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We pose the direct and inverse problem of finding the acoustic wave velocity and pressure, diagonal memory matrix for a reduced canonical system of integro-differential acoustic equations. The problems are replaced by a closed system of Volterra-type integral equations of the second kind with respect to the Fourier transform in the variables x1 and x2 of the solution of the unknowns of the direct problem and the inverse problem. To this system, we then apply a reduction method, a mapping in the space of continuous functions with a weighted norm. Thus, we prove global existence and uniqueness theorems to solve the given problems. [ABSTRACT FROM AUTHOR]

The contribution focuses on the investigation of the transient wave process into a viscoelastic continuum. Mechanical waves are provoked by a blast, as the impact load acts on the internal surface of a structural element (a hollow cylinder). Within the hypothesis of isotropic behavior, the material response is simulated through an incremental modification of the initial elasticity modulus by the predefined viscoelasticity relationship. The presented results include a simulation of stress evolutions at specified locations. [ABSTRACT FROM AUTHOR]

The contribution presents a study of the propagation of waves in a hollow finite cylinder provoked by a transient load. The viscose behavior is superimposed to linear elasticity in terms of the linear Boltzmann-Volterra model. The proposed semi-analytical algorithm includes Laplace transform with respect to time applied to the wave equation with an account for the specified stress-strain relationship, boundary, and initial conditions. The solution is found in the space of images and is then 'projected back' into the space of originals. Alternatively, the wave propagation is investigated by monitoring the stress and the displacement evolution in a specified location within a transient, finite element analysis. Viscoelasticity is modeled by modifying the material parameters according to a predefined time-dependent constitutive relation. Both analyses employ the same geometry and comparable boundary conditions, assuming isotropic viscoelasticity and constant Poisson's ratio. [ABSTRACT FROM AUTHOR]

Microfluidic techniques have the advantages of short reaction time, high throughput, low reagent consumption and high sensitivity. Acoustic field-driven micro-mixers have attracted much attention because of their non-invasive and simple mixing mechanism. However, most acoustic field-driven mixers have a small operating range and achieve high efficiency mixing under harsh conditions. By investigating the mixing of a contraction-expansion flow channel, a straight channel with bubbles in the side cavity and a straight channel with sharp edges on the side walls under different fluid properties, flow rates and acoustic conditions, the mixing characteristics of each flow channel are compared and analyzed. The experimental results show that the microchannels with bubbles and sharp-edge structures have excellent mixing effect under kHz acoustic vibration conditions, and the mixing index can reach more than 0.9. Mixing efficiency increases with increasing voltage, as the more intense vibration creates stronger acoustic streaming and decreases with increasing flow rate, as a faster background flow is more difficult to disrupt. Both structures have good mixing effects below 10 µL/min, and the contraction-expansion flow channels are suitable for mixing of viscoelastic fluids with flow rates above 30 µL/min. Finally, the design ideas of combining bubble and sharp edge structures as well as combing acoustic vibration and fluid viscoelasticity are proposed to provide reference for the design of micro-mixers with large flow ranges. [ABSTRACT FROM AUTHOR]

This contribution deals with the investigation of viscoelastic properties of the collagen matter using the extrusion rheometry. The bovine collagen matter with a high mass fraction of the collagen (8 %) is extracted from the calf skin and is used in the food industry as an edible sausage casing. Experiments were performed using the extrusion rheometer. Samples are pushed through the modified rectangular capillary (called the hole-pressure capillary) by the hydraulically driven piston. Bottom part of the capillary head is equipped with five pressure transducers monitoring pressures along the capillary. Upper capillary head is equipped with one pressure transducer mounted in the groove. The so-called hole pressure value can be measured by these experiments. The hole pressure is then used to evaluate the viscoelastic parameter: first normal stress difference N1. Other possible ways to determine the viscoelasticity of the collagen by extrusion are used in this contribution as well. The exit pressure method uses the pressure value in the position of capillary end. The value is reached by the extrapolation of pressure values from five pressure transducers. The jet swell evaluation method uses the image (photography) of the extruded sample and evaluates the expansion of material when it leaves the capillary. All methods were used to quantify the viscoelastic effect. Methods are compared with each other and their suitability to the collagen matter is discussed. The hole pressure method was found to have a solid data overlap with the exit pressure method (N1≈ 180 000 Pa) at high shear rates (γ ̇ = 3500 s-1). The hole pressure ratio pHole/p2 was found to be between 13 and 19 % during experiments. The N1 values are compared with similar experiments in the literature with some positive similarities. All this despite the fact, that collagen data in this study are evaluated under much bigger shear rates. [ABSTRACT FROM AUTHOR]

The study of non-equilibrium thermal convection in an anisotropic porous layer saturated with a visco-elastic fluid is carried out to find how the anisotropy and non-equilibrium of temperature affect the onset of convection. The small perturbation method is applied to obtain the linearized form of equations are then solved by using the normal mode technique. The viscoelasticity is considered by taking Oldrovd-B momentum equation and two separate expressions are used for the energy equation, out of these one represents solid and anther for fluid medium. The effect of both anisotropy and non-equilibrium on the oscillatory and stationary modes of convection has been studied. Apart from these the effect of conductivity ratio, diffusivity ratio, and Darcy Prandtl number is also observed and presented graphically. The stability of the system is protected by thermal anisotropy and diffusivity ratio. But mechanical anisotropy and conductivity ratio are taking the system in destabilizing mode. [ABSTRACT FROM AUTHOR]

Aiming to estimate the effective behavior of the parts obtained by fused deposition modeling (FDM) in the case of short fiber composite materials, the Mean-field homogenization procedure, introduced in linear elasticity, is here extended to linear thermo-viscoelasticity. The variation of the parameters describing the state of the fibers inside the printing filament is represented by introducing appropriate distribution functions obtained through the statistical analysis of the microstructure. The validation of the procedure is achieved by comparing its predictions with calculations based on full-field Fast-Fourier-Transform homogenization and experiments results from samples treated in autoclave to remove layer-scale porosities from the printed filament. [ABSTRACT FROM AUTHOR]

This paper discusses the castability, mixed viscoelasticity and voids of the propellant based AP/HTPB/TDI with HTPB-TDI binders. Several influential variables were observed including the plasticizer and the order in which the curing agent was added. Then the mixer used is also horizontal and planetary (vertical) mixers in order to obtain the scale up or scale down parameters between mixers. First experiment was mixed propellant formulation in a 5 kg horizontal sigma blade mixer, which was set up at 55°C and 60 rpm. The composition of propellant was 14% of HTPB, 7.5% of Al powder, 77.5% of AP with coarse, medium, fine ratio 1/1/1, and 1% of TDI. Propellant slurry as mixing product was casted into vacuum casting chamber then grained as desired, cured at 60°C for 2 days, was left in room temperature for a week. The propellant was X-Ray scanned and viscosity measurement at 3 rpm spindle speed. This experiment was repeated with the addition of TDI in the first sequence and the end sequence. The experiments were repeated with addition of DOA for 0.5 to 4%. The order of adding TDI in the propellant mixing process affects the viscoelasticity and castability of AP / HTPB / Al composite propellant. The mixing of the TDI at the end process will provide a better mixing space and evenly distributed particle distribution, is better applied to large rockets that require a longer treatment. Scale up and scale down of propellant mixers can use similarity while maintaining the same viscoelasticity and operating conditions. The addition of DOA increase pot life and viscoelasticity, but there are limitations to where the dough can be molded or processed. The maximum addition of DOA is 2.74% to produce a dough with an even distribution of particles with the added bonding agent. [ABSTRACT FROM AUTHOR]

We consider the motion of a multidimensional viscoelastic continuum which subjects some fractional constitutive law. The weak solvability of a corresponding initial-boundary value problem is established. [ABSTRACT FROM AUTHOR]

The study of how the flows of polymer solutions behave in the area of non-linear viscoelasticity provides for a more precise description of how adequate the rheological models are and the rheological properties of the material can be described much better. Non-linear viscoelastic properties demonstrated while studying the behavior of polymer material under large deformations have been under research when studying time curves of shear stresses which are calculated for various amplitudes. This work looks into the possibility to apply the modified Vinogradov and Pokrovskii rheological model in the description of the oscillating shear deformation of polymer fluids with large amplitude. It was stated that when the deformation amplitude increases, shear stresses fail to stay right harmonic curves, which results in a “step” formation on their left front. It means significant non-linear behavior of the sample. Also, non-zero first and second differences of normal stresses are revealed. The obtained theoretical dependences have been compared to the experimental data. The comparison was held via constructing time curves of normalized stresses and by analyzing Lissajous figures. Despite its simplicity, the modified Vinogradov and Pokrovskii rheological model describes adequately the behavior of polymer materials at large periodic deformations. [ABSTRACT FROM AUTHOR]

In this paper, we consider the following second-order abstract semilinear evolution equation with past history and time delay. Under suitable conditions on initial data, we prove the well-posedness by using the semigroup arguments. The stability result is also established defining a suitable Lyapunov functional for a larger class of kernels. An application is also given. [ABSTRACT FROM AUTHOR]

This paper addresses the question: what is the physical origin of friction in a contact of two amorphos, hard, smooth elastic surfaces (as e.g. DLC coatings). The dissipation channels due to viscoelasticity, plasticity or elastic instabilities are not active in such systems. We show that short range adhesion leads to "adhesion instabilities" which cause energy dissipation similarly to the classic elastic instabilities mechanicsm. We argue that the main governing parameter determining friction in such systems is the "Johnson parameter" introduced 1995 by K.H. Johnson. [ABSTRACT FROM AUTHOR]

The correspondence principle of linear viscoelasticity is used to derive the displacement fields due to a single force in an elastic half-space overlying a viscoelastic half-space. The results are valid for arbitrary values of the relaxation time and a change in the rigidity of the two half-spaces. The variation of the viscoelastic displacements with the epicentral distance as well as with the different relaxation time and a change in the rigidity of the two half-spaces are studied and shown graphically. [ABSTRACT FROM AUTHOR]

In paper [2] the theory [1] for a thermo-electro-mechanical simple body B with fading memory is used to set up a thermoelectro viscoelastic theory, which is obtained by a linearization procedure and the Riesz representation theorem. This is done within the Green-Naghdi scheme of thermodynamics. Hence several restrictions on the various relaxation functions are found that extend the ones in [3] for finitely thermoviscoelastic materials. The restrictions are obtained from an internal dissipation inequality that is a consequence of a dissipation inequality. [ABSTRACT FROM AUTHOR]

Bipolar plates (BPPs) are the main component in proton exchange membrane fuel cells. In the last years, different manufacturing processes have been proposed as alternative to the traditional graphite BPPs, including the manufacture of thin stamped BPPs using the rubber pad forming process. In this context, the numerical simulation of the forming process is used to optimize of the process parameters. Thus, in addition to the modelling of the elastoplastic behavior of the metallic sheet, it is also necessary to describe the hyper-viscoelastic behavior of the rubber pad. The main objective of this study is to evaluate the importance of the viscous effect on the global behavior of two different polyurethanes, since the modelling of the viscoelastic behavior is significantly more complex than the hyperelastic one. Uniaxial compression and stress relaxation tests are carried out both experimentally and numerically, considering three loading/unloading velocities. The hyperelastic behavior is described by the Mooney-Rivlin model, while the viscoelasticity is modelled by a series of Maxwell elements. The results show that the viscous effect can be neglected in the numerical modelling of the rubber pad forming, if the rubber hardness value is low. [ABSTRACT FROM AUTHOR]

Most existing nonlinear viscoelastic models are founded on the Finger tensor and its evolution during deformation and flow. In this paper, a new framework for nonlinear viscoelasticity on the basis of a projected velocity gradient tensor is presented. The pairing of the logarithmic strain tensor with the projected velocity gradient tensor is considered the cornerstone of the proposed formulation. The resulting linear relation for strain evolution in affine deformation provides a facile passage to connect small deformation mechanics with large deformation mechanics. Accordingly, proven linear viscoelastic models may now be extended to large deformation. Another salient feature is that the deformation is decoupled into stretching and rotation and each of them has its own evolution equation. The relaxation process is considered coaxial, whereas rotational retardation is included in the formulation to tackle with shear-related rotational softening. In this way, the similarity and difference between rotational deformation and coaxial deformation are explicitly explored. Strain shape functions are added to the linear version of the model to render more realistic nonlinear effects. The overall framework uses three elements for model formulation, namely, a stretch evolution equation, a rotation evolution equation, and a stress law. Example models are provided to illustrate the proper combination of these elements for creation of useful models. A particular model with a fractional relaxation process is used for model testing against typical observations in simple shear. With five model parameters (one for fractionality, two for linear viscoelasticity, one for straining, and the last one for rotation) is able to fit startup shear viscosity of a polystyrene solution in high accuracy. With additional understanding of the role of entropic strain in the relaxation process, simple and unified constitutive equations for modeling general 3D viscoelastic deformation and flow may be developed. [ABSTRACT FROM AUTHOR]

The present study investigates the traversal of torsional wave at a corrugated interface between viscoelastic sandy medium and inhomogeneous half-space. In the lower half-space, the inhomogeneity has been assumed as an exponential variation in density and rigidity with respect to depth. We have taken the method of variable separation to obtain analytical solution of displacement components for layer and half-space. Consequently, we have obtained the wave velocity equation of torsional wave using sufficient boundary conditions. Moreover, the wave velocity equation has been reduced to the pre-established classical relationship of Love wave, when both the media are isotropic and homogeneous, as shown in special case. The comparative study of the influence of various parametric traits viz. sandiness, viscoelasticity, corrugated and inhomogeneity associated with the studied model on the traversal characteristics of torsional wave is one of the salient feature of the current investigation. The study may find its possible applications in various seismic prospecting techniques to analyse the nature of torsional wave propagation in the considered layered media. [ABSTRACT FROM AUTHOR]

An unsteady motion of dusty memory fluid (governed by Walters liquid model B') in horizontal channel with energy transfer has been carried out analytically. Both the channel walls are maintained at same fluidic temperature and concentration. The fluid flow inside the channel is guided by periodical pressure gradient. Viscosity helps to generate heat inside the system through energy dissipation. Dust particle’s motion is governed by Saffman model. Governing equations of motion are solved analytically using perturbation technique. The influence of various flow parameters on Sherwood number, Nusselt number and shearing stress are discussed with the aid of tables. [ABSTRACT FROM AUTHOR]

In this study, heat and mass transfer flow of a viscoelastic (Walter’s liquid-B model) nanofluid over a stretching sheet with slip velocity condition is considered. The governing equations for the model which is non-linear partial differential equations are first transformed by using similarity transformation. Runge-Kutta-Fehlberg (RKF) method is employed to solve the transformed ordinary differential equations. Numerical solutions are obtained for the reduced Nusselt number, the Sherwood number and the skin friction coefficient. It is found that the Walter’s viscoelastic nanofluid provided the higher heat and mass transfer rate compared to the ordinary nanofluid and the presence of the velocity slip reduces the effects of the stretching parameter on the skin friction coefficient. [ABSTRACT FROM AUTHOR]

The experimental and finite modal analysis of floor pan of a conventional car without and with incorporating viscoelastic damper is examined for NVH performance. The 3D model of the floor pan is generated using CATIA V5 software as an initial step, later on it is being meshed precisely using HYPERMESH software and analysed using the software ABAQUS 6.10However, to validate the Finite Element Analysis Experimental modal analysis has been conducted using impact hammer and FFT analyser. Natural frequencies and mode shapes are collected at both the methods and compared together. Thus, found that floor pan with incorporating viscoelastic damper was found that better than without one. [ABSTRACT FROM AUTHOR]

We study a new rheological model (a modification of a known Pokrovski - Vinogradov model). As was shown by numerical simulations, it takes into account nonlinear effects arising in flows of melts and solutions of polymers in domains with a complex boundary geometry. In the case when the main solution is a Poiseuille-type flow in an infinite plane channel (one considers a viscoelastic polymeric fluid) we obtain an asymptotic formula for the distribution of spectrum points of the linear problem. Small perturbations have such the additional property that they are periodic with respect to the variable going along the side of the channel. [ABSTRACT FROM AUTHOR]

A new numerical model for hydrodynamic stability analysis of flows in pipes of constant elliptic cross-section with compliant walls is described. The model is based on the equations governing the propagation of small-amplitude three-dimensional disturbances of the velocity and the pressure as well as Love's equations of two-dimensional shell theory for simulating the viscoelastic properties of the pipe wall. The possible presence of media surrounding the shell is simulated as a set of equidistant springs and dampers. Numerical properties of the model are illustrated with the Poiseuille flow in round and elliptic pipes with compliant walls of constant thickness. [ABSTRACT FROM AUTHOR]

The paper is devoted to the description of the mathematical model of a linear magnetoelectric drive for testing the viscoelastic properties of elastomers, and also to a research of dynamic characteristics of the linear magnetoelectric motor in the mode of providing a given clamping force on the prototype. The paper is an attempt to introduce in the general mathematical model of magnetoelectric drive equations based on multi-loop equivalent circuit of a prototype of the elastomer. Parameters of modulus of elasticity, modulus of viscosity, tangent of angle of mechanical losses are determined on the basis of simulation results. These results were compared with experimental data obtained using the developed laboratory stand for testing the rheological properties of elastomers. [ABSTRACT FROM AUTHOR]

On March 11, 2011, the Tohoku-Oki earthquake (Mw 9.0) occurred on the plate interface of the subducting Pacific plate. During the co- and post-seismic periods, almost all GEONET and seafloor observations detected eastward crustal deformation. However, seafloor observations around main raptured zone moved in a westward direction during the post-seismic period. In this study, we estimate the spatio-temporal slip distribution during the co- and post-seismic periods from observed crustal deformations, such as GEONET and seafloor observations. For inversion, we use a viscoelastic Green's function derived from a three-dimensional finite element method considering the underground structure. The estimated maximum co-seismic slip is approximately 60m from the Japan Trench. Off Iwate prefecture, the moment magnitude and maximum afterslip 2.5 years after the Tohoku-Oki earthquake are approximately 8.13 and 2.0m, respectively. The afterslip distributions differ from the co-seismic slip distribution, and the relationship between the co-seismic and afterslip regions is complementary. [ABSTRACT FROM AUTHOR]

In rheometry rough geometries are often used to prevent wall-slip typically encountered with heterogeneous fluids. However, the fluid does not stand still within the geometry roughness and a residual wall-slip can be often measured. The problem can be treated as the flow through a porous medium that is here investigated when the fluid is non-Newtonian. We use a Boger fluid that is a good model for the Second Order Fluids and a commercial cross-hatched plate as rough geometry. The results obtained are in good qualitative agreement with predictions available in the literature and are promising for the development of a simple procedure to use rough geometries with viscoelastic fluids. [ABSTRACT FROM AUTHOR]

In the frame of a project aiming to the improvement of the properties of biodegradable polyester-based devices designed for bone regeneration, we have prepared and characterized novel composites based on poly(ε-caprolactone) (PCL) and nanohydroxyapatite (HA). In particular, in this paper we report the preparation, the morphology and the rheological properties of nanocomposites obtained with PCL and its less hydrophobic derivative, PCL-MAGMA [(maleic anhydride (MA)-glycidylmethacrylate (GMA)] including different amounts of nanohydroxyapatite. [ABSTRACT FROM AUTHOR]

In the present paper, the preload reduction over the time attributed to the relaxation phenomenon which involves hybrid (composite-metal) bolted joints has been experimentally and numerically investigated. Unlike conventional materials (steel, aluminium, etc.), composite ones, due to the presence of the polymeric matrix with viscoelastic properties, feel more considerably the relaxation phenomenon, which begins at room temperature (around 20 °C). As a result, the loss in the mechanical fastening is a very critical aspect to consider when composite components have to be bolted to other; further investigations are still mandatory to increase the effectiveness of such jointing techniques. The test article consists of two bolted plates made of mild steel and a composite laminate made by E720 770gms 3x1 T EGlass roving 1200 Tex 35% RW, respectively. Under this purpose, the preload reduction in the bolt has been experimentally emphasized by developing a novel strain-gauged bolt. Moreover, the relaxation phenomenon has been numerically simulated by developing a complex Finite Element (FE) model. [ABSTRACT FROM AUTHOR]

Stress relaxation behavior of unidirectional AS4/8852 carbon/epoxy composites at high temperatures has been investigated. Specimens with an off axis angle of 90° and ±45° have been exposed to constant strain and high temperature. The mechanical results have been modeled by Standard Linear Solid model. The obtained parameters have been implemented in a finite element (FEM) model able to predict the residual stresses in carbon fiber reinforced composites, both in the elastic and in the viscoelastic field. [ABSTRACT FROM AUTHOR]

Hyper-viscoelastic polymers have multiple areas of application including aerospace, biomedicine, and automotive. Understanding their mechanical responses is therefore extremely important, particularly because they often exhibit strong rate and temperature dependence, including a low temperature brittle transition. Relationships between the response at various strain rates and temperatures are investigated and a framework developed to predict response at rates where experiments are unfeasible. A mas-ter curve showing the rate dependences of the storage and loss moduli at a reference temperature is constructed using the results of a dynamic mechanical analysis (DMA) experiment. A frequency sweep spanning two decades and a temperature range from pre-glass transition to pre-melt is used. A fractional derivative model is fitted to the experimental data, and this models parameters are used to show how the stress-strain relationships at a desired strain rate could be derived. [ABSTRACT FROM AUTHOR]

On-going research into the complexities of polyurea behavior under shock loading has led to some breakthroughs in the predictive simulation of how this nominally soft polymer responds under high velocity impact conditions. This work expands upon a previously reported modified pressure-dependent viscoelastic constitutive model for polyurea and its performance under ballistic impact. Specifically, we present recent enhancements to the model including nonlinearites in the Hugoniot and improvements in the high-temperature viscoelastic behavior, which substantially improves accuracy and extends the model's range of applicable conditions. These improvements are demonstrated through correlation of computations for a suite of normal and pressure-shear plate impact experiments well documented in the open literature. Additionally, microparticle impact experiments were performed on polyurea using a laser-induced particle impact test (LIPIT) technique. High-speed imaging of the impact mechanics revealed elastic particle rebound at low velocity but penetration at high velocity. Simulation of these LIPIT experiments demonstrates good accuracy of the polyurea model under these conditions as well as provides insight into the mechanisms governing the results observed. [ABSTRACT FROM AUTHOR]

The dynamic response to low amplitude blast waves of four viscoelastic materials has been investigated: P1000, P650, P250/1000 blend, and Dragonshield-BC™. This pressure range is of interest for the protection against blast-induced traumatic brain injury (TBI). A 40-mm-bore gas gun was used as a shock tube to generate planar air blast waves, ranging from 0.5 to 2 bar, that impacted instrumented target assemblies mounted on the gas gun muzzle. Each target assembly consisted of the viscoelastic polymer sample sandwiched between two 6061-T6 Al gauge assemblies containing polyvinylidene fluoride (PVDF) stress gauges. Impedance matching techniques were used to determine the polymer input and output stresses and attenuation. The shock velocity-particle velocity, stress-particle velocity, and stress-strain relationships for each polymer were determined. The experimental technique and analysis methodology are discussed. [ABSTRACT FROM AUTHOR]

Modeling living cells behavior is a delicate mission as it involves alike biological, physical, chemical, electrical aspects. This article presents a semi-analytical modeling for telopodes elongation, based on their appearance and behavior captured from in vitro approaches. The semi-analytical solution is determined solving a system of five coupled differential equations and the results are compared both with the numerical solution and with the results of the laboratory tests. A good agreement is obtained. [ABSTRACT FROM AUTHOR]

We consider the initial-boundary value problem for the velocity distribution of a viscoelastic Poiseuille flow in a rectangular channel. To model viscoelastic properties of the fluid, the generalized fractional Jeffreys' constitutive equation is used. Only the case of thermodynamically compatible constitutive model is considered, in which the orders of the two fractional derivatives are equal. Exact solution is derived in the form of eigenfunction expansion. An explicit compact integral representation of the time-dependent components is obtained by using Laplace transform technique. Asymptotic behaviour is discussed. [ABSTRACT FROM AUTHOR]

In the present paper, the modulation of nonlinear wave in a thin viscoelastic tube filled with inviscid fluid is studied. We assumed that the arterial wall material is an incompressible, isotropic and thin viscoelastic tube and considered blood as approximate equations of an incompressible inviscid fluid. Applying the reductive perturbation method, the Nonlinear Schrödinger (NLS) type equation as the evolution equation is obtained for the modulation of nonlinear wave in such a medium [ABSTRACT FROM AUTHOR]

The hot embossing process has been widely used in the manufacturing of polymer components, especially for the fabrication of micro or nano components. The significant advantage of the hot embossing process compared to the traditional injection moulding process is the excellent effective filling ratio for the high aspect ratio components and large surface structural components. The lack of material behavior modeling and numerical simulation limits the further development the hot embossing process, especially at the micro and nano scales. In this paper, a visco-elastoplastic behavior law has been proposed to describe the amorphous thermoplastic polymer mechanical properties in the hot embossing processing temperature range, which is lightly above their glass transition temperature. Uniaxial compression tests have been carried out in order to investigate the amorphous thermoplastic polymers properties. The material parameters in the visco-elastoplastic model have been identified according to the experimental results. A 3D numerical model has been created in the simulation software, which is based on the finite element method. The numerical simulation of the filling step of the hot embossing process has been effectuated by taking into account the viscous, elastic and plastic behaviors of thermoplastic polymers. The micro hot embossing process has been carried out using horizontal injection compression moulding equipment. A complete compression mould tool, equipped with the heating system, the cooling system, the ejection system and the vacuum system, has been designed and elaborated for this research work. The microfluidic devices based on the amorphous thermoplastic polymers have been successfully elaborated by hot embossing process. Proper agreement between the numerical simulation and the experimental elaboration has been obtained. [ABSTRACT FROM AUTHOR]

In this work, we perform a linear stability analysis on pressure and drag flows of an Upper Convected Maxwell viscoelastic fluid. We use the well-recognised method of expanding the disturbances in Chebyschev polynomials and solve the resulting generalized eigenvalues problem with a collocation spectra method. Both the level of elasticity and the back-pressure vary. In a second stage, recent analytic solutions of viscoelastic fluid flows in slowly varying sections [1] are used to extend this stability analysis to flows in a compression or in a diverging section of a single screw channel, for example a wave mixing screw. [ABSTRACT FROM AUTHOR]

A statically unbalanced rotor in viscoelastic orthotropic supports equipped with an automatic ball balancer (ABB), the axis of symmetry of which does not coincide with the symmetry axis of the rotor, is considered. Based on an analysis of the equations describing the stationary modes of motion of the system, the principal impossibility of complete balancing of the rotor is shown. The possibility of the existence of two types of stationary modes is established, one of which has a constant average amplitude of residual vibration equal to the eccentricity of the ABB. The solution corresponding to this almost balanced mode is constructed analytically. A study is made of its asymptotic stability. [ABSTRACT FROM AUTHOR]

An explicit asymptotic model extracting the contribution of a surface wave to the dynamic response of a viscoelastic half-space is derived. Fractional exponential Rabotnov’s integral operators are used for describing of material properties. The model is derived by extracting the principal part of the poles corresponding to the surface waves after applying Laplace and Fourier transforms. The simplified equations for the originals are written by using power series expansions. Padè approximation is constructed to unite short-time and long-time models. The form of this approximation allows to formulate the explicit model using a fractional exponential Rabotnov’s integral operator with parameters depending on the properties of surface wave. The applicability of derived models is studied by comparing with the exact solutions of a model problem. It is revealed that the model based on Padè approximation is highly effective for all the possible time domains. [ABSTRACT FROM AUTHOR]

Based on Frenkel's Newtonian viscous flow approach [1] and Mackenzie and Shuttleworth's [2] extension accounting for trapped gas, a model for the shrinkage of a gaseous pore in a viscoelastic polymer melt driven by surface tension was developed. The viscoelastic flow of polymeric materials was considered by using a simple viscoelastic model described by Bellehumeur [3]. In the simulation, the viscoelastic relaxation leads to a pronounced increase of the characteristic time of shrinkage but does not alter the final equilibrium gas sphere diameter. A first attempt at including gas diffusion into the polymer melt motivated by the works of Kontopoulou and Vlachopoulos [4] and Gogos [5] will be given. Numerical solutions of our simplified model show that gas diffusion leads to a complete collapse of the pore. The rate of diffusion can change the collapse mechanism and the time-dependence of pore radius and pressure tremendously. [ABSTRACT FROM AUTHOR]

The decrease in tensile strength and increase in specimen weight due to lipid diffusion in a biocompatible thermoplastic elastomer was studied and quantified. Mechanical and viscoelastic properties of poly(styrene-isobutylenestyrene) (SIBS) block copolymer are critical to determine feasibility of certain load bearing in vivo applications. Moreover, changes of these properties due to the presence of lipids must be well understood for long-term bio implantation. Dumbbell specimens were thermoformed via injection molding and weights were recorded. Lipid uptake in the body was simulated by specimen immersion in palm and castor oils at 25 °C and 37 °C. After only 96 hours of immersion at body temperature (37 °C), dumbbell weight increased by 6% and 0.3% for palm oil and castor oil, respectively. These values correspond to a reduction in ultimate tensile strength of approximately 30% and 10%, respectively. These results restrict the use of this biocompatible polymer in certain critical components due to the high concentration of lipids in vivo. Based on these significant and rapid reductions in tensile strength in the presence of lipids, it is of vital importance to fully understand the bio-durability and lipid uptake characteristics of SIBS for future design and performance prediction of implantable devices. Further, the results highlight the necessity of improving lipid resistance in order to fully exploit the biocompatibility of SIBS. [ABSTRACT FROM AUTHOR]

Mucus is mainly composed of water (90-95%) and mucins (2-5%), these mucins being high molecular weight macromolecules forming a 3D cross-linked matrix. It makes it a complex non-Newtonian fluid, displaying viscoplasticity, viscoelasticity, shear-thinning and thixotropy. These properties were qualitatively and separately tested by different authors. Due to the difficulties to collect samples and the extreme sensibility of the tested materials, the results are widely variable. A complete and intrinsically consistent characterization remains to be done to develop a reliable rheological model for future numerical simulations of mucus displacements in airways, in the case of pathologies such as cystic fibrosis. For this purpose, samples of mucus simulants were tested using a controlled stress rheometer. The material consists of macromolecular colloidal gels at different concentrations in macromolecules to mimic the variability in mucin production depending on the disease state and environmental factors. The rheological properties at rest were investigated using small amplitude oscillatory shear tests as a function of stress amplitude, frequency and temperature. They revealed that mucus simulant behaves as a gel within a defined linear viscoelastic region and as a viscoelastic liquid above the yield stress zone. To characterize the behavior of mucus in response to in vivo shearing induced by cough or by air flows produced by clearance helping devices, steady state flow tests have been performed. The steady state flow curves for various polymer concentrations are well fitted using a Herschel-Bulkley law. To account for the time dependent behavior of mucus, 3 intervals thixotropy tests were also implemented to monitor structure breakdown and buildup processes. The combination of all these measurements finally designs a reliable procedure accounting for the rheological complexities of mucus, that now needs to be applied to real mucus for validations. [ABSTRACT FROM AUTHOR]

Highly flexible nanocomposites based on room temperature vulcanizing (RTV) silicon rubber, conductive nano system comprised of high structure carbon black (CB) and graphite nano plates (GNP) were fabricated by solution processing method. Electrical properties and EMI shielding effectiveness of nanocomposites generated by various ratios of CB/GNP were measured and compared with the binary nanocomposites based on SR/CB and SR/GNP. The composites prepared by SR/CB showed higher electrical conductivity and lower percolation threshold than SR/GNP counterparts. The electrical properties of the hybrid composites showed to be governed by the major filler phase. AC conductivity of all binary and ternary composites showed increasing by frequency below percolation threshold without plateau (non-ohmic conduction), whereas above percolation threshold AC conductivity showed plateau within low frequency region, but sudden increase within high frequency region indicating non-ohmic conductivity. The dielectric and electrical analysis suggested that above percolation threshold direct contact between filler particles plays the main mechanism for the electric charge current throughout SR phase for all nanocomposites. EMI shielding effectiveness measurement results within X-band frequency revealed that absorption loss is the dominant mechanism for the attenuation of the intensity of incident electromagnetic wave in all nanocomposites generated by SR/CB, SR/GN, and SR/CB/GN systems. [ABSTRACT FROM AUTHOR]

The paper conserns to the problem of the equilibrium of a two-dimensional viscoelastic body having delaminated thin elastic inclusion. The formulation of the problem contains nonlinear conditions of inequality-type on the fracture curve as on a part of the boundary. This causes formulation of the problem in the form of variational inequality. The existence and uniqueness of solution of the problem is proved. The case of a thin rigid inclusion in a viscoelastic body is considered and it is shown that the problem of rigid inclusion is a limiting case for a family of problems on thin elastic inclusions as the stiffness parameter of the inclusion goes to infinity. [ABSTRACT FROM AUTHOR]

The unique solvability in the classical sense is studied for the initial problems to semilinear degenerate evolution equations with the fractional Caputo derivative. Results are obtained for arbitrary fractional order equations with a nonlinear operator not depending on the unknown function derivatives and depending on t explicitly. The abstract theorems are applied to solvability research for various degenerate in time partial differential equations from the theory of semiconductors. [ABSTRACT FROM AUTHOR]

The strain rate sensitivity index, m-value, is being applied as a common tool to evaluate the impact of the strain rate on the viscoplastic behaviour of materials. The m-value, as a constant number, has been frequently taken into consideration for modeling material behaviour in the numerical simulation of superplastic forming processes. However, the impact of the testing variables on the measured m-values has not been investigated comprehensively. In this study, the m-value for a superplastic grade of an aluminum alloy (i.e., AA5083) has been investigated. The conditions and the parameters that influence the strain rate sensitivity for the material are compared with three different testing methods, i.e., monotonie uniaxial tension test, strain rate jump test and stress relaxation test. All tests were conducted at elevated temperature (470°C) and at strain rates up to 0.1 s-1. The results show that the m-value is not constant and is highly dependent on the applied strain rate, strain level and testing method. [ABSTRACT FROM AUTHOR]

The paper is to describe the method for studying the rheological characteristics of elastomers using a multi-circuit electrical scheme of substitution, the synthesis of which is performed on the basis of experimental data obtained during the mechanical relaxation of loaded test samples at a fixed value of the relative deformation. In analyzing the fast and slow stages of the stress relaxation process in elastomer test specimens with significantly different viscoelastic properties, it is established that the number of relaxation mechanisms in the decomposition of the time dependence into exponentials does not exceed 6 (six). [ABSTRACT FROM AUTHOR]

In viscous materials systems, time and stress dependent instabilities often occur. The evolution of visco-elastic systems under external stress has already been modeled by applying matricial dynamic equations comprehending elasticity and viscosity matrices. In this study we report a novel formulation for such kind of systems as a nonlinear quadratic eigenvalue problem evolving from an already defined adjacency matrix. A four mass-spring damped system is presented as case study. [ABSTRACT FROM AUTHOR]

We study the dynamics of small perturbations to the rest state of a viscoelastic rate type fluid with temperature dependent material parameters. We show that if the material parameters are chosen appropriately, then the quiescent state of the fluid filling an isolated (mechanically, thermally) vessel is a stable state. The outlined analysis explicitly documents the importance of thermodynamic analysis in the development of advanced models for complex fluids. [ABSTRACT FROM AUTHOR]

For a model describing the behaviour of one-dimensional viscoelastic medium, all conservation laws are constructed. From a nonlinear partial differential equation, which describes the model, the conservation laws are derived by using the multipliers method. Furthermore, the Lie point symmetries of the equation and the Lie point symmetries associated with the conserved vectors are determined. Then, the classical symmetries are obtained by the Lie classical method. Finally, in order to find potential symmetries, the associated auxiliary system is obtained. However, applying the classical method to the associated auxiliary system yields to the classical symmetries too. [ABSTRACT FROM AUTHOR]

In this paper we discuss a one parameter modification of the well known fractional Maxwell model of viscoelasticity. Such models appear to be particularly interesting because they describe the short time asymptotic limit of a more general class of viscoelastic models known in the literature as Bessel models. [ABSTRACT FROM AUTHOR]