Mechanical Engineering / Makina Mühendisliği
Permanent URI for this collectionhttps://hdl.handle.net/11147/4129
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Article Citation - WoS: 28Citation - Scopus: 30Bone Marrow Stem Cells Adapt To Low-Magnitude Vibrations by Altering Their Cytoskeleton During Quiescence and Osteogenesis(TUBITAK, 2015) Demiray, Levent; Özçivici, EnginApplication of mechanical vibrations is anabolic to bone tissue, not only by guiding mature bone cells to increased formation, but also by increasing the osteogenic commitment of progenitor cells. However, the sensitivity and adaptive response of bone marrow stem cells to this loading regimen has not yet been identified. In this study, we subjected mouse bone marrow stem cell line D1-ORL-UVA to daily mechanical vibrations (0.15 g, 90 Hz, 15 min/day) for 7 days, both during quiescence and osteogenic commitment, to identify corresponding ultrastructural adaptations on cellular and molecular levels. During quiescence, mechanical vibrations significantly increased total actin content and actin fiber thickness, as measured by phalloidin staining and fluorescent microscopy. Cellular height also increased, as measured by atomic force microscopy, along with the expression of focal adhesion kinase (PTK2) mRNA levels. During osteogenesis, mechanical vibrations increased the total actin content, actin fiber thickness, and cytoplasmic membrane roughness, with significant increase in Runx2 mRNA levels. These results show that bone marrow stem cells demonstrate similar cytoskeletal adaptations to low-magnitude high-frequency mechanical loads both during quiescence and osteogenesis, potentially becoming more sensitive to additional loads by increased structural stiffness.Article Citation - WoS: 8Citation - Scopus: 8Pore Connectivity Effects on the Internal Surface Electric Charge of Mesoporous Silica(Springer, 2019) Şen, Tümcan; Barışık, MuratNano-scale confinements within mesoporous systems develop overlapping electric double layers (EDL) such that the existing theoretical models cannot predict the electric potential distributions and resulting surface charges. In addition, ionic conditions undergo local variation through connections between the pore voids and pore throats. For the first time in literature, we studied the charging behavior of mesoporous silica in terms of the pore to throat size ratio (R-pt) to characterize the pore connectivity effects, in addition to porosity (epsilon) and pore size (H). Both local and average surface charge densities inside mesoporous silica were examined by varying these parameters systematically. Results showed that the magnitude of surface charge density decreased with increasing EDL overlap and decreasing connectivity effects. We formulized this behavior and developed an extended model to predict mesoporous silica's internal charge as a function of porosity, pore size, and pore to throat size ratio.Article Citation - WoS: 7Citation - Scopus: 9Effects of Hybrid Yarn Preparation Technique and Fiber Sizing on the Mechanical Properties of Continuous Glass Fiber-Reinforced Polypropylene Composites(SAGE Publications, 2016) Merter, N. Emrah; Başer, Gülnur; Tanoğlu, MetinIn this study, hybrid yarns were developed by commingling the continuous polypropylene and glass fibers using air jet and direct twist preparation techniques. The non-crimp fabrics were obtained with +/- 45 degrees fiber orientation from these hybrid yarns. The fabrics were prepared with fiber sizings that are compatible and incompatible with polypropylene matrix to investigate the effect of interfacial adhesion on the properties of the thermoplastic composites. Composite panels were produced from the developed fabrics by hot press compression method and microstructural and mechanical properties of the composites were investigated. It was found that type of the hybrid yarn preparation technique and glass fiber sizing applied on the glass fibers have some important role on the properties of the composites. Composites made of fabrics produced by air jet hybrid yarn preparation technique exhibited better results than those produced by direct twist covering (single or double) hybrid yarn preparation techniques. The highest flexural properties (99.1MPa flexural strength and 9.55 GPa flexural modulus) were obtained from the composites manufactured from fabric containing compatible sizing, due to better adhesion at the interface of glass fibers and polypropylene matrix. The composite fabricated from fabric with polypropylene compatible sizing also exhibited the highest peel resistance (interlaminar peel strength value of 5.87N/mm). On the other hand, it was found that hybrid yarn preparation technique and type of the glass fiber sizing have insignificant effect on the impact properties of the glass fiber/polypropylene composites.Article Interfacial Convective Heat Transfer for Randomly Generated Porous Media(Begell House, 2018) Uçar, Eren; Mobedi, Moghtada; Ahmadi, AzitaHeat and fluid flow in 20 random porous media is investigated by using the Monte Carlo (MC) procedure. Each porous medium consists of long square rods distributed randomly in flow direction. The continuity, momentum, and energy equations are solved for a row of porous media representing the entire domain of a random porous medium. The microstructure properties of each random porous medium which are the mean and standard deviations of the Voronoi areas, the nearest neighbor distance and orientation are obtained. The rods in the domain are classified into three groups as blocker, active, and passive rods according to their effects on the penetration of heat in porous media. 'The interfacial convective heat transfer coefficients for each rod and entire porous medium are calculated and plotted for different Reynolds numbers. A characteristic length based on the microstructure properties of the generated porous media is defined, and three correlations relating to the upper limit, lower limit, and mean of the overall interfacial convective heat transfer coefficient are proposed.Article Citation - WoS: 16Citation - Scopus: 24Developing Polymer Composite-Based Leaf Spring Systems for Automotive Industry(Walter de Gruyter GmbH, 2018) Öztoprak, Nahit; Güneş, Mehmet Deniz; Tanoğlu, Metin; Aktaş, Engin; Eğilmez, Oğuz Özgür; Şenocak, Çiler; Kulaç, GedizComposite-based mono-leaf spring systems were designed and manufactured to replace existing mono-leaf metal leaf spring in a light commercial vehicle. In this study, experimentally obtained mechanical properties of different fiber-reinforced polymer materials are presented first, followed by the description of the finite element analytical model created in Abaqus 6.12-1 (Dassault Systemes Simulia Corp., RI, US) using the obtained properties. The results from the finite element analysis are presented next and compared with actual size experimental tests conducted on manufactured prototypes. The results demonstrated that the reinforcement type and orientation dramatically influenced the spring rate. The prototypes showed significant weight reduction of about 80% with improved mechanical properties. The hybrid composite systems can be utilized for composite-based leaf springs with considerable mechanical performance.Article Citation - WoS: 9Citation - Scopus: 10On the Estimation and Optimization Capabilities of the Fatigue Life Prediction Models in Composite Laminates(SAGE Publications, 2018) Deveci, Hamza Arda; Artem, Hatice SeçilIn this study, the estimation and optimization capabilities of the multiaxial fatigue life prediction models, namely, Failure Tensor Polynomial in Fatigue, Fawaz-Ellyin, Sims-Brogdon and Shokrieh-Taheri are investigated comparatively. Fatigue life predictions are obtained for multidirectional graphite/epoxy, glass/epoxy, carbon/epoxy and carbon/PEEK composite laminate data taken from the literature. The prediction study shows that the models can predict the fatigue behavior of the multidirectional laminates at different degrees of proximity. In the optimization, a hybrid algorithm combining particle swarm algorithm and generalized pattern search algorithm is used to search the optimum stacking sequence designs of the laminated composites for maximum fatigue life. The hybrid algorithm shows superior performance in terms of computational time and finding improved global optima compared to the best results presented in the literature. After the capability of the models and the reliability of the algorithm are revealed, several lay-up design problems involving different cyclic loading scenarios are solved. The results indicate that the reliability of the optimization may considerably change according to the used model even if the model may yield reasonable prediction results.Article Citation - WoS: 13Citation - Scopus: 17Design of Dimensionally Stable Composites Using Efficient Global Optimization Method(SAGE Publications Inc., 2019) Aydın, Levent; Aydın, Olgun; Artem, Hatice Seçil; Mert, AliDimensionally stable material design is an important issue for space structures such as space laser communication systems, telescopes, and satellites. Suitably designed composite materials for this purpose can meet the functional and structural requirements. In this paper, it is aimed to design the dimensionally stable laminated composites by using efficient global optimization method. For this purpose, the composite plate optimization problems have been solved for high stiffness and low coefficients of thermal and moisture expansion. Some of the results based on efficient global optimization solution have been verified by genetic algorithm, simulated annealing, and generalized pattern search solutions from the previous studies. The proposed optimization algorithm is also validated experimentally. After completing the design and optimization process, failure analysis of the optimized composites has been performed based on Tsai-Hill, Tsai-Wu, Hoffman, and Hashin-Rotem criteria.Article Citation - WoS: 35Citation - Scopus: 39Development of Graphene Nanoplatelet-Reinforced Az91 Magnesium Alloy by Solidification Processing(Springer Verlag, 2018) Kandemir, SinanIt is a challenging task to effectively incorporate graphene nanoplatelets (GNPs) which have recently emerged as potential reinforcement for strengthening metals into magnesium-based matrices by conventional solidification processes due to their large surface areas and poor wettability. A solidification processing which combines mechanical stirring and ultrasonic dispersion of reinforcements in liquid matrix was employed to develop AZ91 magnesium alloy matrix composites reinforced with 0.25 and 0.5 wt.% GNPs. The microstructural studies conducted with scanning and transmission electron microscopes revealed that fairly uniform distribution and dispersion of GNPs through the matrix were achieved due to effective combination of mechanical and ultrasonic stirring. The GNPs embedded into the magnesium matrix led to significant enhancement in the hardness, tensile strength and ductility of the composites compared to those of unreinforced AZ91 alloy. The strength enhancement was predominantly attributed to the grain refinement by the GNP addition and dislocation generation strengthening due to the coefficient of thermal expansion mismatch between the matrix and reinforcement. The improved ductility was attributed to the refinement of β eutectics by transforming from lamellar to the divorced eutectics due to the GNP additions. In addition, the strengthening efficiency of the composite with 0.25 wt.% GNP was found to be higher than those of the composite with 0.5 wt.% GNP as the agglomeration tendency of GNPs is increased with increasing GNP content. These results were compared with those of the GNP-reinforced magnesium composites reported in the literature, indicating the potential of the process introduced in this study in terms of fabricating light and high-performance metal matrix composites.Article Citation - WoS: 12Citation - Scopus: 13A General Expression for the Stagnant Thermal Conductivity of Stochastic and Periodic Structures(The American Society of Mechanical Engineers(ASME), 2018) Bai, X.; Çelik, Hasan; Mobedi, Moghtada; Nakayama, AkiraA general expression has been obtained to estimate thermal conductivities of both stochastic and periodic structures with high-solid thermal conductivity. An air layer partially occupied by slanted circular rods of high-thermal conductivity was considered to derive the general expression. The thermal conductivity based on this general expression was compared against that obtained from detailed three-dimensional numerical calculations. A good agreement between two sets of results substantiates the validity of the general expression for evaluating the stagnant thermal conductivity of the periodic structures. Subsequently, this expression was averaged over a hemispherical solid angle to estimate the stagnant thermal conductivity for stochastic structures such as a metal foam. The resulting expression was found identical to the one obtained by Hsu et al., Krishnan et al., and Yang and Nakayama. Thus, the general expression can be used for both stochastic and periodic structures.Article Citation - WoS: 1Sol-Derived Hydroxyapatite Ddip-Coating of a Porous Ti6al4v Powder Compact(Al-Farabi Kazakh State National University, 2009) Altındiş, Mustafa; Güden, Mustafa; Ni, ChaoyingA sintered porous Ti6Al4V powder compact with a mean pore size of 63 µm and an average porosity of 37±1% was dip-coated at soaking times varying between 1- and 5-minute using a sol-derived calcium Hydrooxyapatite (HA) powder. The coated compacts were heat-treated at 840 °C. The coating thickness was found to increase with increasing soaking time, from 1.87 µm at 1-minute soaking to 9 µm at 5-minute soaking on the average. It was shown that at increasing soaking times, the originally open pores started to close, while at low soaking times the Ti6Al4V particles were partially coated. The coating layer was shown to be nano porous and the depth of coating was observed to be relatively shallow: only few particles near the compact surface were HA-coated.