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Permanent URI for this collectionhttps://hdl.handle.net/20.500.14288/3

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    Evolution of transformation plasticity in austenite-to-bainite phase transformation: a multi parameter problem
    (Elsevier Science Sa, 2012) Lambers, Hans -Gerd; Maier, Hans Jürgen; Department of Mechanical Engineering; Canadinç, Demircan; Faculty Member; Department of Mechanical Engineering; College of Engineering; 23433
    The current paper presents a thorough experimental analysis of the austenite-to-bainite phase transformation, and provides insight into microstructural changes associated with the observed transformation plasticity (TP). Specifically, the evolution of TP was studied in the presence of several parameters, namely pre-deformation, temperature and superimposed constant stresses, and the observed phenomena were linked to microstructural changes under these circumstances based on experimental data. One major finding is that the evolution of TP strains is governed both by the superimposed external stresses and the temperature dependence of internal stresses introduced by pre-deformation.
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    Contact mechanics between the human finger and a touchscreen under electroadhesion
    (Natl Acad Sciences, 2018) Scaraggi, Michele; Persson, Bo N. J.; N/A; N/A; Department of Mechanical Engineering; Ayyıldız, Mehmet; Şirin, Ömer; Başdoğan, Çağatay; Researcher; 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; 125489
    The understanding and control of human skin contact against technological substrates is the key aspect behind the design of several electromechanical devices. Among these, surface haptic displays that modulate the friction between the human finger and touch surface are emerging as user interfaces. One such modulation can be achieved by applying an alternating voltage to the conducting layer of a capacitive touchscreen to control electroadhesion between its surface and the finger pad. However, the nature of the contact interactions between the fingertip and the touchscreen under electroadhesion and the effects of confined material properties, such as layering and inelastic deformation of the stratum corneum, on the friction force are not completely understood yet. Here, we use a mean field theory based on multiscale contact mechanics to investigate the effect of electroadhesion on sliding friction and the dependency of the finger-touchscreen interaction on the applied voltage and other physical parameters. We present experimental results on how the friction between a finger and a touchscreen depends on the electrostatic attraction between them. The proposed model is successfully validated against full-scale (but computationally demanding) contact mechanics simulations and the experimental data. Our study shows that electroadhesion causes an increase in the real contact area at the microscopic level, leading to an increase in the electrovibrating tangential frictional force. We find that it should be possible to further augment the friction force, and thus the human tactile sensing, by using a thinner insulating film on the touchscreen than used in current devices.
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    The effect of strut protrusion on local shear stress and neointimal hyperplasia
    (Oxford University Press (OUP), 2019) Tenekecioglu, E.; Katagiri, Y.; Torii, R.; Onuma, Y.; Serruys, P. W.; Department of Mechanical Engineering; Pekkan, Kerem; Faculty Member; Department of Mechanical Engineering; College of Engineering; 161845
    N/A
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    Dimensional stability of 51CrV4 steel during bainitic phase transformation under tensile and compressive stresses
    (Elsevier, 2010) Lambers H.-G.; Maier H.J.; Department of Mechanical Engineering; Canadinç, Demircan; Faculty Member; Department of Mechanical Engineering; College of Engineering; 23433
    The effect of the prior austenitization treatment on the stress-strain response of a low alloy 51CrV4 steel in its supercooled austenitic state and its effect on the evolution of transformation plasticity strain during a subsequent isothermal bainitic transformation was investigated. One of the key findings is that the prior austenitization treatment strongly affects the evolution of transformation plasticity strains during the subsequent isothermal bainitic transformation under stress, such that higher transformation plasticity strains are present when the austenite grain size is increased. In addition, higher 0.2% offset yield strengths of the supercooled austenite are present following an incomplete austenitization treatment due to the existence of residual carbides. Overall, the current results clearly lay out the effect of the prior austenitization treatment on the stress-strain response of the supercooled austenite and the evolution of transformation plasticity during bainitic transformation.
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    On the cyclic stability and fatigue performance of ultrafine-grained interstitial-free steel under mean stress
    (Trans Tech Publications Ltd, 2008) Niendorf, Thomas; Maier, Hans J.; Karaman, Ibrahim; Department of Mechanical Engineering; Canadinç, Demircan; Faculty Member; Department of Mechanical Engineering; College of Engineering; 23433
    This paper reports on the fatigue performance of an ultrafine-grained (UFG) interstitial-free (IF) steel deformed at various mean stress levels. The UFG microstructure was achieved using equal channel angular extrusion processing at room temperature (RT) and along an "efficient" route, giving way to the formation of high angle grain boundaries (HAGBs) with a high volume fraction. The current study not only confirms the previous finding that a high volume fraction of HAGBs promotes cyclic stability, but also inquires into the role of mean stress level on the cyclic stability. It is shown that the UFG IF steel exhibits a stable cyclic deformation response in the lowcycle fatigue regime within the medium applied mean stress range of -75 to 75 MPa. The corresponding fatigue lives can still be predicted with the Smith-Watson-Topper approach within this range. Furthermore, the present study demonstrates that the evolution of mean strains with cyclic deformation can be linked to the evolution of mean stresses in strain-controlled loading.
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    Propagation and rupture of elastoviscoplastic liquid plugs in airway reopening model
    (Elsevier, 2022) Bahrani, S. Amir; Hamidouche, Souria; Moazzen, Masoud; Seck, Khady; Duc, Caroline; Grotberg, James B.; Romano, Francesco; Department of Mechanical Engineering; Muradoğlu, Metin; Faculty Member; Department of Mechanical Engineering; College of Engineering; 46561
    The propagation and rupture of mucus plugs in human lungs is investigated experimentally by injecting synthetic mucus in a pre-wetted capillary tube. The rheology of our test liquid is thoroughly characterized, and four samples of synthetic mucus are considered in order to reproduce elastoviscoplastic regimes of physiological interest for airway reopening. Our experiments demonstrate the significant impact of the viscoplasticity and viscoelasticity of mucus. In support to our experiments, we propose a one-dimensional reduced-order model that takes into account capillarity, and elastoviscoplasticity. Our model manages to capture the cross-section averaged dynamics of the liquid plug and is used to elucidate and interpret the experimental evidence. Relying on it, we show that the liquid film thickening due to non-Newtonian effects favors plug rupture, whereas the increase of the effective viscosity due to higher yield stresses hinders plug rupture. As a result of such two effects, increasing the polymeric concentration in the mucus phase leads to a net increase of the rupture time and traveling length. Hence, non-Newtonian effects hinder airway reopening.
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    Strain-controlled bulge test
    (Cambridge University Press (CUP), 2008) Akar, Orhan; Akın, Tayfun; Department of Mechanical Engineering; N/A; Alaca, Burhanettin Erdem; Toga, Kamil Buğra; Faculty Member; Master Student; Department of Mechanical Engineering; College of Engineering; Graduate School of Sciences and Engineering; 115108; N/A
    A closed-loop approach is adopted to implement strain rate Control during the bulge test. Due to the difficulty of measuring strains directly, the technique is based oil the conversion of displacement measurement to the corresponding strains using the plane-strain formulation. The necessary temporal evolution of the midpoint displacement of a rectangular diaphragm is derived under the condition of constant Strain rate and is Imposed as a control criterion. The technique is demonstrated oil 500-nm-thick Au diaphragms by applying, strain rates ranging from 2 x 10(-6) to 2 x 10(-4) s(-1). By measures the corresponding yield strength values, a Strain rate sensitivity of 0.11 is obtained, which is close to what was previously reported oil similar specimens using the microbending test.
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    Dynamic analysis of a novel moving magnet linear actuator
    (IEEE-Inst Electrical Electronics Engineers Inc, 2017) N/A; N/A; N/A; Department of Mechanical Engineering; Hassan, Adnan; Bijanzad, Armin; Lazoğlu, İsmail; 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; 179391
    A novel moving magnet linear actuator is proposed for linear oscillations in the linear resonant compressors for household refrigerators. This paper provides stator and armature design including CAD model and geometric parameters. Furthermore, the working principle of the proposed actuator is explained. The stator assembly is composed of two reversely wound coils, which are electrically excited with single phase ac power and oscillates the radially magnetized armature. With the help of the electromechanical analytical model, the dynamic parameters such as stroke, velocity, and acceleration of the armature are derived. Additionally, the time-dependent current model of the stator winding is proposed. An experimental setup is used to validate these responses at the resonance excitation frequency with the help of sensors. The system kinetics are discussed to estimate the spring, damping, inertial, and magnetic forces. A simulation is executed to estimate the time-domain responses of these dynamic parameters and the effects of excitation frequency are discussed. The force models are experimentally validated at the resonance frequency excitation. In order to evaluate the performance of the proposed actuator, a comparison of the performance parameters, such as efficiency, stroke, current, and mass flow rate is demonstrated with the conventional rotary as well as the linear motors for linear compressor application.
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    A novel mold design for one-continuous permeability measurement of fiber preforms
    (Sage Publications Ltd, 2015) N/A; N/A; Department of Mechanical Engineering; Yalçınkaya, Mehmet Akif; Sarıoğlu, Ayşen; Sözer, Murat; PhD Student; Master 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; 110357
    One-continuous permeability measurement experiments allow measuring permeability of a fiber preform within a range of fiber volume fractions by conducting a single unsaturated (a.k.a. transient) flow experiment on a dry specimen at an initial thickness, and a set of saturated flow experiments on the wetted specimen by varying the thickness of the mold cavity. This approach allows quicker database construction and reduces the effect of inherent variation of fabric structure caused by inconsistent labor on permeability. In this study, the drawbacks of previous mold designs are eliminated by using appropriate sealing, gap thickness adjustment mechanism and features that allow straightforward and reliable manual operation. Experiments for three different fabric types are conducted and the results are discussed. It is mainly observed that the unsaturated permeability is higher than the saturated permeability.
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    Implicit multigrid computations of buoyant drops through sinusoidal constrictions
    (ASME: American Society of Mechanical Engineers, 2004) Gökaltun, Seçkin; Department of Mechanical Engineering; Muradoğlu, Metin; Faculty Member; Department of Mechanical Engineering; College of Engineering; 46561
    Two-dimensional computations of dispersed multiphase flows involving complex geometries are presented. The numerical algorithm is based on the front-tracking method in which one set of governing equations is written for the whole computational domain and different phases are treated as a single fluid with variable material properties. The front-tracking methodology is combined with a newly developed finite volume solver based on dual time-stepping, diagonalized alternating direction implicit multigrid method. The method is first validated for a freely rising drop in a straight channel, and it is then used to compute a freely rising drop in various constricted channels. Interaction of two buoyancy-driven drops in a continuously constricted channel is also presented.