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Viscoelastic Material (viscoelastic + material)
Selected AbstractsResponse of unbounded soil in scaled boundary finite-element methodEARTHQUAKE ENGINEERING AND STRUCTURAL DYNAMICS, Issue 1 2002John P. Wolf Abstract The scaled boundary finite-element method is a powerful semi-analytical computational procedure to calculate the dynamic stiffness of the unbounded soil at the structure,soil interface. This permits the analysis of dynamic soil,structure interaction using the substructure method. The response in the neighbouring soil can also be determined analytically. The method is extended to calculate numerically the response throughout the unbounded soil including the far field. The three-dimensional vector-wave equation of elasto-dynamics is addressed. The radiation condition at infinity is satisfied exactly. By solving an eigenvalue problem, the high-frequency limit of the dynamic stiffness is constructed to be positive definite. However, a direct determination using impedances is also possible. Solving two first-order ordinary differential equations numerically permits the radiation condition and the boundary condition of the structure,soil interface to be satisfied sequentially, leading to the displacements in the unbounded soil. A generalization to viscoelastic material using the correspondence principle is straightforward. Alternatively, the displacements can also be calculated analytically in the far field. Good agreement of displacements along the free surface and below a prism foundation embedded in a half-space with the results of the boundary-element method is observed. Copyright © 2001 John Wiley & Sons, Ltd. [source] Three-dimensional finite-element model of the human temporomandibular joint disc during prolonged clenchingEUROPEAN JOURNAL OF ORAL SCIENCES, Issue 5 2006Miho Hirose In the temporomandibular joint (TMJ), overloading induced by prolonged clenching appears to be important in the cascade of events leading to disc displacement. In this study, the effect of disc displacement on joint stresses during prolonged clenching was studied. For this purpose, finite-element models of the TMJ, with and without disc displacement, were used. Muscle forces were used as a loading condition for stress analysis during a time-period of 10 min. The TMJ disc and connective tissue were characterized as a linear viscoelastic material. In the asymptomatic model, large stresses were found in the central and lateral part of the disc through clenching. In the retrodiscal tissue, stress relaxation occurred during the first 2 min of clenching. In the symptomatic model, large stresses were observed in the posterior part of the disc and in the retrodiscal tissue, and the stress level was kept constant through clenching. This indicates that during prolonged clenching the disc functions well in the asymptomatic joint, meanwhile the retrodiscal tissue in the symptomatic joint is subject to excessive stress. As this structure is less suitable for bearing large stresses, tissue damage may occur. In addition, storage of excessive strain energy might lead to breakage of the tissue. [source] Synthesis and characterization of novel biodegradable aliphatic poly(ester amide)s containing cyclohexane unitsJOURNAL OF POLYMER SCIENCE (IN TWO SECTIONS), Issue 6 2006Hélène A. Lecomte Abstract Polyesters provide a good basis to work on for designing novel biodegradable materials that are also mechanically and thermally resistant. In this study, a series of aliphatic poly(ester amide)s (PEA) based on cyclohexane units was synthesized. Block-copolymers of cyclohexyl sebacate and cyclohexyl sebacamide were produced by controlling the length of the ester block and the amount of amide during a two-step melt/interfacial polycondensation reaction. Films produced from these materials could retain their shape above 373 K due to the physical network of amide hydrogen-bonding. Thermal properties were also evaluated, with various melting and softening points obtained depending on the PEA composition. The determining factor for mechanical properties was the amount of amide introduced, with films containing up to 10 mol % amide showing the best handleability and flexibility. Tensile properties typical of an amorphous viscoelastic material were observed, but with much superior elongation to break achievable (,1700%). These materials were also shown to be hydrolyzable, noncytotoxic, and favorable for cell attachment: they may therefore have a promising future in the area of medical devices or packaging, especially as their properties can be tuned by changing their composition. © 2006 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 44: 1785,1795, 2006 [source] The assessment of fatigue damage on short-fiber-glass reinforced polyamides (PA) through the surface roughness evolutionPOLYMER COMPOSITES, Issue 4 2006J.A. Casado This paper analyses quantitatively the surface damage generated by the fatigue micromechanisms of the polyamide 6.6 (PA 6.6) reinforced with short fibre glass, with a series of specific tests for determining the surface roughness (RA) of the normalised specimens previously fatigue tested. The study has demonstrated that when the viscoelastic material is deformed in the fatigue process at a constant speed (d2,/dN2 = 0), crazing phenomena start to appear, distributed uniformly throughout its core. The effects of this damage, which is permanent and can be detected using sweep electron microscopy techniques (SEM), have been quantified through the measurement of the surface roughness of the specimens. In this way, the evolution of the roughness of the material is established as a qualitative index of the development of the nucleation of crazes and, thus, of irreversible damage, as these emerge towards the surface of the material. POLYM. COMPOS., 27:349,359, 2006. © 2006 Society of Plastics Engineers [source] Overall numerical simulation of extrusion blow molding processPOLYMER ENGINEERING & SCIENCE, Issue 8 2000Shin-Ichiro Tanifuji This paper focuses on the overall numerical simulation of the parison formation and inflation process of extrusion blow molding. The competing effects due to swell and drawdown in the parison formation process were analyzed by a Lagrangian Eulerian (LE) finite element method (FEM) using an automatic remeshing technique. The parison extruded through an annular die was modeled as an axisymmetric unsteady nonisothermal flow with free surfaces and its viscoelastic properties were described by a K-BKZ integral constitutive equation. An unsteady die-swell simulation was performed to predict the time course of the extrudate parison shape under the influence of gravity and the parison controller. In addition, an unsteady large deformation analysis of the parison inflation process was also carried out using a three-dimensional membrane FEM for viscoelastic material. The inflation sequence for the parison molded into a complex-shaped mold cavity was analyzed. The numerical results were verified using experimental data from each of the sub-processes. The greatest advantage of the overall simulation is that the variation in the parison dimension caused by the swell and drawdown effect can be incorporated into the inflation analysis, and consequently, the accuracy of the numerical prediction can be enhanced. The overall simulation technique provides a rational means to assist the mold design and the determination of the optimal process conditions. [source] Maxwell Fluid Model for Generation of Stress,Strain Curves of Viscoelastic Solid Rocket PropellantsPROPELLANTS, EXPLOSIVES, PYROTECHNICS, Issue 4 2010Himanshu Shekhar Abstract Solid rocket propellants are modeled as Maxwell Fluid with single spring and single dashpot in series. Complete stress,strain curve is generated for case-bonded composite propellant formulations by taking suitable values of spring constant and damping coefficient. Propellants from same lot are tested at different strain rate. It is observed that change in spring constant, representing elastic part is very small with strain rate but damping constant varies significantly with variation in strain rate. For a typical propellant formulation, when strain rate is varied from 0.00037 to 0.185 per second, spring constant (K) changed from 5.5 to 7.9,MPa, but damping coefficient (D) varied from 1400 to 4,MPas. For all strain rates, stress,strain curve is generated using developed Maxwell model and close matching with actual test curve is observed. This indicates validity of Maxwell fluid model for case-bonded solid propellant formulations. It is observed that with increases in strain rate, spring constant increases but damping coefficient decreases representing solid rocket propellant as a true viscoelastic material. It is also established that at higher strain rate, damping coefficient becomes negligible as compared to spring constant. It is also observed that variation of spring constant is logarithmic with strain rate and that of damping coefficient follows a power law. The correlation coefficients are introduced to ascertain spring constants and damping coefficients at any strain rate from that at a reference strain rate. Correlation for spring constant needs a coefficient "H," which is function of propellant formulation alone and not of test conditions and the equation developed is K2=(K1 - H)×{ln(d,2/dt)/ln(d,1/dt)}+H. Similarly for damping coefficient (D) also another constant "S" is introduced and prediction formula is given by D2=D1×{(d,2/dt)/(d,1/dt)}S. Evaluating constants "H" and "S" at different strain rates validate this mathematical formulation for different propellant formulations. Close matching of test and predicted stress,strain curve indicates propellant behavior as viscoelastic Maxwell Fluid. Uniqueness of approach is to predict complete stress,strain curves, which are not attempted by any other researchers. [source] A viscoelastic model for the dynamic response of soils to periodical surface water disturbanceINTERNATIONAL JOURNAL FOR NUMERICAL AND ANALYTICAL METHODS IN GEOMECHANICS, Issue 12 2006P. C. Hsieh Abstract In many instances soils can be assumed to behave like viscoelastic materials during loading/unloading cycles, and this study is aimed at setting up a viscoelastic model to investigate the dynamic response of a porous soil layer of finite thickness under the effect of periodically linear water waves. The waves and homogeneous water are described by potential theory and the porous material is described by a viscoelastic model, which is modified from Biot's poroelastic theory (1956). The distributions of pore water pressures and effective stresses of various soils such as silt, sand, and gravel are demonstrated by employing the proposed viscoelastic model. The discrepancies of the dynamic response between the simulations of viscoelastic model and elastic model are found to be strongly dependent on the wave frequency. Copyright © 2006 John Wiley & Sons, Ltd. [source] CHARACTERIZATION OF THE NONLINEAR VISCOELASTIC CONSTITUTIVE PROPERTIES OF MILD CHEDDAR CHEESE FROM INDENTATION TESTSJOURNAL OF TEXTURE STUDIES, Issue 5-6 2005S.M. GOH ABSTRACT A methodology to convert the indentation force,displacement response into the stress,strain properties of nonlinear viscoelastic materials was evaluated. Mild cheddar cheese was used as the test material, and indentation tests were performed using two spherical indenters of different sizes. The indentation tests were performed at different speeds, and the corresponding force,displacement responses were fitted with an analytical solution to obtain the time-dependent constants and the instantaneous force,displacement response. An inverse analysis based on the finite element method was performed to obtain the strain-dependent constants from the instantaneous force,displacement response. The predictions of the viscoelastic stress,strain properties from the indentation tests were compared to independent measurements through uniaxial compression tests, and a reasonable agreement was obtained. [source] Novel epoxy compositions for vibration damping applicationsPOLYMERS FOR ADVANCED TECHNOLOGIES, Issue 10 2004D. Ratna Abstract Three epoxy compositions have been developed by using polyether amine hardeners having varying chain lengths of polyethers. Unlike normal epoxies, the compositions show low glass transition temperatures (0,45°C). Dynamic mechanical analysis and time,temperature superposition of the isotherms indicate that they have broad and high loss factor values over broad frequency and temperature ranges suggesting their application as viscoelastic materials in constrained layer damping of structural vibrations. Copyright © 2004 John Wiley & Sons, Ltd. [source] Fractional calculus applied to radiation dampingPROCEEDINGS IN APPLIED MATHEMATICS & MECHANICS, Issue 1 2003C. Trinks Dipl.-Ing. Separate treatment of the low, and high,frequency part of the dynamic stiffness is essential when approximating the latter in the frequency,domain. In this paper, a doubly,asymptotic rational approximation of the low,frequency part is combined with an analytical interpretation of the asymptotic part leading to a system of fractional differential equations to represent the force,displacement relationship. Here, an analogy between fading memory of viscoelastic materials and radiation damping of unbounded domains is visible. [source] | ||||||||||||||||