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Publication Metadata only A dynamic model of an overhung rotor with ball bearings(Sage, 2011) Şanlıtürk, K. Y.; N/A; Çakmak, Onur; PhD Student; Graduate School of Sciences and Engineering; N/AA ball bearing comprising rolling elements, inner and outer rings, and a cage structure can be described as a multi-body system (MBS). In order to predict the dynamic behaviour and resonance characteristics of a rotor-ball bearing system, it can be modelled and analysed as a MBS with flexible and rigid parts. In this study, a ball bearing is modelled with MBS approach using MSC ADAMS commercial software. The Hertzian theory is used for modelling the contact dynamics between the balls and the rings. The ball bearing model is then assembled with the rotor model which comprised a shaft and a disc positioned at the free end of the shaft. The ball bearing model is used with both flexible and rigid shaft assumptions in order to highlight the differences between the two cases. For the flexible shaft case, the MBS model also included a finite element model of the shaft. As expected, it is necessary to include the flexibility of the shaft in the model in order to to predict the changes in the modal characteristics of the system as a function of the rotor speed. Furthermore, including the gyroscopic effects leads to observe the forward and backward travelling modes with different natural frequencies. The effects of the bearing diametral clearance and localized defects on the inner and outer rings are modelled and analysed using the model developed. Also, the effects of the rotor unbalance on the vibration level of the whole system are examined. A test rig - consisting of two ball bearings, a shaft, and a disc - is also designed and developed so as to validate the theoretical model using experimental data. Order tracking and modal analyses are carried out and Campbell diagrams are obtained. Finally, the theoretical and the experimental results are compared and a refined MBS model is obtained for further analyses.Publication Metadata only A novel approach for monitoring plastic flow localization during in-situ sem testing of small-scale samples(Springer, 2018) Niendorf, Thomas; Weidner, Anja; N/A; Department of Mechanical Engineering; Mirzajanzadeh, Morad; Canadinç, Demircan; PhD Student; Faculty Member; Department of Mechanical Engineering; Koç University Surface Science and Technology Center (KUYTAM) / Koç Üniversitesi Yüzey Teknolojileri Araştırmaları Merkezi (KUYTAM); Graduate School of Sciences and Engineering; College of Engineering; N/A; 23433A novel method is proposed for monitoring the plastic flow localization during in-situ scanning electron microscopy (SEM) testing of small-scale AISI 316 L stainless steel. Stress-strain behavior of the material was obtained using a hybrid numerical-experimental (HNE) approach. By repeatedly illustrating each pair of sequentially taken SEM surface images throughout the deformation history in alternating order in form of a video, location of the material points which are not moving during the deformation can be detected. At the initial stages of deformation these points are located on the geometrical symmetry line of the test sample, however; when uniform straining limit of the material is reached, the locations of the stationary material points reveal the plastic localization regions. The current results clearly prove the feasibility of the presented method in monitoring primary plastic localization events through in-situ SEM tensile testing.Publication Metadata only Effect of the micro-textured piston on the performance of a hermetic reciprocating compressor(Sage) Haque, Umar UI; N/A; Department of Mechanical Engineering; Shahzad, Aamir; Lazoğlu, İsmail; PhD Student; Faculty Member; Department of Mechanical Engineering; Graduate School of Sciences and Engineering; College of Engineering; N/A; 179391A hermetic reciprocating compressor is one of the most critical parts for the energy efficiency of a household refrigerator. Piston-cylinder contact in the hermetic compressor accounts for most of the energy loss. The tribological performance of the piston-cylinder pair can be enhanced by introducing micro-texturing on the piston surface. In this research, an experimental study is presented to tribologically assess the effect of the micro-textured piston on the performance of the hermetically sealed reciprocating compressor. The micro-texture on the piston surface was prepared by the laser surface texturing method. Four different micro-textures were studied: radial micro-grooves, axial micro-grooves, mesh micro-grooves, and micro-dimples. The textures' size, shape, and depth were studied using scanning electron microscopy (SEM) and white light interferometry (WLI) techniques. The results were compared with the non-textured piston compressor. It was found that the radial, axial, and mesh micro-grooves pistons have a negative effect on the coefficient of performance of the hermetic reciprocating compressor. However, the piston with the micro-dimples texture increased the compressor's coefficient of performance by 1%. Refrigerant leakage from the piston-cylinder clearance was also investigated and it was observed that micro-dimples on the piston surface decrease the refrigerant leakage by 35% due to the presence of a continuous oil film between piston and cylinder. The compressor's cooling capacity (Q(c)) was observed to be increased by 1 W in the case of a micro-dimpled piston.Publication Metadata only Forces and temperatures in hard turning(Taylor & Francis Inc, 2006) Kratz, H.; Klocke, F.; Department of Mechanical Engineering; N/A; Lazoğlu, İsmail; Büyükhatipoğlu, Kıvılcım; Faculty Member; Master Student; Department of Mechanical Engineering; Manufacturing and Automation Research Center (MARC); College of Engineering; Graduate School of Sciences and Engineering; 179391; N/AIn precision machining, due to the recent developments in cutting tools, machine tool structural rigidity and improved CNC controllers, hard turning is an emerging process as an alternative to some of the grinding processes by providing reductions in costs and cycle-times. In industrial environments, hard turning is established for geometry features of parts with low to medium requirements on part quality. Better understanding of cutting forces, stresses and temperature fields, temperature gradients created during the machining are very critical for achieving highest quality products and high productivity in feasible cycle times. To enlarge the capability profile of the hard turning process, this paper introduces prediction models of mechanical and thermal loads during turning of 51CrV4 with hardness of 68 HRC by a CBN tool. The shear flow stress, shear and friction angles are determined from the orthogonal cutting tests. Cutting force coefficients are determined from orthogonal to oblique transformations. Cutting forces, temperature field for the chip and tool are predicted and compared with experimental measurements. The experimental temperature measurements are conducted by the advanced hardware device FIRE-1 (Fiberoptic Ratio Pyrometer).Publication Metadata only Friction and wear behaviors of tin coatings under dry and vacuum conditions(Taylor & Francis Inc, 2019) Kara, Levent; Özkan, Doğuş; Sulukan, Egemen; Sert, Yaşar; Sert, Tugay Sonsuz; N/A; Yağcı, Mustafa Barış; Researcher; Koç University Surface Science and Technology Center (KUYTAM) / Koç Üniversitesi Yüzey Teknolojileri Araştırmaları Merkezi (KUYTAM); N/A; N/AIn this study, pure TiN coatings were applied on AISI 52100 steel surfaces with a magnetron sputterer at different direct current (DC) powers and N-2 flows to develop a wear-resistant coating for sliding tribopairs working in atmospheric and vacuum conditions. Wear and friction characteristics of these coatings were investigated with a tribometer under both vacuum and atmospheric conditions. DC power and N-2 flow affected the coating thickness and structure. TiN coatings showed different wear and friction characteristics under ambient and vacuum conditions due to the oxidation level of the surface.Publication Metadata only Incorporation of dynamic strain aging Into a viscoplastic self-consistent model for predicting the negative strain rate sensitivity of hadfield steel(Asme, 2016) N/A; N/A; Department of Mechanical Engineering; Bal, Burak; Gümüş, Berkay; Canadinç, Demircan; PhD Student; PhD Student; Faculty Member; Department of Mechanical Engineering; Graduate School of Sciences and Engineering; Graduate School of Sciences and Engineering; College of Engineering; N/A; N/A; 23433A new multiscale modeling approach is proposed to predict the contributions of dynamic strain aging (DSA) and the resulting negative strain rate sensitivity (NSRS) on the unusual strain-hardening response of Hadfield steel (HS). Mechanical response of HS was obtained from monotonic and strain rate jump experiments under uniaxial tensile loading within the 10(-4) to 10(-1) s(-1) strain rate range. Specifically, a unique strain-hardening model was proposed that incorporates the atomic-level local instabilities imposed upon by the pinning of dislocations by diffusing carbon atoms to the classical Voce hardening. The novelty of the current approach is the computation of the shear stress contribution imposed on arrested dislocations leading to DSA at the atomic level, which is then implemented to the overall strain-hardening rule at the microscopic level. The new model not only successfully predicts the role of DSA and the resulting NSRS on the macroscopic deformation response of HS but also opens the venue for accurately predicting the deformation response of rate-sensitive metallic materials under any given loading condition.Publication Metadata only Left atrial ligation in the avian embryo as a model for altered hemodynamic loading during early vascular development(Journal of Visualized Experiments, 2023) Department of Mechanical Engineering; Sevgin, Börteçine; Çoban, Merve Nur; Karataş, Faruk; Pekkan, Kerem; Department of Mechanical Engineering; Graduate School of Sciences and Engineering; College of EngineeringDue to its four-chambered mature ventricular configuration, ease of culture, imaging access, and efficiency, the avian embryo is a preferred vertebrate animal model for studying cardiovascular development. Studies aiming to understand the normal development and congenital heart defect prognosis widely adopt this model. Microscopic surgical techniques are introduced to alter the normal mechanical loading patterns at a specific embryonic time point and track the downstream molecular and genetic cascade. The most common mechanical interventions are left vitelline vein ligation, conotruncal banding, and left atrial ligation (LAL), modulating the intramural vascular pressure and wall shear stress due to blood flow. LAL, particularly if performed in ovo, is the most challenging intervention, with very small sample yields due to the extremely fine sequential microsurgical operations. Despite its high risk, in ovo LAL is very valuable scientifically as it mimics hypoplastic left heart syndrome (HLHS) pathogenesis. HLHS is a clinically relevant, complex congenital heart disease observed in human newborns. A detailed protocol for in ovo LAL is documented in this paper. Briefly, fertilized avian embryos were incubated at 37.5 degrees C and 60% constant humidity typically until they reached Hamburger-Hamilton (HH) stages 20 to 21. The egg shells were cracked open, and the outer and inner membranes were removed. The embryo was gently rotated to expose the left atrial bulb of the common atrium. Pre-assembled micro-knots from 10-0 nylon sutures were gently positioned and tied around the left atrial bud. Finally, the embryo was returned to its original position, and LAL was completed. Normal and LALinstrumented ventricles demonstrated statistically significant differences in tissue compaction. An efficient LAL model generation pipeline would contribute to studies focusing on synchronized mechanical and genetic manipulation during the embryonic development of cardiovascular components. Likewise, this model will provide a perturbed cell source for tissue culture research and vascular biology.Publication Open Access Multi-scale modeling of the impact response of a strain-rate sensitive high-manganese austenitic steel(Frontiers, 2014) Department of Mechanical Engineering; Canadinç, Demircan; Önal, Orkun; Özmenci, Cemre; Faculty Member; Department of Mechanical Engineering; College of Engineering; 23433; N/A; N/AA multi-scale modeling approach was applied to predict the impact response of a strain rate sensitive high-manganese austenitic steel. The roles of texture, geometry, and strain rate sensitivity were successfully taken into account all at once by coupling crystal plasticity and finite element (FE) analysis. Specifically, crystal plasticity was utilized to obtain the multi-axial flow rule at different strain rates based on the experimental deformation response under uniaxial tensile loading. The equivalent stress – equivalent strain response was then incorporated into the FE model for the sake of a more representative hardening rule under impact loading. The current results demonstrate that reliable predictions can be obtained by proper coupling of crystal plasticity and FE analysis even if the experimental flow rule of the material is acquired under uniaxial loading and at moderate strain rates that are significantly slower than those attained during impact loading. Furthermore, the current findings also demonstrate the need for an experiment-based multi-scale modeling approach for the sake of reliable predictions of the impact response.Publication Metadata only Using haptics to convey cause-and-effect relations in climate visualization(IEEE, 2008) Sen, Omer Lutfi; Department of Mechanical Engineering; Department of Computer Engineering; Başdoğan, Çağatay; Taşıran, Serdar; Yannier, Nesra; Faculty Member; Faculty Member; Master Student; Department of Mechanical Engineering; Department of Computer Engineering; College of Engineering; College of Engineering; Graduate School of Sciences and Engineering; 125489; N/A; N/AWe investigate the potential role of haptics in augmenting the visualization of climate data. In existing approaches to climate visualization, dimensions of climate data such as temperature, humidity, wind, precipitation, and cloud water are typically represented using different visual markers and dimensions such as color, size, intensity, and orientation. Since the numbers of dimensions in climate data are large and climate data need to be represented in connection with the topography, purely visual representations typically overwhelm users. Rather than overloading the visual channel, we investigate an alternative approach in which some of the climate information is displayed through the haptic channel in order to alleviate the perceptual and cognitive load of the user. In this approach, haptic feedback is further used to provide guidance while exploring climate data in order to enable natural and intuitive learning of cause-and-effect relationships between climate variables. As the user explores climate data interactively under the guidance of wind forces displayed by a haptic device, she/he can understand better the occurrence of events such as cloud and rain formation and the effect of climate variables on these events. We designed a set of experiments to demonstrate the effectiveness of this multimodal approach. Our experiments with 33 human subjects show that haptic feedback significantly improves the understanding of climate data and the cause-and-effect relations between climate variables, as well as the interpretation of the variations in climate due to changes in terrain. © 2008 IEEE.