Papers by Keyword: Viscoelasticity

Abstract: Laminated glass is a structural element used extensively in a reconstruction of existing building structures because of its transparency and simplicity. When using laminated glass as a glass staircase, balustrades, transparent flooring, facades or other structural elements, it is advisable to consider the shear interaction of individual glass panes in the cross-section. A conservative approach where the glass panes shear interaction is not considered, is uneconomical. This interaction depends on the properties of polymeric interlayers used in lamination process. Various commercial products based on PVB (polyvinyl butyral), EVA (ethylene vinyl acetate), ionomer, or thermoplastic polyurethane (TPU) are used. Stiffness of polymers depends on temperature and duration of a load. Interlayers exhibit the viscoelastic properties and temperature dependency usually described by the generalized Maxwell model and WLF model (Williams-Landel-Ferry). Parameters of these models are the most effectively determined by Dynamic Mechanical Thermal Analysis (DMTA), where the material is cyclically loaded at different frequencies and temperatures. Material parameters were found by DMTA in shear for PVB type of interlayer Trosifol® BG R20. In addition, the experimental quasi-static loading tests in shear were performed at different loading rates and at various temperatures. These experimental stress-strain diagrams were compared to the theoretical stress-strain relations obtained from Maxwell model with material parameters based on DMTA testing. All tests were performed in Klokner Institute CTU in Prague.

Abstract: The dielectric elastomer (DE) has the advantages of large deformation ability, fast response speed, low price and high energy density. Therefore, DE has great prospects as artificial muscle and flexible robot. The purpose of the research is to clarify the mechanical behavior for acrylate dielectric elastomer by tensile test, fatigue test and viscoelasticity measurement.

Abstract: The subject of this work is to investigate viscoelastic properties such as loss modulus (G ́ ́), storage modulus (G ́), complex shear modulus (G*), complex viscosity (η*) and loss angle () at different temperatures by means of a small-amplitude oscillatory test. These properties allow to provide information about materials structure. For this purpose, we employed a tin-lead alloy (Sn-15%Pb) which exhibits a similar microstructure to aluminum alloys and is the classic alloy for semisolid thixotropic studies. It is interesting to note that the Sn-15%Pb alloy exhibits a slightly decrease in storage modulus (G ́) over the entire frequency (0.01-10Hz) at high temperatures, showing its viscoelastic behavior. In addition, a detailed analysis of master curves (oscillatory tests) was made to relate the semisolid microstructure (solid fraction) with the plateau modulus (G_N⁰) which is directly related with both molecular weight or percolation threshold in polymer and gels science respectively.

Abstract: A new fiber (x) reinforced Dynamic Covalent epoxy-polyurea Interface (x-DC_EPI) shows good mechanical energy transferability of impact and vibration forces. The bonding property of x-DC_EPI interface, engendered between curing, or reactive, epoxy and dynamic polyurea, is controlled by epoxy curing time (t_c). The reaction of curing epoxy, where t_c is a thermodynamic processing parameter, and fast-curing/ dynamic aliphatic polyurea, which lacks polyol in its resin chain extender, is linked to bulk mechanical energy transfer, quantified specifically via the loss modulus of x-DC_EPI. The parameter t_c effectuates designable chemical bond properties within x-DC_EPI. Using Generalized Maxwell models, viscoelastic properties of epoxy, polyurea, and x-DC_EPI are predicted, and results are verified using Dynamic Mechanical Analysis (DMA). The Maxwell models for x-DC_EPI, as a function of t_c, are used in a finite element analysis (ABAQUS) to control performance of dynamically loaded structures.

Abstract: 3D printing methods are developing and they have become popular recently. 3D printing can easily make complex and seamless parts, however, there are questions about their strength. In particular, the strength of the places where the lamination layer joins is important. We performed rolling contact fatigue (RCF) tests in dry conditions using 3D-printed bearing race and observed the fracture behavior and cracks. We found that the main crack is related to the stress moving direction.

Abstract: The outcome of most implant failures is tragic. There is an increasing need to reduce the rate of implant failure. While there has been a lot of progress regarding this problem, a lot still needs to be done. The behaviour of biomaterials had been represented using linear models. Linear models failed to capture some certain behaviours in materials due to the nonlinear nature of biomaterials. More work has been done in an attempt to represent the deformation of these biomaterials using non-linear models, which realised success to a degree. However, providing accurate solutions to these models became a problem. Here, An efficient approximate analytical method, differential transformation method (DTM) is provided for prediction of biomaterial deformation. The results of the solutions are found to be in excellent agreements with the results of the numerical methods. It was observed that at high viscosity, the material exhibit very high resistance to deformation and as it decreases, the material allows more deformation, for longer periods of time. Keywords : Biomaterials; Viscoelasticity; deformation; Differential Transformation Method;

Abstract: A brief review of the mechanical characterization of viscoelastic materials under uniaxial, biaxial and multiaxial stress condition is carried out in this paper. Parametric analytical studies have been done in a simulated tubular specimen with different internal pressure loadings at various material properties. We observed that the relaxation modulus values obtained from the thick-walled hollow cylinder analyses are higher that the traditional uniaxial and biaxial test data under the same strain level. We noticed that during the initial period, the relaxation-modulus values are almost identical and later the relaxation modulus obtained from the thick-walled hollow cylinder analysis is found significantly higher than the uniaxial and the strip biaxial test data. We conjectured that in the initial stage the stress–strain ratios are almost independent of the geometry of the test specimen and subsequently the stress conditions vary according to the shape of the specimen because the relaxation modulus is found geometry dependent when loading time advances. Note that the main objective of this characterization is not to determine the magnitude of the stress actually present in the test specimen, but to help the designer to decide the best geometry in a realistic way according to the industrial applications from the viscoelastic stress relaxation point of view.

Abstract: Rheological model for crumb rubber modified bitumen (CRMB) by the rheological testing and model analyzer was investigated in this paper. The effects of crumb rubber content and temperature on rheological model and viscoelasticity of CRMB were conducted. The results illustrate that rheological model of bitumen is transformed from Dashpot into Burgers at 60°C after bitumen has been modified by crumb rubber. And the elasticity of CRMB is increased and the viscosity is decreased with crumb rubber content rising. For the modified bitumen with 20% crumb rubber (CRMB-20), its rheological model is Burgers among 20°C~80°C, but the model is translated into Maxwell at 100°C. As the temperature increases, both viscosity and elasticity of CRMB-20 are increased. Compared with 20°C, the instantaneous elastic and viscous deformation of CRMB-20 at 100°C are increased by 14.2 times and 80.5 times, respectively.

Abstract: This study aims to investigate the quasi-static response of linear viscoelastic Kirchhoff plates of uniformly varying cross-section subjected to time-dependent loads. Four-parameter solid model is used for defining the linear viscoelastic material behavior. Through an efficient systematic procedure based on the Gâteaux Differential (GD), a functional has been constructed for the analysis. For the analysis, mixed finite element (MFE) method in transformed Laplace-Carson space is used. For transformation of the solutions obtained in the Laplace-Carson domain to the real time domain, Dubner & Abate (D&A) numerical inverse transform technique is employed.

Abstract: The formulation of a new mathematical model of deformed viscoelastic composite is shown in the work. The method of constructing a model is based on commonality problems of determining the effective modules of inhomogeneous elastic composites and viscoelastic bodies, which demonstrate the dependence of the mechanical properties from the duration of load action. Mathematical model of hybrid efficient modules has been constructed on the superposition of expressions for known effective modules of Voigt and Reuss, Hashin and Shtrikman. The new model summarizes the properties of classical approximate approaches. We have shown that the calculations of stress-strain state with the help of new effective characteristics provide a more accurate prediction in comparison with known models.