Scopus İndeksli Yayınlar Koleksiyonu
Permanent URI for this collectionhttps://hdl.handle.net/20.500.12573/395
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Article Crashworthiness Evaluation of 3D-Printed Hybrid-Design Multi-Cell Energy Absorbers Under Lateral Compression for Unmanned Aerial Vehicles(Springer Heidelberg, 2025-11-25) Atahan, M. Gokhan; Zeybek, Halil; Ozturk, SezginEnergy absorbers can be strategically integrated into critical areas of unmanned aerial vehicles to protect their structural integrity and electronic components in the event of an accident. In this study, hybrid-design multi-cell energy absorber configurations were proposed, and their crashworthiness performance and collapse mechanisms were comparatively analyzed. Hybrid energy absorbers were designed considering circular, square, hexagonal, and re-entrant unit cell geometries. The energy absorber configurations were produced via additive manufacturing. Compared to the single-cell circular energy absorber, the hybrid-design multi-cell approach resulted in a higher peak crushing force value, while offering considerable enhancements in other crashworthiness parameters. Configuration 3 is recommended for use in energy absorber applications in unmanned aerial vehicles due to its superior crashworthiness performance. Moreover, in hybrid-design multi-cell energy absorbers, the selection of layer geometries significantly influences deformation capability. Compared to the single-cell circular configuration (Configuration 1), Configuration 3 demonstrated superior crashworthiness performance by increasing the MCF, EA, and SEA values by 7.47, 4.47, and 1.41 times, respectively.Article Citation - WoS: 3Citation - Scopus: 7The Electronic and Optical Properties of an Exciton, Biexciton and Charged Excitons in CdSe/CdTe-Based Multi-Shell Type-II Quantum Dot Nanocrystals(Springer Heidelberg, 2019-09-17) Koc, Fatih; Sahin, MehmetIt has been recently reported that multi-shell type-II quantum dot nanocrystals (QDNCs) have higher quantum yields. Besides these higher quantum yields of multi-shell type-II QDNCs, additional second layer has been a critical influence on the formation mechanisms of the excitonic structures. Understanding of bound and unbound cases of the excitonic structures in multi-shell type-II QDNCs gives some important information for applications. In this study, we have investigated the electronic and optical properties of a single exciton (X), biexciton (XX), and positively and negatively charged excitons (X+ and X-) in CdSe/CdTe-based multi-shell type-II QDNCs. In the study, three different structure compositions, i.e., CdSe/ CdTe, CdSe/CdTe/CdS, and CdSe/CdTe/ZnTe, have been considered. We have observed that CdS and ZnTe materials have drastically changed the electronic and optical properties of the bare CdSe/CdTe type-II QDNCs.Article Citation - WoS: 3Citation - Scopus: 3RF MEMS Variable Attenuators With Improved dB-Linearity(Springer Heidelberg, 2023-02-22) Hah, DooyoungA variable attenuator is one of the essential components in radio frequency (RF) systems, such as automatic gain control amplifiers and full-duplex systems. Variable attenuators based on microelectromechanical systems (MEMS) technology have several advantages over the semiconductor counterparts, including low power consumption and suppressed harmonics. Attenuation can be realized by disruption of signal propagation, which is induced by moving electrodes placed next to a signal line. In this work, the effect of the moving electrodes on the RF characteristics of the variable attenuators is studied via numerical simulation. It is observed that 10 lm of moving electrode displacement can result in 18 dB of attenuation dynamic range at 20 GHz. The similar type of RF MEMS variable attenuators reported previously showed substantial nonlinearity in attenuation-voltage characteristics, which becomes a serious drawback for applications where high-precision attenuation management is required. The main objective of the current study is, therefore, to achieve high dB-linearity, by employing shaped-finger comb-drive actuators in the moving electrode displacement control. In addition, a nonlinear relationship between force and displacement in a clamped-clamped beam spring is taken into account for more accurate device modelling. Through finite element analysis, it is shown that an improvement by a factor of twelve can be obtained in dB-linearity by using a single-comb shaped-finger actuator, compared to standard straight-finger comb-drives. The study also shows that the dB-linearity can be further (2.2 times additionally) improved by utilizing dual-comb shaped finger actuators.Article Citation - WoS: 19Citation - Scopus: 19Investigation of the Effect of Surface Crack on Low-Velocity Impact Response in Hybrid Laminated Composite Plates(Springer Heidelberg, 2020-06) Gunes, Aydin; Sahin, Omer SinanComposite materials can be damaged in the environments in which they are used, due to the loads they are exposed to or due to different effects on the production processes. The formation processes of these damages generally develop as crack formation or progress of the existing crack. For this reason, it is very important to investigate the behavior of the crack that occurs after the dynamic loads to which the composite materials are exposed. In this study, the dynamic behaviors of hybrid laminated composites with different surface crack geometries were investigated. Surface cracks with different crack depth-to-thickness (a/t) and crack depth-to-crack width (a/c) ratios were machined upon hybrid composite laminates and subjected to low-velocity impact tests under 2 m/s, 2.5 m/s and 3 m/s impact velocities. The effect of different surface crack geometries upon variation of contact force versus time, variation of contact force versus displacement and variation of absorbed/rebound energy have been evaluated. The effect of surface crack geometry and impact velocity upon contact stiffness and bending stiffness was also evaluated. Damage formation during impact loading was examined by scanning electron microscopy and optical microscopy. After the evaluations, the damage behaviors caused by the dynamic loads depending on the initial surface crack geometry were examined in detail.Article Citation - WoS: 1Citation - Scopus: 1Electret Vibration Energy Harvesters With Symmetrically Configured Curved-Beam Hinges(Springer Heidelberg, 2024-03) Hah, DooyoungFor vibration energy harvesters, broadband power spectral characteristics are often desirable. One way of achieving broadband spectrum is to employ curved-beam hinges, utilizing their nonlinear spring characteristics. In our previous study, electret-based vibration energy harvesters employing curved-beam hinges were investigated via numerical analysis based on stochastic differential equations and colored-noise inputs. It showed that the harvesters with curved-beam hinges can produce higher power than the ones with ordinary straight beams when the external acceleration is between 0.02g and 0.05g. It was also learned that the straight-beam device, a Duffing oscillator, performs better than the curved-beam device, a Duffing-Holmes oscillator, at higher acceleration (>= 0.1g\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\ge 0.1g$$\end{document}). Since the energy harvesting efficiency is one of the most important performance indicators, continuous search for novel configurations with improved efficiency is needed. For that purpose, a symmetric configuration of curved-beam hinges (a Duffing oscillator) is proposed in this work, in contrast to the previously reported one with an asymmetric configuration (a Duffing-Holmes oscillator). This study shows, via numerical analysis, that the symmetric configuration can produce higher (up to 7.3% more) power outputs for the external acceleration magnitude higher than 0.1g, when compared to an asymmetric configuration. The study results also show that it can produce higher power outputs (up to 4.5 times) than the harvesters employing ordinary straight-beam hinges.Article Citation - WoS: 1Citation - Scopus: 1Dynamics of Fuzzy Difference Equations System With Higher-Order(Springer Heidelberg, 2025-03-19) Topan, Osman; Yazlik, Yasin; Atpinar, SevdaThere are few studies in the literature that focus on two-dimensional higher-order fuzzy difference equations, leaving a considerable gap in our understanding of their behavior and dynamics. This highlights the necessity to investigate this field in order to answer fundamental concerns and broaden its possible uses.This study looks into the existence, uniqueness, boundedness, persistence, and convergence of positive solutions to a two-dimensional system of higher-order fuzzy difference equations. These qualities are crucial to understanding the system's behavior and stability.Theoretical analysis is used to rigorously establish the aforementioned system features. To validate the efficiency and application of the theoretical results, numerical simulations are provided, exhibiting the behavior and supporting the study's findings.Conference Object Citation - WoS: 3Citation - Scopus: 3Analytical Design of MEMS Variable Capacitors Based on Shaped-Finger Comb-Drives(Springer Heidelberg, 2019-02-23) Hah, DooyoungA variable capacitor is one of the widely used components in radio frequency (RF) circuits. Variable capacitors can benefit from the microelectromechanical systems (MEMS) technology, to be equipped with attractive characteristics such as high quality factor and wide tuning range. One of the design goals for MEMS varactors has been to realize linear capacitance-voltage (C-V) characteristics, for which a design method is proposed in this paper, based on shaped-finger comb-drive actuators. The shaped-finger design method, originally developed for a tunable optical filter application by the author, is redeveloped in this work for a linear C-V varactor. Moreover, the conformal mapping method is employed in calculation of capacitances, making the whole design process more time-efficient, being almost all-analytical with the minimum usage of numerical analysis methods. Effects of sense capacitor finger shapes to the optimized drive capacitor finger shapes and the corresponding C-V characteristics are investigated as well. Variable capacitors with the shaped-finger design show linearity factor (LF)-defined as the maximum deviation from the perfect linear relationship-as good as 0.4%, enormously improved from that of the conventional constant-finger-gap devices (LF: 49.9%). Further probed by 3-D numerical analysis, the C-V characteristics of the designed variable capacitor show LF better than 2.62% in the case of constant-gap sense capacitors, and as good as 0.77% in the case of shaped-finger sense capacitors. Versatility of the design method is further demonstrated by presenting a varactor for linear resonant frequency-voltage (f-V) characteristics in voltage-controlled oscillator (VCO) applications. Finally, effects of etch bias, one of common fabrication imperfections, to the linearity of C-V characteristics are studied. The developed analytical design method with shaped fingers can find a wide range of applications where comb-drive actuators are used.