 with an average diameter of 280 nm. The ratio of 1: 3 and 1: 4 was seen as the formation of \beaded" bers and the pealing limit of gelatin over PVA, respectively. Mechanical and water resistance of the produced sca olds was further improved by cross-linking. Core - shell morphology was demonstrated by TEM, SEM, EDS analysis. The secondary structure of the gelatin from collagen and the e ects of the electrospinning were revealed by FTIR and DSC. Approximately 60% of all cross-linked sca olds were degraded in solution using lysozyme enzyme up to day 14.
Development and modeling of a colonoscopy robot
(Thesis (Ph.D.) - Bogazici University. Institute for Graduate Studies in Science and Engineering, 2020., 2020.) Tutcu, Cem.; Samur, Evren.
Colorectal cancer is the second leading mortality cause among all cancer types. Similar to the other cancer types, early detection plays a vital role in the prevention of mortality. Colonoscopy is an endoscopic method that is widely used to screen colon, and remove legions, which is considered to be the most reliable method for detecting colorectal cancer. Conventional colonoscopes are propelled and positioned manually. This operation presents the risk of colon perforation, and patient discomfort due to high reaction forces applied to the colon wall. The conventional approach also often emerges the problem of colonoscope shaft looping inside the convoluted colon that causes loss of haptic feedback from the tip. Due to these post-colonoscopy complications, scans are not performed as frequently as required to mitigate the risks. In this study, a novel robotic solution is proposed for colonoscopy operations that will reduce operational risk, and improve patient comfort which will have an impact to increase colonoscopy scan rate. The robotic system also aims to provide a more ergonomic working environment for the colonoscopist to reduce long term usage complications. This thesis focuses on the colonoscopy robot development; particularly the design of an in-vivo shaft, kinematics and quasi-static modeling of the robot, and a medical application scenario. An experimental study is performed to prove navigation and position control capabilities of the system using a large scale prototype. Experiments showed that wall reaction forces are considerably lower than the conventional colonoscopy. Positioning tests have demonstrated close correlation with a model estimate up to a certain robot body length. This thesis proves the concept of a growing soft robot that can be further developed to be used in colonoscopy in future studies.
Physical based analysis and model reduction of engineering systems
(Thesis (Ph.D.)-Bogazici University. Institute for Graduate Studies in Science and Engineering, 2003., 2003.) Orbak, Ali Yurdun.; Eşkinat, Eşref.; Türkay, Osman S.
There is a need for obtaining low order approximations of high order models of physical systems as low order models result in several advantages including the reduction of computational complexity and improved understanding of the original system structure. Different methods have been suggested in literature for obtaining suitable low order approximations, but these approaches do not reflect the relation between the mathematical model and the physical components of a system. In this thesis, some new approaches are provided for model reduction in the physical domain. The approaches that are presented use the idea of decomposition of physical systems, which is useful for the identification of dominant components or subsystems. The procedures are applied to the physical systems that are represented by bond graphs as they lead to better understanding of the system structure. One of the proposed methodologies exploits the idea of decomposition of physical systems. The proposed decomposition and model reduction procedures are directly implemented on the model providing a better perception of the physical model reduction and a better design point of view. As a second methodology, the determination of subsystems and/or components that influence a given eigenvalue of the overall system has been explored. A set of theorems and definitions are proposed that lead to an efficient procedure for this aim. After the calculation of eigenvectors, effect matrices are produced that indicate the relative importance of physical parameters in a selected eigenvalue. Using these matrices, an efficient physical model reduction procedure is constructed. The advantages of the presented approaches over existing methodologies are emphasized through several examples.
Shape and topology optimization of intertial amplification induced phononic band GAP structures
(Thesis (Ph.D.)-Bogazici University. Institute for Graduate Studies in Science and Engineering, 2018., 2018.) Yüksel, Osman.; Yılmaz, Çetin.
Inertial ampliﬁcation is a novel phononic band gap generation method in which wide vibration stop bands can be obtained at low frequency regions. The engineering importance for this novelty comes from the fact that the phononic band gap structures can be utilized as passive vibration isolators for the low frequency range. In this thesis, primarily the research is focused on the improvements achieved on stop band widths and depths via employment of structural optimization tools. To that end, size, shape and topology optimization studies are conducted on a compliant inertial ampliﬁcation mechanism, then with these compliant unit cell mechanisms, one and two dimensional periodic structures are formed. Consequently, by means of these periodic structures, it is demonstrated that the vibration transmission is inhibited for wide ranges at low frequencies. The work comprises analytical and numerical studies and more import antly experimental validation of the results. Moreover, topology optimization studies performed during the thesis lead to the development of a new fast topology optimiz ation algorithm to obtain structures with maximized fundamental frequency, though this was not originally among the research objectives. Finally, explicit problem formu lations and a comprehensive review on topology optimization are also presented.
Experimentally verified numerical simulation of single crystal growth process with a low melt height and an axial vibration
(Thesis (Ph.D.) - Bogazici University. Institute for Graduate Studies in Science and Engineering, 2018., 2018.) Sheikhi, Aidin.; Balıkçı, Ercan.
This Ph.D. dissertation investigates experimental and numerical crystal growth of antimony-doped germanium (Ge-Sb) single crystals. The investigation is a part of the TUBITAK project 212M030. The single crystal growth of Ge-Sb from the melt is investigated by the Vertical Bridgman (VB), Axial Heat Processing (AHP), and Axial Vibrational Control (AVC) techniques. The effects of method dependent growth parameters on the quality of the grown single crystals have been analyzed. To this end, two different pulling rates (10 mm/h and 20 mm/h), different initial melt heights (5 mm, 10 mm, 14 mm, and 58 mm), and three different sets of vibrational parameters (2 mm amplitude and 0.25 Hz frequency, 0.25 mm amplitude and 1 Hz frequency, and 0.25 mm amplitude and 25 Hz frequency) are applied in the growth of seven different crystals. It is observed that the highest single crystal length with the most homogeneous solute redistribution and the least dislocation density are achieved in an AHP crystal which is grown with the lowest pulling rate (10 mm/h). However, it is determined that an appropriate control of the vibration parameters in the AVC technique makes it possible to achieve almost the same crystal quality with doubled growth rate, so the production yield is decreased. Moreover, global and local numerical simulations are performed in order to investigate the effects of the growth parameters on the convective flow patterns. Also, results of the numerical simulations contribute to make better and more reliable interpretations of the experimental observations. The simulation results provide useful information for the experimentalists to investigate the effects of growth parameters on the temperature and solute distribution, flow pattern, and the interface shape. According to the numerical results, it is possible to clarify how the insertion of the baffle, adjusting the melt height, and optimizing the vibrational parameters of the baffle contribute the thermal and the solutal homogenization in the melt, interface stability, and consequently improved crystal quality.

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