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We show that the detachment of a flat punch from a viscoelastic substrate has a relatively simple behaviour, framed between the Kendall's elastic solution at the relaxed modulus and at the instantaneous modulus, and the cohesive strength limit. We find hardly any dependence of the pull-off force on the details of the loading process, including maximum indentation at preload and loading rate, resulting much simpler than the case of a spherical punch. Pull-off force peaks at the highest speeds of unloading, when energy dissipation is negligible, which seems to be in contrast with what suggested by the theories originated by de Gennes of viscoelastic semi-infinite crack propagation which associated enhanced work of adhesion to dissipation. [ABSTRACT FROM AUTHOR]

The article touches upon an unsteady rectilinear motion of a submarine in a liquid under an ice cover. The ice cover is modeled as a viscoelastic plate. The viscoelastic properties of the ice are described using the Kelvin–Voigt model. The fluid is assumed to be inviscid and incompressible, and its motion is potential. Free surface fluid flow past system of one source and several sinks is used to simulate the motion of Joubert submarine. The solution of this problem is constructed analytically using the Fourier and Laplace transforms. Numerical results for the wave resistance and lift force acting on Joubert submarine are presented for different ice thicknesses, length-to-diameter ratio of a submarine, and the speed of the uniform motion. It is demonstrated that the use of a viscoelastic model for an ice cover results in a significant decrease in the maximum values of wave resistance and lift coefficients compared to the scenario of using an elastic plate model. The results indicate that when a submarine moves at realistic speeds (Fr < 0.7) under a thick ice cover (thicker than a meter), the wave resistance is less than for the same submarine moving under a free surface. The lift force for moving at these speeds under a thick ice cover is directed upwards. [ABSTRACT FROM AUTHOR]

Fabrication of polyethylene (PE) pipe with significantly enhanced hoop strength and burst pressure is essential necessary for high transportation requirements. In this work, biaxially oriented PE/ultrahigh molecular weight polyethylene (PE/UHMWPE) blend pipes were fabricated on self-made pipe solid-phase die drawing apparatus. Due to formation of chain entanglement network and enhanced viscoelasticity, incorporation of UHMWPE chains improved stretchability and biaxial orientation degree of blend pipe. During die drawing, formation of more miscible blend system make long UHMWPE chains be sufficiently stretched under load, and such extended-chain conformation of UHMWPE acted as shish structure and induced short PE chains to array orderly and form kebab crystals. With increasing axial draw ratio, the lamellae remarkably deviated from hoop to axial direction, while the blend pipe exhibited higher crystallinity and orientation degree than that of PE pipe, presenting a more uniform and dense biaxial orientation structure with shish-kebab crystals. As a result, the blend pipe showed roughly 99% increase in tensile strength along hoop direction and 25% increase in burst pressure, respectively. Such biaxial self-reinforced PE/UHMWPE blend pipes exhibit promising application prospect as high-pressure transportation pipelines in civil engineering. [ABSTRACT FROM AUTHOR]

In computational models of microchannel flows, the Helmholtz–Smoluchowski slip velocity boundary condition is often used because it approximates the motion of the electric double layer without resolving the charge density profiles close to the walls while drastically reducing the computational effort needed for the flow model to be solved. Despite working well for straight channel flow of Newtonian fluids, the approximation does not work well for flow involving complex fluids and spatially varying surface potential distribution. To treat these effects using the slip velocity boundary condition, it is necessary to understand how the surface potential and fluid properties affect the slip velocity. The present analysis shows the existence of a modified electroosmotic slip velocity for viscoelastic fluids, which is strongly dependent upon Deborah number and viscosity ratio, and this modification differs significantly from the slip velocity of Newtonian fluids. An augmentation of fluid elasticity results in an asymmetric distribution of slip velocity. Nonintuitively, the modulation wavelength of the imposed surface potential contributes to changing the slip velocity magnitude and adding periodicity to the solution. The proposed electroosmotic slip velocity for viscoelastic fluid can be used in computational models of microchannel flows to approximate the motion of the electric double layer without resolving the charge density profiles close to the walls. [ABSTRACT FROM AUTHOR]

This paper is a continuation of two recent publications on crack growth in viscoelastic media. It provides further theoretical results for large strains that enable prediction of crack opening displacement for comparison with experimental data in the region of the singularity. In order to achieve good agreement with experiment it was necessary to account for far-field viscoelasticity. Additionally, it is found that with large deformation throughout the singularity, the deformation consists of simple shearing and stretching normal to the crack plane. Thus, there is no significant displacement parallel to the crack plane; such simplicity exists for materials that stiffen or soften at high strains if the stress obeys a power law in strain at high strains. This finding means that, despite the frame-dependence of the theory, there is no local rotation in the singularity to affect the stress. [ABSTRACT FROM AUTHOR]

The rheology of sputum is viewed as a powerful emerging biophysical marker for monitoring muco-obstructive pulmonary diseases such as cystic fibrosis (CF) and non-CF bronchiectasis (NCFB). However, there is no unified practice to process sputa from collection to analysis, which can lead to highly variable, and sometimes inconsistent results. The main objective of this study is to bring light into the handling of sputum samples to establish a standardised and robust protocol before rheological measurements. Sputum collected from 22 CF and 10 NCFB adults, was divided into control (vortexed and fresh: non-heated and non-frozen) and three treated conditions (either non-vortexed, heated or frozen). In addition, 6 CF expectorations were used to study the dynamics of ageing over 24 h. Sputum's mechanical properties were measured with a rotational rheometer to obtain their properties at rest, elastic ( G ′ ) and viscous moduli ( G ′ ′ ), and at the onset of flow, critical deformation ( γ c ) and critical stress ( σ c ). We demonstrate that heating sputum is completely destructive while freezing sputa at - 80 ∘ C has no discernible effect on their rheology. We also show that the variability of rheological measurements largely resulted from the sample's macroscopic heterogeneity, and can be greatly reduced by non-destructive vortex homogenisation. Finally, we observed contrasted ageing effects as a fonction of purulence: while the viscoelasticity of purulent samples reduced by half within 6 h after collection, semi-purulent samples did not evolve. These results guide towards a robust unified protocol for simple sputum handling in rheometry. We therefore suggest to vortex and snap freeze sputum samples immediately after collection when direct testing is not possible. [ABSTRACT FROM AUTHOR]

A ternary blend of poly(L-lactic acid) (PLA), poly(ε-caprolactone) (PCL) and poly(vinyl acetate) (PVAc) was melt compounded with an aim to obtain novel biodegradable blends with a balance of performance. We systematically studied the relationship between the microstructure and macroscopic properties of the ternary blends. PVAc was miscible with both PLA an PCL. It was interesting that PVAc was selectively located in the PLA matrix of PLA/PCL/PVAc ternary blends, which significantly affected the crystallization behavior, mechanical and rheological properties, and enzymatic hydrolysis rate of the blends. The inhibited crystallization of PLA was achieved, whereas the crystallization of PCL was not affected by incorporating PVAc. Unexpectedly, increases of 10%, 17% and 26% were achieved in the yield strength, breaking strength and modulus of the ternary blend with 20 wt% PVAc compared to PLA/PCL binary blend, while the elongation at break maintained above 300%. Moreover, the rheological properties was enhanced and enzymatic hydrolysis rate was decelerated by introduction of PVAc. The excellent stiffness − toughness balance coupled with the enhanced melt viscoelasticity of the blends will help to expand the property range and processing methods of biodegradable polymers. [ABSTRACT FROM AUTHOR]

Intra-articular viscoelastic supplements are commonly administered by musculoskeletal radiologists for the treatment of symptomatic osteoarthritis (OA). This article provides an overview of the putative mechanism of action of the agents, a brief review of the evidence base underlying the practice, a commentary on some of the major society guidelines regarding the treatment, and a description of the adverse events that are associated with intra-articular hyaluronic acid administration. [ABSTRACT FROM AUTHOR]

The prime thrust of this proposed mathematical model is to investigate on Love type wave propagation in a visco-porous piezoelectric waveguide with complex fluid loaded on its surface, with loosely bonded common interfaces. To describe the fluid viscoelasticity the Maxwell and Kelvin–Voigt equations are incorporated, with the aid of varying Deborah number. Solving the equation of motion for different media, namely, complex fluid layer, visco-porous piezoelectric waveguide layer, and porous half-space, a direct Sturm–Liouville problem has been formulated, which is a salient feature of this analysis. Employing suitable boundary conditions an accurate generalized complex dispersion equation is derived. To validate the results, comparison has been done with a pre-established study as a particular case, and a strong agreement is revealed. Effect of several parameters viz. fluid viscosity (glycerol concentration), fluid layer thickness, waveguide layer thickness, and bonding parameters, on the phase velocity and attenuation of Love type wave has been delineated by means of graphs. The effect of glycerol concentration and thickness of the complex fluid layer is among the key outcome of the study. It is established that Kelvin–Voigt fluid model is most appropriate when the glycerol concentration is high, as compared to other models, in the case of rapid simulation, while the Newtonian fluid model may be preferred for problems involving varying thickness of the medium. The findings of this study may be utilized in the optimal design and development of viscosity sensors Love wave fluid sensing devices. [ABSTRACT FROM AUTHOR]

There is a growing need to develop accurate nonlinear viscoelastic material models and tools that could describe the complex nonlinear behavior of being developed composite material systems at their simulations at high loads and temperatures or to move the industry towards more sustainable bio-based alternatives. Developing such models in 3D formulation for materials with changing material state due to physical aging, change of crystallinity or degree of cure is also imperative since the loading is mostly multiaxial in real-live applications and manufacturing processes. The present paper contains the derivation of two material models with varying complexity and accuracy. The incremental procedure for implementation of the nonlinear viscoelastic material model within the numerical analysis was presented. Two relatively simple simulations with the incremental methodology developed were performed, namely triaxial mechanical loading and thermal stress development during the manufacturing process. The results obtained showed significant differences between stresses calculated using 1D and 3D simulations. Significantly higher stresses obtained in 3D simulations demonstrated the necessity of 3D models. [ABSTRACT FROM AUTHOR]

A fracture energy-based constitutive model of concrete in the framework of continuum damage mechanics is formulated. Elastic, viscoelastic, and viscoplastic mechanisms are defined in a fictitious undamaged material state, the so-called effective configuration. A linear spring and a linear dashpot characterize the viscoelastic response of concrete. The viscoplastic behavior is also described using a linear spring, a nonlinear dashpot, and a slider with constant frictional resistance. The nonlinear dashpot of the viscoplastic body is formulated using a logarithmic function so that the model can reproduce valid strength magnifications under a wide range of strain rates. As a result, a consistency viscoplastic approach is obtained wherein, in contrast to the so-called overstress viscoplastic laws, the rate effects are induced in the yield surface of the model. A fracture energy-based regularization is employed to adjust the rate of damage growth to obtain mesh-objective results. The directional degradation of concrete is also characterized by a frame-independent tensorial description of damage. Next, a fully implicit return-mapping algorithm based on the Newton–Raphson scheme is proposed. The presented model is then assessed by validating its results with a series of experimental tests. In addition, the mixed-mode fracture of concrete is investigated under different strain rates, verifying the experimentally observed transition of the failure mode from a ductile flexural to a brittle diagonal failure. [ABSTRACT FROM AUTHOR]

Magnetorheological finishing (MRF) is an important technique to achieve the surface precision of difficult-to-cut materials. In this paper, a wheel-type vibration-magnetorheological compound finishing is proposed in terms of reducing the unidirectional scratch caused by the wheel-type magnetorheological finishing tool and further improving the convergence rate of surface roughness. The vibration-magnetorheological coupling was realized through utilizing designed magnetorheological finishing (MRF) wheel and a nonresonant vibrational device (NRVD). Through the theoretical and experimental analysis, the surface roughness has been verified improved through increasing the normal and tangential forces, which are associated with introducing 2D vibration. The flow and viscoelastic models of the MRP fluid were established based on hydrodynamic lubrication and viscoelasticity theories. Finally, the feasibility of the proposed finishing method was verified by the results of improving surface roughness through designing reasonable processing experiment. [ABSTRACT FROM AUTHOR]

We present a complete characterization of the (uniform) exponential stabilization for a class of viscoelastic models under small delay perturbations. The main ingredient under consideration is the notion of admissible kernels. While in the standard literature it is mostly common to request a exponential/general kernel as a sufficient condition for the exponential/general stability of the whole viscoelastic system under study, here our objective is to employ the much more general concept of admissible kernels and prove that it is not only sufficient but also a necessary assumption for exponential stability in linear viscoelasticity under small delay perturbations. [ABSTRACT FROM AUTHOR]

Abstract—This paper studies instability of a heavy inclusion in the Earth's upper layers by the linear theory method for small perturbations. The existence of such inclusions with increased density is associated with chemical inhomogeneity or phase transitions. The viscoelasticity of the geomaterial is described by the Maxwell rheological model. Two layouts of the inclusion with increased density are considered. The heavy inclusion in the cold upper elastic layer of the crust does not change its location under small perturbations, i.e., it is stable according to the linear theory. The heavy inclusion which is located in the hot viscous crustal layer underlying the upper cold layer, is unstable (slowly sinking into the underlying viscous mantle layers). [ABSTRACT FROM AUTHOR]

Histotripsy is a technique that uses sound waves to create cavitation microbubbles which are used in the liquefaction of tumor tissue cells. High-intensity ultrasonic waves perform the production of small vacuum microbubbles or microcavities in the liquid during the low-pressure cycle. In this work, we present the theoretical study and mathematical modeling of microbubble dynamics in histotripsy, based on the modified Keller–Miksis equation in viscoelastic biotissues. The neo-Hookean model of viscoelastic tissue (i.e., viscosity and Young modulus) is used in the formulation of the proposed study. Effect of variation in viscosity, tissue elasticity modulus and the polytropic exponent is taken into account which is applicable for the case of the rapid evolution of bubble size during histotripsy. The model is analytically solved by the modified Plesset–Zwick method, based on the initial and boundary conditions of microbubble. The given results suggest that our proposed strategy to determine the radius of microbubble cavitation with the different viscoelasticity effects through the process of microbubble dynamics plays a highly significant role on histotripsy in hydrogel. Finally, these findings enhance our comprehension of how histotripsy affects tissue with various mechanical qualities and offer a solid foundation for investigating the physical parameters in tissue fractionation. [ABSTRACT FROM AUTHOR]

The explicit formulas for calculating the components of the effective tensor of relaxation kernels of a two-phase layered medium with a periodic microstructure are derived. The first phase of such a medium consists of an isotropic viscoelastic material and the second phase consists of a viscous incompressible fluid. In particular, it is established that the components of this tensor depend on the volume fraction of the fluid inside a periodicity cell and do not depend on the number of layers and the distances between them. [ABSTRACT FROM AUTHOR]

We investigate the impact of viscoelastic tidal deformation of the Moon on the motion of a polar orbiter. The dissipative effects in the Moon's interior, i.e., tidal phase lags, are modeled as Fourier series sampled at given frequencies associated with linear combinations of Delaunay arguments, the fundamental parameters describing the lunar motion around the Earth and the Sun. We implement the tidal model to evaluate the temporal lunar gravity field and the induced perturbation on the orbiter. We validate the numerical scheme via a frequency analysis of the perturbed orbital motion. We show that, in the case of the Lunar Reconnaissance Orbiter at a low altitude of less than 200 km, the main lunar tides and hence the potential Love numbers around the monthly and some multiple frequencies are dynamically separable. The omission of those effects in practice introduces a position error at the level of a few decimeters within 10 days. [ABSTRACT FROM AUTHOR]

The problems of prediction of relaxation and deformation conditions of performance of polymeric textile materials were dealt with. Mathematical models of processes of relaxation and deformation of the studied materials are based on spectral-temporal theory of viscoelasticity. Practical use of methods of fiducial prediction of these processes of polymeric nonwoven materials is simplified by their digitization. [ABSTRACT FROM AUTHOR]

Issues with evaluation of the degree of reliability of the mathematical modeling and prediction of viscoelastic and plastic processes of polymeric textile materials based on integral adequacy criteria of mathematical modeling of the viscoelasticity of these materials are examined. The criteria were an evaluation of deviations of integral convolutions of polymeric textile relaxation and creep nuclei from the theoretically based unit value. The less the integral convolutions differed from the unit value, the greater the adequacy of the developed mathematical models of relaxation and creep were, meaning that the numerical prediction of the viscoelastic and plastic processes of the studied polymeric textiles were more accurate. [ABSTRACT FROM AUTHOR]

Nanocomposite hydrogels were elaborated by the addition of citrated magnetic nanoparticles (MNPs) in sodium alginate aqueous solutions ionically crosslinked by in situ release of calcium ions from calcium carbonate (CaCO3) with gradual hydrolysis of d-glucono-δ lactone (GDL). The sol-gel transition was studied by time-resolved mechanical spectroscopy (TRMS) in the linear viscoelastic region. The power law frequency dependence of the storage and loss moduli allowed to determine the gelation time (tg), the power law relaxation exponent (Δ), and the gel stiffness (S) at the critical gel (gel at tg) for different calcium and MNP concentrations. The effect of an applied magnetic field on these parameters was also studied for the first time. The obtained results show an effect of the concentration of both calcium and MNPs on the kinetics (tg) and properties at the critical gel (S and Δ) obtaining faster kinetics and harder critical gels for higher calcium and lower MNP concentrations. Moreover, the application of the magnetic field allows to modulate the viscoelastic properties before the gel point, but no effect was observed on the structural properties of the critical gel. Finally, this work highlights how the shear viscoelastic, compressive, and swelling properties of totally gelled nanocomposite hydrogels can be successfully modulated when MNPs are introduced in the calcium alginate matrices with a good agreement between all these properties and with the properties of the critical gels. [ABSTRACT FROM AUTHOR]

In this paper, the propagation of a Rayleigh-type wave is explored in a half-space of an incompressible nematic elastomer with a uniform director aligned orthogonal to the surface. The nematic elastomer is idealized so as to fit within the framework of linear viscoelasticity theory. The governing equations of nematic elastomers are subjected to the Tiersten-type impedance boundary conditions. An explicit secular equation of the Rayleigh wave is obtained which depends upon the non-dimensional anisotropy parameter, impedance parameters, frequency, rubber relaxation time, director rotation times, and the dynamic soft elasticity of nematic elastomers. The numerical computations of the Rayleigh wave speed are restricted for the case of ideal nematic rubbers. The Rayleigh wave speed is illustrated graphically to observe the effects of non-dimensional anisotropy parameter, frequency, impedance parameters, rubber relaxation time, and director rotation times. [ABSTRACT FROM AUTHOR]

The study presents the experimental results of measurements of the relaxation time of cryogels formed from various colloid-dispersed systems based on a poly(vinyl alcohol) aqueous solution. It is shown that the value of relaxation time, which characterizes the relationship between the viscosity and elasticity of real bodies, describes the rheological properties of materials more objectively than the traditional assessment of their physical and mechanical characteristics only by the elasticity modulus of the samples. [ABSTRACT FROM AUTHOR]

Morphogenesis, tissue regeneration, and cancer invasion involve transitions in tissue morphology. These transitions, caused by collective cell migration (CCM), have been interpreted as active wetting/de-wetting transitions. This phenomenon is considered based on a model system as wetting of a cell aggregate on a rigid substrate, which includes cell aggregate movement and isotropic/anisotropic spreading of a cell monolayer around the aggregate depending on the substrate rigidity and aggregate size. This model system accounts for the transition between 3D epithelial aggregate and 2D cell monolayer as a product of: (1) tissue surface tension, (2) surface tension of substrate matrix, (3) cell–matrix interfacial tension, (4) interfacial tension gradient, (5) viscoelasticity caused by CCM, and (6) viscoelasticity of substrate matrix. These physical parameters depend on the cell contractility and state of cell–cell and cell–matrix adhesion contacts, as well as the stretching/compression of cellular systems caused by CCM. Despite extensive research devoted to study cell wetting, we still do not understand the interplay among these physical parameters which induces an oscillatory trend of cell rearrangement. This review focuses on these physical parameters in governing the cell rearrangement in the context of epithelial aggregate wetting/de-wetting, and on modeling approaches aimed at reproducing and understanding these biological systems. In this context, we not only review previously published biophysical models for cell rearrangement caused by CCM, but also propose new extensions of those models to point out the interrelation between cell–matrix interfacial tension and epithelial viscoelasticity and the role of the interfacial tension gradient in cell spreading. [ABSTRACT FROM AUTHOR]

In this paper, shaped fibers which are especially useful for radiative cooling have been firstly deposited by Electrohydrodynamic direct-writing based on Micro-Weissenberg effect (MWE). At first, The process of pumping viscoelastic fluids (polyethylene oxide) in micro-tube (non-circle nozzle) based on Micro-Weissenberg effect has been studied to reveal the mechanism of Micro-Weissenberg effect. We have found that the pumping based on MWE have obvious periodic fluctuation when the flow rate is small. Then the process of steel pins without small diameter pipes pumping viscoelastic fluid which is a continuous, less wall and controllable pumping manner have been studied in detail. It is found that the pumping based on MWE mainly stem from an interesting capillarity between two pins and the viscoelasticity of fluid. So compare with the traditional Electrohydrodynamic direct-writing, the shaped fibers have been more easily deposited by Electrohydrodynamic direct-writing based on MWE due to the capillarity and the shear thinning in Micro-Weissenberg effect. [ABSTRACT FROM AUTHOR]

To improve the production yield rate, reliability is one of the important indicators of electronic packaging products. In past research, however, the influence of the fabrication process was rarely taken into consideration. In this thesis, mold flow analysis software Moldex3D is used to develop a series analysis procedure for IC package products. The effects of many factors, including process, structure, and materials were being taken into account. Especially for epoxy molding compound, namely EMC, is studied on its properties during the molding and post-mold cure (PMC) processes. This paper adopted P–V-T-C equations, which consider both volume shrinkage due to thermal mismatch and chemical shrinkage to predict the amount of warpage and residual stresses after the mold filling process. Next, dual shift factor model for viscoelastic analysis was used to model the PMC process and predict the amount of warpage and residual stresses after PMC. And the influence of different PMC process conditions and loading conditions on the warpage results is discussed. The residual stresses after PMC simulation are set as the initial conditions for reliability analysis and then the stress distribution after two thermal cycles is analyzed. It is observed that the deformed shape of the simulation and experiment results after PMC were consistent. Both are concave downwards. In comparison with experiment results, the error of warpage simulation results was between 10% and 50%. The biggest error was found in the short direction. During two thermal cycles, it is can be found that the maximum stress of the lead frame is 505.7 MPa and the location of the possible failure is at the top left of the die. In addition, when considering or not considering the process-induced residual stress in the thermal cycle analysis, the stress states are very different. [ABSTRACT FROM AUTHOR]

This study investigates the size-dependent wave propagation behaviors under the thermoelectric loads of porous functionally graded piezoelectric (FGP) nanoplates deposited in a viscoelastic foundation. It is assumed that (i) the material parameters of the nanoplates obey a power-law variation in thickness and (ii) the uniform porosity exists in the nanoplates. The combined effects of viscoelasticity and shear deformation are considered by using the Kelvin-Voigt viscoelastic model and the refined higher-order shear deformation theory. The scale effects of the nanoplates are captured by employing nonlocal strain gradient theory (NSGT). The motion equations are calculated in accordance with Hamilton's principle. Finally, the dispersion characteristics of the nanoplates are numerically determined by using a harmonic solution. The results indicate that the nonlocal parameters (NLPs) and length scale parameters (LSPs) have exactly the opposite effects on the wave frequency. In addition, it is found that the effect of porosity volume fractions (PVFs) on the wave frequency depends on the gradient indices and damping coefficients. When these two values are small, the wave frequency increases with the volume fraction. By contrast, at larger gradient index and damping coefficient values, the wave frequency decreases as the volume fraction increases. [ABSTRACT FROM AUTHOR]

Due to the increasing use of passive absorbers to control unwanted vibrations, many studies have been done on energy absorbers ideally, but the lack of studies of real environmental conditions on these absorbers is felt. The present work investigates the effect of viscoelasticity on the stability and bifurcations of a system attached to a nonlinear energy sink (NES). In this paper, the Burgers model is assumed for the viscoelasticity in an NES, and a linear oscillator system is considered for investigating the instabilities and bifurcations. The equations of motion of the coupled system are solved by using the harmonic balance and pseudo-arc-length continuation methods. The results show that the viscoelasticity affects the frequency intervals of the Hopf and saddle-node branches, and by increasing the stiffness parameters of the viscoelasticity, the conditions of these branches occur in larger ranges of the external force amplitudes, and also reduce the frequency range of the branches. In addition, increasing the viscoelastic damping parameter has the potential to completely eliminate the instability of the system and gradually reduce the amplitude of the jump phenomenon. [ABSTRACT FROM AUTHOR]

PTFE and filled PTFE are used in sealing applications such as rotary valves, quarter-turn ball valves and pipeline gaskets, among others. Many times, these seals are designed to operate below their yield point such that plastic effects are negligible and viscoelastic effects dominate. This may lead to leakage since these devices are placed in service environments that are subjected to vibration. In seals made of viscoelastic materials, this vibration can lead to separation of the seal faces due to the delayed recovery of the material. In this paper, the isothermal viscoelastic dynamic response of a PTFE seal subjected to a harmonic input and static preload from an ideally rigid opposing face is examined. The model is a hybrid that combines the Golla–Hughes–McTavish finite element approach, a delayed recovery creep model and a penalty method contact model. Parameters for the models are taken from experimental data using a Dynamic Mechanical Analyzer test for pure PTFE material. Results for a simple application show face separation magnitude as a function of frequency and harmonic displacement amplitude. Results show that face separation occurs in PTFE seals even for small amplitude harmonic vibrations and that this is due to delayed recovery at low frequencies and to the viscoelastic damping at higher frequencies. A threshold of static preload is also found above which there is no separation and a leakage estimation is presented. [ABSTRACT FROM AUTHOR]

We investigate indentation of a linear viscoelastic half-space with periodically distributed heterogeneities by a smooth, rigid, axisymmetric indenter of an arbitrary profile. For this, we first show how such materials may be homogenized directly in the time-domain to yield a homogeneous,anisotropic, linear viscoelastic medium. For the layered half-space that we then focus upon homogenization leads to a transversely isotropic, homogeneous, viscoelastic medium. We then replace the layered half-space in the original indentation problem by its homogenized counterpart. For axisymmetric indenters, for loadings in which the contact area is non-decreasing, we solve this indentation problem analytically by extending the correspondence principle of Graham (in Q Appl Math 26:167–174, 1968. https://doi.org/10.1090/qam/99860). Finally, we compare the results of homogenization with direct finite element computations of both displacement and force controlled indentation tests. We find that the contact pressure, the depth of penetration and the displacement of the top surface of the layered half-space are well approximated by those obtained through homogenization, and the approximation improves as the layer thickness is reduced, which is precisely when FE computations become harder. The latter aspect makes the development of the present approach useful. [ABSTRACT FROM AUTHOR]

Ink transfer is achieved by the squeezing and rotation of two rollers, one of which is a roller coated with a viscoelastic rubber layer. In this study, the force caused by ink hydrodynamic pressure on the rubber roller during the rotation is considered, the viscoelasticity of ink and rubber is equivalently substituted by spring-damping unit, and a rolling squeezing theoretical model of two rollers is established to study the node deformation of the rubber roller. The validity of the theoretical model is verified by experiments. Results show that the deformation of the rubber roller is greatly affected by the roller rotational speed, rubber layer thickness, and nip width. The rotational speed and nip width have a significant influence on the stiffness-damping of the ink layer and the ink force, and the rubber layer thickness has a remarkable effect on the squeezing deformation between the rollers. [ABSTRACT FROM AUTHOR]

In this article, we consider a viscoelastic plate equation with past history, nonlinear damping, and logarithmic nonlinearity. We prove explicit and general decay rate results of the solution to the viscoelastic plate equation with past history. Convex properties, logarithmic inequalities, and generalized Young's inequality are mainly used to prove the decay estimate. [ABSTRACT FROM AUTHOR]

The ensuing pilot investigation sheds new light on characterizing tumoral and non-tumoral human skin mechanical properties that will not only assist the dermatologist's diagnosis but also could constitute the creation of an Artificial Intelligence database for upcoming research. A modern, non-invasive, and contact-free methodology—UNDERSKIN—was developed, and hinges upon Fourier transform computations that permit the analysis of surface wave dispersion with a specific skin inversion model and viscoelastic model. It yields a detailed look at how particle movements of the medium propagate throughout its near sub-surface, hence a novel knowledge of the mechanical responses of skin tumors. The research results display the tumors' viscoelastic responses alongside their respective healthy skin outcomes for each skin layer as well as the dermatologist's touch analysis. Although dermatologists are capable of sensing and having a fair overall assessment of what they are palpating, they are unable heretofore to quantify it and inform where the firmness or softness derives from, which it is necessary to be acquainted with so as to perform an accurate diagnosis, prognosis, treatment, future surgery, and teledermatology. [ABSTRACT FROM AUTHOR]

We discuss the inverse problem of successively finding two unknowns (a one-dimensional integral operator kernel and a two-dimensional wave propagation velocity) for the viscoelasticity equation in a weakly horizontally inhomogeneous medium. The direct initial boundary value problem for the displacement function contains zero initial data and the Neumann boundary condition of special form. Additional information consists in the Fourier transform of the displacement function at . We assume that the unknown functions are expanded in an asymptotic power series in a small parameter. We prove theorems on the global unique solvability and stability of the inverse problem solution. [ABSTRACT FROM AUTHOR]

The paper presents the measurement results of the internal friction high temperature background for the VZh171 refractory alloy before and after its nitridation. The obtained data are used to study mechanisms of the alloy transition from the elastic to viscoelastic state at elevated temperatures. The nitridation process leads to a rise of the transition temperature from 571 K (initial state) to 763 K (after nitridation). The activation energy of the internal friction high temperature background is 0.63±0.01 and 0.59±0.01 eV before and after the bulk nitridation, respectively. The activation energy of processes responsible for the formation of viscoelastic properties, is ≈2.52 and ≈2.36 eV before and after the bulk nitridation, respectively. These values correspond to the self-diffusion activation energy in nickel, the main component of the alloy matrix. The presence of the dispersed nitride phase in the matrix decreases the activation energy of viscoelastic processes by about 6%. Possible reasons are considered for changes in the nitridation-induced transition temperature and activation energy of viscoelastic processes at moderately high temperatures. [ABSTRACT FROM AUTHOR]

Background: Characterizing the mechanical behavior of materials at elevated temperatures is critical for the design and development of polymer systems for use in complex operating conditions. The commonly used Dynamic Mechanical Analysis (DMA) does not apply to the investigation of the three-dimensional properties of materials. A combination of a mechanical testing system and an environmental chamber with extensometer measurements is more suitable for this purpose. However, the bi-axial extensometer suffers errors in measuring strains in thermoplastic specimens at elevated temperatures due to its characteristics. Objective: This brief technical note analyzes the source of measurement errors with an extensometer and proposes a robust and straightforward experimental procedure for three-dimensional mechanical testing at high temperatures. Methods: Two temperature and physical aging effects characterization experiments on polycarbonate at 120 ∘ C were performed as a case study. Thermal and penetration drifts were analyzed and corrected in the axial and transverse measurements. Results: The corrected experimental results for the two tests are nearly identical, attesting to the reproducibility of the proposed procedure. Furthermore, the bulk-related strain computed using the corrected strain increases monotonically with time, consistent with the thermodynamic principle, thus demonstrating the reliability of experimental results. Conclusions: The methodology described in this work can serve as a protocol to guide the three-dimensional thermo-mechanical testing with a bi-axial extensometer. [ABSTRACT FROM AUTHOR]

Background: Mechanical characterization of materials that solely relies on global responses may overlook important local behavior that significantly affects the characterization of material properties. Field displacements such as from digital image correlation (DIC) can provide high-fidelity experimental data, which combined with finite element method (FEM) can form DIC-FEM inverse method that can better account for complex mechanical properties of materials. Despite its capability, the DIC-FEM inverse method has been mainly applied to an elastic-dominant regime even though inelastic deformation is important in many engineering materials. Specifically, the DIC-FEM inverse method has not been fully extended to viscoelastic materials due to the complex representation of the time-dependent modulus. Objective: This study aimed at establishing a DIC-FEM inverse framework to identify constitutive properties of homogeneous elastic and viscoelastic materials. Methods: Two example materials (i.e., polyetheretherketone (PEEK) and a viscoelastic fine aggregate matrix (FAM) with a bituminous binder) were selected for the elastic and viscoelastic investigation, respectively. Both were experimentally tested using three-point bending incorporated with DIC. FEM simulated the experiment and the Nelder-Mead nonlinear optimization algorithm was implemented to solve the inverse problem. Results: The DIC-FEM inverse method successfully identified Young's modulus of an example linear elastic PEEK and the linear viscoelastic relaxation modulus of FAM. Conclusions: The resulting DIC-FEM inverse method is applicable to various materials with inelastic deformation and can be extended to localized behavior induced by microstructure heterogeneity and fracture. [ABSTRACT FROM AUTHOR]

Polymers are often used for enhanced oil recovery (EOR) and in situ site remediation. Polymers with high specific surface area and porosity can be used to remove contaminants from water for water remediation. The additional viscosity improves mobility control as compared to water. Polymers are also non-Newtonian (having shear-dependent viscosity) and might be viscoelastic. However, the effects of non-Newtonian rheology on fracture opening and fluid leakoff morphology is largely unexplored as compared to Newtonian fluids. In this study, we inject Newtonian, shear thinning and viscoelastic fluids into a sand-filled Hele–Shaw cell to study fluid invasion and fracturing patterns of sand and injected fluids. The experimental results demonstrate complex fracture patterns depending on injection velocity, fluid viscosity, boundary conditions, and fluid rheology. We use digital image correlation to demonstrate that the fracturing mechanism in sand is associated with shear dilation at the tip of fracture and fracture offshoots. The presence of stress-free boundaries causes the fracture opening to be nearly perpendicular to the minimum principal stress, and the peak injection pressure to increase as compared to the fixed–displacement boundaries. We observe two fluid flow regimes: capillary-dominated flow and viscous drag-dominated flow. When compared with Newtonian fluids, shear-thinning fluids cause thinner fracture openings and more leakoff. Injecting viscoelastic fluid into sand creates a more irregular leakoff pattern, with fluid sometimes leaking mostly at the fracture tip for high injection rates. Finally, we propose a dimensionless parameter to quantify fluid invasion and fracturing patterns of sand and injected fluids and show that the ratio of fracture to leakoff area increases with the parameter for inelastic fluids. However, we observe the opposite behavior for viscoelastic fluids, which we attribute to shear-thickening behavior flowing through variable pore throats. Highlights: There is a lack of research on the injection of viscoelastic polymer into sand. This paper studies the injection of non-Newtonian polymers into sand. Polymer injectivity is sometimes higher than expected due to polymer-driven fractures. This paper quantifies the fracture and leakoff of power–law non-Newtonian polymers. This study shows for first time shear thickening of viscoelastic polymers in a fractured porous medium and demonstrate the leakoff morphology of viscoelastic fluids. [ABSTRACT FROM AUTHOR]

A theory of viscoelastic crack growth that accounts for global and local (crack-tip) viscoelasticity, as well as continuum nonlinearity, is used to analyze stable and unstable crack growth data on carbon black-filled rubber. First, we briefly review relevant results from a recently published linear theory that compares the effect of various models of the failure zone (fracture process zone) on crack growth. Then, it is shown that an extension of this theory to include nonlinearity is in very good agreement with experimental data on crack opening displacement and crack speed for rubber with three different volume fractions of carbon black. [ABSTRACT FROM AUTHOR]

This work focuses on the linear viscoelastic behaviour at length scales shorter than an entangled strand and attempts to connect it to the nonlinear behaviour (from data in literature) in fast extensional flow where such short length scales play an important role. We have measured the linear viscoelasticity at high frequencies in the transition to glassy regime for a series of polystyrene (PS) melts and solutions. When the samples are compared at the same Tref-Tg, where Tref is the reference temperature and Tg is the glass transition temperature, their third crossover points (near the glassy regime) of G′ and G″ plots are located at the same frequency. While the unentangled styrene oligomer behaves as a solvent in the flow and rubber plateau regimes, it contributes the same as the PS melts in the transition regime, unless the chain length is close to a Kuhn segment. In fast extensional flow, using τ0 as the time scale and G0 as the modulus scale, where τ0 and G0 are both associated with the third crossover point, the normalized steady-state extensional viscosity seems to depend on the number of Kuhn segments per chain, but not on the PS fraction. This is probably due to the contribution from the styrene oligomer solvent. However, fracture may happen before the steady state if the stretched polymer chains are close to their theoretical maximum stretch ratio. [ABSTRACT FROM AUTHOR]

Skin is a living organ which can grow in a specific situation. In some cases, a defect like a wound or congenital nevus can bother the patients. Accordingly, the defect needs to be repaired with adjacent skin, and the growth phenomenon will be necessary in these conditions. Some studies have been done to model the growth of skin; however, none of them considered the viscoelastic property which is an inherent characteristic of the skin. Therefore, the main goal of this paper is to develop a nonlinear viscoelastic formulation for growing skin. The formulation is derived in such a way that the pure growth phenomenon can be recovered when the effect of viscoelasticity is ignored. Moreover, to deal with highly nonlinear formulation, a nonlinear finite element formulation in Total Lagrangian framework is developed. Furthermore, implicit trapezoidal time integration method is employed for evolution equations obtained from viscoelastic and growth contributions. Finally, some examples are provided to investigate the performance of the developed formulation and the good results compared to those available in the literature are obtained. [ABSTRACT FROM AUTHOR]

The flexural wave propagation in a micro-beam mounted on viscoelastic foundation is considered in the present work. The mechanical behavior of the micro-beam is modeled by nonlocal strain gradient elasticity with the size-effects taken into account. The constraint is modeled by the Pasternak foundation with the rheology behavior modeled by fractional-order viscoelasticity. The main concerns are the dispersion and attenuation feature of five kinds of possible flexural waves derived from the governing equations of the presented micro-beam model. The influences of the nonlocal effects, strain gradient effects and the fractional-order viscoelastic effects are investigated by a parameter study in the numerical examples. This research is helpful for the design of micro-sensor, micro-actuator and energy-harvest devices in micro electric-mechanic system (MEMS). [ABSTRACT FROM AUTHOR]

In this article, a theoretical framework for problems involving fractional equations of hyperbolic type arising in the theory of viscoelasticity is presented. Based on the Galerkin method, a variational problem of the fractionary viscoelasticity is studied. An appropriate functional setting is introduced in order to establish the existence, uniqueness and a priori estimates for weak solutions. This framework is developed in close concordance with important physical quantities of the theory of viscoelasticity. [ABSTRACT FROM AUTHOR]

Cells are very sensitive to the shear stress (SS). However, undesirable SS is generated during physiological process such as collective cell migration (CCM) and influences the biological processes such as morphogenesis, wound healing and cancer invasion. Despite extensive research devoted to study the SS generation caused by CCM, we still do not fully understand the main cause of SS appearance. An attempt is made here to offer some answers to these questions by considering the rearrangement of cell monolayers. The SS generation represents a consequence of natural and forced convection. While forced convection is dependent on cell speed, the natural convection is induced by the gradient of tissue surface tension. The phenomenon is known as the Marangoni effect. The gradient of tissue surface tension induces directed cell spreading from the regions of lower tissue surface tension to the regions of higher tissue surface tension and leads to the cell sorting. This directional cell migration is described by the Marangoni flux. The phenomenon has been recognized during the rearrangement of (1) epithelial cell monolayers and (2) mixed cell monolayers made by epithelial and mesenchymal cells. The consequence of the Marangoni effect is an intensive spreading of cancer cells through an epithelium. In this work, a review of existing literature about SS generation caused by CCM is given along with the assortment of published experimental findings, to invite experimentalists to test given theoretical considerations in multicellular systems. [ABSTRACT FROM AUTHOR]

The components of the stress-strain state of rubber shock absorbers under viscoelastic deformation are investigated. The contraction of rubber shock absorber under the action of a vertical force and a combination of vertical and shear forces is calculated. The dependence of the contraction of the structure on time was obtained. The specific mechanical properties inherent in rubber, such as the weak compressibility of the material and deformation viscoelasticity deformation, the mathematical modeling of which involves significant mathematical difficulties, were taken into account. In the numerical calculation, the weak compressibility of rubber was modeled using a finite element moment scheme for weakly compressible materials, the essence of which is the triple approximation of displacement fields, strain components and volume change function. For the mathematical modeling of the viscoelastic nature of strains, the hereditary Boltzmann–Volterra theory with exponential relaxation kernel is used. On the basis of the Lagrange variational principle, a system of resolving integral equations of viscoelasticity is obtained taking into account the weak compressibility of the material, and an iterative procedure for its solution is proposed. The modified Newton–Kantorovich method is used to solve a nonlinear boundary value problem. The numerical solution is obtained by the finite element method in the case of viscoelastic deformation of rubber material. The calculation is performed for the case when the rubber layer is vulcanized to metal plates. The influence of the viscoelastic properties of the rubber on the value of rubber shock absorber contraction for two cases of mechanical loading is investigated. Calculations were carried out for two rubber grades: 2959 and 1562. [ABSTRACT FROM AUTHOR]

The 3D nonaxisymmetric local stability loss problem is studied for a hollow cylinder, made of a viscoelastic composite material, under the axial compression. The criterion of an infinitesimal initial imperfection is employed, and the evolution of this imperfection is investigated by 3D geometrically nonlinear exact equations of viscoelasticity theory. The corresponding nonlinear problem is solved by the boundary perturbation method. The material of the cylinder is modeled as a viscoelastic transversely isotropic medium. Numerical results for the critical time and critical compressive forces are presented, discussed, and compared with those obtained by approximate shell theories. [ABSTRACT FROM AUTHOR]

The promising phase-field method has been intensively studied for crack approximation in brittle materials. The realistic representation of material degradation at a fully evolved crack is still one of the main challenges. Several energy split formulations have been postulated to describe the crack evolution physically. A recent approach based on the concept of representative crack elements (RCE) in Storm et al. (The concept of representative crack elements (RCE) for phase-field fracture: anisotropic elasticity and thermo-elasticity. Int J Numer Methods Eng 121:779–805, 2020) introduces a variational framework to derive the kinematically consistent material degradation. The realistic material degradation is further tested using the self-consistency condition, which is particularly compared to a discrete crack model. This work extends the brittle RCE phase-field modeling towards rate-dependent fracture evolution in a viscoelastic continuum. The novelty of this paper is taking internal variables due to viscoelasticity into account to determine the crack deformation state. Meanwhile, a transient extension from Storm et al. (The concept of representative crack elements (RCE) for phase-field fracture: anisotropic elasticity and thermo-elasticity. Int J Numer Methods Eng 121:779–805, 2020) is also considered. The model is derived thermodynamic-consistently and implemented into the FE framework. Several representative numerical examples are investigated, and consequently, the according findings and potential perspectives are discussed to close this paper. [ABSTRACT FROM AUTHOR]

Understanding tissue rheology is critical to accurately model the human heart. While the elastic properties of cardiac tissue have been extensively studied, its viscous properties remain an issue of ongoing debate. Here we adopt a viscoelastic version of the classical Holzapfel Ogden model to study the viscous timescales of human cardiac tissue. We perform a series of simulations and explore stress–relaxation curves, pressure–volume loops, strain profiles, and ventricular wall strains for varying viscosity parameters. We show that the time window for model calibration strongly influences the parameter identification. Using a four-chamber human heart model, we observe that, during the physiologically relevant time scales of the cardiac cycle, viscous relaxation has a negligible effect on the overall behavior of the heart. While viscosity could have important consequences in pathological conditions with compromised contraction or relaxation properties, we conclude that, for simulations within the physiological range of a human heart beat, we can reasonably approximate the human heart as hyperelastic. [ABSTRACT FROM AUTHOR]

Blast polishing offers an operator-friendly solution to many of the previously encountered polishing difficulties. However, the process lacks studies into the control of key parameters, one of which is viscoelasticity (particularly present in biological-based abrasive medias). Together with analytical-empirical models, a vibrational spring-dashpot model is presented, which characterizes the impact force, contact time, and damping ratio/coefficient of polishing media upon impact; as well as the effects of damping on contact parameters: stress, deformation, and area of contact. These are compared to experimentally gathered results for verification of the model. Impact force is shown to decrease dramatically with increasing hydration while increasing linearly with an increase in kinetic energy. Experimental findings reveal that 50% wet contact exhibits a 340% reduction in force magnitude compared to dry contact. Contact time results show an exponential increase with an increase in hydration. Research findings also show that higher hydration levels result in lower damping ratios and that higher kinetic energies (related to higher hydration levels) tend toward a decrease in damping ratio. Similarly, media damping coefficients decrease with both hydrational increases and kinetic energy increases. Results show that contact stress is reduced at higher hydration levels, which is mainly due to higher contact areas, and hence it was noticed that an increase in hydration prevents occurrence of chipping and brittle failure on the workpiece surface. Contact stress is shown to reduce by 325% from a 10% wet contact to a 50% wet contact. A high hydration of 30 to 50%, a high impinging velocity of value 31.4 m/s and above, a low stand-off distance of value 20 mm and below, a 45° polishing angle, and a polishing time of 20 to 40 min provide the most optimal parameters for efficient polishing to achieve a mirror finish on an additive manufactured Ti-6Al-4V component. The findings stipulated provide a base on which to further characterize the process and aim to promote further development and optimization of the blast polishing process. [ABSTRACT FROM AUTHOR]

Magnetorheological elastomers (MREs) have attracted significant attention as potential intelligent materials for vibration absorption and damping in tank engines. To increase the force of contact between the matrix and the particles, a novel graphene-coating material was created. The graphene-coated iron (G-Fe) used in this study was synthesized at a high temperature and then combined with natural rubber to create MRE samples. The magnetic resonance effect of MRE samples was examined using a rheometer fitted with a magnetic field. The data indicate that by adding G-Fe at a concentration of 6% to MREs, their magnetorheological (MR) effect is dramatically boosted, while its Payne effect is significantly lowered. This is probably because adding G-Fe can reduce the initial modulus as the particles can move more easily in the magnetic field. However, the high levels of G-Fe may adversely affect the movement of particles in the matrix. The aging properties of the MRE samples were tested, and the results indicated that the surface layer of G-Fe is graphene. It has excellent compatibility with rubber and has little effect on the thermal oxygen aging of MREs compared with Carbonyl Iron Powder (CIP). Therefore, doping with G-Fe improves the thermal oxygen aging performance of MREs. [ABSTRACT FROM AUTHOR]