Makine Mühendisliği Bölümü Koleksiyonu

Permanent URI for this collectionhttps://hdl.handle.net/20.500.11779/1944

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  • Article
    Increasing and Other Subsequence Problems for Random Interval Sequences
    (Elsevier, 2026) Arslan, Ilker; Islak, Umit
    Various relations for comparison of intervals of real numbers are introduced, and the expected length of the corresponding longest increasing subsequence is analyzed. When intervals are randomly generated by taking the minimum and maximum of two independent uniform random variables, we prove that the expected length of the longest increasing subsequence grows on root the order of 3 n. We also investigate the asymptotic behavior of the expected length under alternative comparison relations and random interval models. Discussions on other subsequence problems for interval sequences are included.
  • Article
    Citation - WoS: 3
    Citation - Scopus: 3
    Powder metallurgy processing of seven/eight component multi-phase (HfTiZr-Mn/Mo/W/Cr/Ta)B<sub>2</sub> high entropy diboride ceramics
    (Elsevier, 2024) Suzer, Ilayda; Akbari, Amir; Ates, Semih; Bayrak, Kuebra Gurcan; Mertdinc-Ulkuseven, Siddika; Arisoy, C. Fahir; Agaogullari, Duygu; Öveçoğlu, M. Lutfi
    This study aims to show the possibility of synthesizing seven- and eight-component high entropy diboride (HEB) ceramics using high energy ball milling-assisted spark plasma sintering (SPS). Metal boride powders, synthesized in laboratory conditions from metal oxide-boron oxide-magnesium powder blends, were combined equimolarly as seven and eight components containing systems. Afterwards, hybridized powders were mechanically alloyed (MA) for 6 h and subjected to spark plasma sintering (SPS) at 2000 degrees C and under 30 MPa. Detailed phase analysis and physical, microstructural, and mechanical characterization of the samples were performed. in the sintered products, the main phase belongs to the HEB, and also low amounts of Hf/Zr oxides and secondary phases (W or Ti-rich) occurred. The highest hardness was observed at the (HfTiZrMoWCrTa)B-2 with 25 GPa, and the lowest hardness was seen at the (HfTiZrMnCrMoWTa)B2 with 17 GPa. Also, the highest wear resistance was calculated for the (HfTiZrMnCrMoTa)B-2 as 6.05 x 10(-7) mm(3)/Nm. Additionally, (HfTiZrMnMoWTa)B-2 and (HfTiZrMnMoCrTa)B-2 have the highest and lowest Archimedes' densities, with 7.94 g/cm(3) and 6.91 g/cm(3), respectively.
  • Book Part
    Citation - Scopus: 4
    Jute and Luffa Fibers: Physical, Acoustical, and Mechanical Properties
    (Elsevier, 2022) Yüce, Hüseyin; Genç, Garip; Körük, Hasan
    In this study, the physical, acoustical, and mechanical properties of jute and luffa fibers are presented. It should be noted that, although the main materials of jute and luffa fibers are cellulose, hemicellulose, and lignin, the mechanical properties of jute and luffa fibers and the acoustical properties of jute and fiber samples can be quite different. It is worth noting that jute fibers are bidirectional while luffa fibers are random-oriented. Furthermore, the diameters and lengths of these two fibers can be quite different. One problem with these natural fibers is that very different values for their diameter, length, and density have been reported in the literature. Therefore, the diameter, length, and density of a huge number of jute and luffa fibers were measured using precise equipment to determine their average values and deviations in this study. In addition to the results of these measurements, the results of a comprehensive literature review on the physical, acoustical, and mechanical properties of jute and fibers and their green samples (i.e., fiber samples without any resin) were presented in this study. © 2023 Elsevier Ltd. All rights reserved.
  • Book Part
    Citation - Scopus: 3
    Effects of Machining on the Acoustic and Mechanical Properties of Jute and Luffa Biocomposites
    (Elsevier, 2023) Genç, Garip; Körük, Hasan
    After their production, biocomposite structures do not always have the final shape or dimensions required for their purpose, hence, they need machining. However, the effects of machining on the acoustic and mechanical properties of many biocomposites are still not well known. The effects of machining on the acoustic and mechanical properties of jute and luffa biocomposites are revealed in this chapter. To do this, the sound absorption coefficients (or SACs) and transmission losses (or TLs) of jute and epoxy and luffa and epoxy composite samples, with and without a turning process, are determined using the impedance tube method. The loss factors and Young’s moduli of the jute and epoxy and luffa and epoxy composite samples, with and without a milling process, are identified using experimental and theoretical modal analyses. The results show that, when the samples are machined, the sound absorption coefficients reduce by 3%-7%, the transmission loss levels increase by 6-11dB, and the damping levels and Young’s moduli reduce by 0.1%-0.5% and 3%-4%, respectively. © 2023 Elsevier Ltd. All rights reserved.
  • Book Part
    Citation - Scopus: 1
    Prediction of the Sound Absorption Performance of Porous Samples Including Cellulose Fiber-Based Structures
    (Elsevier, 2022) Körük, Hasan
    The mathematical models for predicting the sound absorption coefficients (SACs) of porous samples are first presented, then they are used to predict the SACs of some porous structures, and their performances are evaluated. First of all, the parameters needed for the calculation of the SACs of a porous sample are briefly introduced. After that, the mathematical models for the prediction of acoustic properties are presented. These models include (i) simple empirical models such as Delany-Bazley and its modified versions, (ii) rigid-frame models such as Johnson-Champoux-Allard and Johnson-Champoux-Allard-Lafarge, and (iii) deformable-frame models such as Biot-Allard. After that, the estimation of the parameters needed in the mathematical models is presented. Then, the aforementioned models are used to predict the SACs of some porous samples including cellulose fiber-based structures, and their performances are evaluated in detail. © 2023 Elsevier Ltd. All rights reserved.
  • Article
    Citation - WoS: 8
    Citation - Scopus: 10
    Identification of the Viscoelastic Properties of Soft Materials Using a Convenient Dynamic Indentation System and Procedure
    (Elsevier, 2022) Körük, Hasan,; Beşli, Ayça,; Koç, Ömer Hayati,; Yurdaer, Berk Salih,; Yurdaer, Salih Berk; Koc, Hayati Omer
    The responses of soft structures such as tissue depend on their viscoelastic properties. Therefore, the knowledge of the elastic and damping properties of soft materials is of great interest. This paper presents the identification of the viscoelastic properties of soft materials using a convenient dynamic indentation system and procedure. Using an electromagnet, a force is applied to a rigid sphere located at the soft-material interface and the dynamic response of the sphere is recorded using a high-speed camera. The recorded video is processed to identify the displacement of the sphere as a function of time. The dynamic response of the sphere located at the soft-material interface is predicted using an analytical model that considers the shear modulus and density of the soft sample, the radiation damping due to shear waves, and the radius and density of the sphere. By matching the measured and predicted steady-state displacements of the sphere, the shear modulus of the soft sample is determined. The viscous damping ratio of the soft sample is identified by using an equivalent viscous damping ratio for the soft sample in the analytical model and matching the measured and predicted oscillation amplitudes of the sphere. Experiments and analyzes are performed using gelation phantoms with different mechanical properties, spheres of different materials and sizes, and different force levels to verify the system and procedure. Three experiments are performed for each gelation phantom, sphere, and external force, and the repeatability of the results is presented. The results show that the dynamic indentation system and procedure presented in this study can be conveniently used to determine the viscoelastic properties of soft materials in practical applications.
  • Book Part
    Citation - Scopus: 3
    19 - Identification of the Elastic and Damping Properties of Jute and Luffa Fiber-Reinforced Biocomposites
    (Elsevier, 2022) Genç, Garip; Saygılı, Yusuf; Körük, Hasan; Şanlıtürk, Yusuf Kenan; Sanliturk, Kenan Y.
    Although there are many studies in the literature on the static mechanical properties of biomaterials such as tensile strength, the dynamic mechanical properties of biomaterials such as modal loss factors have not been investigated in detail. In this study, the Young’s moduli and damping (or loss factors) of some jute and luffa fiber-reinforced biocomposites are investigated. The effects of fiber/resin ratio and thickness on the mechanical properties of the jute and luffa composites are identified via an experimental approach. For this purpose, acoustic and structural frequency response functions of some homogeneous and hybrid jute and luffa composite plates with different fiber/resin ratios and thicknesses are measured. By analyzing the measured frequency response functions using the circle-fit method, the modal frequencies and loss factors of the homogeneous and hybrid composite plates are determined. By assuming that the homogeneous plates are isotropic, the same plates are modeled using the finite element method, and by comparing the experimental and theoretical natural frequencies, the elastic properties of the homogeneous plates are determined. In addition, the same homogeneous plates are modeled by considering an anisotropic material model, and the associated material properties are determined. By using the identified material properties, the finite element models of the hybrid composite plates are developed, and by comparing their experimental and theoretical natural frequencies, the identified elastic material properties are evaluated and validated.
  • Book Part
    Citation - Scopus: 6
    18 - Acoustic and Mechanical Properties of Biofibers and Their Composites
    (Elsevier, 2022) Koç, Büşra; Genç, Garip; Körük, Hasan
    In this study, the acoustic and mechanical properties of many biofibers and their composites are presented. First, the sound absorption coefficients and transmission losses of commonly used natural fibers and their composites are presented, by clearly reporting the thickness of the samples, for three different frequency ranges (<500 Hz: low, 500–2000 Hz: medium, and >2000 Hz: high). In addition, the sound absorption coefficients (for low- and medium-frequency ranges) and noise reduction coefficients of some 40-cm-thick samples are overlaid in order to directly compare their performances. Second, the physical properties, such as the density, diameter, and length of biofibers, and mechanical properties, such as the damping (or loss factor) and Young’s modulus of biofibers and their composites, are presented in detail. For comparison purposes, the acoustic and mechanical properties of some conventional materials, such as carbon and glass fibers, are included in the tables and figures. Finally, the effects of some parameters, such as pretreatment, fiber diameter, fiber/matrix ratio, moisture content, manufacturing and machining parameters/techniques, and measurement conditions/methods, on the acoustic and mechanical properties of natural materials are presented. Furthermore, current applications and potential usage areas of natural fibers are briefly discussed.
  • Article
    Citation - WoS: 34
    Citation - Scopus: 37
    Development of a New Solar, Gasification and Fuel Cell Based Integrated Plant
    (Elsevier, 2021) Dinçer, İbrahim; Karapekmez, Aras
    Despite its shortcomings, fossil-based fuels are still utilized as the main energy source, accounting for about 80% of the world's total energy supply with about one-third of which comes from coal. However, conventional coal-fired power plants emit relatively higher amounts of greenhouse gases, and the derivatives of air pollutants, which necessitates the integration of environmentally benign technologies into the conventional power plants. In the current study, a H2–CO synthesis gas fueled solid oxide fuel cell (SOFC) is integrated to the coal-fired combined cycle along with a concentrated solar energy system for the purpose of promoting the cleaner energy applications in the fossil fuel-based power plants. The underlying motivation of the present study is to propose a novel design for a conventional coal-fired combined cycle without altering its main infrastructure to make its environmentally hazardous nature more ecofriendly. The proposed SOFC integrated coal-fired combined cycle is modeled thermodynamically for different types of coals, namely pet coke, Powder River Basin (PRB) coal, lignite and anthracite using the Engineering Equation Solver (EES) and the Ebsilon software packages. The current results show that the designed hybrid energy system provide higher performance with higher energy and exergy efficiencies ranging from 70.6% to 72.7% energetically and from 35.5% to 43.8% exergetically. In addition, carbon dioxide emissions are reduced varying between 18.31 kg/s and 30.09 kg/s depending on the selected coal type, under the assumption of 10 kg per second fuel inlet.
  • Article
    Citation - WoS: 28
    Citation - Scopus: 29
    Development of a New Integrated Energy System With Compressed Air and Heat Storage Options
    (Elsevier, 2020) Javania, Nader; Dinçer, İbrahim; Karapekmez, Aras; Javani, Nader
    The present study investigates a biomass driven power plant integrated with compressed air and thermal energy storage subsystems. Compressed air energy storage system exploits the pressurized air at non-peak periods to be used in peak times when there is a need for extra energy. Thermal energy storage systems including phase change material, allow the solar subsystem to operate independently in order to produce hot air when solar irradiation is insufficient. The energy stored in the present system is then supplied to both the gasifier and combustion chamber in order to achieve a higher combustion efficiency. Three different phase change materials (PCMs) are investigated and their efficiencies are comparatively evaluated. Among the considered PCMs, LiNO3 is the most suitable material for the considered system with 82% energy efficiency and 84% exergy efficiency. The current study also aims at designing a renewable energy based power plant which operates continuously through using storage subsystems and is more environmental benign compared to fossil fuel based conventional systems. In this regard, wet wood (CH1.46O0.64N0.002) with 15% moisture content is selected as a fuel instead of fossil-based fuels in order to reduce the greenhouse gas emissions and eliminate the dep endency on fossil fuels. A comprehensive thermodynamic analysis is conducted to evaluate the entropy generations, exergy destructions, and energy and exergy efficiencies. The highest overall energy and exergy efficiencies are obtained as 28.58% and 24.08% in the discharging period, respectively.