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ÖgeMechanical properties of boron nanotubes( 20211105)Boron nanotubes (BNTs) which can be considered as structural analogs of carbon nanotubes (CNTs) and boron nitride nanotubes (BNNTs) offer remarkable mechanical, electrical, and chemical properties. As the building unit of BNTs, boron, the fifth element in the periodic table, is the lightest elemental substance that can form interatomic covalent bonds possessing multiple bonding states, which in turn provides a variety of allotropes with diverse physical and chemical properties. BNTs exhibit metallic behavior regardless of their chirality and diameters, which renders them extremely attractive in the design of novel electronic nanodevices, such as fieldeffect transistors, lightemitting diodes, field emission displays. In these applications, mechanical properties play a significant role since the mechanical strain is usually employed to adjust the electronic properties of the BNTs. Therefore, mechanical properties, such as tensile strength and elastic Young's modulus, of the boron nanotube structures hold significant importance. In literature, most of the theoretical studies regarding the boron nanotubes are based on the firstprinciples density functional theory calculations. As an alternative approach, reactive molecular dynamics can provide accurate and quick results depending on the accuracy of the force field. Furthermore, unlike density functional theory calculations, molecular dynamics can be used to investigate large systems. In the present study, boron nanotubes are simulated using reactive molecular dynamics simulations. Although this method has been extensively practiced for borophene, to the best of our knowledge, it has not been used to simulate BNTs yet. We created 10 different BNTs with different vacancy ratios ranging between 0 and 0.33 in two different chiral directions, zigzag and armchair. Simulations are conducted for different diameters, lengths, and aspect ratios using four different strain rates and three different temperatures, 1, 300, and 600 K. We conducted tensile tests to inspect the mechanical properties. Mechanical properties and thermal stabilities of BNTs are highly dependent on their vacancy ratio, atomic configuration, and chirality. Our results indicate that BNTs with exhibit highly anisotropic behavior. Young moduli and ultimate tensile stress of nanotubes are generally two times higher in the zigzag direction, yet the ultimate tensile strain is two times higher in the armchair direction, except for some configurations. Stiffness and strength in general decrease while the vacancy ratio and temperature increase. The potential energy difference per atom due to the bond order is the main root of the defect formation. Some structures exhibit plastic behavior owing to stable bond formations during tensile. We believe that our study will drive further research for BNTs using classical molecular dynamics since it will allow largescale simulation and modeling. Their vacancies can be exploited for several applications such as hydrogen storage. Thermal properties, nanocomposites with BNTs can be subject to future studies.

ÖgeAnalysis of tube spinning process(Graduate School, 202107)Tube spinning is a continuous bulkforming process used to produce seamless, cylindrical, conical and contoured tubes. Over the last six decades, tube spinning process has been applied in a wide range of engineering applications; especially in automotive, aerospace and nuclear industry. However, the process has seen little change, and despite a large volume of literature investigating the process, understanding of the process mechanics is limited and the key references have been published about 50 years ago. This thesis aims to provide an insight into tube spinning process and its process mechanics, looking into mechanism of deformation, subsequently using this insight to explain phenomena observed in tube spinning. To do this, tube spinning was investigated in detail using three different theoretical investigation methods. To understand the effects of process parameters on tube geometry in terms of tube geometry: bellmouth, material accumulation and wave phenomena, a set of physical trials was performed with varying process parameters: feed ratio, reduction ratio, staggered spinning and lubrication. The results show that bellmouth, material accumulation and wave increase with an increase in feed ratio and reduction ratio, number of passes and decrease when staggered roller setup is used. However, the results indicate that there is no relationship between friction (lubrication) and dimensional accuracy. Also, to give insights into the mechanics of tube spinning process, numerical and analytical modelling approaches have been developed. The process is investigated using a numerical model with an implicit time integration and Lagrangian scheme. Full tube model and a set of fast angular segment tube models have been developed and validated against physical trials in terms of tube profile and forming forces. According to the tube profile comparison between the model and physical trial, the results show that the model agrees with the physical trial to within %97.

ÖgeHigh cycle fatigue life estimation in frequency domain using multi input multi output QT matrix method(Graduate School, 20191219)Component failures due to vibrational fatigue damage is a common problem in automotive industry. There are many studies in literature to predict the fatigue life of vehicle components. It is reported that frequency domain fatigue life estimation methods (FDFEMs) have advantages over time domain approaches. In this study, FDFEMs are applied to vibrational fatigue problems having multi inputs and multi outputs (MIMO). In particular, high cycle fatigue life estimation problem was considered and SN diagrams were mainly employed in analyses. Inputoutput relations are obtained in frequency domain using the QT matrix method that enables using a reduced order model in frequency domain. The QT matrix method for MIMO system representation is convenient to obtain transfer functions between selected input and output degrees of freedom (DOFs) considering the crosscorrelations among the DOFs. Following, popular FDFEMs such as Narrow Band Approximation, Tovo Benasciutti, Zhao and Baker, and Dirlik Methods are applied to some critical components in a truck chassis based on MIMO QT matrix models. The FDFEMs considered in this study can be classified as narrow band (namely, Narrow Band Approximation) and wide band (namely, Tovo Benasciutti, Zhao and Baker, and Dirlik Methods) processes. Comparisons are made with the Ncode solutions and experimental results. It is concluded that the FDFEMs associated with the MIMO QT matrix method yield accurate fatigue life estimations and their CPU times are much less than those of full order models. In this thesis, it is analyzed how to get the transfer functions between accelerations of input DOFs and accelerations of critical DOFs of some critical parts. QT matrix method is a convenient method to obtain transfer functions of critical locations on a vehicle using some chassis input locations, from inputs' power spectral density (PSD) to outputs' PSD. Acceleration values at each input point are measured by an accelerometer. Output PSDs are obtained by QT matrix method and turned into stress values. QT matrix method which is entegrated with FDFEMs can be applied to random excitation and MIMO problems in frequency domain. Due to the accuracy of results and consideration of crosscoupling effects, QT matrix method was preferred in this study. Due to the fact that knowledge of input force is not neccesary, operational modal analysis can be implemented. Therefore, it is cheaper and faster compared to experimental modal analyses. The Frequency Response Functions (FRFs) used in the formulations of QT matrix method were obtained by finite element models. To this end, Nastran software was used to obtain FRFs. Nonetheless, the FRFs could be obtained by experimental modal tests in which multiple shakers should be employed. Applicability of this study is in realtime and onboard for heavy duty truck is investigated. The instrumentation of a heavy duty truck produced by Otosan was completed, accelerometers were attached to the critical locatons of the truck chassis. Then, the Raspberry Pi controller was used for computational tasks. The thesis was a part of a Tubitak Ardeb project focusing on merely vibrational fatigue damage. Besides the truck use case, this study is feasible also for other complicated structures such as airplanes, ships, buses and otomobiles. All algorithms for this study, first, was written in Matlab for Ardunio. Later, this algorithms were converted to Python codes for Raspberry Pi as Python is free software, which means cost reduction. The developed approach was applied to a heavy duty truck chassis. The critical components that were examined in this study were the exhaust muffler bracket and fuel tank bracket. Experimental verifications were made on test roads and vibration test bench excited by shakers. The validation of computational models, mathematical derivations and programs were made by making comparisons with numerical results of Ncode software and experimental tests. It is observed that although the input excitations (namely, forces) were wide band processes, the chassis dynamic behaves like a filter and the PSDs of stresses at selected locations on the chassis were found to be narrow band processes. It was surprising that the fatigue life estimations at critical locations obtained by the Dirlik method which was originally developed for wide band processes yielded larger errors than those of the Narrow Band Approximation which was originally developed for narrow band processes. It is also noteworthy that the computational load of the Narrow Band Approximation is lower than that of the Dirlik method.