Dissertations / Theses on the topic 'Viscoelastic Response'
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1
Feng, Jie. "On the viscoelastic response of laminated composites." Thesis, National Library of Canada = Bibliothèque nationale du Canada, 1999. http://www.collectionscanada.ca/obj/s4/f2/dsk2/ftp01/MQ45219.pdf.
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Lu, Chun-Yi David. "Theory of viscoelastic response in bilayer systems." Thesis, University of Cambridge, 1995. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.389849.
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Chang, Tsu-Sheng. "Seismic Response of Structures with Added Viscoelastic Dampers." Diss., Virginia Tech, 2002. http://hdl.handle.net/10919/29915.
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Several passive energy dissipation devices have been implemented in practice as the seismic protective systems to mitigate structural damage caused by earthquakes. The solid viscoelastic dampers are among such passive energy dissipation systems. To examine the response reducing effectiveness of these dampers, it is necessary that engineers are able to conduct response analysis of structures installed with added dampers accurately and efficiently. The main objective of this work, therefore, is to develop formulations that can be effectively used with various models of the viscoelastic dampers to calculate the seismic response of a structure-damper system.
To incorporate the mechanical effect from VE dampers in the structural dynamic design, it is important to use a proper force-deformation model to correctly describe the frequency dependence of the damper. The fractional derivative model and the general linear model are capable of capturing the frequency dependence of viscoelastic materials accurately. In our research, therefore, we have focused on the development of systematic procedures for calculating the seismic response for these models.
For the fractional derivative model, we use the G1 and L1 algorithms to derive various numerical schemes for solving the fractional differential equations for earthquake motions described by acceleration time histories at discrete time points. For linear systems, we also develop a modal superposition method for this model of the damper. This superposition approach can be implemented to obtain the response time history for seismic input defined by the ground acceleration time history. For random ground motion that is described stochastically by the spectral density function, we derive an expression based on random vibration analysis to compute the mean square response of the system.
It is noted that the numerical computations involved with the fractional derivative model can be complicated and cumbersome. To alleviate computation difficulty, we explore the use of a general linear model with Kelvin chain analog as a physical representation of the damper properties. The parameters in the model are determined through a curve fitting optimization process. To simplify the analytical work, a self-adjoint system of state equations are formulated by introducing auxiliary displacements for the internal elements in the Kelvin chain. This self-adjoint system can then be solved by using the modal superposition method, which can be extended to develop a response spectrum approach to calculate the seismic design response for the structural system for seismic inputs defined by design ground response spectra.
Numerical studies are carried out to demonstrate the applicability of these formulations. Results show that all the proposed approaches provide accurate response values, and the response reduction effects of the viscoelastic dampers can be evaluated to assess their performance using these models and methods. However, the use of a general linear model of the damper is the most efficient. It can capture frequency dependence of the storage and loss moduli as well as the fractional derivative model. The calculation of the response by direct numerical integration of the equations of motion or through the use of the modal superposition approach is significantly simplified, and response spectrum formulation for the calculation of seismic response of design interest can be conveniently formulated.Ph. D.
4
Pandey, Anurag V. "Nonlinear viscoelastic response of a thermodynamically metastable polymer melt." Thesis, Loughborough University, 2011. https://dspace.lboro.ac.uk/2134/9096.
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Ultra High Molecular Weight Polyethylene (UHMw-PE) is an engineering polymer that is widely used in demanding applications because of its un-paralleled properties such as high abrasion resistance, high-modulus and high-strength tapes and fibres, biaxial films etc. In common practice, to achieve the uniaxial and the biaxial products, the solution processing route is adopted to reduce the number of entanglements per chain, such as found in Dyneema(R) from DSM(R). Another elegant route to reduce the number of entanglements to ease solid-state processing is through controlled polymerisation using a single-site catalytic system. In this theses, how different polymerisation condition, such as temperature and time control molecular weight and the resultant entangled state in synthesised disentangled UHMw-PE is addressed. Linear dynamic melt rheology is used to follow entanglement formation in an initially disentangled melt. With the help of rheological studies, heterogeneity in the distribution of entanglements along the chain length and the crystal morphology produced during polymerisation is considered. For the understanding of influence of large shear flow on melt dynamics large amplitude oscillatory shear (LAOS) is used and the non-linear viscoelastic regime is explored. A remarkable feature of overshoot in loss (viscous) modulus with increasing deformation (strain) in UHMw-PE melt in the LAOS is observed. This observation is characteristic of colloidal systems. The role of entanglement density in the amorphous region of the synthesised disentangled UHMw-PE (semi-crystalline polymers) on the melting and crystallisation is presented. To understand the effect of topological differences on melting behaviour, nascent entangled, nascent disentangled and melt-crystallised samples have been used. The role of superheating on the melting process is also addressed. Preliminary results on characteristic melting time of a crystal using TM-DSC are also presented.
5
Yang, Guanwen Zhu Da-Ming. "Probing the viscoelastic response of polymer films using atomic force microscopy." Diss., UMK access, 2005.
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Thesis (M.S.)--Dept. of Physics. University of Missouri--Kansas City, 2005."A thesis in physics." Typescript. Advisor: Da-Ming Zhu. Vita. Title from "catalog record" of the print edition Description based on contents viewed June 27, 2006. Includes bibliographical references (leaves 50-52). Online version of the print edition.
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Deigan, Richard James. "Modeling and experimental investigations of the shock response of viscoelastic foams." College Park, Md. : University of Maryland, 2007. http://hdl.handle.net/1903/7141.
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Thesis (Ph. D.) -- University of Maryland, College Park, 2007.Thesis research directed by: Mechanical Engineering. Title from t.p. of PDF. Includes bibliographical references. Published by UMI Dissertation Services, Ann Arbor, Mich. Also available in paper.
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Pravincumar, Priyanka. "Viscoelastic response of cells snd the role of actin cytoskeletal remodelling." Thesis, Queen Mary, University of London, 2012. http://qmro.qmul.ac.uk/xmlui/handle/123456789/3357.
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The mechanical properties of living cells provide useful information on cellular structure and function. In the present study a micropipette aspiration technique was developed to investigate the viscoelastic parameters of isolated articular chondrocytes. The Standard Linear Solid (SLS) and the Boltzmann Standard Linear Solid (BSLS) models were used to compute the instantaneous and equilibrium moduli and viscosity based on the response to an aspiration pressure of 7 cm of water. The modulus and viscosity of the chondrocytes increased with decreasing pressure rate. For example, the median equilibrium moduli obtained using the BSLS model increased from 0.19 kPa at 5.48 cmH2O/s to 0.62 kPa at 0.35 cmH2O/s. Cell deformation during micropipette aspiration was associated with an increase in cell volume and remodelling of the cortical actin visualised using GFP-actin. Interestingly, GFP-actin transfection inhibited the increase in cell moduli observed at the slower aspiration rate. Thus actin remodelling appears to be necessary for the pressure rate-dependent behaviour. A hypothesis is proposed explaining the role of actin remodelling and interaction with the membrane in regulating cell mechanics. Further studies investigated a mechanical injury model of cartilage explants which resulted in significant increases in all three viscoelastic parameters. Treatment with IL-1β also increased the instantaneous moduli of cells treated in explants but there was no difference in equilibrium moduli or viscosity. IL-1β treatment in monolayer had no effect on cell mechanics suggesting that previously reported changes in actin associated with IL-1β may be lost during cell isolation or trypsinisation. Separate studies demonstrated increases in chondrocyte moduli and viscosity during passage indicating changes in cell structure-function associated with de-differentiation in monolayer. In conclusion, this study has developed an optimised micropipette aspiration technique which was successfully used to quantify chondrocyte viscoelastic behaviour and to elucidate the underlying role of actin dynamics and response to pathological stimuli and in vitro culture.
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Gomez, Consarnau Rafael J. "A Simplified Methodology for Validating the Hyper-Viscoelastic (HVE) Dynamic Response." Thesis, California State University, Long Beach, 2018. http://pqdtopen.proquest.com/#viewpdf?dispub=10837944.
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This thesis presents a mathematical modeling process for characterizing a hyperelastic material with viscous response under dynamic loading conditions. The model is designed with the advantage of performing only one compressive dynamic test in order to provide the requisite parameters to fully determine the hyper-viscoelastic response. This is achieved in both deformations and contact forces, using digital image correlation and force sensors. Experiments performed at strain rates ranging from 10–3–10 2 s–1 correlate with computational simulations at the same loading rates up to 80% compression. The validity of the fit and prediction is assessed using MATLAB along with ABAQUS finite element software.
The results provided by this novel methodology, i.e. the mathematical model using non-homogeneous deformations and the subsequent dynamic experimental techniques, proves that this approach is a more effective alternative to the current standards used to characterize the mechanical response of hyperelastic, viscoelastic, and hyper-viscoelastic materials.
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Brinson, L. C. Knauss Wolfgang Gustav. "Time-temperature response of multi-phase viscoelastic solids through numerical analysis /." Diss., Pasadena, Calif. : California Institute of Technology, 1990. http://resolver.caltech.edu/CaltechETD:etd-10292003-112909.
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Sen, Ozge. "Transient Dynamic Response Of Viscoelastic Cylinders Enclosed In Filament Wound Cylindrical Composites." Phd thesis, METU, 2005. http://etd.lib.metu.edu.tr/upload/12606412/index.pdf.
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In this study, transient dynamic response of viscoelastic cylinders enclosed in filament wound cylindrical composites is investigated. Thermal effects, in addition to mechanical effects, are taken into consideration. A generalized thermoelasticity theory which incorporates the temperature rate among the constitutive variables and is referred to as temperature-rate dependent thermoelasticity theory is employed. This theory predicts finite heat propagation speeds.
The body considered in this thesis consists of n+1-layers, the inner layer being viscoelastic, while the outer fiber reinforced composite medium consist of n-different generally orthotropic, homogeneous and elastic layers. In each ply, the fiber orientation angle may be different. The body is a hollow circular cylinder with a finite thickness in the radial direction, whereas it extends to infinity in the axial direction. The multilayered medium is subjected to uniform time-dependent dynamic inputs at the inner and/or outer surfaces. The body is assumed to be initially at rest. The layers are assumed to be perfectly bonded to each other.
The case in which the inner surface of the viscoelastic cylinder is a moving boundary is further investigated in this study. This is similar to the solid
propellant rocket motor cases. The solid propellant is modelled as a viscoelastic material which in turn is modelled as standard linear solidwhereas, the rocket motor case is a fiber-reinforced filament wound cylindrical composite. Method of characteristics is employed to obtain the solutions. Method of characteristics is suitable because the governing equations are hyperbolic. The method is amenable to numerical integration and different boundary, interface and initial conditions can be handled easily.
11
Heras, Segura Mariona. "Vibration Characteristics of Axially Graded Viscoelastic Beams." Miami University / OhioLINK, 2019. http://rave.ohiolink.edu/etdc/view?acc_num=miami1556911763120726.
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Chittur, Krishna Murthy Anantha Narayan. "Application of visco-hyperelastic devices in structural response control." Thesis, Virginia Tech, 2005. http://hdl.handle.net/10919/32910.
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Structural engineering has progressed from design for life safety limit states to performance based engineering, in which energy dissipation systems in structural frameworks assume prime importance. A visco-hyperelastic device is a completely new type of passive energy dissipation system that not only combines the energy dissipation properties of velocity and displacement dependent devices but also provides additional stability to the structure precluding overall collapse.
The device consists of a viscoelastic material placed between two steel rings. The energy dissipation in the device is due to a combination of viscoelastic dissipation from rubber and plastic dissipation due to inelastic behavior of the steel elements. The device performs well under various levels of excitation, providing an excellent means of energy dissipation. The device properties are fully controlled through modifiable parameters.
An initial study was conducted on motorcycle tires to evaluate the hyperelastic behavior and energy dissipation potential of circular rubber elements, which was preceded by preliminary finite element modeling. The rubber tires provided considerable energy dissipation while displaying a nonlinear stiffening behavior. The proposed device was then developed to provide additional stiffness that was found lacking in rubber tires.
Detailed finite element analyses were conducted on the proposed device using the finite element software package ABAQUS, including parametric studies to determine the effect of the various parameters of device performance. This was followed by a nonlinear dynamic response history analysis of a single-story steel frame with and without the device to study the effects of the device in controlling structural response to ground excitations. Static analyses were also done to verify the stabilizing effects of the proposed device. Results from these analyses revealed considerable energy dissipation from the device due to both viscoelastic as well as plastic energy dissipation.
Detailed experimental analyses on the proposed device, finite element analyses of the device on multistory structures have been put forth as the areas of future research. It may also be worthwhile to conduct further research, as suggested, in order to evaluate the use of scrap tires which is potentially a very valuable structural engineering material.Master of Science
13
Byrne, Katherine. "The viscoelastic response of single biological molecules to thermal noise by atomic force spectroscopy." Thesis, University of Leeds, 2006. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.432315.
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Bryant, James William Jr. "Prediction of Linear Viscoelastic Response of the Loss Shear Modulus of Polymer-Modified Binders." Thesis, Virginia Tech, 1999. http://hdl.handle.net/10919/31719.
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Current mathematical models, developed on straight asphalt binders, are inadequate to characterize the frequency dependence of response of polymer-modified asphalt binders. In an earlier study at Virginia Tech, mathematical models were developed to predict the storage and loss shear moduli of polymer-modified binders. However the model developed for the loss shear moduli is limited at high frequency (G" less than or equal to 10^7.5 Pa). This thesis presents a statistical modeling of loss shear modulus of polymer (random copolymers and thermoplastic block copolymers) modified binder. Data from dynamic mechanical analysis on modified binders, at temperatures between 5 and 75°C and frequencies ranging from 0.06 to 0188.5 rad/s, were reduced to dynamic master curves of moduli, and used to develop the model. Twenty-one polymer-binder blends prepared and tested earlier at Virginia Tech where included in the study. Realistic characterization of loss shear moduli values was obtained using the Gompertz statistical model. The model was validated by using mean square error of prediction (MSEP) in which a second set of frequency data was input in the model to obtain the moduli values, which were compared to the measured data of the second set. Although this model was successfully tested for shear loss modulus prediction of polymer-modified binders, caution should be exercised when it is applied, as such a model should be able to predict the storage modulus for a known phase angle.Master of Science
15
Marko, Julius. "Influence of damping systems on building structures subject to seismic effects." Queensland University of Technology, 2006. http://eprints.qut.edu.au/16318/.
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In order to control the vibration response of high rise buildings during seismic events, energy absorbing passive damping devices are most commonly used for energy absorption. Today there are a number of types of manufactured dampers available in the market, which use a variety of materials and designs to obtain various levels of stiffness and damping. Some of these include friction, yielding, viscoelastic and viscous dampers. These dampers are usually installed between two load bearing elements (walls or columns) in new buildings. In existing buildings, which require retrofitting, they could be installed in cut-outs of shear walls, as evidenced from recent investigations. An effective damping system can result in higher levels of safety and comfort, and can also lead to considerable savings in the total cost of a building. This thesis treats seismic mitigation of multistorey buildings using embedded dampers. Three types of damping mechanisms, viz, friction, viscoelastic, and combined friction-viscoelastic were investigated. Finite element methods were employed in the analysis using the program ABAQUS version 6.3. A direct integration dynamic analysis was carried out to obtain the damped and undamped responses of the structure in terms of deflections and accelerations at all storeys in order to evaluate the effectiveness of the damping system in mitigating the seismic response. The damping mechanisms have been modelled as (i) a linear spring and dash-pot in parallel for the viscoelastic damper, (ii) a contact pair with friction parameter for a friction damper and (iii) a hybrid damper consisting of both a viscoelastic and a friction damper. The earthquake events used in this study have been applied as acceleration time-histories at the base of the structure in the horizontal plane. Concrete material properties were chosen to represent the model as many high-rise buildings are constructed by using reinforced concrete. Several medium and high-rise building structures with embedded dampers in different configurations and placed in various locations throughout the structure were subjected to different earthquake loadings. Influence of damper type and properties, configuration and location were investigated. Results for the reduction in tip deflection and acceleration for a number of cases demonstrate the feasibility of the technique for seismic mitigation of these structures for a range of excitations, even when the dominant seismic frequencies match the natural frequency of the structure. Results also provide information which can be used for optimal damper placement for seismic mitigation.
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Drake, Daniel Adam. "Viscoelastic characterization of vapor-grown carbon nanofiber/vinyl ester nanocomposites using a response surface methodology." Thesis, Mississippi State University, 2013. http://pqdtopen.proquest.com/#viewpdf?dispub=1536065.
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The effects of vapor-grown carbon nanofiber (VGCNF) weight fraction, applied stress, and temperature on the viscoelastic responses (creep strain, creep rate, and creep compliance) of VGCNF/vinyl ester (VE) nanocomposites were studied using a central composite design (CCD). The nanocomposite test articles were fabricated by high shear mixing, casting, curing, and post-curing in an open face mold under a nitrogen environment. Short-term creep/creep recovery experiments were conducted at prescribed combinations of temperatures (23.8 - 69.2 C), applied stresses (30.2 - 49.8 MPa), and VGCNF weight fractions (0.00 - 1.00 parts of VGCNF per hundred parts of resin, phr) determined from the CCD. The response surface models (RSMs) for predicting these viscoelastic responses were developed using the least squares method and an analysis of variance procedure. The response surface estimates indicate that increasing the VGCNF weight fraction marginally increases the creep resistance of the VGCNF/VE nanocomposite at low temperatures (i.e., 23.8 - 46.5 C). However, increasing the VGCNF weight fraction for temperatures greater than 50 C decreased the creep resistance of these nanocomposites. The latter response may be due to a decrease in the nanofiber-to-matrix adhesion as the temperature is increased. The RSMs for creep strain, creep rate, and creep compliance revealed the interactions between the VGCNF weight fraction, stress, and temperature on the creep behavior of thermoset polymer
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Ssozi, Eva Nantongo. "The effect of viscoelastic deformation in pipe cracks on leakage response to variations in pressure." Master's thesis, University of Cape Town, 2014. http://hdl.handle.net/11427/13263.
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Includes bibliographical references.Water is an important and increasingly scarce resource in the world today. Unfortunately, a lot of water is lost through leakage since most distribution systems are deteriorating. Therefore research in leakage management is necessary in order to improve the utilization of water resources. Leakage may be reduced by managing the water pressure in water distribution systems. One of the important factors affecting the pressure-leakage relationship is pipe material behaviour (Van Zyl & Clayton 2007; Greyvenstein & Van Zyl 2007). The pressure – leakage relationship has been described by several relationships such as the Torricelli equation, the Fixed and Variable Area Discharge (FAVAD) concept and the conventional equation. Pipe material behaviour affects leakage parameters in the pressure-leakage relationship such as the leak area and the leak exponent (Cassa et al. 2010). For this project, the pressure-leakage relationships in High Density Polyethylene (HDPE) and Polyvinylchloride (PVC) pipes are investigated. HDPE and PVC are polymeric materials and therefore exhibit a viscoelastic response to applied stress and applied strain. Viscoelastic responses include creep, relaxation, hysteresis and time dependency. When these pipes experience stresses due to water pressures, failure and fracture may occur as leaks. The viscoelastic properties of these materials therefore affect how the leaks respond to pressure change. The effect of viscoelastic deformations in leaks was investigated using Finite Element Analysis (FEA) software, Abaqus. Round holes and longitudinal cracks were represented as individual leaks in HDPE and PVC pipes in Abaqus. Pressure was applied to each pipe model for different time periods, and the deformed leak areas were obtained. Further analysis was carried out to determine the effects of pressure on leak parameters such as the gradient of the leak area-pressure relationship, leak exponent and the leakage number. The analysis shows that viscoelastic deformations have an effect on the pressure-leakage relationship. A linear relationship exists between the leak area and pressure for all time periods investigated and therefore gradients could be obtained. Deformed leak areas, gradients and leak exponents all increased with time and therefore confirmed that the time dependency of viscoelastic materials affected the pressure-leakage relationship. The leakage exponents for both materials were found to vary between 0.5 and 1.5 for both HDPE and PVC. HDPE also exhibited higher leak exponents, gradients and larger deformed leak areas than PVC for the same leaks. It was also found that leakage in viscoelastic materials may be analysed using the leakage number, developed for elastically deforming materials by Van Zyl & Cassa (2013).
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Meredith, Steven L. "CHARACTERIZATION OF A VISCOELASTIC RESPONSE FROM THIN METAL FILMS DEPOSITED ON SILICON FOR MICROSYSTEM APPLICATIONS." DigitalCommons@CalPoly, 2009. https://digitalcommons.calpoly.edu/theses/53.
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Understanding the mechanisms that control the mechanical behavior of microscale actuators is necessary to design an actuator that responds to an applied actuation force with the desired behavior. Micro actuators which employ a diaphragm supported by torsional hinges which deform during actuation are used in many applications where device stability and reliability are critical. The material response to the stress developed within the hinge during actuation controls how the actuator will respond to the actuating force. A fully recoverable non-linear viscoelastic response has been observed in electrostatically driven micro actuators employing torsional hinges of silicon covered with thin metal films. The viscoelastic response occurs over a time period of 50 minutes at an operating temperature of 35°C. This viscoelastic phenomenon is similar to that reported in articles addressing anelastic behavior associated with viscous grain boundary slippage and dislocation bowing. In order to investigate this viscoelastic response as a function of metal film composition and thickness, bi-layer torsional hinge actuators consisting of Si with a deposited metal layer were designed to exhibit similar stress levels as the electrostatically driven micro actuators. The test devices were fabricated using common semiconductor fabrication techniques. The actuators were micromachined by deep etching 100mm diameter, 425µm thick, double side polished, single crystal (100) wafers to create a 4.5µm thick device layer. Subsequent etching of the device layer released the fixed-fixed torsional hinge test actuators. Physical vapor depositions of Au, Al and Al-Ti in two different thicknesses (100nm, and 150nm) were deposited in order to investigate the impact of metal film thickness and composition on the viscoelastic response. Grain sizes of the deposited films were estimated using backscattered electron images. Rotational actuation of the test actuators was achieved by using a modified Ambios XP-1 surface profiler that applies a constant force of 0.28mN while measuring the displacement of the actuator with respect to time. The viscoelastic response was observed in the test devices with Au and Al thin films indicating that this phenomenon is attributable to the stresses induced on the torsional hinge. Results indicate that the viscoelastic response was not observed in AlTi thin films consisting of 0.3at% titanium. Two theoretical models are presented that discuss the mechanism associated with the viscoelastic response as well as a method for inhibiting these mechanisms by the addition of an alloying element to form a second phase precipitate.
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Marko, Julius. "Influence of damping systems on building structures subject to seismic effects." Thesis, Queensland University of Technology, 2006. https://eprints.qut.edu.au/16318/1/Julius_Marko_Thesis.pdf.
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In order to control the vibration response of high rise buildings during seismic events, energy absorbing passive damping devices are most commonly used for energy absorption. Today there are a number of types of manufactured dampers available in the market, which use a variety of materials and designs to obtain various levels of stiffness and damping. Some of these include friction, yielding, viscoelastic and viscous dampers. These dampers are usually installed between two load bearing elements (walls or columns) in new buildings. In existing buildings, which require retrofitting, they could be installed in cut-outs of shear walls, as evidenced from recent investigations. An effective damping system can result in higher levels of safety and comfort, and can also lead to considerable savings in the total cost of a building.
This thesis treats seismic mitigation of multistorey buildings using embedded dampers. Three types of damping mechanisms, viz, friction, viscoelastic, and combined friction-viscoelastic were investigated. Finite element methods were employed in the analysis using the program ABAQUS version 6.3. A direct integration dynamic analysis was carried out to obtain the damped and undamped responses of the structure in terms of deflections and accelerations at all storeys in order to evaluate the effectiveness of the damping system in mitigating the seismic response. The damping mechanisms have been modelled as (i) a linear spring and dash-pot in parallel for the viscoelastic damper, (ii) a contact pair with friction parameter for a friction damper and (iii) a hybrid damper consisting of both a viscoelastic and a friction damper. The earthquake events used in this study have been applied as acceleration time-histories at the base of the structure in the horizontal plane. Concrete material properties were chosen to represent the model as many high-rise buildings are constructed by using reinforced concrete.
Several medium and high-rise building structures with embedded dampers in different configurations and placed in various locations throughout the structure were subjected to different earthquake loadings. Influence of damper type and properties, configuration and location were investigated. Results for the reduction in tip deflection and acceleration for a number of cases demonstrate the feasibility of the technique for seismic mitigation of these structures for a range of excitations, even when the dominant seismic frequencies match the natural frequency of the structure. Results also provide information which can be used for optimal damper placement for seismic mitigation.
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Gahvari, Fariborz. "Modeling of the linear viscoelastic response of polymer modified asphalt binders at intermediate and high temperatures." Diss., This resource online, 1996. http://scholar.lib.vt.edu/theses/available/etd-08272007-163901/.
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Cotton, John Rowland. "A viscoelastic model for the response of a step change in velocity of the human otolith organs." Thesis, Virginia Tech, 1989. http://hdl.handle.net/10919/44113.
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The otolith organs are housed in the inner ear and are responsible for sensing accelerations
and gravity. These inertial sensing systems are modeled in this thesis as a three
material system consisting of a rigid otoconial plate attached to the skull by a gel layer, surrounded
by a viscous endolymph fluid. The gel layer is considered to be a viscoelastic solid,
and modeled as a simple Kelvin element. The governing differential equations are derived
and nondimensionalized, yielding three nondimensional parameters: nondimensional density,
R, nondimensional viscosity, M, and nondimensional elasticity, 6. The equations are
solved using uÌ nite difference techniques on a digital computer. By comparing the modelâ s
response with previous biological research, values for the nondimensional parameters are
found. The value of R is 0.75 and the value of 6 is between 0.3 and 0.075. While the value
of M is placed between 5. and 10., results indicate that to properly model the long time
response of the otolith, a single and constant value for viscosity is not feasible.Master of Science
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Reubush, Stacey Diane. "Effects of Storage on the Linear Viscoelastic Response of Polymer-Modified Asphalt at Intermediate to High Temperatures." Thesis, Virginia Tech, 1999. http://hdl.handle.net/10919/30838.
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The design and construction of roads with longer service lives is a priority of civil engineers. The selection of appropriate highway materials with respect to climatic and loading conditions may significantly increase the lifespan of pavements. One material receiving interest in the area of improved roadway performance is polymer-modified binder. The complex behavior of polymer-modified binders, particularly over time, is not yet well-understood by engineers. Therefore, an experimental study was performed to determine the effects of four years of storage at room temperatures (23°C) on the dynamic mechanical properties of polymer-modified binders at intermediate and high temperatures. A typical paving grade (AC-20) and three elastomeric modifiers, each at three concentrations were used. Initial tests were performed in 1995 to evaluate the effects of short-term aging as simulated by the Rolling Thin Film Oven Test (RTFOT) procedure. This study encompasses a second phase of testing occurring after the modified binders were stored at ambient room temperature (23°C) for four years. The study found that significant changes affecting the dynamic response of binders occur during long term storage at a temperature of 23°C. These changes are dependent on the type and concentration of modifier and may be beneficial. Additionally, four mathematical models describing the dynamic response of binders were evaluated and found to be variable in their ability to accurately predict response of modified binders. Most of these models are not well suited for prediction of the response of stored binders.Master of Science
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Toosizadeh, Nima. "Time-dependent assessment of the human lumbar spine in response to flexion exposures: in vivo measurement and modeling." Diss., Virginia Tech, 2013. http://hdl.handle.net/10919/19274.
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Among several work-related injuries, low back disorders (LBDs) are the leading cause of lost workdays, and with annual treatment costs in excess of $10 billion in the US. Epidemiological evidence has indicated that prolonged and/or repetitive non-neutral postures, such as trunk flexion, are commonly associated with an increased risk of LBDs. Trunk flexion can result in viscoelastic deformations of soft tissues and subsequent mechanical and neuromuscular alterations of the trunk, and may thereby increase LBD risk. While viscoelastic behaviors of isolated spinal motion segments and muscles have been extensively investigated, in vivo viscoelastic responses of the trunk have not, particularly in response to flexion exposures. Further, most biomechanical efforts at understanding occupational LBDS have not considered the influence of flexion exposures on spine loads.
Four studies were completed to characterize viscoelastic deformation of the trunk in response several flexion exposures and to develop and evaluate a computational model of the human trunk that accounts for time-dependent characteristics of soft tissues. Participants were exposed to prolonged flexion at different trunk angles and external moments, and repetitive trunk flexion with different external moments and flexion rates. Viscoelastic properties were quantified using laboratory experiments and viscoelastic models. A multi-segment model of the upper body was developed and evaluated, and then used to estimate muscle forces and spine loads during simulated lifting tasks before and after prolonged trunk flexion at a constant angle and constant external moment. Material properties from the earlier experiments were used to evaluate/calibrate the model.
Experimental results indicated important effects of flexion angle, external moment, and flexion rate on trunk viscoelastic behaviors. Material properties from fitted Kelvin-solid models differed with flexion angle and external moment. Nonlinear viscoelastic behavior of the trunk tissues was evident, and predictive performance was enhanced using Kelvin-solid models with ≥2 iii retardation/relaxation time constants. Predictions using the multi-segment model suggested increases in spine loads following prolonged flexion exposures, primarily as a consequence of additional muscle activity. As a whole, these results help to characterize the effects of trunk flexion exposures on trunk biomechanics, contribute to more effective estimates of load distribution among passive and active components, enhance our understanding of LBD etiology, and may facilitate future controls/interventions.Ph. D.
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Green, Vicki L. "Investigation of Structural Responses for Flexible Pavement Sections at the Ohio-SHRP Test Road." Ohio University / OhioLINK, 2008. http://rave.ohiolink.edu/etdc/view?acc_num=ohiou1213280487.
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Iyer, Srinivasan. "On the characterization of the viscoelastic response of a class of materials using acousto-ultrasonics: A pattern recognition approach." Thesis, University of Ottawa (Canada), 1993. http://hdl.handle.net/10393/6545.
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This thesis describes the experimental work carried out at the University of Ottawa for characterizing linear viscoelastic materials using acousto-ultrasonic wave propagation technique. The objective of the research program is to asses the feasibility of utilizing acousto-ultrasonics in the characterization of mechanical response and strength prediction of solid polymers as a class of engineering materials. The basic principles of acousto-ultrasonic technique, the instrumentation involved, and the factors affecting waveform measurements are described. A review on the analysis of the resultant acousto-ultrasonic waveform using statistical pattern recognition methodology is presented. The time-dependent properties of creep, and stress-relaxation of linear viscoelastic materials are reviewed. Also, fatigue, macromechanical defects and yielding of polymers are discussed. Experimental results obtained in a series of acousto-ultrasonic characterization tests performed on Polyvinylchloride (PVC) and Polycarbonate (PC) specimens are presented. Good correlation was obtained between the acousto-ultrasonic parameter (AUP) and stress in material during a time-dependent stress relaxation testing for a particular strain level. The acousto-ultrasonic parameter (AUP) was seen to decrease with increasing stress levels during uniaxial tensile loading. Also, induced fatigue states in the material and macromechanical defect states were seen to have good correlation with the AUP. Statistical pattern recognition classifiers were designed for all characterization experiments performed and tested with unknown samples and found to be very effective. Acousto-Ultrasonics (AU), combined with statistical pattern recognition methodology applied in real time environment, proves to be an efficient non-destructive quantitative evaluation process for the class of solid polymers investigated.
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Wang, FeI. "A Numerical and Experimental Study of the Thermodynamically Consistent Nonlinear Viscoelastic Response of an Expoxy Polymer in the Glassy Regime." University of Cincinnati / OhioLINK, 2006. http://rave.ohiolink.edu/etdc/view?acc_num=ucin1163791491.
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Coggins, M. Denise. "Bi-layered viscoelastic model for a step change in velocity and a constant acceleration stimulus for the human otolith organs." Thesis, This resource online, 1996. http://scholar.lib.vt.edu/theses/available/etd-02132009-172020/.
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Krausser, Johannes. "Non-affine lattice dynamics of disordered solids." Thesis, University of Cambridge, 2018. https://www.repository.cam.ac.uk/handle/1810/280686.
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This thesis provides a study of different aspects of the mechanical and vibrational properties of disordered and amorphous solids. Resorting to the theoretical framework of non-affine lattice dynamics the attention is focused on the analysis of disordered networks and lattices which serve as tractable model systems for real materials. Firstly, we discuss the static elastic response and the vibrational spectra of defective fcc crystals. The connection to different types of microstructural disorder in the form of bond-depletion and vacancies is described within the context of the inversion symmetry breaking of the local particle configurations. We identify the fluctuations of the local inversion symmetry breaking, which is directly linked to the non-affinity of the disordered solid, as the source of different scalings behaviours of the position of the boson peak. Furthermore, we describe the elastic heterogeneities occurring in a bond-depleted two- dimensional lattice with long-range interactions. The dependence of the concomitant correlations of the local elastic moduli are studied in detail in terms of the interaction range and the degree of disorder. An analytical scaling relation is derived for the radial part of the elastic correlations in the affine limit. Subsequently, we provide an argument for the change of the angular symmetry of the elastic correlation function which was observed in simulations and experiments on glasses and colloids, respectively. Moving to the dynamical behaviour of disordered solids, a framework is developed based on the kernel polynomial method for the approximate computation of the non- affine correlator of displacement fields which is the key requirement to describe the linear viscoelastic response of the system within the quasi-static non-affine formalism. This approach is then extended to the case of multicomponent polymer melts and validated against molecular dynamics simulations at low non-zero temperatures. We also consider the dynamical behaviour of metallic glasses in terms of its shear elasticity and viscosity. A theoretical scheme is suggested which links the repulsive strength of the interatomic potential to the viscoelasticity and fragility in metallic glasses in the quasi-affine limit.
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Herrenbrück, Martin Johannes [Verfasser], Roman [Akademischer Betreuer] Lackner, and Fabian [Akademischer Betreuer] Duddeck. "Finite-Element Based Determination of Response Spectra of Viscoelastic Materials Subjected to Low-Velocity Impact Loading / Martin Johannes Herrenbrück. Gutachter: Roman Lackner ; Fabian Duddeck. Betreuer: Roman Lackner." München : Universitätsbibliothek der TU München, 2013. http://d-nb.info/1043587640/34.
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Al, Haddad Aiham Emil. "Wind-induced Vibration Control of Tall Timber Buildings : Improving the dynamic response of a 22-storey timber building." Thesis, Linnéuniversitetet, Institutionen för byggteknik (BY), 2016. http://urn.kb.se/resolve?urn=urn:nbn:se:lnu:diva-57005.
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Plans for construction of the tallest residential timber building has driven the Technical Research Institute of Sweden (SP), Linnaeus University, Växjö and more than ten interested companies to determine an appropriate design for the structure. This thesis presents a part of ongoing research regarding wind-induced vibration control to meet serviceability limit state (SLS) requirements. A parametric study was conducted on a 22-storey timber building with a CLT shear wall system utilizing mass, stiffness and damping as the main parameters in the dynamic domain. Results were assessed according to the Swedish Annex EKS 10 and Eurocode against ISO 10137 and ISO 6897 requirements. Increasing mass, stiffness and/or damping has a favorable impact. Combination scenarios present potential solutions for suppressing wind-induced vibrations as a result of higher efficiency in low-increased levels of mass and damping.
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Kress, Oliver Herbert [Verfasser], Tanya [Akademischer Betreuer] Ostapenko, Jörg [Gutachter] Enderlein, and Simone [Gutachter] Techert. "The Viscoelastic Response of Liquid Crystalline Fibers Formed By Bent-core Molecules : From Microscopic Ordering to Macroscopic Behavior / Oliver Herbert Kress ; Gutachter: Jörg Enderlein, Simone Techert ; Betreuer: Tanya Ostapenko." Göttingen : Niedersächsische Staats- und Universitätsbibliothek Göttingen, 2019. http://d-nb.info/1200209176/34.
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Ko, Chan Uk. "Effect of surface treatment on the mechanical properties of the polysulfone-Al/Li bonded system including thin film studies of moisture intrusion and the viscoelastic response of the interphase region." Diss., Virginia Polytechnic Institute and State University, 1988. http://hdl.handle.net/10919/53556.
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An investigation of polysulfone-Al/Li alloy interaction involved single lap shear joints and wedge samples following an FPL etch, sulfuric acid anodization (SAA) and phosphoric acid anodization (PAA). The study of the Al/Li surfaces involved the determination of the elemental composition and morphological features of the pretreated adherend before bonding and following failure. When thermoplastic polysulfone (PSF) was either thermally pressed or primed onto the microporous surface, the PSF indeed penetrated into the porous oxide and thereby provided a mechanical means of adhesion. The wedge test results for the adherend pretreated by PAA and SAA were superior to those for the FPL etched adherend. The failure path for the FPL etched samples was at the adhesive/oxide interface whereas the failure path for the SAA and PAA samples was within the adhesive but with occasional divergence of the crack into the oxide. The porous oxides on Al/Li alloy formed after PAA and SAA treatment were shown to undergo dramatic changes in morphology on short term (<90 hrs) exposure to 71 C and 100% R.H. environment. The mechanism of failure was due to moisture which caused slight hydration of the Al/Li oxide and subsequent debonding of the PSF from the oxide layer. Lithium was not concentrated at the surface in the PAA treated Al/Li alloy as shown by AES depth profiling. The effect of lithium on the durability of the bonded alloy is considered minimal. Along these lines, cyclic loading, use of primers, and infrared spectroscopy studies have been carried out.
The mode of moisture intrusion into the polysulfone-Al/Li oxide interphase region is discussed. Specifically, water molecules diffuse into the polysulfone rather than transporting along the interface. Moisture then attacks the oxide interface.
Thin polysulfone coatings on pretreated aluminum surfaces were characterized utilizing dynamic mechanical thermal analysis (DMTA), and dielectric thermal analysis (DETA) to detect changes in the molecular motions and structural transitions in the polysulfone-aluminum interphase. The order of the loss peak temperature of the polysulfone is, PSF coating on a porous Al > PSF coating on a smooth Al > neat PSF film. The activation energy of relaxation is also lower for neat PSF when compared to the thin film cast onto a smooth Al or a porous PAA Al substrate. The loss peak temperature shift and the higher activation energy associated with the coated films can be explained by the entropy being reduced when the chains are laid down in two dimensions. Thus studies of polymer properties in the interphase region will contribute to the understanding of the adhesive-adherend interaction.Ph. D.
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Molavi, Tabrizi Amirhossein. "Elastic and Viscoelastic Responses of Anisotropic Media Subjected to Dislocation Sources." University of Akron / OhioLINK, 2015. http://rave.ohiolink.edu/etdc/view?acc_num=akron1448218517.
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Deuel, Christopher R. "Modeling viscoelastic responses of the head/neck system during pilot ejection." Thesis, This resource online, 1994. http://scholar.lib.vt.edu/theses/available/etd-12042009-020255/.
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Ben, Hassine Mouna. "Modélisation du vieillissement thermique et mécanique d’une protection externe en EPDM de jonctions rétractables à froid." Thesis, Paris, ENSAM, 2013. http://www.theses.fr/2013ENAM0048/document.
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L'objectif de cette thèse est l'étude des conséquences de la thermo-oxydation sur la structure chimique et le comportement mécanique d'un Ethylène Propylène Diène Monomère (EPDM). Afin de déterminer les modifications à différentes échelles structurales, quatre formulations modèles sont étudiées : la gomme pure, les matrices vulcanisées stabilisée et non stabilisée et l'élastomère industriel. L'ensemble des échantillons est vieilli entre 70 et 170°C dans l'air ou sous vide puis caractérisé par divers outils analytiques. La thermogravimétrie donne accès aux variations de masse résultant de l'incorporation d'oxygène et l'émission de composés volatils. L'analyse infrarouge permet de suivre les évolutions des espèces chimiques. Les essais de gonflement, de chromatographie et de spectrométrie mécanique permettent de calculer les nombres de coupures de chaînes et d'actes de réticulation à chaque instant. Sur la base de ces résultats, un modèle cinétique général de thermo-oxydation de la matrice EPDM est proposé et en partie validé. Les conséquences du vieillissement thermique sur le comportement mécanique de l'élastomère industriel sont mises en évidence par des essais de traction uniaxiale et de multi-relaxation à température ambiante et vitesse de déformation initiale de 10-3 s-1. L'impact du vieillissement thermique sur les propriétés ultimes et les réponses à l'équilibre et hors équilibre est examiné. Un critère prédictif de rupture basé sur la mécanique de la rupture est proposé. Enfin, le couplage vieillissement thermique - contrainte mécanique est étudié par des essais relaxation de contraintes continues entre 130 et 170°C dans l'air. Les modifications de la microstructure pendant le vieillissement thermique sont intégrées dans les équations constitutives du modèle mécanique macroscopique afin de proposer un outil de prédiction du comportement à long terme de l'élastomère industriel. La simulation numérique montre une bonne adéquation avec les résultats expérimentauxThe aim of this work is to study the consequences of the thermal oxidation on the chemical structure and mechanical behavior of an Ethylene-Propylene-Diene Monomer (EPDM). In order to determine the structural changes at different scales, four model formulations have been considered: free additive gum, stabilized and unstabilized vulcanized matrix and industrial rubber. All samples were aged between 70 and 170°C in air or vacuum and characterized by several analytical tools. Thermogravimetry gives access to weight variations due to oxygen grafting and volatile compounds release. Infrared analysis is used to follow chemical species evolutions. Swelling tests, chromatographic and mechanical spectrometry tests allow calculating the number of chain scission and cross-linking events at any time. Based on these results, a general kinetic model is proposed and partially validated for EPDM matrix thermal oxidation. The consequences of thermal ageing on the mechanical behavior of the industrial rubber are pointed out by monotonic tensile and stress relaxation tests at room temperature and a 10-3 s-1- initial strain rate. The impact of thermal ageing on ultimate properties and equilibrium and non-equilibrium response are examined. Finally, the coupling between thermal ageing and mechanical stress is studied by continuous stress relaxation tests between 130 et 170°C in air. The microstructural modifications during thermal ageing are introduced into the constitutive equations of the macroscopic mechanical model in order to propose a predictif tool of the long time behavior of the industrial rubber. The numerical simulation is in good agreement with experimental results
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Heitz, Jean-François. "Propagation d'ondes en milieu non linéaire : applications à la reconnaissance des sols et au génie parasismique." Grenoble 1, 1992. http://www.theses.fr/1992GRE10120.
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La propagation d'ondes en milieu non lineaire est etudiee sur les plans theorique, experimental et numerique. Des applications touchant les domaines du genie parasismique et de la reconnaissance in situ des sols sont proposes. Apres un etat des connaissances acquises a ce jour sur le comportement du sol sous sollicitation dynamique, un modele de comportement viscoelastique non lineaire est introduit dans l'equation fondamentale de la dynamique. L'equation du mouvement du sol obtenue exhibe au second membre un terme de source contenant l'ensemble des termes caracteristiques lies a la non-linearite du sol. Une analyse dans l'espace des frequences montre que le comportement non lineaire deviatoire du sol est a l'origine de modifications du contenu spectral au cours de la propagation par rapport au contenu frequentiel de la sollicitation. Une methode iterative de resolution de l'equation du mouvement est utilisee. A chaque iteration, la solution est obtenue explicitement par l'utilisation de transformations fonctionnelles integrales appropriees. Deux essais dynamiques in situ en surface et en puits avec excitation harmonique sont interpretes sur la base de l'approche theorique precedente. Le premier essai a permis essentiellement de mettre en evidence in situ le comportement non lineaire du sol sous sollicitation sinusoidale. Pour le second essai, des calculs simulent la reponse du sol a distance du puits et une methode d'identification de parametres caracteristiques du comportement non lineaire du sol est proposee. Une autre application de l'approche theorique proposee permet d'etudier les effets de site unidimensionnels caracteristiques du comportement non lineaire d'un horizon de sol reposant sur un demi-espace et subissant une sollicitation transitoire de type sismiques. Une extension du calcul en configuration bidimensionnelle est ensuite proposee
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Zhang, Zaicheng. "Nano-rheology at soft interfaces probed by atomic force microscope." Thesis, Bordeaux, 2020. http://www.theses.fr/2020BORD0157.
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Des progrès récents dans les études expérimentales et théoriques ont montré que l'écoulement d’un liquide à l'échelle micro/nano se comporte différemment de celui à l'échelle macroscopique. À l'échelle microscopique, les propriétés des surfaces sont prédominantes pour le comportement d'écoulement proche des parois. Pour un confinement élevé, non seulement les propriétés physico-chimie des parois sont importantes, mais leur comportement élastique doit également être pris en compte. Dans cette thèse, nous avons utilisé l'AFM dynamique à sonde colloïdal pour étudier l’écoulement confiné sur des surfaces molles (bulles d'air et échantillons PDMS) et nous avons montré que:• A l'interface air-eau, la présence de faible trace de tensioactifs modifie le flux à proximité des interfaces de manière drastique, ce qui conduit à une réponse viscoélastique. Les forces visqueuses et élastiques agissant sur la sphère sont extraites de la mesure du mouvement de la sphère. En raison de la contamination par l'agent tensioactif, la force visqueuse présente un passage d’une condition aux limites non-glissante sur la bulle à une condition aux limites de glissement parfait en augmentant la fréquence d’oscillation de la sphère. La force élastique apparaît également avec une valeur comparable à la force visqueuse.• A faible distance, la pression visqueuse induite par la vibration de la sonde colloïdale déforme la surface de la bulle et donne lieu à l'interaction visco-capillaire. Une excitation par bruit thermique ou une excitation acoustique externe sont utilisées pour induire les vibration la sonde AFM. Pour expliquer nos mesures, nous avons développé un modèle simplifié basé sur un ressort-dissipateur en série et nous avons également effectué la résolution numérique de l'équation de Navier-Stokes combinée à l'équation de Laplace-Young. L'ajustement de nos résultats expérimentaux nous permet de mesurer la tension superficielle de l'interface de la bulle sans contact.• Le levier AFM est un outil puissant pour sonder le mouvement thermique de l'interface de la bulle hémisphérique. Le spectre de telles oscillations thermiques nanométriques de la surface de la bulle présente plusieurs pics de résonance et révèle que la ligne de contact de la bulle hémisphérique est fixée sur le substrat. La viscosité de surface de l'interface due à la contamination par le tensioactif est obtenue à partir de l'analyse de ces pics.• Une force de portance élasto-hydrodynamique agit sur une sphère se déplaçant à proximité et le long d'un substrat mou dans un liquide visqueux. La force de portance est sondée en fonction de la taille de l’écoulement, pour diverses vitesses de la sphère, viscosités du liquide et rigidité de l'échantillon. À grande distance, les résultats expérimentaux sont en excellent accord avec un modèle développé à partir de la théorie de la lubrification. À petite distance, une saturation de la force de portance est observée et une loi d'échelle pour cette saturation est donnée et discutéeRecent progresses in experimental and theoretical studies have shown that the liquid flow at micro/nano scale behaves differently from that at macroscale. At microscale, surface properties are predominant for the flow behavior at the boundaries. For high confinement, not only the physico-chemistry of the confining surfaces are important, their elastic behavior should also be taken into account. In this thesis, we used the dynamic colloidal AFM to probe the confined flow at soft surfaces (Air bubbles and PDMS samples) and we have shown that:• At the air-water interface, the presence of surfactant impurities modifies the flow near the interfaces in a drastic manner, which leads to the viscoelastic responses. The viscous and elastic forces acting on the sphere are extracted from the measurement of the sphere motion. Due to the surfactant contamination, the viscous force shows a crossover from non-slip to full slip boundary conditions and the elastic force also appears with a comparable value to viscous force.• At small distance, the viscous pressure induced by the colloidal probe vibration deforms the bubble surface and gives rise to the visco-capillary interaction. Thermal noise excitation or external acoustic excitation are used to drive the AFM probe. To explain our measurements, we have developed a simplified model based on a spring-dashpot in series and we have also performed numerical solution of the Navier-Stokes equation combined with Laplace-Young equation. Fitting our experimental results allow us to measure the surface tension of bubble interface without contact.• The AFM cantilever is a powerful tool to probe the thermal motion of the hemispherical bubble interface. The spectrum of such nanoscale thermal oscillations of the bubble surface presents several resonance peaks and reveals that the contact line of the hemispherical bubble is fixed on the substrate. The surface viscosity of the bubble interface due to the surfactant contamination is obtained from the analysis of these peaks.• An elastohydrodynamic lift force is acting on a sphere moving near and along a soft substrate within a viscous liquid. The lift force is probed as a function of the gap size, for various driving velocities, liquid viscosities, and sample stiffnesses. At large distance, the experimental results are in excellent agreement with a model developed from the soft lubrication theory. At small gap distance, a saturation of the lift force is observed and a scaling law for this saturation is given and discussed
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Champagne, Jonathan. "Modélisation physique du comportement mécanique linéaire et non-linéaire des élastomères renforcés." Thesis, Université Paris sciences et lettres, 2020. http://www.theses.fr/2020UPSLM043.
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L’ajout de charges dans les élastomères, en particulier les nano-silices dans les silicones, améliore sensiblement les propriétés mécaniques (rigidité, dissipation, contrainte à rupture,...). En contrepartie, cela introduit dans le comportement mécanique des dépendances et des non-linéarités absentes dans la matrice pure. Le lien entre la microstructure complexe du matériau et ses propriétés mécaniques macroscopiques n’est pas encore clairement établi. C’est pourquoi, nous commençons par construire un modèle stochastique décrivant la réponse mécanique linéaire des élastomères renforcés. Ce modèle est basé sur le concept de ponts vitreux. La microstructure représentative du matériau est formée par une distribution d’orientations et de longueurs de ponts, i.e. de régions de chaînes polymères confinées entre agrégats. Localement, la réponse mécanique du pont peut être vitreuse ou caoutchoutique selon le confinement, la température et l’amplitude de déformation. Les propriétés mécaniques macroscopiques de la distribution sont calculées par une méthode d’homogénéisation auto-cohérente. Pour chaque matériau, la distribution de ponts peut être identifiée à partir des mesures expérimentales du renforcement mécanique en régime linéaire.Le modèle stochastique nous permet aussi d’élucider l’origine physique de l’équivalence température-pression mise en évidence sur la rigidité. Alors que la température engendre la disparition des ponts vitreux (adoucissement), nous montrons que la pression, elle, en crée de nouveaux (rigidification). Le concept de ponts vitreux nous permet alors d’établir une nouvelle loi d’équivalence température-pression pour les élastomères renforcés. En outre, nous montrons que cette théorie décrit aussi, en première approximation, les propriétés mécaniques non-linéaires (effet Payne) de nos matériaux. Pour finir, nous mettons en œuvre dans un code de calcul éléments finis, un modèle de comportement macroscopique à base physique. Bien que le cadre thermodynamique soit classique, les valeurs des paramètres matériaux sont issues de surfaces de réponses qui sont identifiées à partir du modèle stochastiqueThe addition of fillers into a polymer matrix, such as nano-silica into silicone rubbers, brings an outstanding enhancement in the mechanical properties (stiffness, hysteresis, ultimate strength,...). In return, many dependencies or nonlinearities appear in the mechanical behaviour which do not exist in the pure rubber. The relation between the microstructure and the macroscopic mechanical properties is not yet clearly understood.Thus, we start with the development of a stochastic model describing the linear mechanical response of filled rubbers. This model is based on the glassy bridges theory. The typical microstructure of the system is composed by a bridges lengths and orientations distribution, i.e. confined polymer chains between aggregates. The local mechanical response of a single bridge can be glassy or rubbery depending on the confinement degree, the temperature and the strain amplitude. The macroscopic mechanical properties of the distribution are calculated by the mean of a self-consistent homogenization. For all our materials, the bridges distribution has been identified based on the linear mechanical reinforcement measurements.The stochastic model also provides the physical origin of the pressure-temperature superposition highlighted on the stiffness. The temperature makes the glassy bridges disappear (softening) while the pressure creates new ones (hardening). The glassy bridges theory allows us to suggest a new pressure-temperature superposition law for filled elastomers.Then, we show that the theory is also capable, to a first approximation, to describe the nonlinear mechanical properties (Payne effect) of our samples. Finally, we implement the constitutive equations of a macroscopique behaviour into a finite element software. Although the thermodynamical framework is standard, the material parameters are given by responses surfaces which are identified based on the stochastic model
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Zhu, Weidong. "Nonlinearly viscoelastic response of glassy polymers." Thesis, 2002. https://thesis.library.caltech.edu/6345/1/Zhu_w_2002.pdf.
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This thesis consists of three chapters. After a brief introduction on the general aspects of polymer characterization and viscoelasticity in the first chapter, all major features of this research project are described in the following two chapters.
The second chapter deals exclusively with the nonlinearly thermo-mechanical creep behavior of (bisphenol A) polycarbonate under pure shear loading at different
temperatures (0 °C to 140 °C). The shear creep in the linearly viscoelastic range was measured with a torsiometer for reference purposes and a master curve, along with a shift factor curve, were deduced. While the master curve is well defined with no detectable deviation, the shift factor can be represented by two straight line segments interrupted at the β transition temperature of polycarbonate. The shear creep tests in the nonlinearly
viscoelastic range were conducted on an Arcan specimen geometry at different temperatures and under different stress levels, utilizing digital image correlation for the
recording of the creep strains. The difference between the nominal stress and the actual stress distribution in the Arcan specimen was explored via numerical simulations
(ABAQUS) by assuming linear quasi-elastic and quasi-plastic analysis in place of the as yet uncertain material characterization. Isochronal plots were created from the creep data. Nonlinearly viscoelastic behavior starts to take effect near 1% strain at the temperatures considered. The applicability of the stress-clock representation for material characterization has been explored and is found to be dubious, at best, for this material. The "yield-like" behavior of polycarbonate has been examined in terms of the isochronal stress-strain response and a corresponding "yield-like shear stress" has been determined
to be a monotonically decreasing function of the temperature, but again with an interruption at the β transition temperature. Time-temperature trade-off as practiced for "time-temperature shifting" at small strains does not apply in the nonlinear domain. The results are generally in agreement with those found for Poly(Methyl Methacrylate), thus fostering the idea that the present results can be generalized -with additional work- to other amorphous polymers.
The third chapter focuses on the role of volumetric strain in nonlinearly viscoelastic behavior of polycarbonate. The creep responses of (bisphenol A) polycarbonate at 80 °C under combined two-dimensional shear and tensile/compressive
stress states were measured on Arcan specimens in the nonlinearly viscoelastic regime. Of particular interest is the influence of the dilatational deformation component on the nonlinearly viscoelastic creep behavior. Because the nonlinear material response determines the stress distribution under fixed deformation or load, but is not known a priori, a re-estimation of the latter is essential to verify or adjust the stress state(s). This is
accomplished by approximating isochronal stress-strain relations derived from shear creep behavior, encompassing the nonlinear domain, by a classical incremental elastoplastic
material description at appropriate times. Inasmuch as the two-dimensional character of the test configuration places limits on accessing three-dimensional information, a coherent representation of the results in terms of maximum shear and/or octahedral representation is examined. It is found that the creep behavior under shear and normal stress or deformation imposition differ significantly: when viewed as a response in terms of a maximum shear description, there are material responses under combined loading when either one or the other dominates. Once the response is formulated in terms of an octahedral description the representation becomes less sensitive to normal vs. shear
behavior. Within the precision underlying the measurements it is found that the shear and normal strain components accumulate under creep in nearly constant ratios. However,
under this scenario it is demonstrated quite clearly that the addition of negative dilatational stress (or deformation) to pure shear leads to distinctly lower creep rates. The converse is true, if positive dilatational stresses are added, though not monotonically so.
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Swenson, Eric Dexter. "Efficient frequency response analysis of structures with viscoelastic materials." Thesis, 2006. http://hdl.handle.net/2152/2936.
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Lin, Yen Shiu, and 林彥旭. "Frequency Response Function of composite beam with viscoelastic layer." Thesis, 1998. http://ndltd.ncl.edu.tw/handle/90167963988653947510.
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碩士國立臺灣大學
機械工程學系研究所
86
In this thesis, we investigate the dynamic characteristics of an elastic bea m completely covered by a viscoelastic layer both theoretically and experiment ally. The fourth-order differential equations of motion are derived for the tr ansverse vibration of thecomposite beam. Not only the transverse strain and st ress of the elastic and viscoelastic layer are included in the equation of mot ion,but also the longitudinal strain and stress are included. An eigenvaluepro blem corresponding to the differential equation is solved to obtainthe require d dynamic characteristics. Numerical analysis and experimental measurements i s illustrated by a cantilever beam in transverse vibration. Galerkin Method is used to find the eigen solution. The dynamic behavior of the composite beam is illustrated with frequency response functions.
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Palmeri, Alessandro, and N. Makris. "Response analysis of rigid structures rocking on viscoelastic foundation." 2008. http://hdl.handle.net/10454/602.
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In this paper the rocking response of slender/rigid structures stepping on a viscoelastic foundation is revisited. The study examines in depth the motion of the system with a non-linear analysis that complements the linear analysis presented in the past by other investigators. The non-linear formulation combines the fully non-linear equations of motion together with the impulse-momentum equations during impacts. The study shows that the response of the rocking block depends on the size, shape and slenderness of the block, the stiffness and damping of the foundation and the energy loss during impact. The effect of the stiffness and damping of the foundation system along with the influence of the coefficient of restitution during impact is presented in rocking spectra in which the peak values of the response are compared with those of the rigid block rocking on a monolithic base. Various trends of the response are identified. For instance, less slender and smaller blocks have a tendency to separate easier, whereas the smaller the angle of slenderness, the less sensitive the response to the flexibility, damping and coefficient of restitution of the foundation.
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Hsieh, Huan-Sheng, and 謝煥昇. "Dynamic Responses of Secondary Systems in Structures with Added Viscoelastic Dampers by Response Spectrum Analysis." Thesis, 2007. http://ndltd.ncl.edu.tw/handle/84155976855422006668.
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碩士國立臺灣大學
土木工程學研究所
95
This paper utilizes complex modal analysis technique to deal with a non-classical damping composite system, which consist of a primary and a secondary systems. The outcome is presented by a response spectrum which is more convenient in the application of design. The interaction, tuning and non-classical damping between sub-systems must be taken into account in the structural analysis of composite system. The present study obtains the modal characteristics of the composite system by simple analysis method. The method offers a less numerical labor than the ordinary one. In regard to a composite system with added viscoelastic dampers, the study focuses on each individual sub-system and modifies the modal shapes by two phases. By using so, the analytical complexity is reduced substantially.
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Escobedo, Torres Javier. "Solution methods for the dynamic response of structures with viscoelastic materials /." Diss., 1997. http://gateway.proquest.com/openurl?url_ver=Z39.88-2004&rft_val_fmt=info:ofi/fmt:kev:mtx:dissertation&res_dat=xri:pqdiss&rft_dat=xri:pqdiss:9734877.
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Tarana, Darmawan Saputra, and 陳秋萬. "STUDY ON IMPACT LOAD RESPONSE OF CONCRETE BEAM USING VISCOELASTIC MODEL." Thesis, 2015. http://ndltd.ncl.edu.tw/handle/48250851448673984137.
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碩士國立臺灣科技大學
營建工程系
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The main goal of this study is to investigate the dynamic response of viscoelastic material due to impact load from free fall of steel ball, in which, the dynamic responses in the time and frequency domain from experimental program and numerical simulation using ABAQUS commercial software were conducted. Six mortar beam specimens of 1001001000 mm were casted for the test to obtain the acceleration. Experimental variables included two curing ages of 14 and 21 days for mortar and two different diameters of 40 and 50 mm for dropping steel balls. The results of time domain response show that the damping values from logarithmic decrements of concrete specimens were varied from 0.026 to 0.044. The result of Fast Fourier Transform (FFT) with 40 mm diameter of steel ball test shows that maximum acceleration at age of 14 days is 1.44 m/s2 at first dominating frequency 1678.4 Hz and 3738.4 Hz for second dominating frequency. The dominating frequencies value is very important when there is a need to select proper accelerometer for further investigation. The age of beam influenced energy dissipation and strain energy of beam model. The result shows that from 14 days to 21 days, the loss factor of beam were reduced. The implementation of creep in ABAQUS with relaxation data was successfully performed in order to study about dynamic response of viscoelastic material. The loss factor and phase angle between stress and strain were obtained and listed.
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Brinson, L. C. "Time-temperature response of multi-phase viscoelastic solids through numerical analysis." Thesis, 1990. https://thesis.library.caltech.edu/4297/1/Brinson_lc_1990.pdf.
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A numerical model has been constructed for the study of the properties of multi-phase viscoelastic composites. The model utilizes the dynamic correspondence principle of viscoelasticity in a finite element program to solve boundary value problems simulating uniaxial tension or simple shear and obtains the global complex Young's or shear moduli of the composite.
Each phase of the composite is considered to be thermorheologically simple. The resulting modulus properties of the composite however, are thermorheologically complex and this investigation examines the nature of time-temperature behavior of multi-phase composite materials. The specific composite considered throughout this study contains viscoelastic inclusions embedded in a different viscoelastic matrix material. The deviation of the composite moduli from thermorheologically simple behavior of the matrix material is shown to occur at frequencies and temperatures where the glass-to-rubber transition of the included phases are reached.
Properties of polystyrene and polybutadiene are used to investigate the thermorheological complexity (non-shiftability) of a Styrene-Butadiene-Styrene (SBS) block copolymer. To achieve congruence of the results with experimental data, it is necessary to consider a transition phase of properties "intermediate" to those of styrene and butadiene. Using accurate physical information on the individual phase properties and on the interphase region, it is possible to utilize the numerical model to predict long term properties of multi-phase composites from short term laboratory data. Lacking detailed information on the properties of a particular phase (e.g., the interphase), but knowing the time dependent properties for the composite material at a broad range of temperatures, it is also possible to use the numerical tool to solve an inverse problem and determine the unknown properties of the phase in question.
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Kress, Oliver Herbert. "The Viscoelastic Response of Liquid Crystalline Fibers Formed By Bent-core Molecules." Doctoral thesis, 2018. http://hdl.handle.net/21.11130/00-1735-0000-0005-12B3-C.
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Hu, Yu-Jen, and 許昱人. "A Study of the Effect of Viscoelastic Damper on the Seismic Response of Buildings." Thesis, 1993. http://ndltd.ncl.edu.tw/handle/27058923638512028632.
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Maurício, Marta Cadete. "Dynamic Analysis of Structures with Viscoelastic Damping Treatments: Complex Modulus Identification and Transient Response." Master's thesis, 2017. https://repositorio-aberto.up.pt/handle/10216/106444.
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Chen, Yung-Ju, and 陳永儒. "Dynamic Response Study of a Full Scale Five Story Steel Frame with Viscoelastic Dampers." Thesis, 1997. http://ndltd.ncl.edu.tw/handle/01292948641545755260.
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