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    Development of a design for a crash energy management system for use in a railway passenger car
    (SAGE Publications Ltd, 2016) Meran, Ahmed Partovi; Baykasoğlu, Cengiz; Muğan, Ata; Toprak, Tuncer
    A design approach for a crash energy management (CEM) system for a N13-type railway passenger car used by the Turkish State Railway Company is developed in this paper. The components of the CEM system are honeycomb-structured boxes, primary energy absorbers, shear bolts, a sliding sill mechanism and a fixed sill mechanism that are located in the passenger-free space at the end of the passenger car. In order to investigate the benefits provided by the CEM system, a full-scale railway passenger car collision with a rigid wall is simulated by using dynamic/explicit finite element (FE) methods. The crushing force, secondary impact velocity, acceleration and velocity curves, and deformation modes are computed to allow a comparison of the crashworthiness performance of a passenger car equipped with the proposed CEM system with that of a conventional passenger car. Comparisons of FE analysis results show that a passenger car incorporating the CEM system has a superior crashworthiness performance to that of the conventional passenger car. © 2014 Institution of Mechanical Engineers.
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    Effect of attachment types and number of implants supporting mandibular overdentures on stress distribution: A computed tomography-based 3D finite element analysis
    (Elsevier Ltd, 2015) Arat Bilhan, Selda; Baykasoğlu, Cengiz; Bilhan, Hakan; Kutay, Ömer; Muğan, Ata
    The objective of this study was to calculate stresses in bone tissue surrounding uncoupled and splinted implants that are induced by a bite force applied to the mandible and to determine whether the number of mandibular overdenture supporting implants in mandibular bone influence the stress distribution. A human adult edentulous mandible retrieved from a formalin fixed cadaver was used to define the geometry of finite element (FE) model and the FE model was verified with experimental measurements. Following the FE model validation, three different biting situations were simulated for the 2-, 3- and 4-implant retentive anchor as well as bar attachment overdentures under vertical loading of 100. N. As a result of the analyses, it was concluded that an increment in implant number and the splinted attachment type tended to cause lower stresses and the use of two single attachments seems to be a safe and sufficient solution for the treatment of mandibular edentulism with overdentures. © 2014 Elsevier Ltd.
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    Effects of lithium doping on hydrogen storage properties of heat welded random CNT network structures
    (Elsevier Ltd, 2016) Baykasoğlu, Cengiz; Öztürk, Zeynel; Kırca, Mesut; Çelebi, Alper Tunga; Muğan, Ata; To, Albert C.
    This paper presents the effects of lithium doping on the hydrogen storage capability of heat welded random carbon nanotube (CNT) network structures having different cross-link densities at room temperature. Cluster based and atom based doping strategies are taken into consideration in the current simulations. Moreover, different doping ratios are used in our calculations to clarify the effect of doping ratio on hydrogen uptake behavior of CNT networks. The network structures are generated by using a cyclic stochastic algorithm and covalently bonded couplings are created by applying the heat welding method via molecular dynamic simulations. Hydrogen storage capacity of the Li-doped CNT networks is investigated using Grand Canonical Monte Carlo (GCMC) simulations. The simulation results show that hydrogen storage capacity is appreciably enhanced with the presence of lithium atoms and increases as the doping ratio increases. Besides, atom based doping method is found to be more efficient in hydrogen uptake than cluster based doping method under the same doping ratio. Furthermore, our results also show that the cross-link density is a critical parameter and hydrogen storage capability of Li-doped CNT network material can be improved with the appropriate choice of cross-link density. © 2016 Hydrogen Energy Publications LLC.
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    Failure analysis of graphene sheets with multiple Stone-Thrower-Wales defects using molecular mechanics based nonlinear finite element models
    (Hitit Üniversitesi Fen Bilimleri Enstitüsü, 2018-06-09) Baykasoğlu, Cengiz; Muğan, Ata
    Experimental studies show that Stone-Thrower-Wales (STW) defects generally exist in graphene sheets (GSs) and these defects considerably affect the fracture strength of GSs. Thus, prediction of failure modes of GSs with STW defects is useful for design of graphene based nanomaterials. In this paper, effects of multiple STW defects on fracture behavior of GSs are investigated by employing molecular mechanics based nonlinear finite element models. The modified Morse potential is used to define the non-linear characteristic of covalent bonds between carbon atoms and geometric nonlinearity effects are considered in models. Different tilting angles of STW defects are considered in simulations. The analysis results showed that the fracture strength of GSs strongly depends on tilting angle of multiple STW defects and the STW defects cause significant strength loss in GSs. The crack initiation and propagation are also studied and brittle failure characteristics are observed for all samples. The results obtained from this study provide some insights into design of GS based-structures with multiple STW defects.
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    Hydrogen storage in heat welded random CNT network structures
    (Elsevier Ltd, 2015) Öztürk, Zeynel; Baykasoğlu, Cengiz; Çelebi, Alper Tunga; Kırca, Mesut; Muğan, Ata; To, Albert C.
    The objective of this study is to investigate hydrogen storage capability of heat welded random carbon nanotube (CNT) network structures. To achieve this objective, different three-dimensional random CNT network structures are generated by using a stochastic algorithm and molecular dynamic simulations. The interaction of CNT networks with hydrogen molecules is then examined via grand canonical Monte Carlo calculations. Hydrogen adsorption capacity of CNT networks having an arbitrarily natured morphology, adjustable porous structure and large surface ratio is investigated. The results show that if cross link density of random CNT networks decreases, hydrogen storage capability of CNT networks increases in terms of the gravimetric capacity. It is observed that random CNT networks could uptake 8.85 wt.% hydrogen at 77 K and this result is very comparable with the results reported in literature where generally ideal ordered nanostructures having no topological irregularities are considered. © 2014 Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
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    Rollover crashworthiness analysis of a railroad passenger car
    (2013) Baykasoğlu, Cengiz; Muğan, Ata; Sünbülo?lu, Emin; Bozda?, Ergün; Aruk, Fatih; Toprak, Tuncer
    Rollover of railroad passenger cars is one of the most serious accidents having high rates of injuries and fatalities. Therefore, the railroad passenger car structures should satisfy the rollover crashworthiness requirements in order to protect the occupants. In this paper, rollover crashworthiness of a railroad passenger car called N13-type used by Turkish State Railways is investigated. To assess the rollover crashworthiness, rollover simulations of the full scale and detailed passenger car models onto a rigid ground are completed by using a non-linear explicit finite element code, Abaqus. The numerical model is validated with static and modal experimental measurements before running rollover analyses of the railroad passenger car. Good agreement is observed between tests and finite element results, which indicate that the numerical model accurately represents the real structure. Simulations are performed in accordance with the regulation of ECE R66. Based on this standard, two different rollover scenarios are considered. In the first scenario, lateral rollover simulation is performed at the standard angular velocity due to ECE R66 regulation. The second scenario is performed at the double of this angular velocity to investigate the crashworthiness behaviour under exaggerated conditions. As a result of the analyses, it is observed that the residual space remains undamaged during and after the rollover, and passenger car is able to pass the required regulations. © 2013 Taylor and Francis.
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    Tensile behavior of heat welded CNT network structures
    (Elsevier, 2014) Çelebi, Alper Tunga; Kırca, Mesut; Baykasoğlu, Cengiz; Muğan, Ata; To, Albert C.
    In this study, a computational investigation on the mechanical properties of random carbon nanotube (CNT) networks is reported. For this purpose, atomistic models of 3-D CNT network structures are generated by an automated stochastic algorithm that is capable of randomly inserting CNTs into a design space while controlling several topological parameters including cross-link density, distance between cross-links, as well as length and chirality of CNTs. During the atomistic model generation process, heat welding is applied to the close contacts to form a welded CNT network with covalently-bonded junctions. Following the generation of atomistic models, molecular dynamics (MD) simulations of uniaxial tensile loading experiments are performed to investigate the basic deformation mechanisms of CNT networks as well as to obtain mechanical properties such as the Young's modulus, yield strength and ultimate strain values. The effects of cross-link density on the mechanical performance of CNT network materials are obtained by employing network specimens with different cross-link densities. © 2014 Elsevier B.V. All rights reserved.
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    Thermal-stress analysis of ceramic laminate veneer restorations with different incisal preparations using micro-computed tomography-based 3D finite element models
    (Elsevier Ltd, 2017) Çelebi, Alper Tunga; İçer, Esra; Mert Eren, Meltem; Baykasoğlu, Cengiz; Muğan, Ata; Yıldız, Esra
    Main objective of this study is to investigate the thermal behavior of ceramic laminate veneer restorations of the maxillary central incisor with different incisal preparations such as butt joint and palatinal chamfer using finite element method. In addition, it is also aimed to understand the effect of different thermal loads which simulates hot and cold liquid imbibing in the mouth. Three-dimensional solid models of the sound tooth and prepared veneer restorations were obtained using micro-computed tomography images. Each ceramic veneer restoration was made up of ceramic, luting resin cement and adhesive layer which were generated based on the scanned images using computer-aided design software. Our solid model also included the remaining dental tissues such as periodontal ligament and surrounding cortical and spongy bones. Time-dependent linear thermal analyses were carried out to compare temperature changes and stress distributions of the sound and restored tooth models. The liquid is firstly in contact with the crown area where the maximum stresses were obtained. For the restorations, stresses on palatinal surfaces were found larger than buccal surfaces. Through interior tissues, the effect of thermal load diminished and smaller stress distributions were obtained near pulp and root-dentin regions. We found that the palatinal chamfer restoration presents comparatively larger stresses than the butt joint preparation. In addition, cold thermal loading showed larger temperature changes and stress distributions than those of hot thermal loading independent from the restoration technique. © 2017