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This article proposes a numerical routine to predict the residual stresses developing in an epoxy component during its curing. The scaling of viscoelastic properties with the temperature and the degree of conversion is modeled, adopting a mathematical formulation that considers the concurrent effects of curing and structural relaxation on the epoxy's viscoelastic relaxation time. The procedure comprises two moduli: at first, the thermal–kinetical problem is solved using the thermal module of Ansys and a homemade routine written in APDL, then the results in terms of temperature and the degree of conversion profiles are used to evaluate the viscoelastic functions, and the structural problem is solved in the mechanical module of Ansys, allowing the residual stresses calculation. The results show that the residual stresses mainly arise during cooling and scale with the logarithm of the Biot number. [ABSTRACT FROM AUTHOR]

This paper analyzes the effect of crosslinking reactions on a thermoset polymer's viscoelastic properties. In particular, a numerical model to predict the evolution of epoxy's mechanical properties during the curing process is proposed and implemented in an Ansys APDL environment. A linear viscoelastic behavior is assumed, and the scaling of viscoelastic properties in terms of the temperature and degree of conversion is modeled using a modified version of the TNM (Tool–Narayanaswamy–Mohynian) model. The effects of the degree of conversion and structural relaxation on epoxy's relaxation times are simultaneously examined for the first time. This formulation is based on the thermo-rheological and chemo-rheological simplicities hypothesis and can predict the evolution of epoxy's relaxation phenomena. The thermal–kinetic reactions of curing are implemented in a homemade routine written in APDL language, and the structural module of Ansys is used to predict the polymer's creep and stress relaxation curves at different temperatures and degrees of conversion. [ABSTRACT FROM AUTHOR]

The relaxation time and frequency spectra are vital for constitutive models and for insight into the viscoelastic properties of polymers, since, from the spectra, other material functions used to describe rheological properties of various polymers can be uniquely determined. In recent decades the non-integer order differential equations have attracted interest in the description of time-dependent processes concerning relaxation phenomena. The fractional Maxwell model (FMM) is probably the most known rheological model of non-integer order. However, the FMM spectrum has not yet been studied and used to describe rheological materials. Therefore, the goal of the present paper was to study the applicability of the relaxation spectrum of FMM to the description of the relaxation spectra of polymers. Based on the known integral representation of the Mittag-Leffler two-parameter function, analytical formulas describing relaxation time and frequency spectra of FMM model were derived. Monotonicity of the spectra was analyzed and asymptotic properties were established. Relaxation frequency spectrum grows for large frequencies with a positive power law, while the relaxation time spectrum decays for large times with a negative power of time. Necessary and sufficient conditions for the existence of the local extrema of the relaxation spectra were derived in the form of two trigonometric inequalities. A simple procedure for checking the existence or absence of the spectra extrema was developed. Direct analytical formulas for the local extrema, minima, and maxima are given in terms of model fractional and viscoelastic parameters. The fractional model parameters, non-integer orders of the stress and strain derivatives of FMM uniquely determine the existence of the spectrum extrema. However, the viscoelastic parameters of the FMM, elastic modulus, and relaxation time affect the maxima and minima of the relaxation spectra and the values of their local peaks. The influence of model parameters on their local extrema was examined. Next, the applicability of the continuous–discrete spectrum of FMM to describe Baumgaertel, Schausberger and Winter (BSW) and unimodal Gauss-like relaxation spectra, commonly used to describe rheological properties of various polymers, was examined. Numerical experiments have shown that by respective choice of the FMM parameters, in particular by respective choice of the orders of fractional derivatives of the stress and strain, a good fit for the relaxation modulus experiment data was obtained for polymers characterized both by BSW and Gauss-like relaxation spectra. As a result, a good approximation of the real spectra was reached. Thus, the viscoelastic relaxation spectrum of FMM, due to the availability of the two extra degrees of freedom (non-integer orders of the stress and strain derivatives), provides deep insights into the complex behavior of polymers and can be applied for a wide class of polymers with unimodal relaxation spectra. [ABSTRACT FROM AUTHOR]

This article addresses a novel anomalous relaxation model with the new general fractional derivative of the Sonine kernel. This operator is considered in the sense of general fractional derivative proposed in the work [Yang et al., General fractional derivatives with applications in viscoelasticity, Academic Press, New York, USA, 2020]. The solution of the mathematical model is obtained with the aid of Laplace transform. The comparison among the classical and anomalous relaxation models is discussed in detail. This result is proposed as a mathematical tool to model the anomalous relaxation behavior of the complex materials. [ABSTRACT FROM AUTHOR]

As a key tool in hydraulic fracturing, the packer plays a vital role in the process of unconventional oil and gas production. Its sealing performance has a significant impact on production. The packer is in high-temperature and high-pressure (HTHP) environment for a long time. It is easy to produce significant stress relaxation, which affects the sealing performance of the packer. In this paper, rubber foundation tests at 150 °C temperature were carried out. The hyperelastic–viscoelastic model of the rubber material was determined. The influence of the stress relaxation on its sealing performance was investigated. In addition, the effects of setting load, height, inner diameter, and outer diameter on the sealing performance were systematically investigated. The findings show that under HTHP, the stress relaxation phenomenon leads to a 6.9% decrease in peak shear stress, 10.3% decrease in peak von Mises stress, and 6.7% decrease in peak contact stress, reducing the risk of damage to the packing element and leading to a decrease in its sealing performance. In addition, it was found that increasing the setting load can significantly increase the contact stress, which also increases the risk of damage. When increasing the height, the peak shear stress and peak contact stress do not change much but can significantly reduce the peak von Mises stress. When increasing the inner diameter, the peak shear stress and peak contact stress show a trend of first increasing and then decreasing, and the peak von Mises stress has been decreasing. Increasing the outer diameter can better improve the force situation, increase the contact stress, and make the contact stress distribution more uniform. It is necessary to consider the influence of stress relaxation on the sealing performance of the packer under HTHP; meanwhile, this paper has guiding significance for the design of packer structures. [ABSTRACT FROM AUTHOR]

This contribution aims at developing a formulation for the large viscoelastic deformation of hyperelastic beams and frames under various loading and boundary conditions. To do so, the kinematics of deformation in two-dimensional space is formulated and basic kinematics and kinetic quantities are introduced. The quasi-linear viscoelasticity theory is employed to capture the time-dependent behavior of the underlying material. The corresponding time integration scheme and the consistent tangent moduli are then presented. Because of the highly nonlinear nature of governing equations at the large regime of deformations including time dependency, a nonlinear finite element formulation in the total Lagrangian framework is developed. Several numerical examples are provided to investigate the applicability of derived formulations. It is observed that the formulation can successfully capture the relaxation and creep phenomena in visco-hyperelastic beams and frames. [ABSTRACT FROM AUTHOR]

• Steady state description of crack propagation during DCB test with viscoelastic-viscoplastic-damage interface is achieved. • Macroscopic Gc vs da/dt master curves are obtained numerically for various adhesive layer rheological behavior. • A dual failure criterion is introduced to reproduce both toughtening and embrittlement phenomena. Adhesively bonded joints are known to suffer from creep and relaxation phenomena due to the viscous nature of the adhesive. These effects are detrimental to the durability of structural joints as they can lead to progressive and slow crack propagation rate phenomena and possible delayed failure. Master curve approaches are often used to assess the durability of structural assemblies, but the physical justification for such approaches requires additional investigation to more accurately assess the coupling between structural effects and the possible complex long-term rheological behaviour of the adhesive layer. Following similar approaches developed to study the steady-state crack propagation regime in bulk materials, an Eulerian description of crack propagation along a viscous adhesive layer during a double cantilever beam test is implemented. Taking into account viscoelastic-viscoplastic and various hardening rules, some master curves can be obtained describing the evolution of the crack propagation rate as a function of the stationary loading conditions. With such a model, the role of adhesive layer behaviour and joint geometry on crack propagation conditions can be discussed, as well as the applicability of such a master curve approach. [ABSTRACT FROM AUTHOR]

The phase-field approach has proven to be a powerful tool for the prediction of crack phenomena. When it is applied to inelastic materials, it is crucial to adequately account for the coupling between dissipative mechanisms present in the bulk and fracture. In this contribution, we propose a unified phase-field model for fracture of viscoelastic materials. The formulation is characterized by the pseudo-energy functional which consists of free energy and dissipation due to fracture. The free energy includes a contribution which is related to viscous dissipation that plays an essential role in coupling the phase-field and the viscous internal variables. The governing equations for the phase-field and the viscous internal variables are deduced in a consistent thermodynamic manner from the pseudo-energy functional. The resulting model establishes a two-way coupling between crack phase-field and relaxation mechanisms, i.e. viscous internal variables explicitly enter the evolution of phase-field and vice versa. Depending on the specific choice of the model parameters, it has flexibility in capturing the possible coupled responses, and the approaches of recently published formulations are obtained as limiting cases. By means of a numerical study of monotonically increasing load, creep and relaxation phenomena, rate-dependency of failure in viscoelastic materials is analysed and modelling assumptions of the present formulation are discussed. [ABSTRACT FROM AUTHOR]

Articular cartilage heals poorly but experiences mechanically induced damage across a broad range of loading rates and matrix integrity. Because loading rates and matrix integrity affect cartilage mechanical responses due to poroviscoelastic relaxation mechanisms, their effects on cartilage failure are important for assessing and preventing failure. This paper investigated rate- and integrity-dependent crack nucleation in cartilage from pre- to post-relaxation timescales. Rate-dependent crack nucleation and relaxation responses were obtained as a function of matrix integrity through microindentation. Total work for crack nucleation increased with decreased matrix integrity, and with decreased loading rates. Critical energy release rate of intact cartilage was estimated as 2.39 ± 1.39 to 2.48 ± 1.26 kJ m−2 in a pre-relaxation timescale. These findings showed that crack nucleation is delayed when cartilage can accommodate localized loading through poroviscoelastic relaxation mechanisms before fracture at a given loading rate and integrity state. [ABSTRACT FROM AUTHOR]

In the paper, the general decay of energy solutions for a type III thermoelastic coupled system with distributed delay is studied. The coupling is via the acoustic boundary conditions. Our result is obtained under a class of generality of the relaxation function g: R+ --> R+ satisfying the inequality W) < for all t > 0, where £ and 'ri are functions satisfying some specific properties. This work extends previous works with thermoelasticity of type III and improves earlier results in the literature. [ABSTRACT FROM AUTHOR]

The problem of large-density variations in supercooled and ambient water has been widely discussed in the past years. Recent studies have indicated the possibility of nanometer-sized density variations on the subpicosecond and picosecond time scales. The nature of fluctuating density heterogeneities remains a highly debated issue. In the present work, we address the problem of possible association of such density variations with the dynamics of terahertz longitudinal acoustic-like modes in liquid water. Our study is based on the fact that the subpicosecond dynamics of liquid water are essentially governed by the structural relaxation. Using a mode coupling theory approach, we found that for typical values of parameters of liquid water, the dynamic mechanism coming from the combination of the structural relaxation process and the finiteness of the amplitude of terahertz longitudinal acoustic-like mode gives rise to a soliton-like collective mode on a temperature-dependent nanometer length scale. The characteristics of this mode are consistent with the estimates of the amplitudes and temperature-dependent correlation lengths of density fluctuations in liquid water obtained in experiments and simulations. Thus, the fully dynamic mechanism could contribute to the formation and dynamics of fluctuating density heterogeneities. The soliton-like collective excitations suggested by our analysis may be relevant to different phenomena connected with supercooled water and can be expected to be associated with some ultrafast biological processes. [ABSTRACT FROM AUTHOR]

A quantitative theory of hydrodynamic interactions in unentangled polymer melts and concentrated solutions is presented. The study is focussed on the pre-Rouse transient time regimes (t < τR, the Rouse relaxation time) where the hydrodynamic response is governed mainly by the viscoelastic effects. It is shown that transient viscoelastic hydrodynamic interactions are not suppressed (screened) at large distances and are virtually independent of polymer molecular mass. A number of transient regimes of unusual and qualitatively different behavior of isotropic and anisotropic hydrodynamic response functions are elucidated. The regimes are characterized in terms of two main length-scale dependent characteristic times: momentum spreading time τi ∝ r4/3 and viscoelastic time τ* ∝ r4. It is shown that for t > τi the viscoelastic hydrodynamic interactions can be described in terms of the time or length scale dependent effective viscosity which, for t < τR and/or for r < Rcoil, turns out to be much lower than the macroscopic 'polymer' viscosity ηm. The theory also involves a quantitative analysis of the length-scale dependent stress relaxation in polymer melts. The general predictions for hydrodynamic interactions in thermostated systems with Langevin friction are obtained as well. [ABSTRACT FROM AUTHOR]

Plasma application for environmental improvement is desirable, and it is worthwhile to clarify the behavior of OH radicals in nonthermal plasma. Under atmospheric-pressure humid air, the time evolutions and spatial distribution of relative density and rotational temperature of OH radicals are measured in pulsed positive corona discharge using laser-induced fluorescence with a tunable optical parametric oscillator laser. The density of OH radicals generated by discharge when 28 kV is applied is estimated to be about 1×1015cm-3 at 3 μs after discharge. The OH density increases with humidity. The rotational temperature rises after discharge. The rate of temperature rise increases with humidity. This phenomenon arises from fast vibration-to-translation energy relaxation of H2O. The spatial distributions of OH rotational temperature indicate that the temperature rises in the secondary streamer channel. [ABSTRACT FROM AUTHOR]

For branched polymers, the curvilinear motion of the branch point along the backbone is a significant relaxation source but details of this motion have not been well understood. This study conducts multi-chain sliplink simulations to examine effects of the spatial fluctuation and curvilinear hopping of the branch point on the viscoelastic relaxation. The simulation is based on the primitive chain network model that allows the spatial fluctuations of sliplink and branch point and the chain sliding along the backbone according to the subchain tension, chemical potential gradients, drag force against medium, and random force. The sliplinks are created and/or disrupted through the motion of chain ends. The curvilinear hopping of the branch point along the backbone is allowed to occur when all sliplinks on a branched arm are lost. The simulations considering the fluctuation and the hopping of the branch point described well the viscoelastic data for symmetric and asymmetric star polymers with a parameter set common to the linear polymer. On the other hand, the simulations without the branch point motion predicted unreasonably slow relaxation for asymmetric star polymers. For asymmetric star polymers, further tests with and without the branch point hopping revealed that the hopping is much less important compared to the branch point fluctuation when the lengths of the short and long backbone arms are not very different and the waiting time for the branch point hopping (time for removal of all sliplinks on the short arm) is larger than the backbone relaxation time. Although this waiting time changes with the hopping condition, the above results suggest a significance of the branch point fluctuation in the actual relaxation of branch polymers. [ABSTRACT FROM AUTHOR]

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]

According to the strain energy density function for finite deformation of viscoelastic material, the relaxation function of the Maxwell mode, and the deformation gradient tensor of the bubble, a stress equation for finite deformation of a protein bubble is derived. By using the above stress equation and the dynamics equation of the bubble, an equation describing the relation between the relative deformation rate of the inner radius and time is developed for finite deformation. Based on this equation and the nonlinear properties of finite deformation, the effect of the pressure difference, the thickness and viscosity of the film, the initial size of the bubble, and the initial gas pressure in the bubble on the radial deformation of the protein bubble are analyzed by numerical simulation. The results show that during the contraction of the bubble, an oscillation with degressive amplitude is associated. The variation tendencies of vibration frequency and amplitude and the balance size of the bubble are closely related to the applied load, the viscoelasticity of the film, and the initial bubble size. Increasing the thickness and viscosity of protein film or decreasing the initial bubble size can counteract the vibration of the bubble wall and thus, can enhance the load-bearing capacity of the protein bubble. [ABSTRACT FROM AUTHOR]

Thermoviscoelastic linear-response functions are calculated from the master equation describing viscous liquid inherent dynamics. From the imaginary parts of the frequency-dependent isobaric specific heat, isothermal compressibility, and isobaric thermal expansion coefficient, we define a “linear dynamic Prigogine-Defay ratio” ΛTp(ω) with the property that if ΛTp(ω)=1 at one frequency, then ΛTp(ω) is unity at all frequencies. This happens if and only if there is a single-order-parameter description of the thermoviscoelastic linear responses via an order parameter (which may be nonexponential in time). Generalizations to other cases of thermodynamic control parameters than temperature and pressure are also presented. [ABSTRACT FROM AUTHOR]

We study the viscoelectroelastic behavior of heterogeneous piezoelectric solids, focusing on the connection between heterogeneity and coupled mechanical and electrical relaxations. Our approach is based on the existence of a correspondence between quasistatic viscoelectroelasticity and static piezoelectricity when linear constitutive response exists. We couple this correspondence principle with micromechanics models to predict the overall behavior of heterogeneous piezoelectric solids in terms of microstructural details. We devote specific attention to a class of two-phase materials consisting of a lossless piezoelectric phase embedded in a lossy (mechanically and electrically) matrix and obtain closed form expressions for the effective complex electroelastic moduli. Numerical results are presented and discussed, and qualitative agreement with experiment is observed. © 2001 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Abstract White blood cells (WBCs) are one of the critical components whose number has to be reduced before blood transfusion, failing which adverse transfusion effects may occur in patients. However, due to the extremely low concentration of residual WBCs (r-WBCs) in WBC-depleted blood, it is difficult to quantify r-WBCs accurately without using expensive and voluminous instruments. Therefore, the development of a continuous cell concentration technique is required to produce a countable number of cells from rare cells, which cannot normally be detected. In this paper, we present a viscoelastic microfluidic device for sheathless, continuous concentration of WBCs. The device performance was evaluated using polystyrene particles with different sizes at various flow rate conditions in a non-Newtonian fluid compared to a Newtonian fluid. Large particles with a blockage ratio higher than 0.1 were tightly focused at the center and collected at the center outlet with a 98% collection ratio. Meanwhile, the viscosity effect of lysed blood samples with various hematocrits was considered. Finally, diluted WBCs with various dilution ratios were concentrated by ~18-fold and continuous concentration of WBCs in lysed blood samples was performed using a non-electrical powered hand pump sprayer. Without using an external power source, center-focused WBCs were collected at the center outlet at approximately 150 μl/min and the final number of WBCs was increased to 1.8 × 104 cells/ml from undetectable levels. Graphical abstract fx1 Highlights • A viscoelastic microfluidic device for white blood cell concentration is proposed. • Size-based flow characteristics of particles were examined at different flow rates. • 10 µm particles were focused at the center and collected with a 98.7% collection ratio. • White blood cells with various dilution ratios were concentrated by 18-fold. • Using a non-electrical powered hand pump, white blood cells were concentrated to 1.8 × 104 cells/ml from undetectable levels. [ABSTRACT FROM AUTHOR]

Delayed capillary break-up of viscoelastic filaments presents scientific and technical challenges relevant for drop formation, dispensing, and adhesion in industrial and biological applications. The flow kinematics are primarily dictated by the viscoelastic stresses contributed by the polymers that are stretched and oriented in a strong extensional flow field resulting from the streamwise gradients created by the capillarity-driven squeeze flow. After an initial inertiocapillary (IC) or viscocapillary (VC) regime, where elastic effects seem to play no role, the interplay of capillarity and viscoelasticity can lead to an elastocapillary (EC) response characterized by exponentially-slow thinning of neck radius (extensional relaxation time is determined from the delay constant). Less frequently, a terminal visco-elastocapillary (TVEC) response with linear decay in radius can be observed and used for measuring terminal, steady extensional viscosity. However, both IC/VC-EC and EC-TVEC transitions are inaccessible in devices that create stretched necks by applying a step strain to a liquid bridge (e.g., capillary breakup extensional rheometer). In this study, we use dripping-onto-substrate rheometry to obtain radius evolution data for unentangled polymer solutions. We deduce that the plots of transient extensional viscosity vs. Hencky strain (scaled by the respective values at the EC-TVEC transition) emulate the functional form of the birefringence-macromolecular strain relationship based on Peterlin's theory. We quantify the duration and strain between the IC/VC-EC and the EC-TVEC transitions using measures we term elastocapillary span and elastocapillary strain increment and find both measures show values directly correlated with the corresponding variation in extensional relaxation time. [ABSTRACT FROM AUTHOR]

In the present work, the study of variation of relaxation time (τ) with viscosity of the medium (η) is carried out on four polar samples: 2-Nitroaniline, 4-Bromoaniline, 4-Chloroaniline, 4-Chlorophenol, and also on the binary mixture of 2-Nitroaniline + 4-Bromoaniline at room temperature by using microwave bench operating at a frequency of 9.59 GHz. In this regard, the different parameters like dielectric constant (--> ε ′ -->), dielectric loss (--> ε ″ -->), relaxation time (τs), macroscopic steady state viscosity (ηs), dynamic viscosity (ηd), and viscoelastic relaxation time (τve) were determined for all the systems. It is observed that the relaxation time (τs) increases with the increase in the viscosity of the medium for all the systems. Plots of log(τs) versus log(ηs) for all the systems show that variation of relaxation time is found to be nonlinear in the higher viscosity regions. This suggests the failure of Debye's theory at these regions. Further, the nonlinear behaviour of relaxation time with the viscosity is explained by using the viscoelastic model suggested by Barlow et al. (Proc. R. Soc. A 309, 473 (1969). doi:10.1098/rspa.1969.0053). It is also observed that macroscopic steady state viscosity (ηs) values are greater than the dynamic viscosity (ηd), and viscoelastic relaxation time (τve) values were found to be lower compared to the relaxation time (τs). These results suggest that the effective frictional resistance experienced by the molecules during reorientation is lower and the measured values of macroscopic steady state viscosity (ηs) are frequency dependent. [ABSTRACT FROM AUTHOR]

By analytically continuing the stress tensor evolution equation used in our previous studies of static phenomena in fluids and solving the equation, we have derived some analytic formulas for dynamic viscosities as functions of the amplitude and frequency of oscillating shear rate. They show markedly nonlinear rate dependencies different from those predicted by the linear theory. A set of approximate dynamic viscosity formulas (the first approximation) is used to derive a rule similar to the Cox–Merz rule, which is seen to be valid in the low frequency regime in the present approach. It is shown that the real and imaginary parts of the dynamic complex viscosity can be put into forms which depend on a reduced shear rate and a reduced frequency only, and therefore there exist corresponding states for the material functions. The formulas predict power laws in the high shear rate and frequency regimes which are quite reminiscent of those holding for some polymer solutions. [ABSTRACT FROM AUTHOR]

In order to reduce the transfer of sound and vibrations in structures such as timber buildings, thin elastomer layers can be embedded between their components. The influence of these elastomers on the response of the structures in the low frequency range can be determined accurately by using conforming hexahedral finite elements. Three-dimensional mesh generation, however, is yet a non-trivial task and mesh refinements which may be necessary at the junctions can cause a high computational effort. One remedy is to mesh the components independently from each other and to couple them using the mortar method. Further, the hexahedral mesh for the thin elastomer layer itself can be avoided by integrating its elastic behavior into the mortar formulation. The present paper extends this mortar formulation to take damping into account such that frequency response analyses can be performed more accurately. Finally, the proposed method is verified by numerical examples. [ABSTRACT FROM AUTHOR]

Purpose The purpose of this paper is to propose the knowledge of thermal transport of magneto hydrodynamic non-Newtonian fluid flow over a melting sheet in the presence of exponential heat source.Design/methodology/approach The group of PDE is mutated as dimension free with the assistance of similarity transformations and these are highly nonlinear and coupled. The authors solved the coupled ODE’s with the help of fourth-order Runge–Kutta based shooting technique. The impact of dimensionless sundry parameters on three usual distributions of the flow was analyzed and bestowed graphically. Along with them friction factor, heat and mass transfer rates have been assessed and represented with the aid of table.Findings Results exhibited that all the flow fields (velocity, concentration and temperature) are decreasing functions of melting parameter. Also the presence of cross-diffusion highly affects the heat and mass transfer performance.Originality/value Present paper deals with the heat and mass transfer characteristics of magnetohydrodynamics flow of non-Newtonian fluids past a melting surface. The effect of exponential heat source is also considered. Moreover this is a new work in the field of heat transfer in non-Newtonian fluid flows. [ABSTRACT FROM AUTHOR]

Abstract: The problem of propagation of acoustic waves in a liquid with gas bubbles covered by a viscoelastic shell has been considered. The dependence of the wave number on the perturbation frequency has been found. The low-frequency asymptotes for the phase velocity and the attenuation coefficient have been deduced. The dependence of the equilibrium sound velocity on the perturbation frequency and size of covered bubbles has been established and illustrated. A comparison of the theory with the known experimental data is presented. [ABSTRACT FROM AUTHOR]

Drug release from hydroxypropyl methylcellulose (HPMC) hydrophilic matrix tablets is controlled by drug diffusion through the gel layer of the matrix-forming polymer upon hydration, matrix erosion or combination of diffusion and erosion mechanisms. In this study, the relationship between viscoelastic properties of the gel layer of swollen intact matrix tablets and drug release was investigated. Two sets of quetiapine fumarate (QF) matrix tablets were prepared using the high viscosity grade HPMC K4M at low (70 mg/tablet) and high (170 mg/tablet) polymer concentrations. Viscoelastic studies using a controlled stress rheometer were performed on swollen matrices following hydration in the dissolution medium for predetermined time intervals. The gel layer of swollen tablets exhibited predominantly elastic behavior. Results from the in vitro release study showed that drug release was strongly influenced by the viscoelastic properties of the gel layer of K4M tablets, which was further corroborated by results from water uptake studies conducted on intact tablets. The results provide evidence that the viscoelastic properties of the gel layer can be exploited to guide the selection of an appropriate matrix-forming polymer, to better understand the rate of drug release from matrix tablets in vitro and to develop hydrophilic controlled-release formulations. [ABSTRACT FROM AUTHOR]

Many constitutive relationships have been derived to model the viscoelastic behaviors of materials, but limited works are done to point out which model is the most suitable one in certain conditions. In this article, we present the detailed comparisons of the classic rheological and fractional derivative models to make up for the deficiency. First, creep and stress relaxation tests of carbon black (CB)–filled rubber are carried out at the room temperature for 12 h, and frequency-sweep tests are conducted at a temperature ranging from Tg − 10°C to Tg + 100°C under tension loading. The master curve of storage modulus over a wide range of frequency covering about 21 decades is constructed according to the time/frequency–temperature superposition (TTS/fTS). And then the classic models (Maxwell, Kelvin, and their generalized models) and fractional derivative models (4, 5, 6, and 10 parameters Zener models) are used to fit the test data. The fitting results show that the classic generalized models and fractional Zener models are quantitatively equivalent. The classic generalized models with several parameters are much simpler and are able to predict the viscoelastic behaviors for short-term or narrow frequency conditions, while the fractional derivative models are suitable for wide frequency conditions. [ABSTRACT FROM AUTHOR]

ABSTRACT: In this study, we quantified the thermal and solid‐state mechanical and viscoelastic properties of isotactic polypropylene (i‐PP) homopolymer and poly(propylene‐1‐butene) copolymer films having a 1‐butene ratio of 8, 12, and 14 wt %, depending on the comonomer content. The uniaxial tensile creep and stress‐relaxation behaviors of the samples were studied in a dynamic mechanical analyzer at different temperatures. The creep behaviors of the samples were modeled with the four‐element Burger equation, and the long‐term creep strains were predicted with the time–temperature superposition method. The short‐term mechanical properties of the samples were also determined with tensile and impact testing at room temperature. We found that the Young's modulus and ultimate strength values of the samples decreased with increasing amount of 1‐butene in the copolymer structure. On the other hand, the strain at break and impact strength values of the samples improved with increasing amount of 1‐butene. Creep analysis showed that i‐PP exhibited a relatively lower creep strain than the poly(propylene‐co‐1‐butene)s at 30 °C. However, interestingly, we discovered that the temperature increase resulted in different effects on the creep behaviors. We also found that short‐chain branching improved the creep resistance of polypropylene at relatively high temperatures. © 2018 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2018, 135, 46350. [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]

Switchable ionic liquids (SILs) are novel green non-aqueous absorbents for energy-efficient post-combustion CO 2 capture. However, their regeneration efficiency and viscosity still need to be improved. SILs can be activated by some alkylol amines to improve their regeneration efficiency and reduce viscosity. In this work, mixtures of the SIL 1,8-diazabicyclo-[5.4.0]-undec-7-ene (DBU)/ethanol and 2-amino-2-methyl-1-propanol (AMP) were used for CO 2 capture. The constitution of the absorbent was optimized. Absorption and regeneration were investigated, and the detailed reaction mechanisms for CO 2 capture were analyzed using 13 C NMR and FTIR. The results demonstrated that CO 2 could be absorbed to form carbonates in two ways. On the one hand, CO 2 reacted with DBU and ethanol to directly form carbonates. On the other hand, CO 2 initially reacted with AMP to form carbamates, and then, the carbamates reacted with ethanol to form carbonates. The CO 2 loading of the optimized absorbent was 2.98 mol kg −1 . The saturated absorbent began to desorb at 80 °C, and the regeneration efficiency reached 99.3% when the temperature rose to 120 °C. After five cycles, the regeneration efficiency remained over 90%. Compared to DBU/ethanol, the regeneration efficiency of the DBU/AMP/ethanol mixtures was significantly better, while the viscosity was greatly reduced, which is beneficial for the industrial application. [ABSTRACT FROM AUTHOR]

By integrating a triboelectric nanogenerator (TENG) and a thin film dielectric elastomer actuator (DEA), the DEA can be directly powered and controlled by the output of the TENG, which demonstrates a self-powered actuation system toward various practical applications in the fields of electronic skin and soft robotics. This paper describes a method to construct a physical model for this integrated TENG-DEA system on the basis of nonequilibrium thermodynamics and electrostatics induction theory. The model can precisely simulate the influences from both the viscoelasticity and current leakage to the output performance of the TENG, which can help us to better understand the interaction between TENG and DEA devices. Accordingly, the established electric field, the deformation strain of the DEA, and the output current from the TENG are systemically analyzed by using this model. A comparison between real measurements and simulation results confirms that the proposed model can predict the dynamic response of the DEA driven by contact-electrification and can also quantitatively analyze the relaxation of the tribo-induced strain due to the leakage behavior. Hence, the proposed model in this work could serve as a guidance for optimizing the devices in the future studies. [ABSTRACT FROM AUTHOR]

Time-dependent behavior and factors affecting preload relaxation in a carbon/epoxy composite bolted joint under resonance were studied. The effect of viscoelasticity of composite material on bolt relaxation was studied quantitatively through modal analysis from the perspective of energy dissipation and stiffness degradation. Damping ratio and resonance frequency were utilized to characterize the effects of preload relaxation on structural dynamic response. The loss of preload was found to decrease with increasing initial preloads over a 10 h vibration fatigue. However, an increase in preload loss occurred as exciting frequency increases. Vibration fatigue damage was found to result in decaying stiffness and amplitude responses of the bolted joints, along with an increase in damping ratio. As a proof-of-concept study, a beam-like specimens with and without bolted joints were comparatively excited to ascertain their respective dynamic responses; results revealed that relaxation in bolted joints could be attributed to the conjunct mechanisms between viscoelastic behavior of polymer matrix composites and interface friction for different contact surfaces, where such relaxation behavior was mainly due to viscoelasticity of the joint materials. [ABSTRACT FROM AUTHOR]

We derive the full set of macroscopic equations necessary to describe the dynamics of systems with active polar order in a viscoelastic or elastic background. The active polar order is manifested by a second velocity, whose non-zero modulus is the polar order parameter and whose direction is the polar preferred direction. Viscoelasticity is described by a relaxing strain field allowing for a straightforward change from transient to permanent elasticity. Relative rotations of the elastic structure with respect to the polar direction are taken into account. The intricate coupling between active polar order and (transient) elasticity leads to a combined relaxation of the polar order parameter and the strains. The rather involved sound spectrum contains a specific excitation due to a reversible coupling between elasticity and polar order. Effects of chirality are also considered. [ABSTRACT FROM AUTHOR]

The aim of the present study was to define biomechanical parameters of the healthy human abdominal aorta, usable to develop materials for the aortic phantom production. Such phantoms used in the training of endovascular treatment must describe the same morphology and mechanical behavior properties as the patient's aorta. To accurately identify these biomechanical parameters, ex vivo experiments in uniaxial tensile and dynamic simple shear tests were performed on six human healthy abdominal aortas (6 males, between 12 and 69 years old). A solid generalized Maxwell model including Yeoh expression for the elastic part was used to describe the hyper-viscoelastic behavior of the aorta. The results obtained from uniaxial tensile tests show an exponential-like increase in stiffness, which can be described by three hyperelastic parameters ( C 1 , C 2 and C 3 ). From dynamic shear experiments, the viscous part of the global biomechanical behavior was expressed in a specific angular-frequency range (1 to 315 rad/s). Three Maxwell elements ( β 1 , β 2 , and β 3 ) put on three constant times ( τ 1 = 0.003 s , τ 2 = 0.03 s , and τ 3 = 0.3 s ) respectively, were necessary to describe it. As this relatively high number of viscoelastic parameters may be difficult to control in the development of materials, we suggest defining the viscous behavior with the global viscosity η 0 that combines the viscoelastic contributions of each Maxwell element. In conclusion, four biomechanical parameters: C 1 , C 2 , C 3 and η 0 , must be considered for the development of materials used in the aortic phantom production. [ABSTRACT FROM AUTHOR]

An effective optimization method is proposed in this paper to determine the basic characteristics of non-linear visco-elastic elements used in passive vibration reduction systems. The developed method is examined by performing experimental investigations on the exemplary vibration reduction system, i.e., the passive seat suspension. The shaping of vibro-isolation properties is analyzed for different spectral classes of excitation signals. [ABSTRACT FROM AUTHOR]

Elaborate chemical design is of utmost importance in order to slow down the relaxation dynamics in single-molecule magnets (SMMs) and hence improve their potential applications. Much interest was devoted to the study of distinct relaxation processes related to the different crystal fields of crystallographically independent lanthanide ions. However, the assignment of the relaxation processes to specific metal sites remains a challenging task. To address this challenge, a new asymmetric Dy2 SMM displaying a well-separated two-step relaxation process with the anisotropic centers in fine-tuned local environments was elaborately designed. For the first time a one-to-one relationship between the metal sites and the relaxation processes was evidenced. This work sheds light on complex multiple relaxation and may direct the rational design of lanthanide SMMs with enhanced magnetic properties. [ABSTRACT FROM AUTHOR]

To analyze the stresses and strains generated during the solidification of glass-forming materials, stress and volume relaxation must be predicted accurately. Although the modeling attributes required to depict physical aging in organic glassy thermosets strongly resemble the structural relaxation in inorganic glasses, the historical modeling approaches have been distinctly different. To determine whether a common constitutive framework can be applied to both classes of materials, the nonlinear viscoelastic simplified potential energy clock (SPEC) model, developed originally for glassy thermosets, was calibrated for the Schott 8061 inorganic glass and used to analyze a number of tests. A practical methodology for material characterization and model calibration is discussed, and the structural relaxation mechanism is interpreted in the context of SPEC model constitutive equations. SPEC predictions compared to inorganic glass data collected from thermal strain measurements and creep tests demonstrate the ability to achieve engineering accuracy and make the SPEC model feasible for engineering applications involving a much broader class of glassy materials. [ABSTRACT FROM AUTHOR]

ABSTRACT Electrospun thermoplastic polyurethane (TPU) nanofibers are known to contract considerably (∼40%) on heating up to ∼90 °C. This study investigates this thermomechanical behavior and the TPU shape memory capabilities. The shape memory effect was first studied in TPU films as a model system by applying classical thermomechanical cycles (programming and recovery). The films were able to fix the applied deformation during long-term storage at room temperature, well above the material's calorimetric glass transition temperature and in the absence of a percolated structure of hard domains. Structural analysis (Fourier transform infrared, differential scanning calorimeter, and dynamic mechanical analysis) revealed broad thermal transitions indicating the presence of a mixed phase of hard segments dispersed in the soft segment matrix. Using a linear viscoelastic model together with time-temperature superposition, the shape memory effect was attributed to the thermoviscoelastic properties of TPU. In particular, the mixed phase was found to give rise to a very broad relaxation spectrum dominated by long relaxation times, which explains the suppression of strain recovery at room temperature. Finally, the electrospinning process was examined and was found to be similar to a programming cycle characterized by the strong elongation flow accompanied by massive solvent evaporation, whereas the contraction effect was interpreted as the recovery phase in a shape memory perspective. Thus, the contraction of electrospun TPU mats may be considered to be an electrospinning-induced shape memory effect. © 2015 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2015, 53, 1590-1602 [ABSTRACT FROM AUTHOR]

The morphological, dynamic and rheological characteristics in the viscoelastic phase separation (VPS) of sheared polymer solutions are investigated by three-dimensional (3D) numerical simulations of viscoelastic model. The simulations are accelerated by graphic process unit (GPU) to break through the limitation of computation power. Firstly, the morphological and dynamic characteristics of VPS under shear are presented by comparing with those in classic phase separation (CPS). The results show that the phase inversion and phase shrink take place in VPS under shear. Then, the roles of bulk and shear relaxation moduli in VPS are investigated in details. The bulk relaxation modulus slows down the phase separation process under shear, but not affects the dynamic path of VPS. The dynamic path can be divided into three stages: freezing stage, growth stage and stable stage. The second overshoot phenomenon in the shear stress is observed, and explained by the breakdown and reform of string structures. The shear modulus affects morphology evolution in the late stage of VPS under shear. [ABSTRACT FROM AUTHOR]

Structural relaxations in humid silk fibers exposed to tensile stress have been reported to take place on a very wide range of time scales from a few milliseconds to several hours. The time-dependence of the measured tensile force following a quasi-instantaneously applied external strain on the fibers can be understood in terms of a fractional viscoelastic relaxation function introducing memory effects by which the mechanical state of a fiber depends on its tensile history. An analog fractional relaxation also gives rise to the subdiffusion observed on picosecond time scales, which governs the mobility of the amorphous polymer chains and adsorbed water on the molecular level. The reduction of the subdiffusive memory effect in stretched fibers compared to native fibers is consistent with the higher order of the polymers in the stretched state. [ABSTRACT FROM AUTHOR]

We demonstrate an optopneumatic piston based on glass capillaries, a mixture of PDMS–carbon nanopowder, silicone and mineral oil. The fabrication method is based on wire coating techniques and surface tension-driven instabilities, and allows for the assembly of several pistons from a single batch production. By coupling the photothermal response of the PDMS–carbon mixture with optical excitation via an optical fiber, we demonstrate that the piston can work either as a valve or as a reciprocal actuator. The death volume of the pistons was between 0.02 and 1.56 μL and the maximum working frequency was around 1 Hz. Analysis of the motion during the expansion/contraction of the piston shows that this machine can be described by a phenomenological equation analogous to the Kelvin–Voight model used in viscoelasticity, having elastic and viscous components. [ABSTRACT FROM AUTHOR]

Using a picosecond pump-probe ultrasonic technique, we study the propagation of high-amplitude, laser-generated longitudinal coherent acoustic pulses in the viscoelastic fragile glass former DC704. We observe an increase of almost 10% in acoustic pulse propagation speed at the highest optical pump fluence which is a result of the supersonic nature of nonlinear propagation in the viscous medium. From our measurement, we deduce the nonlinear acoustic parameter of the glass former in the gigahertz frequency range across the glass transition temperature. [ABSTRACT FROM AUTHOR]

Characterizing the mechanical response of the human amnion is essential to understand and to eventually prevent premature rupture of fetal membranes. In this study, a large set of macroscopic and microscopic mechanical tests have been carried out on fresh unfixed amnion to gain insight into the time-dependent material response and the underlying mechanisms. Creep and relaxation responses of amnion were characterized in macroscopic uniaxial tension, biaxial tension and inflation configurations. For the first time, these experiments were complemented by microstructural information from nonlinear laser scanning microscopy performed during in situ uniaxial relaxation tests. The amnion showed large tension reduction during relaxation and small inelastic strain accumulation in creep. The short-term relaxation response was related to a concomitant in-plane and out-of-plane contraction, and was dependent on the testing configuration. The microscopic investigation revealed a large volume reduction at the beginning, but no change of volume was measured long-term during relaxation. Tension–strain curves normalized with respect to the maximum strain were highly repeatable in all configurations and allowed the quantification of corresponding characteristic parameters. The present data indicate that dissipative behavior of human amnion is related to two mechanisms: (i) volume reduction due to water outflow (up to ∼20 s) and (ii) long-term dissipative behavior without macroscopic deformation and no systematic global reorientation of collagen fibers. [ABSTRACT FROM AUTHOR]

A jet of a Newtonian liquid impacting on a wall at right angle spreads as a thin liquid sheet which preserves the radial symmetry of the jet. We report that for a viscoelastic jet (solution of polyethylene glycol in water) this symmetry can break; close to the wall, the jet cross section becomes faceted and radial steady liquid films (wings) form, which connect the cross-section vertices to the sheet. The number of wings increases with increasing the viscoelastic relaxation time of the solution, but also with increasing jet velocity and decreasing distance from the jet nozzle to the wall. We propose a mechanism for this surprising destabilization of the jet shape, which develops perpendicularly to the direction expected for a buckling mechanism, and explain these dependencies. We also discuss the large-scale consequences of the jet destabilization on the sheet spreading and fragmentation, which show through the faceting of hydraulic jumps and of suspended (Savart) sheets. [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]