Mechanical Engineering / Makina Mühendisliği
Permanent URI for this collectionhttps://hdl.handle.net/11147/4129
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Article Citation - WoS: 3Citation - Scopus: 3Quasi-Static and Dynamic Brazilian Testing and Failure Analysis of a Deer Antler in the Transverse To the Osteon Growth Direction(Elsevier, 2023) Orhan, Mehmet; Sarıkaya, Mustafa Kemal; Taşdemirci, Alper; Tuncer, Can; Güden, MustafaThe transverse tensile strength of a naturally fallen red deer antler (Cervus Elaphus) was determined through indirect Brazilian tests using dry disc-shape specimens at quasi-static and high strain rates. Dynamic Brazilian tests were performed in a compression Split-Hopkinson Pressure Bar. Quasi-static tensile and indirect Brazilian tests were also performed along the osteon growth direction for comparison. The quasi-static transverse tensile strength ranged 31.5–44.5 MPa. The strength increased to 83 MPa on the average in the dynamic Brazilian tests, proving a rate sensitive transverse strength. The quasi-static tensile strength in the osteon growth direction was however found comparably higher, 192 MPa. A Weibull analysis indicated a higher tensile ductility in the osteon growth direction than in the transverse to the osteon growth direction. The microscopic analysis of the quasi-static Brazilian test specimens (tensile strain along the osteon growth direction) revealed a micro-cracking mechanism operating by the crack deflection/twisting at the lacunae in the concentric lamellae region and at the interface between concentric lamellae and interstitial lamellae. On the other side, the specimens in the transverse direction fractured in a more brittle manner by the separation/delamination of the concentric lamellae and pulling of the interstitial lamellae. The detected increase in the transverse strength in the high strain rate tests was further ascribed to the pull and fracture of the visco-plastic collagen fibers in the interstitial lamellae. This was also confirmed microscopically; the dynamically tested specimens exhibited flatter fracture surfaces. © 2023 Elsevier LtdArticle Citation - WoS: 9Citation - Scopus: 11Experimental Investigation of Spray Characteristics of Ethyl Esters in a Constant Volume Chamber(Springer, 2022) Ulu, Anılcan; Yıldız, Güray; Özkol, Ünver; Rodriguez, Alvaro DiezAbstract: Biodiesels are mainly produced via the utilization of methanol in transesterification, which is the widespread biodiesel production process. The majority of this methanol is currently obtained from fossil resources, i.e. coal and natural gas. However, in contrast with methanol, biomass-based ethanol can also be used to produce biodiesels; this could allow the production line to become fully renewable. This study aimed to investigate the spray characteristics of various ethyl ester type biodiesels derived from sunflower and corn oils in comparison to methyl esters based on the same feedstocks and reference petroleum-based diesel. Spray penetration length (SPL) and spray cone angle (SCA) were experimentally evaluated in a constant volume chamber allowing optical access, under chamber pressures of 0, 5, 10 and 15 bar and injection pressures of 600 and 800 bar. Sauter mean diameter (SMD) values were estimated by using an analytical correlation. Consequently, ethyl esters performed longer SPL (2.8–20%) and narrower SCA (5.1–19%) than diesel under ambient pressures of 5 and 10 bar. Although the SMD values of ethyl esters were 48% higher than diesel on average, their macroscopic spray characteristics were very similar to those of diesel under 15 bar chamber pressure. Moreover, ethyl esters were found to be very similar to methyl esters in terms of spray characteristics. The differences in SPL, SCA and SMD values for both types of biodiesels were lower than 4%. When considering the uncertainty (± 0.84%) and repeatability (±5%) ratios, the difference between the spray characteristics of methyl and ethyl esters was not major.Article Citation - WoS: 26Citation - Scopus: 27Electric Charge of Nanopatterned Silica Surfaces(Royal Society of Chemistry, 2019) Özçelik, H. Gökberk; Barışık, MuratThe most recent technologies employ nanoscale surface patterning or roughening in order to engineer desired properties on a surface. Electrokinetic properties at the interface of such surfaces and ionic liquids show different behavior to the well-known theoretical descriptions. Basically, the ionic distribution on the surface differs due to electrical double layer overlap effects in the pits and curvature effects at the tips of surface structures. Generally, the charge density of a surface is assumed to be a material property and surface roughness effects are overlooked in most of the literature. In contrast, we properly calculated the local surface charges based on surface chemistry at the corresponding local ionic concentration (charge regulation) for various surface roughness and solution conditions. The results showed that the surface charge density of silica decreased at the pits but increased at the tips of surface patterns. Even for the simplest case of self-repeating surface structures, the average of local surface charges becomes lower than the theoretical predictions. Based on numerical calculations, a phenomenological model was developed as an extension to the existing flat surface theory, which can successfully predict the average surface charge on a nano patterned surface as a function of the surface pattern size, ionic concentration and pH.Article Citation - WoS: 14Citation - Scopus: 14Size Dependent Surface Charge Properties of Silica Nano-Channels: Double Layer Overlap and Inlet/Outlet Effects(Royal Society of Chemistry, 2018) Şen, Tümcan; Barışık, MuratTransport inside nano-channels and tubes is highly dependent on their surface charge properties. While previous studies assume that the charge density of a surface is a material property and independent of confinement size, this study properly characterized the surface charge of a nanochannel as a function of channel height and length under various solution conditions. By calculating the local surface charge based on local ionic concentrations, the surface charge of a nano-channel was studied by considering the effects of both overlapping electrical double layers (EDLs) and inlet/outlet regions. First, the surface charge of silica decreased with the increase in EDL overlap, which is characterized by the ratio of EDL thickness to channel height. Second, the local surface charge showed variation at the inlet/outlet regions where the channel’s electrokinetics was in development. We defined a general entrance length as a function of EDL thickness for the electrokinetically developing part of different cases, after which the surface charge reached its equilibrium value and remained constant. Based on such length scales, we extended the existing theory to include nano-effects. A phenomenological model was developed, which can predict the average nano-channel surface charge as a function of EDL thickness, pH, channel length and channel height.Article Citation - WoS: 44Citation - Scopus: 47Electric Field Controlled Transport of Water in Graphene Nano-Channels(American Institute of Physics, 2017) Çelebi, Alper Tunga; Barışık, Murat; Beşkök, AliMotivated by electrowetting-based flow control in nano-systems, water transport in graphene nano-channels is investigated as a function of the applied electric field. Molecular dynamics simulations are performed for deionized water confined in graphene nano-channels subjected to opposing surface charges, creating an electric field across the channel. Water molecules respond to the electric field by reorientation of their dipoles. Oxygen and hydrogen atoms in water face the anode and cathode, respectively, and hydrogen atoms get closer to the cathode compared to the oxygen atoms near the anode. These effects create asymmetric density distributions that increase with the applied electric field. Force-driven water flows under electric fields exhibit asymmetric velocity profiles and unequal slip lengths. Apparent viscosity of water increases and the slip length decreases with increased electric field, reducing the flow rate. Increasing the electric field above a threshold value freezes water at room temperature.Article Citation - WoS: 6Citation - Scopus: 7An Entropy-Based Analysis of Lane Changing Behavior: An Interactive Approach(Taylor and Francis Ltd., 2017) Koşun, Çağlar; Özdemir, SerhanObjectives: As a novelty, this article proposes the nonadditive entropy framework for the description of driver behaviors during lane changing. The authors also state that this entropy framework governs the lane changing behavior in traffic flow in accordance with the long-range vehicular interactions and traffic safety. Methods: The nonadditive entropy framework is the new generalized theory of thermostatistical mechanics. Vehicular interactions during lane changing are considered within this framework. The interactive approach for the lane changing behavior of the drivers is presented in the traffic flow scenarios presented in the article. According to the traffic flow scenarios, 4 categories of traffic flow and driver behaviors are obtained. Through the scenarios, comparative analyses of nonadditive and additive entropy domains are also provided. Results: Two quadrants of the categories belong to the nonadditive entropy; the rest are involved in the additive entropy domain. Driving behaviors are extracted and the scenarios depict that nonadditivity matches safe driving well, whereas additivity corresponds to unsafe driving. Furthermore, the cooperative traffic system is considered in nonadditivity where the long-range interactions are present. However, the uncooperative traffic system falls into the additivity domain. The analyses also state that there would be possible traffic flow transitions among the quadrants. This article shows that lane changing behavior could be generalized as nonadditive, with additivity as a special case, based on the given traffic conditions. Conclusions: The nearest and close neighbor models are well within the conventional additive entropy framework. In this article, both the long-range vehicular interactions and safe driving behavior in traffic are handled in the nonadditive entropy domain. It is also inferred that the Tsallis entropy region would correspond to mandatory lane changing behavior, whereas additive and either the extensive or nonextensive entropy region would match discretionary lane changing behavior. This article states that driver behaviors would be in the nonadditive entropy domain to provide a safe traffic stream and hence with vehicle accident prevention in mind.Article Citation - WoS: 24Citation - Scopus: 28Atomic Density Effects on Temperature Characteristics and Thermal Transport at Grain Boundaries Through a Proper Bin Size Selection(American Institute of Physics, 2016) Vo, Truongquoc; Barışık, Murat; Kim, BohungThis study focuses on the proper characterization of temperature profiles across grain boundaries (GBs) in order to calculate the correct interfacial thermal resistance (ITR) and reveal the influence of GB geometries onto thermal transport. The solid-solid interfaces resulting from the orientation difference between the (001), (011), and (111) copper surfaces were investigated. Temperature discontinuities were observed at the boundary of grains due to the phonon mismatch, phonon backscattering, and atomic forces between dissimilar structures at the GBs. We observed that the temperature decreases gradually in the GB area rather than a sharp drop at the interface. As a result, three distinct temperature gradients developed at the GB which were different than the one observed in the bulk solid. This behavior extends a couple molecular diameters into both sides of the interface where we defined a thickness at GB based on the measured temperature profiles for characterization. Results showed dependence on the selection of the bin size used to average the temperature data from the molecular dynamics system. The bin size on the order of the crystal layer spacing was found to present an accurate temperature profile through the GB. We further calculated the GB thickness of various cases by using potential energy (PE) distributions which showed agreement with direct measurements from the temperature profile and validated the proper binning. The variation of grain crystal orientation developed different molecular densities which were characterized by the average atomic surface density (ASD) definition. Our results revealed that the ASD is the primary factor affecting the structural disorders and heat transfer at the solid-solid interfaces. Using a system in which the planes are highly close-packed can enhance the probability of interactions and the degree of overlap between vibrational density of states (VDOS) of atoms forming at interfaces, leading to a reduced ITR. Thus, an accurate understanding of thermal characteristics at the GB can be formulated by selecting a proper bin size.Article Citation - WoS: 67Citation - Scopus: 82Microhardness and Fracture Toughness of Dental Materials by Indentation Method(John Wiley and Sons Inc., 2005) Şakar-Deliormanlı, Aylin; Şakar Deliormanlı, Aylin Müyesser; Güden, Mustafa; Güden, MustafaThe main objective of this study was to measure the fracture toughness of the human teeth enamel using the microindentation technique and to compare the results calculated from the equations developed for Palmqvist and radial-median cracks. Vickers microhardness measurements of dental ceramic (alumina) and human teeth were performed using indentation fracture method. The fracture toughness of enamel was calculated using different equations reported in the literature. Vickers microhardness of the sintered alumina specimen (98.8% theoretical density) was measured to be 14.92 GPa under 9.8N indentation load. Three equations based on the radial-median cracks were found to be applicable for the fracture toughness determination of the enamel. Results show that indentation fracture method is adequate to measure microhardness and fracture toughness of dental materials. However the calculation of fracture toughness depended on the nature of the cracks and also on the location of the indentation. Therefore, it is necessary to identify the crack profile and to select the appropriate equation for accurate fracture toughness values.Article Citation - WoS: 37Citation - Scopus: 42Effects of Compaction Pressure and Particle Shape on the Porosity and Compression Mechanical Properties of Sintered Ti6al4v Powder Compacts for Hard Tissue Implantation(John Wiley and Sons Inc., 2007) Güden, Mustafa; Çelik, Emrah; Hızal, Alpay; Altındiş, Mustafa; Çetiner, SinanSintered Ti6Al4V powder compacts potentially to be used in implant applications were prepared using commercially available spherical and angular powders (100-200 mum) within the porosity range of 34-54%. Cylindrical green powder compacts were cold compacted at various pressures and then sintered at 1200 degrees C for 2 h. The final percent porosity and mean pore sizes were determined as functions of the applied compaction pressure and powder type. The mechanical properties were investigated through compression testing. Results have shown that yield strength of the powder compacts of 40-42% porosity was comparable with that of human cortical bone. As compared with previously investigated Ti powder compacts, Ti6Al4V powder compacts showed higher strength at similar porosity range. Microscopic observations on the failed compact samples revealed that failure occurred primarily by the separation of interparticle bond regions in the planes 45 degrees to the loading axis. Effects of compaction pressure and particle shape on the porosity and compression mechanical properties of sintered Ti6Al4V powder compacts for hard tissue implantation.