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We examine the question of whether fluids and crystals are differentiated on the basis of their zero frequency shear moduli or their limiting zero frequency shear viscosity. We show that while fluids, in contrast with crystals, do have a zero value for their shear modulus, in contradiction to a widespread presumption, a crystal does not have an infinite or exceedingly large value for its limiting zero frequency shear viscosity. In fact, while the limiting shear viscosity of a crystal is much larger than that of the liquid from which it is formed, its viscosity is much less than that of the corresponding glass that may form assuming the liquid is a good enough glass former. [ABSTRACT FROM AUTHOR]

Background: Cartilage elasticity changes with cartilage degeneration. Hence, cartilage elasticity detection might be an alternative to traditional imaging methods for the early diagnosis of osteoarthritis. Based on the wave propagation measurement, Shear wave elastography (SWE) become an emerging non-invasive elasticity detection method. The wave propagation model, which is affected by tissue shapes, is crucial for elasticity estimating in SWE. However, wave propagation model for cartilage was unclear.Methods: This study aimed to establish a wave propagation model for the cartilage-bone structure. We fabricated a cartilage-bone structure, and studied the elasticity measurement and wave propagation by experimental and numerical Lamb wave method (LWM).Results: Results indicated the wave propagation model satisfied the lamb wave theory for two-layered structure. Moreover, a near field region, which affects wave speed measurements and whose occurrence can be prevented if the wave frequency is larger than one critical frequency, was observed.Conclusion: Our findings would provide a theoretical foundation for further application of LWM in elasticity measurement of cartilage in vivo. It can help the application of LWM to the diagnosis of osteoarthritis. [ABSTRACT FROM AUTHOR]

Results of experimental investigation of the low-frequency shear elasticity of liquids depending on the angle of shear strain are presented. A cluster model of the liquid structure is suggested to explain the low-frequency viscoelastic relaxation. [ABSTRACT FROM AUTHOR]

The focus of this research is on linear and nonlinear melt rheological investigations of polylactide (PLA) and nanographite platelet (NGP) based bionanocomposites. The linear steady and dynamic shear viscoelastic behaviors of the samples is analyzed to study the effects of NGP filler contents on the rheological properties of the composites at terminal and zero-shear viscosity regions whilst the nonlinear shear behaviors are investigated to predict and validate the extensional rheological properties of neat PLA and PLA/NGP composites. Uniaxial extensional measurements are performed to probe the impacts of nanofiller contents along with extensional strain rates on linear viscoelastic envelope (LVE) and nonlinear elongational strain hardening behaviors of the samples. The modeling of strain-hardening behavior of neat PLA and its composites is accomplished by means of analyzing their dynamic moduli, steady shear viscosity, relaxation spectrum and damping function based on Papanastasiou-Scriven-Macosko-Luo-Tanner (PSMLT) form of Kaye-Bernstein-Kearsley-Zapas (K-BKZ) constitutive equation. [ABSTRACT FROM AUTHOR]

The article deals with a study which developed an experimental method for determining the viscoelastic behavior of a sedimentary rock. The viscoelastic behavior of a sedimentary rock was investigated through a series of impact tests. It relates how the frequency dependence of the viscoelastic storage modulus and loss modulus is obtained. It describes the rock viscosity and its effect on stress wave propagations in the sedimentary rock.

We determine the form of the complex shear modulus G* in soft sphere packings near jamming. Viscoelastic response at finite frequency is closely tied to a packing's intrinsic relaxational modes, which are distinct from the vibrational modes of undamped packings. We demonstrate and explain the appearance of an anomalous excess of slowly relaxing modes near jamming, reflected in a diverging relaxational density of states. From the density of states, we derive the dependence of G* on the frequency and distance to the jamming transition, which is confirmed by numerics. [ABSTRACT FROM AUTHOR]

In this paper, we consider a multi-dimensional system of thermoelasticity type III with a viscoelastic damping acting on a part of the boundary. We establish a general decay result, from which the usual exponential and polynomial decay rates are only special cases. [ABSTRACT FROM AUTHOR]

Increased stiffness of reactive glial cells may impede neurite growth and contribute to the poor regenerative capabilities of the mammalian central nervous system. We induced reactive gliosis in rodent retina by ischemia-reperfusion and assessed intermediate filament (IF) expression and the viscoelastic properties of dissociated single glial cells in wild-type mice, mice lacking glial fibrillary acidic protein and vimentin (GFAP-/-Vim-/-) in which glial cells are consequently devoid of IFs, and normal Long-Evans rats. In response to ischemia-reperfusion, glial cells stiffened significantly in wild-type mice and rats but were unchanged in GFAP-/-Vim-/- mice. Cell stiffness (elastic modulus) correlated with the density of IFs. These results support the hypothesis that rigid glial scars impair nerve regeneration and that IFs are important determinants of cellular viscoelasticity in reactive glia. Thus, therapeutic suppression of IF up-regulation in reactive glial cells may facilitate neuroregeneration. [ABSTRACT FROM AUTHOR]

Abstract: The aim of this work is the study of general properties of phenomenological and state coefficients in a thermomechanical model for viscoanelastic media proposed by one of us (VC), in which cross-effect between viscous and inelastic flow is neglected. In particular, after the specification of the low and high frequencies region, under physical considerations characterized by limiting the values of some functions connected to mechanical phenomena, generally valid behaviour for phenomenological and state coefficients as function of the frequency are deduced. It has been possible, from the entropy production, to determine some characteristics of the coefficients as function of frequency in order that this principle is satisfied. The application of the so obtained results to a polymeric material as PolyIsobutilene in these regions in which, in a first approximation, the medium shows a linear behaviour, confirms our deductions. [Copyright &y& Elsevier]

Chrome-free leather such as glutaraldehyde-tanned leather behaves very differently from chrome-tanned leather. Information regarding its viscoelasticity has not been previously reported. Hysteresis and stress relaxation are two essential properties associated with viscoelasticity. We have designed a cyclic tensile test to measure these properties to gain insight into the structural difference between chrome-free and chrome-tanned leather. Observations revealed that chrome-free leather tanned with glutaraldehyde has a higher hysteresis than chrome-tanned leather. Stress relaxation experiments, on the other hand showed chrome-free leather has very similar relaxation curves as chrome-tanned leather. Both leathers demonstrate a rapid decrease in stress for the first few seconds followed by a much slower decay thereafter. The chrome-free leather, however, has a greater initial stress than chrome-tanned leather, indicating a higher stiffness than chrome-tanned leather. Moreover, observations showed the viscoelasticity of leather was affected significantly by its fatliquor content. A decrease of loading energy in a cyclic stress–strain experiment resulted from higher fatliquor content in leather. [ABSTRACT FROM AUTHOR]

Abstract: The mechanical properties of tissues are essential in diagnosing medical pathologies, with the tissue''s viscoelastic properties being of particular interest. However, current technology is inadequate in its capability to measure accurately the viscoelastic properties of living tissues, thus limiting progress in the medical field. To overcome the limitations of the current technology, this paper develops the design of a new mechanical resonator device to measure the viscoelastic properties of tissues. The device will be minimally invasive and measure quantitatively the localized properties of tissues in a living state. To achieve the design of the resonator, extensive modeling and analysis of biological tissues are performed. The resonator system is modeled using both analytical lumped-mass models and finite element models. Simulations validate the resonator as an effective measurement technique for viscoelastic tissue properties. [Copyright &y& Elsevier]

Abstract: We are concerned with the shock structure in viscoelasticity of rate type. The existence of a shock profile, which in the vanishing relaxation limit becomes an elastic shock, is demonstrated by an explicit construction of the solution of viscoelastic equations provided that the equilibrium stress–strain relation is governed by a parabolic law. For a more general stress–strain law which does not differ substantially from a parabolic one, the solution is obtained from a perturbation expansion. [Copyright &y& Elsevier]

Based on the premise that the addition of glass beads (GB) could hardly influence the linear viscoelasticity in low frequency (ω) region for homogeneous polymer systems, the dynamic rheological behaviors of unfilled and filled poly(methyl methyacrylate) (PMMA)/poly(styrene-co-acrylonitrile) (SAN) blends were studied in order to explore the effect of GB on the phase-separation of binary polymer matrix. Results show that GB has an induced effect on the phase-separation, which embodies that the phase-separation temperature (Ts) of PMMA/SAN blend filled with GB is lower than that of the unfilled system. The higher content of GB, the higher is the "secondary plateau" of ω in the terminal region of storage modulus (G′) versus ω plot. The "secondary plateau" appearing in the terminal region is attributed to the phase-separation of PMMA/SAN blends and it becomes more flat for filled polymer blends under the same conditions. However, it is suggested that this kind of "induced effect" is related to the GB content; the higher content of GB particles might enhance the interaction between the particles and polymer matrix. Moreover, it is found that the addition of GB also has an influence more or less on the morphology and domain size of polymer matrix. It is believed that the plot of dynamic viscosity (η′) versus the loss viscosity (η") is sensitive to examine the effect induced by GB on the phase-separation of binary polymer matrix. [ABSTRACT FROM AUTHOR]

An approach is proposed to determine the dissipation function in viscoelasticity theory. The first and second laws of thermodynamics and naive and integrated viscoelastic models are considered by the example of linear viscoelasticity [ABSTRACT FROM AUTHOR]

Abstract: We have developed and tested a new model suitable for the prediction of linear viscoelasticity (LVE) from knowledge of molecular structure for arbitrary mixtures of polydisperse star and linear entangled molecules. The model is based on key ingredients of tube models: reptation, fluctuations and constraint release. We impose no artificial time scale separation between fluctuations and reptation but rather allow both relaxation processes to proceed simultaneously. Early fluctuations are treated in a new and simple way, based on thermal energy threshold considerations. Because the modelled systems are arbitrary mixtures of linear and star molecules, we cannot obtain analytical solutions but have to resort to a time-marching algorithm to predict the relaxation modulus. The model has been tested on a wide range of literature data. Excellent predictions are obtained if the constraint release solvent influence on relaxation processes is treated consistently by using an extension of the “Graessley criterion”. The solvent must be included only if the corresponding constraint release characteristic time is short compared to the considered relaxation process. [Copyright &y& Elsevier]

Polymer composites reinforced by carbon nanotubes have been extensively researched for their strength and stiffness properties. Unless the interface is carefully engineered, poor load transfer between nanotubes (in bundles) and between nanotubes and surrounding polymer chains may result in interfacial slippage and reduced performance. Interfacial shear, although detrimental to high stiffness and strength, could result in very high mechanical damping, which is an important attribute in many commercial applications. We previously reported evidence of damping in nanocomposites by measuring the modal response (at resonance) of cantilevered beams with embedded nanocomposite films. Here we carry out direct shear testing of epoxy thin films containing dense packing of multiwalled carbon nanotube fillers and report strong viscoelastic behaviour with up to 1,400% increase in loss factor (damping ratio) of the baseline epoxy. The great improvement in damping was achieved without sacrificing the mechanical strength and stiffness of the polymer, and with minimal weight penalty. Based on the interfacial shear stress (∼0.5 MPa) at which the loss modulus increases sharply for our system, we conclude that the damping is related to frictional energy dissipation during interfacial sliding at the large, spatially distributed, nanotube–nanotube interfaces. [ABSTRACT FROM AUTHOR]

An analytic method for inverting the constitutive compliance equations of viscoelasticity for resins is developed. These equations describe the HWKK/H rheological model, which makes it possible to simulate, with a good accuracy, short-, medium- and long-term viscoelastic processes in epoxy and polyester resins. These processes are of first-rank reversible isothermal type. The time histories of deviatoric stresses are simulated with three independent strain history functions of fractional and normal exponential types. The stiffness equations are described by two elastic and six viscoelastic constants having a clear physic meaning (three long-term relaxation coefficients and three relaxation times). The time histories of axiatoric stresses are simulated as perfectly elastic. The inversion method utilizes approximate constitutive stiffness equations of viscoelasticity for the HWKK/H model. The constitutive compliance equations for the model are a basis for determining the exact complex shear stiffness, whereas the approximate constitutive stiffness equations are used for determining the approximate complex shear stiffness. The viscoelastic constants in the stiffness domain are derived by equating the exact and approximate complex shear stiffnesses. The viscoelastic constants are obtained for Epidian 53 epoxy and Polimal 109 polyester resins. The accuracy of the approximate constitutive stiffness equations are assessed by comparing the approximate and exact complex shear stiffnesses. The constitutive stiffness equations for the HWKK/H model are presented in uncoupled (shear/bulk) and coupled forms. Formulae for converting the constants of shear viscoelasticity into the constants of coupled viscoelasticity are given as well. [ABSTRACT FROM AUTHOR]

A new approach is proposed to extract the elastic and viscoelastic properties of a polymeric film on a hard substrate using a sharp indentation relaxation test. In the theoretic formulation, a sharp indentation relaxation test on a film/substrate system was interpreted by its equivalent flat-ended punch indentation relaxation test. The radius of the equivalent flat-ended punch was determined by the residual depth of the sharp indentation. A semi-analytical solution for a flat-ended punch relaxation test was derived. Sharp indentation relaxation tests on a polymethyl methacrylate film and a polycarbonate film on a glass substrate were performed. The semi-analytical solution and the experimental results were used to extract the elastic-viscoelastic parameters of the films. The extracted elastic-viscoelastic parameters of both films were found to be in a good agreement with data sheets of the films. The proposed approach provides a simple and yet feasible way to extract "film-only" elastic and viscoelastic properties. [ABSTRACT FROM AUTHOR]