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Öğe Başlıksız(Elsevier, 2017) Bhullar, Sukhwinder Kaur; Rana, Deepti; Lekesiz, Hüseyin; Bedeloğlu, Ayşe; Ko, Junghyuk; Cho, YonghyunThe main objective of this study was to fabricate poly (e-caprolactone) (PCL)-based auxetic nanofiber membranes and characterize them for their mechanical and physicochemical properties. As a first step, the PCL nanofibers were fabricated by electrospinning with two different thicknesses of 40 pm (called PCL thin membrane) and 180 pm (called PCL thick membrane). In the second step, they were tailored into auxetic patterns using femtosecond laser cut technique. The physicochemical and mechanical properties of the auxetic nanofiber membranes were studied and compared with the conventional electrospun PCL nanofibers (non-auxetic nano fiber membranes) as a control. The results showed that there were no significant changes observed among them in terms of their chemical functionality and thermal property. However, there was a notable difference observed in the mechanical properties. For instance, the thin auxetic nanofiber membrane showed the magnitude of elongation almost ten times higher than the control, which clearly demonstrates the high flexibility of auxetic nanofiber membranes. This is because that the auxetic nanofiber membranes have lesser rigidity than the control nanofibers under the same load which could be due to the rotational motion of the auxetic structures. The major finding of this study is that the auxetic PCL nanofiber membranes are highly flexible (10-fold higher elongation capacity than the conventional PCL nanofibers) and have tunable mechanical properties. Therefore, the auxetic PCL nanofiber membranes may serve as a potent material in various biomedical applications, in particular, tissue engineering where scaffolds with mechanical cues play a major role.Öğe Başlıksız(Inderscience Enterprises Ltd, 2017) Yıldız, Betül Sultan; Lekesiz, HüseyinThis paper deals with a new product design framework for the optimal structural design applications. The proposed approach called hybrid charged system search (CSS) and Nelder-Mead algorithm (HCSSNM) offers significant improvements in performance over existing methods in the field of computer-aided design optimisation. The HCSSNM based on the CSS and Nelder-Mead algorithm is used for optimisation of a vehicle suspension arm which is critically important for the vehicle safety. As observed in the results, the HCSSNM can be used effectively to reduce the mass of the vehicle components while maintaining its fatigue durability.Öğe Başlıksız(Gruppo Italiano Frattura, 2016) Demir, O.; Iric, S.; Ayhan, A. O.; Lekesiz, HüseyinIn this study, to investigate and understand the nature of fracture behavior properly under in-plane mixed mode (Mode-I/II) loading, three-dimensional fracture analyses and experiments of compact tension shear (CTS) specimen are performed under different mixed mode loading conditions. Al 7075-T651 aluminum machined from rolled plates in the L-T rolling direction (crack plane is perpendicular to the rolling direction) is used in this study. Results from finite element analyses and fracture loads, crack deflection angles obtained from the experiments are presented. To simulate the real conditions in the experiments, contacts are defined between the contact surfaces of the loading devices, specimen and loading pins. Modeling, meshing and the solution of the problem involving the whole assembly, i.e., loading devices, pins and the specimen, with contact mechanics are performed using ANSYS (TM). Then, CTS specimen is analyzed separately using a submodeling approach, in which three-dimensional enriched finite elements are used in FRAC3D solver to calculate the resulting stress intensity factors along the crack front. Having performed the detailed computational and experimental studies on the CTS specimen, a new specimen type together with its loading device is also proposed that has smaller dimensions compared to the regular CTS specimen. Experimental results for the new specimen are also presented.Öğe Başlıksız(Iop Publishing Ltd, 2017) Lekesiz, Hüseyin; Bhullar, Sukhwinder Kaur; Karaca, A. A.; Jun, M. B. G.Smart materials in auxetic form present a great potential for various medical applications due to their unique deformation mechanisms along with durable infrastructure. Both analytical and finite element (FE) models are extensively used in literature to characterize mechanical response of auxetic structures but these structures are mostly thick enough to be considered as bulk material and 3D inherently. Auxetic plates in very thin form, a. e. foil, may bring numerous advantages such as very light design and better biodegradability when needed. However, there is a gap in literature on mechanical characterization of auxetic thin plates. In this study, structural analysis of very thin auxetic plates under uniaxial loading is investigated using both FE method and experimental method. 25 mu m thick stainless steel (316L) plates are fabricated with reentrant texture for three different unit cell dimensions and tested under uniaxial loading using universal testing machine. 25 and 50 mu m thick sheets with same cell dimensions were analyzed using implicit transient FE model including strain hardening and failure behaviors. FE results cover all the deformation schemes seen in actual tests and total deformation level matches with test results. Effect of plate thickness and cell geometry on auxetic behavior is discussed in detail using FE results. Finally, based on FE analysis results, an optimum geometry for prolonged auxetic behavior, high flexibility and high durability is suggested for future potential applications.Öğe Başlıksız(Carl Hanser Verlag, 2016) Yıldız, Betül Sultan; Lekesiz, Hüseyin; Yıldız, Ali RızaThe fuel and cost efficiency in automotive industry is related to the designing of light-weight vehicle components. Main aim of this paper is to show effectiveness of two new optimization algorithms in solving structural design optimization problems. The gravitational search algorithm (GSA) and charged system search algorithm (CSS) are applied to the optimum design of a vehicle component. The results show that both GSA and CSS have a better search capability than the genetic algorithms to find optimum solutions.Öğe Başlıksız(Springer, 2013) Lekesiz, HüseyinThe simple method developed by Kachanov (1985) for multiple interacting cracks in homogenous medium is extended to predict complex stress intensity factor for multiple split type interface cracks. Calculations are implemented for two equal cracks and infinite row of periodic cracks at the interface between two dissimilar isotropic materials. Results for infinite row of cracks are compared against the exact analytical solution provided by Sih (1973). The approximate method leads to the results very close the exact solution for crack density up to 0.90 (relative error is less than 3.8% for real part of stress intensity factor) and material dissimilarity does not have a major influence on the error. For crack densities higher than 0.90, the influence of material dissimilarity is more evident and the error increases as material dissimilarity increases. The promising match between the approximate and exact method proves the capability of the approximate method for solving other interacting interface crack problems, such as multiple penny-shaped interface cracks, in which the solution is not obtained in the literature yet.Öğe Başlıksız(Sage Publications Inc, 2014) Lekesiz, HüseyinThe objective of this study was to evaluate the potential of a survival prediction method for the assessment of ceramic dental restorations. For this purpose, fast-fracture and fatigue reliabilities for 2 bilayer (metal ceramic alloy core veneered with fluorapatite leucite glass-ceramic, d.Sign/d.Sign-67, by Ivoclar; glass-infiltrated alumina core veneered with feldspathic porcelain, VM7/In-Ceram Alumina, by Vita) and 3 monolithic (leucite-reinforced glass-ceramic, Empress, and ProCAD, by Ivoclar; lithium-disilicate glass-ceramic, Empress 2, by Ivoclar) single posterior crown restorations were predicted, and fatigue predictions were compared with the long-term clinical data presented in the literature. Both perfectly bonded and completely debonded cases were analyzed for evaluation of the influence of the adhesive/restoration bonding quality on estimations. Material constants and stress distributions required for predictions were calculated from biaxial tests and finite element analysis, respectively. Based on the predictions, In-Ceram Alumina presents the best fast-fracture resistance, and ProCAD presents a comparable resistance for perfect bonding; however, ProCAD shows a significant reduction of resistance in case of complete debonding. Nevertheless, it is still better than Empress and comparable with Empress 2. In-Ceram Alumina and d.Sign have the highest long-term reliability, with almost 100% survivability even after 10 years. When compared with clinical failure rates reported in the literature, predictions show a promising match with clinical data, and this indicates the soundness of the settings used in the proposed predictions.Öğe Başlıksız(2018) Lekesiz, HüseyinYarı-hermetik soğutma kompresörleri, arıza durumunda kolay müdahale kolaylığı sağlarlar. Bu kompresörlerde kullanılan krank milleri hem hafif hem de dayanıklı olmalıdır. Bu sebeple, doğru tasarım ve doğru malzeme seçimi çok önemlidir. Bu kompresörler yüksek hızda çalıştığından, zamana bağlı dinamik analiz kaçınılmaz olmaktadır. Ancak bu yöntemle problem çözümü çok uzamaktadır ve yakınsama sağlamak zorlaşmaktadır. Özellikle yataklardaki yağ basıncı değişimi vs gibi yapıdaki tüm dinamik etkiler dikkate alındığında problem iyice karmaşıklaşmaktadır. Probleme hızlı ancak statik yaklaşımdan daha doğru bir çözüm elde etmek amacıyla bu çalışmada sanki-statik yaklaşım kullanılmıştır. Kinematik ve kinetik analiz sonucu elde edilen kuvvetler yapıya belli kabuller altında statik olarak uygulanmıştır. Ortaya çıkan maksimum gerilme krank milinin yağlama plakasına bağlanan ucundaki fatura dibinde olup, bu literatürdeki hasar örnekleriyle uyuşmaktadır. Analizler sonucunda atalet etkisi %1,7 civarında olup, ele alınan krank mili için sanki-statik yaklaşımın oldukça doğru bir yaklaşım olduğu gösterilmiştir.Öğe Başlıksız(SAE International, 2019) Karaduman, A.; Lekesiz, Hüseyin; Yildiz, A.Diaphragm spring design has a critical importance on engagement and disengagement mechanism for a clutch system. The release bearing load characteristic of diaphragm springs determines the quality of disengagement. Release bearing load may change significantly especially under dynamic conditions compared to static conditions. For instance, a significant drop in release bearing load may be experienced under high engine velocities. This is an undesirable situation in terms of clutch performance. Drop in release bearing load may cause not to be transmitting reliably torque from engine to gearbox since minimum release bearing load is not ensured. Drop in release bearing load increases incrementally in proportion to the engine speed increase. This problem has not been addressed in technical literature. Therefore, new design of diaphragm spring becomes necessary to avoid weakening of clutch performance as the engine speed increases which leads to highly dynamic conditions. In this study, the effect of some design parameters for diaphragm springs such as outer radius and embossing are investigated using nonlinear finite element modeling. Three different outer radius with and without embossing were analyzed under boundary conditions representing actual diaphragm tests. Based on the results of FEA, it is concluded that unwanted load drops increases as the outer radius increases while embossing also leads to higher load drop. On the other hand, embossing increases the stiffness of diaphragm fingers and therefore a smaller deformation and a more long-lasting clutch load performance. As a result, an optimum design is required as the diaphragm outer radius increases for a robust disengagement behavior. © 2019 SAE International. All Rights Reserved.Öğe Başlıksız(Elsevier Science Inc, 2018) Demir, Oguzhan; Ayhan, Ali O.; Iric, Sedat; Lekesiz, HüseyinIn this study, in-plane mixed mode-I/II fatigue crack growth simulations and experiments are performed for the Al 7075-T651 aluminum alloy which is widely used in the aerospace industry. Tests are carried out under different mode mixity ratios to evaluate the applicability of a fracture criterion developed in a previous study to mixed mode-I/II fatigue crack growth tests. Results obtained from the analyses and experiments are compared with existing and developed criteria in terms of crack growth lives. Compact Tension Shear (CTS) specimens, which enable mixed mode loading with loading devices under different loading angles, are used in the simulations and experiments. In an effort to model and simulate the actual conditions in the experiments, crack surfaces of fractured specimens are scanned, crack paths are modeled exactly, and contacts are defined between the contact surfaces of a specimen and the loading device for each crack propagation step in the analyses. Having computed the mixed mode stress intensity factors from the numerical analyses, propagation life cycles are predicted by existing and the developed mixed mode-I/II criteria and then compared with experimental results. (C) 2018 Chinese Society of Aeronautics and Astronautics. Production and hosting by Elsevier Ltd.Öğe Başlıksız(2013) Lekesiz, Hüseyin; Katsube, N.; Rokhlin, S.I.; Seghi, R.R.An effective spring stiffness approximation is proposed for a hexagonal array of coplanar penny shaped cracks located at the interface between two dissimilar solids. The approximation is based on the factorization of the solution on the material dissimilarity factor, the crack interaction factor and the effective spring stiffness solution for non-interacting cracks in a homogeneous material. Such factorization is exact and was validated for 2D collinear cracks between two dissimilar solids. The crack interaction factor is obtained using a recently developed model for stress intensity factors for an array of coplanar penny shaped cracks in a homogeneous material; also the material dissimilarity function recently obtained for non-interacting penny shaped crack at the interface between two dissimilar materials is employed. The obtained solution is useful for an assessment by ultrasonic measurements of the interface stiffness in bonded structures for monitoring the interfacial microdamage growth due to mechanical loading and environmental factors. © 2013 Elsevier Ltd. All rights reserved.Öğe Başlıksız(2013) Lekesiz, Hüseyin; Katsube, N.; Rokhlin, S.I.; Seghi, R.R.The effect of crack interactions on stress intensity factors is examined for a periodic array of coplanar penny-shaped cracks. Kachanov's approximate method for crack interactions [Kachanov, M.; 1987. Elastic solids with many cracks: a simple method of analysis. International Journal of Solids and Structures 23 (1), 23-43] is employed to analyze both hexagonal and square crack configurations. In approximating crack interactions, the solution converges when the total truncation number of the cracks is 10 9. As expected, due to high density packing crack interaction in the hexagonal configuration is stronger than that in the square configuration. Based on the numerical results, convenient fitting equations for quick evaluation of the mode I stress intensity factors are obtained as a function of crack density and angle around the crack edge for both crack configurations. Numerical results for the mode II and III stress intensity factors are presented in the form of contour lines for the case of Poisson's ratio ? = 0.3. Possible errors for these problems due to Kachanov's approximate method are estimated. Good agreement is observed with the limited number of results available in the literature and obtained by different methods. © 2012 Elsevier Ltd. All rights reserved.
