M.S. Theses
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Item Investigation of mechanical twinning via measurements of temperature and strain fields(Thesis (M.S.) - Bogazici University. Institute for Graduate Studies in Science and Engineering, 2023., 2023) Erman, Sefer Can.; Aydıner, C. Can.Wrought magnesium exhibits deformation complexity with high load path dependency and plastic anisotropy. Lightweight magnesium AZ31 is widely used in literature for twinning investigations due to the ease of {1012}(1011) tensile twin activation under favorable loading conditions. Coordinated propagation of twinning leads to strain localization across the aggregate in the form of macroscopic shear bands. When the load is reversed after twin growth, material undergoes detwinning that is crystallographic orientation transformation of twinned grains to the original position. The contributions of this thesis built on the previous OM-DIC studies are composed of two independent channels. First, a notched sample design is used to guide conjugate shear bands into predetermined diagonal corridors and enforce their overlap at a prefixed location to study the physics of the overlap with OM-DIC. Secondly, the energetic aspects of material deformation with emphasis on twinning and detwinning have been studied by in situ infrared thermography. The primary goal of this part is to reveal the stored energy in the material over cyclic deformation that entails twinning/detwinning and slip plasticity regimes. Absolute temperature measurements with IRT conducted are successfully achieved by attaching an undeformed (free hanging) reference material next to the actual sample and the temperature data considered is the differential between deformed and undeformed material. Further, the reference material is also thermally insulated from the sample during by inserting a thermal isolator for more robust results. In the big picture, these experimental results will help the validation of higher fidelity crystal plasticity models for magnesium.Item Experimental investigation and system design of dishwasher heat pump system with R600a(Thesis (M.S.) - Bogazici University. Institute for Graduate Studies in Science and Engineering, 2023., 2023) Şeren, Erdoğan Mert.; Bedir, Hasan.In this thesis study, the effect of R600a/R290 zeotropic mixture on system performance was investigated in detail on a heat pump system adapted to the dishwasher and using pure R600a as the refrigerant. In this context, five different compositions of the mixtures were investigated experimentally, with the R290 composition in the refrigerant mixture ranging from 10% to 50% by mass. In experimental studies, energy consumption, power consumption, cycle time, heating capacity, and heating efficiency were mainly compared with the base system using pure R600a. Fundamentally, a gradual decrease in the dishwasher cycle time was noted, with the percentage of R290 increasing and the cycle time in the experiment using 50% R290 in the mixture decreased by 54 minutes (28.4%) compared to the base experiment with a cycle time of 190 minutes. Although increasing R290 ratios in the mixture offered a higher power consumption value, it increased the heating capacity, so increased power consumption did not always bring high energy consumption with it. In the experiment where a 60% R600a and 40% R290 mixture was applied, a cycle time of 160 minutes was obtained, and it was observed that the increase in heating capacity provided optimum heating efficiency by surpassing power consumption. In parallel, the energy consumption decreased by 10 Wh compared to the base experiment with an energy consumption value of 470 Wh. Also, this study created a sub-model for each component of the heat pump dishwasher, and then all models were combined to form a heat pump dishwasher cycle model. The dynamic behavior of the dishwasher has been validated for both pure R600a and the R600a/R290 mixtures. While the maximum deviation in the cabinet temperature of the dishwasher was 1.5 °C, the cycle time variance was 2 minutes throughout the cycle. In addition, although the maximum deviation in power consumption was 5.2%, a maximum deviation of 2.9% was observed in the energy consumption value. Thus, the outputs of the model and the experiment were relatively comparable.Item Investigation of power turbine casing thermal environment using one dimensional thermal-fluid network solver(Thesis (M.S.) - Bogazici University. Institute for Graduate Studies in Science and Engineering, 2023., 2023) Uyav, Ömer.; Bedir, Hasan.The gas turbine engine operates in high-temperature, instantly changing power demand and various atmospheric conditions. Therefore, turbine inlet temperature and material selection are among the most critical parameters to maintain its integrity and satisfy power output. However, the higher the turbine inlet temperature is, the less operational time is. Thus, cooling the engine components is crucial regarding the durability, integrity, and reliability of both rotating and structural parts and for controlling blade tip clearances. In addition, thermal and secondary air systems design collectively play a key role in the successful case cooling, as they determine the heat load and cooling flow distribution. The first tool of engineers for an initial concept design is a 1D simulation to create a new cooling system. However, this step can be challenging for designers due to the absence of a 1D modeling methodology in the literature. Therefore, this paper aims to present a 1D network modeling methodology for any turbine geometry at the preliminary design stage. In this sense, a test campaign is carried out to obtain the temperature distribution of a generic power turbine casing using thermocouples and pressure sensors. In addition, a 1D thermo-fluid steady-state model is established. The results of the model are validated against test data. Additionally, a sensitivity analysis is performed, which helps to determine critical geometric locations for the base model and to measure how the model is affected by boundary conditions. Furthermore, another casing geometry with a different cooling system is designed to examine temperature distribution variation on the casing by changing the conduction path of components. Finally, comprehensive comparisons of the geometries with advantages and disadvantageous are considered, and the result of the sensitivity analysis is evaluated.Item Finite element analysis of elastomeric bearings under cyclic shear loading(Thesis (M.S.) - Bogazici University. Institute for Graduate Studies in Science and Engineering, 2023., 2023) Biçer, Berkay.; Özüpek, Şebnem.The study is concerned with the analysis of high damping rubber isolation bearings under seismic excitations. High damping rubber (HDR) plays an important role in seismic isolation systems due to its good damping capability and long service life. Accurate representation of its mechanical behavior is essential for the simulation of the high damping rubber bearings (HDRB). The emphasis of the study is on the formulation of a constitutive model for HDR that accounts for large strains, cyclic loading, hysteresis and rate dependence while being computationally efficient. The calibration of the constitutive model and its validity are done using experimental data available in the literature. The constitutive model for HDR is used in the analysis of HDRB. 2D and 3D finite element models of bearing are developed and the response for compression and shear loadings are predicted. The sensitivity of the response to various parameters is investigated. The agreement of the predictions with the limited test data on HDRB is encouraging. Bearing stiffness and damping ratio are calculated within ranges available in the literature. The effect of loading rate on the hysteresis amount follows the trend observed in experiments. The methodology developed in the thesis can be applied to different isolators. The computational model can be used to understand the isolator’s behavior in detail without numerous tests and procedures.Item Broadband ventilated acoustic metamaterial design with coupled space-coiled resonators(Thesis (M.S.) - Bogazici University. Institute for Graduate Studies in Science and Engineering, 2023., 2023) Karakoyun, Ahmet.; Yılmaz, Çetin.In this thesis, broadband ventilated acoustic metamaterial design with coupled space-coiled resonators is studied. The main aim is to determine acoustic metamaterial designs, which provide high level of sound attenuation in a wide frequency range and results in minimum air pressure loss during airflow. The number and dimensions of the resonators are optimized for this aim. In order to accomplish coupling with more than two resonators, i.e., channels, an analytical model of the design having one resonator is obtained by using the transfer matrix method. Consequently, transmission loss characteristics are compared with the finite element model for verification. After verification for one resonator, the analytical model of the design having two resonators is obtained by using parallel connection of transfer matrices. Once the derivation regarding parallel connection of transfer matrices is done, a new methodology for the analytical model of the design having more than two resonators is proposed. By using this methodology, optimization is conducted by changing the number and/or the dimensions of the coupled resonators to accomplish sound isolation in the widest frequency range for a given transmission loss constraint. After determining the proper number and dimensions of the resonators, the finite element model of the proposed design is constructed. By using genetic algorithm, the analytical resonator lengths are updated to determine the lengths in the finite element model. Parametric studies are conducted to show the dependence of isolation bandwidth on target transmission loss and channel dimensions. It is shown that for low transmission loss targets such as 10 dB, a two channel design can provide both large isolation bandwidth and low pressure loss. On the other hand, for high transmission loss targets such as 50 dB or 60 dB, a three or four channel design can provide both wider isolation bandwidth and lower pressure loss than a two channel design.Item Phonon mean free path - thermal conductivity relatıon of AlxGa1−xN and β-Ga2O3 semiconductors(Thesis (M.S.) - Bogazici University. Institute for Graduate Studies in Science and Engineering, 2022., 2023) Ghanizadeh, Pegah.; Dönmezer Akgül, Fatma Nazlı.Ultrawide-bandgap (UWBG) semiconductors like AlxGa1−xN and β- Ga2O3 emerge as a promising option for advancing next- generation high-power electronic devices. Al- GaN preserves significant attention due to its unique capability of tuning the bandgap from 3.4 (eV) to 6 eV, enabling a nonlinear increase in the critical breakdown field. β-Ga2O3, with a wide bandgap of 4.8 eV , surpasses GaN and has cost-effective substrates, making it appealing for high-power electronics. However, field-effect transistors (FET) and Schottky-barrier diodes based on AlxGa1−xN and β-Ga2O3 have shown superior performance to GaN, indicating their potential for overcoming this challenge. The pressing issue of local heat build-up and narrowing thermal pathways in such high-performance small scales devices is a significant challenge.To optimize the performance and ensure reliable operation, efficient dissipation of heat generated in the device is essential. This can be done by understanding the thermal transport of these systems at a short-length scale, in this case, lattice vibrations (i.e., phonons). One of the critical properties that characterize this behaviour is the phonon mean free path (MFP). This research offers a detailed analysis of phonon mean free path accumulation spectra in β-Ga2O3, and AlxGa1−xN alloys with different Al fractions at different lattice temperatures by utilizing ab-initio and lattice dynamics calculations based on density functional theory (DFT) along with the Boltzmann transport equation (BTE). Our results indicated that the normalized cumulative thermal conductivity of alloys is notably reduced compared to that observed in pure systems. This effect is particularly pronounced for larger mean free paths (MFPs).Item Approximate lumped parameter model for bioheat equation(Thesis (M.S.) - Bogazici University. Institute for Graduate Studies in Science and Engineering, 2023., 2023) Akay, Simay.; Ertürk, Hakan.Lumped parameter modeling is a good alternative for analytical and numerical models, providing simple, and accurate results in many applications, including analysis of biological systems in the human body. However, the use of lumped parameter models in the bioheat transfer is very limited, as the thermal resistances are only valid for systems with no heat generation and biological tissues often have metabolic heat production. This study shows that the lumped parameter models can be used for examining the bioheat transfer in the human body. For this purpose, the arms, legs, and head were modeled and temperatures of the tissues within these body parts are determined by using analytical, numerical, and lumped parameter modeling. The models are examined for resting and exercise conditions. 4- layered cylindrical and spherical models are used for representing the limbs and the head, respectively. These models include the blood flow, convection, radiation, and perspiration. Comparison of the results obtained with analytical, numerical, and lumped parameter methods showed that the deviations between the results of different methods are not significant. Maximum error for the limbs during rest is determined as 0.4°C, and it is not more than 1°C, during exercise. The error for the head is slightly higher, but does not exceed 1.5°C. Maximum deviations are observed at the fat and bone tissue for limbs and head, respectively. The errors for skin and core are much lower. Furthermore, the effects of thermoregulatory systems such as blood flow, sweating, radiation, and convection are also examined throughout the analysis. It is observed that the effect of blood flow is very dominant at the core, but other mechanisms are more effective at the outermost layers than the core instead. Lumped parameter method will be more useful when more complex systems with pulsating blood flow and transient changes are considered.Item Effects of geometric features and size on the scattering behavior of nanoparticles for independent scattering regime(Thesis (M.S.) - Bogazici University. Institute for Graduate Studies in Science and Engineering, 2023., 2023) Yalçı, Yasin Alperen.; Ertürk, Hakan.With the fast development of nanotechnology in recent years, the detection and characterization of nanoparticles have grown in importance. Due to their effectiveness and robustness, optical techniques are widely employed for nanoparticle detection and characterization. Although there are several studies on absorption parameters, there are comparatively few studies on the influence of size and shape on scattering parameters. This research intends to address this void by examining scattering parameters for gold nanoparticles of varying size and shape. For this inquiry, COMSOL is used to develop a finite element model that permits working with various forms and sizes. The results indicate that the impacts of edges and a higher aspect ratio result in a higher scattering efficiency and peak resonance wavelength. It has also been shown that the redshift of the resonance wavelength is mainly due to the increase of aspect ratio. The investigation of scattering dispersion reveals that the scattering over polar angle is symmetric at resonance wavelengths but asymmetric at other wavelengths. However, shape factors such as edges and aspect ratio change exacerbate the asymmetries of scattering at polar angles. The effects of the presence of corners can be distinguished only by examining the dispersion of the scattered light. The statistical analysis is carried out for the determination of impact ratios of shape parameters on the scattering efficiency, the resonance wavelength and the asymmetry of the scattering. These investigations demonstrate that the shape and size of gold nanoparticles may be determined by analysing scattering properties, and this work offers the data necessary for this estimation within specific constraints.Item Relative positioning for underwater navigation of multi-vehicle systems(Thesis (M.S.) - Bogazici University. Institute for Graduate Studies in Science and Engineering, 2023., 2023) Gönül, Göksel Berker.; Samur, Evren.Precise and accurate navigation is a significant problem for Autonomous Underwater Vehicle (AUV) applications. Due to limited electromagnetic penetration, GPS sensors are not an option for determining the position of an AUV. The most common method is called dead reckoning, which uses inertial measurement units (IMUs) with accelerometers and gyroscopes. Bias errors, misalignment and noisy measurements are common problems with IMUs. These errors make position information to be unreliable over long periods of time. Position errors grow rapidly due to velocity and angular random walk. The problem is further complicated when more than one vehicle is involved in a series of underwater tasks. Although conventional acoustic systems such as long baseline and ultra-short baseline have been developed to overcome the navigation problem, they are logistically complex and only cover a pre-deployed area. The process of deploying transponders and initialising them with precisely known locations makes the systems expensive and inappropriate for low cost AUVs. In order to eliminate growing errors of dead reckoning method, a system architecture with a navigation algorithm for N vehicles is proposed. This architecture makes use of a simple acoustic measurement system and a filtering algorithm. The simplicity of this acoustic measurements reduces the logistic complexities of conventional systems greatly. A leader-follower formation control is implemented, where there is a leader AUV and N-1 follower AUVs. The system obtains relative positions between the leader and its followers, rather than absolute positions, to keep the formation intact. The follower AUVs position themselves according to the leader. This allows coordinated movements such as synchronised turns and manoeuvres without breaking the formation.Item Revısiting the scattering regime map based on transport scattering coefficient(Thesis (M.S.) - Bogazici University. Institute for Graduate Studies in Science and Engineering, 2023., 2023) Taufiq, Aristo.; Ertürk, Hakan.Radiative transfer in media consisting of randomly dispersed particles is commonly found in various scientific and engineering applications. The norm for analyzing such systems is to use the independent scattering assumption, where the radiative characteristics of the system are derived from the superposition of the individual particles’ radiative characteristics. One criterion for the validity of this assumption is that the particles are sparsely distributed. If this is not the case, estimation of properties for dependent scattering necessitates a more rigorous analysis. Historically, researchers have established regime maps that are used to identify if the independent scattering assumption is valid. However, recent experiments in the literature have shown that the well-established regime map may have shortcomings and predicts certain cases to be within the validity of the independent scattering assumption, although they are not. In this paper, a new regime map is sought considering the effect of particle refractive index. The study is established through numerical simulations using the static structure factor that accounts for the dependent scattering. The approach is validated with an experimentally verified method based on the T-matrix method, considering only the incoherent component of the scattered electric field, which represents the scattering of electromagnetic waves propagating through a dense medium. Coherent and incoherent coefficients were overlooked in recent studies, resulting in erroneous interpretations of the location of the demarcation line and the effect of the refractive index. To rectify this, the present study uses the transport scattering coefficient as a single intrinsic characteristic to define the transition between independent and dependent scattering. The transport scattering coefficient has been proven to correlate well with reflectance, unlike the scattering coefficient and the asymmetry parameter.Item Modelling of necking and primary particle polydispersity in soot aggregates and their effects of the scattering matrix(Thesis (M.S.) - Bogazici University. Institute for Graduate Studies in Science and Engineering, 2023., 2023) Yazıcı, Halil İbrahim.; Ertürk, Hakan.Soot has complex morphological characteristics such as aggregate and primary particle polydispersity, and necking. Modelling these structures and their e↵ects on the optical behavior is challenging attributed to the complex morphology and the corresponding formation mechanism. In this study, a comprehensive analysis of the impacts of necking and polydispersity in aggregate and primary particle size is conducted to identify their e↵ects on scattering matrix elements. Aggregate representations are generated via tunable fractal generation algorithms using particle-cluster and clustercluster aggregation, based on typical morphological parameters of flame-soot. Di↵erent necking models are evaluated quantitatively based on measured scattering matrix elements derived from an experimental study in the literature. Polydispersity in aggregate and primary particle size are implemented through log-normal distribution functions. Scattering matrix elements are calculated via discrete dipole approximation and multisphere T-matrix method. The impacts of the considered structures on the scattering matrix elements are presented, and the volume variation-based approach is evaluated for representing the impact of necking. According to the results, the impacts of necking and primary particle polydispersity are similar and dominated by the resultant volume change, whereas that of aggregate polydispersity is more complicated. Accordingly, the volume variation-based approach performs well in representing the e↵ect of necking on the scattering matrix for the specified cases. The deficiency of the volume variationbased approach is discussed as well, and an enhancement is proposed to increase its accuracy.Item Development of a bipropellant rocket engine with a focus on the injector(Thesis (M.S.) - Bogazici University. Institute for Graduate Studies in Science and Engineering, 2022., 2022) İskender, Enes Oğuz.; Çelik, Murat.Access to space relies on powerful propulsion systems to take a spacecraft out of the Earth’s atmosphere. Currently, chemical propulsion systems are the only propulsion method that are able to provide the thrust levels required to carry out these missions. Among the chemical propulsion systems, bipropellant liquid rocket engines provide the highest specific impulse. In this study, a bipropellant liquid rocket engine, and specifically its injector, is designed, manufactured and tested. The used injector is a fixed area triple impinging injector. The tests include water tests, open injector cold flow tests, open injector hot flow tests and an engine hot fire test. BUSTLab (Boğaziçi University Space Technologies Laboratory) Liquid Rocket Engine is a pressure - fed liquid oxygen - ethanol (75% ethanol - 25% water) engine. The design point for the engine is 5.65kN thrust, 254s of specific impulse, 1.25 oxidizer to fuel ratio, total mass flow rate of 2.27kg/s at 30bar chamber pressure. The engine is hot fire tested at full thrust for 12 seconds. During this test, 0.83kg/ s mass flow rate of oxidizer and 0.89kg/s mass flow rate of fuel are measured which gives 1.72kg/s total mass flow rate and 0.93 oxidizer to fuel ratio. Chamber pressure and thrust measurements during this test are inconclusive due to sealing and load cell issues. With the measured mass flow rates, the chamber pressure is calculated to be 21.2bar and the thrust is calculated as 3.62kN.Item A numerical approach for predicting hemodynamic characteristics of 3D aorta geometry under pulsatile turbulent blood flow conditions using fluid-structure interaction(Thesis (M.S.) - Bogazici University. Institute for Graduate Studies in Science and Engineering, 2022., 2022) Saat, Ahmet.; Atalık, Salim Kunt.Cardiovascular diseases are the leading cause of death all around the world and harm the society in terms of economically, socially, and psychologically. Hence diagnosing cardiovascular diseases as early as possible has become vital circumstance. Since clinicians need reliable and fast numerical approaches for their urgent pre-surgery decisions, individualised risk prediction and virtual treatment planning, CFD has become widespread in biomedical especially in cardiovascular medicine. The main aim of current study is to provide insight to hemodynamic characteristics of 3-D aorta geometry with pulsatile turbulent blood flow. In line with this purpose, blood and vessel mechanism has been evaluated through numerical fluid-structure interaction (FSI) analysis that couples computational fluid dynamics (CFD) and finite element analysis (FEA). Besides, effects of turbulence modelling, viscous effects and solid domain parameters such as artery thickness, elastic modulus and Poisson’s ratio on hemodynamic characteristics have been investigated. The investigations are carried out by using twelve turbulence models, two Non- Newtonian models and different solid domain values to compare output parameters such as oscillatory shear index, velocity field characteristics, von-Mises stress and displacement. Results have shown that SST k-omega with low-Re corrections model seem to be better capable of predicting hemodynamic characteristics. Proposed computational model can be considered as an initial work for the digital twin of cardiovascular system which is described as the realistic virtual model.Item Band structure calculation of 3D ultrawide elastic metamaterials with embedded inertial amplification mechanisms(Thesis (M.S.) - Bogazici University. Institute for Graduate Studies in Science and Engineering, 2022., 2022) Tetik, Zafer Gökay.; Yılmaz, Çetin.In this study, phononic band structure of three dimensional (3D) ultrawide elastic metamaterials with embedded inertial amplification mechanisms are obtained. In order to achieve that, inertial amplification mechanisms with different sizes and geometries are considered by applying periodic boundary conditions, also known as Bloch’s boundary conditions, to the unit cells. First, typical wave propagation problems in one dimensional, two dimensional, and three dimensional periodic structures studied in the literature are investigated and benchmark studies are performed by COMSOL Multiphysics and ABAQUS/MATLAB programs. In this way, these models are tested and verified so that the phonon band structure of the 3D elastic metamaterials with embedded inertial amplification mechanisms can be calculated accurately. Inertial amplification mechanisms have complex geometries and their computational costs can be very high. Thus, analyses are done by using both COMSOL Multiphysics and ABAQUS/MATLAB programs. Also, the comparison of the results obtained using these programs with the FRF results of the 3×2 octahedron array enables the determination of the most accurate model. It is very likely to encounter many problems when applying Bloch’s theorem to a complex system such as a 3D elastic metamaterial with embedded inertial amplification mechanisms. Among many problems, four possible problems are explained and their solutions are presented. Thus, it is shown that the band structure of any geometry can be easily obtained, regardless of the complexity of the geometry. To sum up, in the literature, the widest band gap in 3D is achieved by this method, and the band gap is found to be in between 6.37 - 90.26 Hz, with a ratio of the upper limit to the lower limit of 14.17. Hence, it is demonstrated that the 3D elastic metamaterial with embedded inertial amplification mechanisms impedes waves coming from all directions in a very wide frequency range.Item Design, manufacturing and testing of an ethanol(Thesis (M.S.) - Bogazici University. Institute for Graduate Studies in Science and Engineering, 2022., 2022) Uç, İbrahim Safa.; Çelik, Murat.Rocket engines are getting more popular day by day with the involvement of private companies in the space industry. Reusable rockets and lander vehicles are gaining more importance with the upcoming technologies regarding space exploration. Throttleable rocket engine is one of the key components for these rockets and vehicles. In this thesis, a 5 kN pressure fed and water cooled rocket engine was designed, manufactured and tested as a starting point for developing a throttleable and regeneratively cooled rocket engine. %75 concentrated ethanol-water mixture is used as the fuel and liquid oxygen is used as the oxidizer. Engine is designed to work at 30 bar chamber pressure for 1.01 kg/s and 1.26 kg/s fuel and oxidizer mass flow rates, respectively. A preliminary design procedure for the thrust chamber is explained. Cooling channel geometry, and heat transfer between the hot combustion gases and the coolant flow are investigated. Cavitating venturies are used for the mass flow rate control. Their designs and tests are discussed in detail. A test stand is developed to test the engine. Overview of the test stand and pressure control systems are explained. Engine firing test is conducted and achieved mass flow rates are found out to be lower than the expected. 0.89 kg/s fuel and 0.83 kg/s oxidizer mass flow rates are achieved at the final complete thrust chamber test. Possible causes for this situation are investigated.Item Crack propagation analysis in elastomeric isolation bearings(Thesis (M.S.) - Bogazici University. Institute for Graduate Studies in Science and Engineering, 2022., 2022) Tıknaz, Emirhan.; Özüpek, Şebnem.This thesis study focuses on the crack propagation analysis in elastomeric isolation bearings. One benchmark problem related to crack propagation analysis in a single-edge notch specimen and two problems related to interface crack modelling were studied. The results such as reaction force, J integral and strain energy values were compared with the findings from literature. In the benchmark problem, conventional FEM and extended finite element method (XFEM) were used for single edge notch specimen. The strain energy values determined from conventional FEM and XFEM were in good agreement. Advantages and limitations of XFEM were investigated and it was found that J integral is not calculated in crack propagation modeling using XFEM. Therefore, a variable that would allow calculation of energy release rate was investigated. It was determined that for small crack advances, dissipated energy values obtained from the XFEM are very close to those based on J integral values calculated from FEM. 2D axisymmetric and 3D FE models of a circular elastomeric isolation bearing containing interface cracks and subjected to compression and shear loading were analyzed. For both models, the effects of fillet radius at sharp corners and coefficient of friction on the convergence of the FE analysis were investigated and optimization of these parameters to overcome convergence difficulties was accomplished. In the analysis, three different models were analyzed to improve the run time of the computation while maintaining the accuracy. J integral and reaction force for several stationary cracks were found to be in good agreement with the results obtained from the literature. In the 3D model, partial convergence was achieved for compression. For the converged combined loading the change of the J integral with the crack size followed the correct trend.Item A two parameter characterization of edge cracked NiTi shape memory alloy under plane strain conditions(Thesis (M.S.) - Bogazici University. Institute for Graduate Studies in Science and Engineering, 2022., 2022) Merde, Oğuzhan Fikri.; Alkan, Sertan.Shape memory alloys (SMAs) are metallic systems that exhibit reversible, diffusionless, martensitic phase transformation. Employing finite element analyses, the stress fields and crack tip constraints generated are examined for a NiTi SMA which exhibits superelastic behavior. For this purpose, a single edge cracked configuration satisfying plane strain conditions is subjected to uniform loading. Both pure Mode I and mixed mode (Mode I + Mode II) configurations are elaborated by changing the crack inclination angle. As a novel step, a multi-parameter fracture mechanics approach is adapted to characterize the dependence of stress field components on both asymptotic r−1/2 and radial ro terms around the crack tip. This task is accomplished by generating closed-form fitting expressions for stress components via nonlinear leastsquare regression of the full field data from finite element analyses. It has been shown that ro term plays a significant role on the stress field around the crack tip in NiTi SMAs. In characterization of crack tip constraint in NiTi, stress triaxiality parameter, Q, is utilized in the present work. To quantify the behavior of Q, the material characteristics of NiTi such as transformation start and end stresses, hardening modulus and transformation strain are varied under both pure Mode I and mixed mode configurations. The results show that martensitic transformation has an effect of stress constraint relaxation effect reflected by the decrease of Q parameter. Meanwhile promotion of transformation start stress is found to have a strong contribution in constraining crack tip, the transformation end stress is observed to have negligible effect.Item Optimum design of stiffened composite cylindrical shells with a cutout for maximum buckling strength(Thesis (M.S.) - Bogazici University. Institute for Graduate Studies in Science and Engineering, 2022., 2022) Değer, Sezer.; Sönmez, Fazıl Önder.Thin-walled composite cylindrical shells have low resistance to buckling and out-of-plane deformations. Introducing a cutout to these structures reduces the load-carrying capacity of such structures drastically. One effective way to recover the load- carrying capacity lost due to a hole is to place stiffeners around the hole. The objective of this study is to find an optimum reinforcement for thin-walled composite cylindrical shells with a cutout to maximize the buckling load and minimize the additional mass due to the reinforcement. A finite element model of a thin-walled composite cylinder with an opening is created and validated using the results of an experimental and numerical study. Then hat-type stiffeners are applied around the cutout. A parametric study is carried out to determine the effect of each stiffener parameter on the buckling strength of the structure and to choose suitable upper and lower limits for optimization. A modified simulated annealing algorithm is used to find the global optimum reinforcement design. Both the FEA and the optimizations are carried out using ANSYS Parametric Design Language (APDL). In the first step , the optimum designs are obtained for stiffeners placed in the axial direction at certain distances to the center of the cutout by varying only the cross-sectional parameters and the length of the stiffener. In the second stage, the distance to the hole center is also optimized. Finally, using the optimum stiffener dimensions optimization is performed by placing additional small stiffeners on the top and bottom of the cutout. Significant improvements are achieved by using optimum stiffener designs. The most effective parameters are found to be the stiffener length, stiffener distance to the center of the opening and stiffener height.Item Analysis of crack initiation in edge cracked NiTi shape memory alloy plate(Thesis (M.S.) - Bogazici University. Institute for Graduate Studies in Science and Engineering, 2022., 2022) Baykara, Onat.; Alkan, Sertan.Shape Memory Alloys (SMAs) are getting more known among scientist and the engineers day by day. Therefore, their industrial, military and also daily life applications are increasing. Their unique properties make them very useful under special circumstances, however also because of that, there are some parameters (such as fracture), behaviors that need to be discovered. This thesis is designed to investigate the fracture behavior of the Shape Memory Alloys. Their super elastic behavior and martensitic transformation, especially around crack tip, is modeled on ABAQUS software followed by some MATLAB fitting evaluations in order to implement data to conventional fracture criterions. The different material properties are changed and their effects on crack angles are studied. As a result of simulations, effect of every material property and crack angle change is discussed on the last chapter. The results are interpreted as the martensitic transformation effects the crack angle according to MTS, Det and Ip crack initiation criterions due to strain hardening.Item Plasmonic enhancement of absorption efficiency of nanoparticles for photothermal therapy applications(Thesis (M.S.) - Bogazici University. Institute for Graduate Studies in Science and Engineering, 2022., 2022) Tahmaz, Ege Şükrü.; Ertürk, Hakan.Even though humanity is in a golden age when it comes to medical wonders, can cer is still one of the most common and deadly diseases in the world. One new treatment method is called photothermal therapy, which is the thermal ablation of cancer cells by the intravenous injection of plasmonic nanoparticles. Plasmonic nanoparticles absorb the incident light, converting almost all to heat, and increasing the temperature of the environment. But since the nanoparticles are taken intravenously, their placements in the tissue are randomized. In this thesis, the effects of adding dielectric nanoparticles to a plasmonic nanoparticle system on absorption are studied. Nanorods, nanocones, and bipyramid nanoparticles are considered as the dielectric nanoparticles due to their elongated shape. Effects of geometric parameters, such as the radius, on the absorption of the system are studied separately. Optimal parameters of each dielectric nanoparticle are identified. Multiple simulations are completed for each dielectric nanoparticle type where the nanoparticles is placed randomly in a control volume for an approximation of the randomly scattered nature of the nanoparticles in PTT. Nanocones are found to be the best dielectric nanostructure for improving absorption when the orientation of the dielectric nanoparticle can be controlled, with a 228.5% increase in absorption efficiency. However, the non-symmetric nature of the nanocone diminishes the ab sorption improvement greatly, with only a 68.1% improvement in absorption when the particles are placed randomly. Comparisons between the nanorods, nanocones and the bipyramid nanoparticles show that the slanted shaped dielectric nanoparticles are more suitable for PTT applications, while the symmetric geometry of the bipyramid nanoparticles provides more consistent improvements.