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The biaxial response of extruded PEEK films at conditions relevant to thermoforming has been investigated extensively using a combination of load-controlled (bulge test) and displacement-controlled (biaxial stretcher) experiments. Results from bulge testing yielded average and maximum strain rate ranges of 2.5-5 and 5-18 [s.sup.-1] respectively, across the forming temperature range. In-plane biaxial characterization highlighted the anisotropic nonlinear viscoelastic behavior of the films with strong dependence on the yield and strain hardening behavior on the temperature and strain history at conditions equivalent to the forming process. The combined approach to material characterization highlights the pros and cons of each test method, the complementary nature of the data generated, and the need to use both methods to have a complete data set for developing accurate material models and validated numerical simulations of thermoforming., INTRODUCTION Thermoforming is commonplace for the large-scale manufacture of thin-walled, polymeric products due to the repeatability of the process for complex parts, with the advantage of relatively cheap production costs. [...]

The three-dimensional generalized thermoviscoelastic shock plate problem is presented. Four thermoviscoelastic relaxations are used during this study, three of them are due to the generalized thermoelasticity models and the fourth is due to the viscosity parameter. The plate is thermally isolated at its bottom surface while its upper one is under a thermal shock. The transient thermal shock plate problem is presented according to a unified theory of generalized thermoelasticity. The classical coupled thermoelasticity model is used and two of its generalizations, namely Green-Lindsay and Lord-Shulman models, are also used. Normal mode analysis is adopted to get an analytical general solution of the present plate problem. The distributions of all variables are investigated along the plate directions. Different thermoelasticity theories are compared to present suitable conclusions. Some special findings are pointed out to show the effects of viscosity and other thermal relaxation parameters on thermoelastic interactions. Numerical results are plotted and tabulated to serve as benchmark results for future comparisons. Key words: thermoviscoelasticity, Lord-Shulman (L-S) model, three-dimensional solution, shock problem. Nous presentons le probleme 3D generalise d'une plaque soumise a un choc thermo- viscoelastique. Quatre relaxations thermo-viscoelastiques sont utilisees dans cette etude, trois qui sont dus aux modeles generalises de thermo-viscoelasticite et la quatrieme est du au parametre de viscosite. La plaque est isolee thermiquement a sa base et sa partie superieure est soumise a un choc thermique. Le regime transitoire de la plaque sous le choc est presente selon une theorie unifiee de la thermoelasticite. Nous utilisons le modele classique de la thermoelasticite, avec deux de ses generalisations, celle due a Green-Lindsay et l'autre a Lord-Shulman. Nous adoptons une analyse par modes normaux, afin d'obtenir une solution analytique generale au probleme de cette plaque. Toutes les variables sont examinees dans toutes les parties de la plaque. Differentes theories de thermoelasticite sont comparees pour affiner les conclusions. Nous soulignons des points speciaux pour illustrer les effets de la viscosite et d'autres parametres de relaxation thermique sur les interactions thermoelastiques. Les resultats numeriques sont presentes sous forme de graphiques et de tables, afin de servir de reference pour de futures comparaisons. [Traduit par la Redaction] Mots-cles : thermo-viscoelasticite, modele Lord-Schulman (L-S), solutions 3D, probleme de choc, thermiquement isole., 1. Introduction The viscoelastic response is of interest in several studies in the literature. According to generalized Hooke's law, all materials deviate in different ways, for example, by exhibiting viscous-like [...]

Experimental data are reported on poly(lactic acid) (PLA) in tensile loading-unloading tests and relaxation tests under stretching and retraction at temperatures ranging from room temperature up to 50[degrees]C. Two characteristic features of the time-dependent response of PLA are revealed: (i) with a decrease in minimum stress under retraction at a fixed temperature, relaxation curves change their shape from monotonically decaying with time (simple relaxation), to non-monotonic (mixed relaxation) to monotonically increasing (inverse relaxation) and (ii) with an increase in temperature, inverse relaxation after unloading down to the zero stress evolves into mixed relaxation with a pronounced shift of the peak position to smaller relaxation times. Constitutive equations are derived for the mechanical behavior of PLA, and adjustable parameters in the stress-strain relations are found by fitting the observations. Ability of the model to predict the time-dependent response under cyclic deformation is confirmed by numerical simulation., INTRODUCTION This article deals with experimental investigation and constitutive modeling of the time-dependent behavior of poly(lactic acid) (PLA) at temperatures T ranging from room temperature up to 50[degrees]C (slightly below [...]

Polypropylene and ethylene-propylene-diene terpolymer (PP/EPDM) blends were melt compounded in a new mixing device, designed in our laboratory under the trademark of RMX®, which predominantly generates elongational flows. Dispersion of the EPDM minor phase in PP was carried out in both RMX® and in an internal mixer (Haake Rheomix 600) at equivalent specific mixing energies and the resultant morphologies obtained by SEM were analyzed and compared. A better dispersive mixing efficiency of the RMX® mixer, i.e., lower [D.sub.n] and [D.sub.v] of the dispersed EPDM phase was observed. The impact of elongational flow was more pronounced for blends having a high viscosity ratio p, indicating an enhanced droplet break-up mechanism, which was attributed to the combination of high shear rates inside the mixing element and important elongational flows in the convergent/divergent zones. The morphology of the blends was correlated with their linear viscoelastic properties by using the Palierne model. Very good agreement was found for the PP/EPDM 80/20 blends but for higher EPDM content, the Palierne model failed to describe the rheological behavior, which was attributed to percolation of the minor phase with increasing the concentration. Higher elasticity at low frequencies was observed for blends processed in the RMX®, which was attributed to a higher generated interfacial area. POLYM. ENG. SCI., 54:1444-1457, 2014. © 2013 Society of Plastics Engineers, INTRODUCTION Polymer blending (or melt mixing) is a common route to obtain materials with specific properties based on each of the components used. From an industrial point of view, it [...]

The dynamical interactions caused by a line heat source moving inside a homogeneous isotropic thermo-microstretch viscoelastic half space, whose surface is subjected to a thermal load, are investigated. The formulation is in the context of generalized thermoelasticity theories proposed by Lord and Shulman (J. Mech. Phys. Solid, 15, 299 (1967)) and Green and Lindsay (Thermoelasticity, J. Elasticity, 2, 1 (1972)). The surface is assumed to be traction free. The solutions in terms of displacement components, mechanical stresses, temperature, couple stress, and microstress distribution are procured by employing the normal mode analysis. The numerical estimates of the considered variables are obtained for an aluminium-epoxy material. The results obtained are demonstrated graphically to show the effect of moving heat source and viscosity on the displacement, stresses, and temperature distribution. PACS No.: 44.10.+i. Resume : Nous etudions les interactions dynamiques causees par une source de chaleur lineaire se deplacant dans un demiespace d'un materiau viscoelastique micro-etirable a chaud isotrope et homogene, dont la surface est soumise a une charge thermique. Nous formulons le tout dans le cadre des theories de thermo-elasticite de Lord et Shulman (J. Mech. Phys. Solid, 15, 299 (1967)), ainsi que de Green et Lindsay (Thermoelasticity, J. Elasticity, 2,1 (1972)). La surface est supposee sans traction. L'utilisation de l'analyse en modes normaux permet de presenter les solutions en fonction des composantes de deplacement, des efforts mecaniques, du second gradient et de la distribution des micro-efforts. Les valeurs numeriques obtenues ici sont pour un materiau d'aluminium epoxy et sont presentees graphiquement pour illustrer l'effet du mouvement de la source et de la viscosite sur le deplacement, les efforts et la distribution de la temperature. [Traduit par la Redaction], 1. Introduction Micromorphic theory introduced by Eringen and Suhubi [1] and Eringen [2] treats a material body as a continuous collection of deformable point particles. Physically, the particles are point [...]

The poly ethylene terephthalate near the glass transition temperature highlights a strongly non linear elastic and viscous behaviour when biaxially stretched at high strain rates representative of the injection stretch blow moulding process. A non linear visco-hyperelastic model, where characteristics are coupled to the temperature, has already been identified from equi-biaxial tension experimental results. The weak form of the mechanical part of the model is presented and implemented into a finite element code developed in the Matlab environment and validated by comparing numerical simulation of equi-biaxial testing with the analytical solution in the isothermal case. Considering the thermal aspects, an experimental study, where PET sheets are heated using infrared (IR for short) lamps is also presented. The modeling of the IR radiation of the sheet helps to identify the thermal properties of the PET. The thermal model is then implemented in the finite element code, coupled to the 2D viscoelastic model. A discussion is made to justify the accuracy of the assumption made on homogeneity of the temperature field through the thickness. The simulation of the 20 plane stress equibiaxial test shows the important influence of the thermal aspects and the coupled thermo-mechanical software is used to quantify the self-heating phenomenon in the case of the biaxial elongations of PET sheets at high strain rates. POLYM. ENG. SCI., 53:2683-2695, 2013. © 2013 Society of Plastics Engineers, INTRODUCTION The injection stretch blow molding (ISBM) process that is managed at a temperature near or slightly above the glass transition temperature ([T.sub.g]) involves multiaxial large strains at high strain [...]

An attempt was made to explore the effects of the interchange reactions on the viscoelastic behavior of binary blends based on poly(ethylene terephthalate) (PET)/poly(ethylene naphthalate) (PEN) and their nanocomposites. It was seen that with an increase in the number of extrusion runs and mixing temperature, the extent of reaction (X) and degree of randomness (RD) both increased, whereas the average sequence block lengths values were decreased. On the contrary, the blend composition did not play a significant role on X and RD values. Addition of nanoclay inhibited the transreactions in PET/PEN blends. The absence of crystallization peaks implied that the crystalline structure was destroyed as a result of blending and an amorphous system was created possibly due to the transreactions simultaneously with the formation of random copolymers inhibiting the crystallization process. The rheological investigations showed that the addition of PEN into the PEN/PET blends enhanced the storage modulus, loss modulus, and complex viscosity. The viscosity upswing observed at low-frequency region in the case of nanocomposite systems evidently confirmed the occurrence of transreactions. Nonetheless, a significant increment in the viscoelastic properties was perceived in the presence of nanoclay corroborating the proper nanoclay distribution throughout the PET/PEN blend system. POLYM. ENG. SCI., 53:2556-2567, 2013. © 2013 Society of Plastics Engineers, INTRODUCTION The aromatic polyesters of unique properties are of substantial importance due to the desirable performance. They are well-suited materials for various applications because of their low permeability to gases, [...]

This paper presents an integrated experimental-numerical effort to more accurately model the damage characteristics of asphalt mixtures and pavement structures than conventional elastic and (or) linear viscoelastic approaches can. To this end, Schapery's nonlinear viscoelastic constitutive model was implemented into a finite element software via user defined subroutine (user material, or UMAT) to analyze an asphalt pavement subjected to heavy truck loads. Then, a series of creep and recovery tests were conducted at various stress levels and at different temperatures to obtain the stress-dependent and temperature-sensitive viscoelastic material properties of asphalt mixtures. With the viscoelastic material properties characterized and the UMAT code, a typical pavement structure subjected to repeated heavy truck loads was modeled with the consideration of the effect of material nonlinearity with a realistic tire loading configuration. Three-dimensional finite element simulations of the pavement structure present significant differences between the linear viscoelastic approach and the nonlinear viscoelastic modeling in the prediction of pavement performance with respect to rutting and fatigue cracking. The differences between the two approaches underline the importance of proper and more realistic characterization of pavement materials and should be addressed in the process of performance-based pavement design. Key words: nonlinear viscoelasticity, finite element method, asphalt, pavement, damage, performance modeling. Cet article presente une approche numerique experimentale integree visant a modeliser plus precisement les dommages aux melanges bitumineux et aux structures des chaussees que les approches viscoelastiques lineaires et-ou elastiques conventionnelles ne peuvent le faire. Le modele constitutif viscoelastique non lineaire de Schapery a ete implante dans un logiciel aux elements finis par un sous-programme configure par l'utilisateur (UMAT) afin d'analyser une chaussee bitumineuse soumise a des charges de poids-lourds. Ensuite, une serie d'essais de recuperation apres fluage a ete realisee a divers niveaux de contraintes et a differentes temperatures pour obtenir les proprietes viscoelastiques du materiel dependantes de la contrainte et sensibles a la temperature des melanges bitumineux. En se basant sur les proprietes viscoelastiques du materiel caracterisees et le code UMAT, une structure de chaussee typique soumise a des charges repetees de poids-lourds a ete modelisee en tenant compte de l'effet de la non-linearite du materiel sous une configuration realiste des charges de pneus. Des simulations tridimensionnelles par elements finis de la structure de chaussee montrent des differences importantes entre l'approche viscoelastique lineaire et la modelisation viscoelastique non lineaire dans la prevision du rendement de la chaussee en ce qui a trait a l'ornierage et a la fissuration par fatigue. Les differences entre les deux approches soulignent l'importance d'une caracterisation adequate et plus realiste des materiaux des chaussees et elles devraient etre abordees lors du processus de conception des chaussees basee sur le rendement. [Traduit par la Redaction] Mots-cles: viscoelasticite non lineaire, methode par elements finis, asphalte, chaussee, dommage, modelisation du rendement., Introduction Trucking is a key component of freight transportation in the US. Trucks moved 67% of the total tonnage and 65% of the total value of US shipments in 2009. [...]

Based on the experimental observations, a cyclic nonlinear viscoelastic constitutive model was proposed to describe the uniaxial time-dependent ratcheting of polyetherimide (PEI) polymer under tension-compression and tension-tension cyclic loading. The model was constructed by extending the nonlinear viscoelastic Schapery model (Schapery, Polyrn. Eng. Sci., 9, 295 (1969)). The extension emphasized the changes of parameter functions used in the original model, which enabled the model to describe the ratcheting of polymer material. Comparing the simulations with corresponding experimental results, the capability of the extended model to predict the uniaxial time-dependent ratcheting of PEI was verified. It is shown that the extended model can reasonably describe the uniaxial time-dependent retcheting of the polymer under the tension-compression and tension-tension cyclic loading with different peakholdings, stress rates, and stress levels. POLYM. ENG. SCI., 52:1874-1881, 2012. © 2012 Society of Plastics Engineers, INTRODUCTION Time-dependent ratcheting of polymer materials is of great importance in the safety design and life assessment of polymer structure components. In the last decades, the cyclic deformation of polymer [...]

As process capabilities become more advanced, the need to predict flow phenomena at a smaller scale increases significantly. Viscoelastic secondary flows in square ducts were simulated using a finite volume approach. Single mode and multimode Giesekus and Phan-Thien Tanner (PTT) models were implemented and were able to reproduce full three-dimensional (3D) flow through a square duct. Results for low density poly-ethylene (LDPE), polystyrene, and polycarbonate are all in agreement with experiments [Dooley, Viscoelastic flow effects in multilayer polymer coextrusion, PhD The sis, Technische Universiteit Eindhoven (2002)] as well as numerical results using a finite element method (FEM) and a meshless radial function method (RFM), [Lopez et al., SPE ANTEC Tech. Pap. (2010)]. The mathematical model presented here has shown the potential to model full 3D flow in more complex geometries. POLYM. ENG. SCI., 52:1715-1723, 2012. © 2012 Society of Plastics Engineers, INTRODUCTION In the last decade, researchers have been able to successfully simulate viscoelastic flows in extruding channel geometries in two-dimensional and three-dimensional space, but the techniques are still emerging from [...]

In this paper the closed form expressions for the transverse vibrations of a homogenous, isotropic, thermally conducting, Kelvin-Voigt type viscothermoelastic thin beam with variable thickness, based on Euler-Bernoulli theory have been derived. The effects of relaxation times, thermomechanical coupling, surface conditions, and beam dimensions on energy dissipation induced by thermoelastic damping in microelectromechanical systems resonators are investigated for beams under clamped and simply supported conditions. Analytical expressions for deflection, temperature change, frequency shifts and thermoelastic damping in the beam have been derived. Some numerical results with the help of MATLAB programming software in the case of silicon nitride have also been presented. The computer simulated results with respect to the damping factor and frequency shift have been presented graphically. PACS Nos: 46.40.Ff, 46.70.De, 46.35.+z, 46.25.Hf, 62.20.D-, 81.40.Jj, 07.05.Tp, 85.85.+j Dans le cadre de la theorie de Euler-Bernouilli, nous obtenons ici des expressions en forme fermee pour les vibrations transverses d'une poutre mince d'un materiau visco-thermoelastique du type Kelvin-Voigt, conducteur de chaleur, homogene isotrope et d'epaisseur variable. Nous etudions les effets des temps de relaxation, du couplage thermomecanique, des conditions de surface et des dimensions de la poutre sur la dissipation d'energie induite par l'amortissement thermoelastique dans des resonateurs systemes microelectromecaniques pour des poutres fixees ou simplement supportees. Nous obtenons des resultats analytiques pour la deflexion, la variation en temperature, le deplacement de frequence et l' amortissement thermoelastique dans la poutre. Nous presentons aussi certains resultats de simulation numerique du nitrure de silicium obtenus en utilisant MATLAB. Nous presentons graphiquement les resultats de simulation numerique concernant le facteur d'amortissement et le deplacement de frequence. [Traduit par la Redaction], 1. Introduction It is well known that in many cases, nonuniform beams may achieve a better distribution of strength and weight than uniform beams and sometimes can satisfy special architectural [...]

Residual stresses and thermoviscoelastic deformation of a laminated film utilized for film insert molding was investigated through measurement of thermal expansion coefficient (CTE) and relaxation modulus. Thermoviscoelastic deformation of the film was also analyzed with numerical analysis by applying measured relaxation modulus, CTE, and residual stress to finite element method (FEM). Stress relaxation of the pristine film showed significantly different behavior from that of the unannealed film during annealing. Effects of the CTE and relaxation modulus on the thermoviscoelastic deformation were predicted by considering thermal shrinkage and structural relaxation. Moreover, numerical results on thermoviscoelastic deformation were in good agreement with experiments when initial stress distribution in the solid specimen was applied to the numerical analysis. POLYM. ENG. SCI., 52:1121-1127, 2012. © 2011 Society of Plastics Engineers, INTRODUCTION Film insert molding (FIM), known as in-mold decorating, is one of the various coating technologies to perform surface modification of injection molded parts. In general, a film is inserted [...]

Influence of filler network on Payne effect and modulus recovery for vapor grown carbon nanofiber (VGCF)/poly-styrene (PS) composites with VGCF content above electrical percolation threshold was studied by using simultaneous measurements of viscoelasticity and electrical conductivity. The strain softening seems to be closely related to breakdown of filler network. Recovery tests of modulus and electrical conductivity by means of time sweep indicate that the reformation of deformed VGCF network structure could not be completed in several hours. Compared with recovery behavior of carbon black (CB) and silica (Si[O.sub.2]) network, the reformation of VGCF network appears more difficult. Moreover, solidification of composites exerts some effect on modulus recovery. The filler network disrupted by small strain can be perfectly recovered by matrix solidification while the initial filler structure collapsing at large strain is only partially restored. POLYM. ENG. SCI., 52:643-648, 2012. © 2011 Society of Plastics Engineers, INTRODUCTION Performances of filled polymers are greatly related to the characteristic and the dispersion state of fillers. The relationship between filler structure and mechanical properties has been investigated intensively in [...]

Polymer chains consisting of water-soluble polyacryl-amides and N-isopropylacrylamide (NIPAM), hydro-phobically modified with low amounts of N, N-dialkyla-crylamides (N, N-dioctylacrylamide) have been prepared via free radical micellar polymerization, using a hydrophobic initiator derived from 4,4'-azobis(4-cyanopenta-noic acid) containing a long linear chain of 16 (C16) carbon atoms. This procedure resulted in polyacryl-amides containing hydrophobic groups along the chain as well as at the chain ends. These polymers are named 'combined associative polymers* and include within their structure a water-soluble monomer (acrylamide), a thermosensitive monomer (NIPAM) and a hydrophobic monomer. The polymers were characterized by different techniques, also studying the effect of adding a surfactant anion such as sodium dodecylsul-fate. The viscoelastic properties as a function of temperature of these associative polymers were investigated using steady-flow and oscillatory experiments considering the relaxation time (7'R) and the plateau modulus (G0). The effect of concentration of acrylamide and NIPAM on the viscosity of the associative polymer solutions was investigated. POLYM. ENG. SCI., 51:2473-2482, 2011. ©2011 Society of Plastics Engineers, INTRODUCTION The hydrophobic modified water-soluble polymers are a class of partially ordered complex systems that have captured the scientific interest in recent years (1-3). These materials are composed of a [...]

Although the mechanical properties of ligament and tendon are well documented in research literature, very few unified mechanical formulations can describe a wide range of different loadings. The aim of this study was to propose a new model, which can describe tendon responses to various solicitations such as cycles of loading, unloading, and reloading or successive relaxations at different strain levels. In this work, experiments with cycles of loading and reloading at increasing strain level and sequences of relaxation were performed on white New Zealand rabbit Achilles tendons. We presented a local formulation of thermodynamic evolution outside equilibrium at a representative element volume scale to describe the tendon's macroscopic behavior based on the notion of relaxed stress. It was shown that the model corresponds quite well to the experimental data. This work concludes with the complexity of tendons' mechanical properties due to various microphysical mechanisms of deformation involved in loading such as the recruitment of collagen fibers, the rearrangement of the microstructure (i.e., collagens type I and III, proteoglycans, and water), and the evolution of relaxed stress linked to these mechanisms. [DOI: 10.1115/1.4002552] Keywords: mechanical properties, tendon, Achilles tendon, loading cycles, relaxation, relaxed stress

The liver harvested from a donor must be preserved and transported to a suitable recipient immediately for a successful liver transplantation. In this process, the preservation period is the most critical, since it is the longest and most tissue damage occurs during this period due to the reduced blood supply to the harvested liver and the change in its temperature. We investigate the effect of preservation period on the dynamic material properties of bovine liver using a viscoelastic model derived from both impact and ramp and hold experiments. First, we measure the storage and loss moduli of bovine liver as a function of excitation frequency using an impact hammer. Second, its time- dependent relaxation modulus is measured separately through ramp and hold experiments performed by a compression device. Third, a Maxwell solid model that successfully imitates the frequency- and time-dependent dynamic responses of bovine liver is developed to estimate the optimum viscoelastic material coefficients by minimizing the error between the experimental data and the corresponding values generated by the model. Finally, the variation in the viscoelastic material coefficients of bovine liver are investigated as a function of preservation period for the liver samples tested 1 h, 2 h, 4 h, 8 h, 12 h, 24 h, 36 h, and 48 h after harvesting. The results of our experiments performed with three animals show that the liver tissue becomes stiffer and more viscous as it spends more time in the preservation cycle. [DOI: 10.1115/1.4002489]

A visco-elastohydrodynamic model of a hydraulic rod seal during transient operation has been developed. The model includes analyses of the macro- and microscale deformation mechanics and contact mechanics, and the microscale fluid mechanics. Viscoelasticity enters the analysis through the deformation mechanics and through the contact mechanics. A hybrid finite element-finite volume computational framework is developed to solve the highly coupled governing equations. Viscoelasticity is seen to affect the leakage and friction characteristics of the seal through its effects on the changing fluid pressure and contact pressure distributions as the rod velocity and sealed pressure change during a cycle. Compared with purely elastic behavior, viscoelasticity increases the fluid pressure and the contact pressure significantly in the sealing region closest to the sealed end, shifts the fluid pressure peaks away from the sealed end during the instroke, and enhances the cavitation during the outstroke. It results in thicker fluid films and produces a significant increase in the Poiseuille flow during the instroke. [DOI: 10.1115/1.4002542] Keywords: reciprocating seal, rod seal, hydraulic seal, elastohydrodynamics, viscoelasticity, finite element

The complex micro-/nanostructure of native cartilage-to-bone insertion exhibits gradations in extracellular matrix components, leading to variations in the viscoelastic and biomechanical properties along its thickness to allow for smooth transition of loads under physiological movements. Engineering a realistic tissue for osteochondral interface would, therefore, depend on the ability to develop scaffolds with properly graded physical and chemical properties to facilitate the mimicry, of the complex elegance of native tissue. In this study, polycaprolactone nanofiber scaffolds with spatially controlled concentrations of [beta]-tricalcium phosphate nanoparticles were fabricated using twin-screw extrusion-electrospinning process and seeded with MC3T3-E1 cells to form osteochondral tissue constructs. The objective of the study was to evaluate the linear viscoelastic and compressive properties of the native bovine osteochondral tissue and the tissue constructs formed in terms of their small-amplitude oscillatory shear, unconfined compression, and stress relaxation behavior. The native tissue, engineered tissue constructs, and unseeded scaffolds exhibited linear viscoelastic behavior for strain amplitudes less than 0.1%. Both native tissue and engineered tissue constructs demonstrated qualitatively similar gel-like behavior as determined using linear viscoelastic material functions. The normal stresses in compression determined at 10% strain for the unseeded scaffold, the tissue constructs cultured f or four weeks, and the native tissue were 0.87 [+ or -] 0.08 kPa, 3.59 [+ or -] 0.34 kPa, and 210.80 [+ or -] 8.93 kPa, respectively. Viscoelastic and biomechanical properties of the engineered tissue constructs were observed to increase with culture time reflecting the development of a tissuelike structure. These experimental findings suggest that viscoelastic material functions of the tissue constructs can provide valuable inputs for the stages of in vitro tissue development. [DOI: 10.1115/1.4001884] Keywords: functionally graded, interface, osteochondral, scaffold, tissue engineering, electrospinning

While the individual tissues of the vertebral joint demonstrate viscoelastic properties, the global viscoelastic properties of the lumbar vertebral joint are not well established. This study investigated how changes in displacement rate influenced the mechanical response of the porcine cervical spine (a surrogate or model for the human lumbar spine) exposed to acute anterior shear failure loading. Thirty porcine cervical spine specimens (15 C3-C4 and 15 C5-C6) were placed under a 1600 N compressive load and subsequently loaded in anterior shear to failure at one of three randomly assigned displacement rates (1 mm/s, 4 mm/s, or 16 mm/s). Ultimate anterior shear force, ultimate displacement, average stiffness, and energy stored until failure were calculated. Load rate in the elastic region was also calculated to compare the load rates used in this study to those used in previous studies. Changes in displacement rate affected the C3-C4 and C5-C6 specimens differently. C5-C6 specimens tested at 16 mm/s had an ultimate force that was 28% and 23% higher than at 1 (p = 0.0215) and 4 mm/s (p = 0.0461), respectively. The average stiffness to failure of the C5-C6 specimens tested at 16 mm/s was 52% higher than at 4 mm/s (p = 0.0289). No such differences were found for the C3-C4 specimens. An increase in the anterior shear displacement rate did not necessarily demonstrate viscoelasticity of the vertebral joint. Specimen intervertebral levels were affected differently by changes in anterior shear displacement rate, which may have been a result of anatomical and postural differences between the two levels. Future studies should further investigate the effect of displacement rate on the spine and the inconsistencies between different specimen levels. [DOI: 10.1115/1.4001885]

Because of its efficiency in analyzing complex viscoelastic problems, the finite-element (FE) analysis has been widely used to identify the time--and rate-dependent effects of viscoelastic materials on various structural conditions. When performing the FE analysis on a viscoelastic structure, most FE programs require fundamental material properties, shear and bulk moduli, of the given viscoelastic material as their input. However, the shear and bulk modulus tests are difficult to perform, so they have been commonly estimated from a single material test on the basis of the assumption that the Poisson's ratio of viscoelastic materials is a time- independent constant. Such an assumption, however, might not be suitable because the Poisson's ratio of the viscoelastic materials is also a function of time. Therefore, this study developed computation algorithms for determining the time-dependent Poisson's ratio and shear and bulk moduli of asphalt mixtures, which have been well recognized as a viscoelastic material, by employing the indirect tension testing system. The shear and bulk moduli determined by the developed approach appear to be reasonable and realistic. Their applicability and reliability were also verified by comparing experimental data to the results of the FE analysis performed on the same circular specimen as that used in the indirect tension creep test. DOI: 10.1061/(ASCE)EM.1943-7889.0000151 CE Database subject headings: Viscoelasticity; Poisson ratio; Material properties; Finite element method; Creep. Author keywords: Viscoelasticity; Poisson ratio; Material properties; Finite element method; Creep.

This study investigates the energy relations and dissipations in viscoelastic pipeline under fluid transients. The investigation is carried out analytically using energy analysis and Fourier transform techniques for viscoelastic waterhammer governing equations. The analytical results show that the viscoelastic term in waterhammer models is wrongly referred to in literature as being wave damping/dissipation when in actual fact it is the work done by the fluid on the pipe and vice versa. The energy dissipation is actually occurred in the pipe-wall due to the viscoelastic material strain. Moreover, the energy transfer/exchange between the fluid and pipe-wall and energy dissipation in the pipe-wall due to viscoelasticity effect is relating to the ratio of the pipe viscoelastic frequency and the fluid wave frequency. [DOI: 10.1115/1.4000915] Keywords: viscoelasticity, pipe system, fluid transients, energy analysis, wave dissipation

Generally speaking, impacts are events of very short duration and a common problem in machine dynamics. During impact, kinetic energy is lost due to plastic deformation near the contact area and excitation of waves. Macromechanically, these kinetic energy losses are often summarized and expressed by a coefficient of restitution, which is then used for impact treatment in the analysis of the overall motion of machines. Traditionally, the coefficient of restitution has to be roughly estimated or measured by experiments. However, more recently finite element (FE) simulations have been used for its evaluation. Thereby, the micromechanical plastic effects and wave propagation effects must be understood in detail and included in the simulations. The plastic flow, and thus the yield stress of a material might be independent or dependent of the strain-rate. The first material type is called elastic-plastic and the second type is called elastic-viscoplastic. In this paper, the influence of viscoplasticity of aluminum and steel on the impact process and the consequences for the coefficient of restitution is analyzed. Therefore, longitudinal impacts of an elastic, hardened steel sphere on aluminum AL6060 rods and steel S235 rods are investigated numerically and experimentally. The dynamic material behavior of the specimens is evaluated by split Hopkinson pressure bar tests and a Perzyna-like material model is identified. Then, FE impact simulations and impact experiments with laser-doppler-vibrometers are performed. From these investigations it is shown that strain-rate effects of the yield stress are extremely small for impacts on aluminum but are significant in impacts on steel. In addition, it is demonstrated that it is possible to evaluate for both impact systems the coefficient of restitution numerically, whereas for the aluminum body a simple elastic-plastic material model is sufficient. However, for the steel body an elastic-viscoplastic material model must be included. [DOI: 10.1115/1.4000912] Keywords: impact, coefficient of restitution, aluminum, steel finite elements, experiment, strain-rate effect, plasticity

This paper proposes a new approach for the reduction in the model-order of linear multiple-degree-of freedom viscoelastic systems via equivalent second-order systems. The assumed viscoelastic forces depend on the past history of motion via convolution integrals over kernel functions. Current methods to solve this type of problem normally use the state-space approach involving additional internal variables. Such approaches often increase the order of the eigenvalue problem to be solved and can become computationally expensive for large systems. Here, an approximate reduced second-order approach is proposed for this type of problems. The proposed approximation utilizes the idea of generalized proportional damping and expressions of approximate eigenvalues of the system. A closed-form expression of the equivalent second-order system has been derived. The new expression is obtained by elementary operations involving the mass, stiffness, and the kernel function matrix only. This enables one to approximately calculate the dynamical response of complex viscoelastic systems using the standard tools for conventional second-order systems. Representative numerical examples are given to verify the accuracy of the derived expressions. [DOI: 10.1115/1.4000913]

The behavior of a one-dimensional mass-rubber band oscillator is investigated experimentally. The data show clear evidence for viscoelastic behavior and can be interpreted in terms of a simple oscillator model consisting of a mass connected to a four parameter viscoelastic element. The model displays the observed crossover in the dynamic response. The success and limitations of the model and the pedagogical relevance of the experiment are discussed. [c] 2010 American Association of Physics Teachers. [DOI: 10.1119/1.3276052]

All characterization of asphalt mixtures must begin with the properties of the mixture in an undamaged state. Subsequent tests of the mixture in different stress or strain states at a level to cause damage can be referred to this undamaged state to assess the degree to which the mixture has been changed. All forms of such change from an original state by such damaging processes as fatigue, plasticity, healing, moisture damage, and aging can only be properly assessed by comparison with an accurately measured undamaged state. This is particularly the case with the use of pseudostrain dissipated energy to characterize the departure of a material from an original linear viscoelastic state. This paper presents a new test and data analysis protocol based on linear viscoelasticity theory to characterize the master curves of the viscoelastic properties of an asphalt mixture. Instead of running a relaxation modulus test in which controlling the test apparatus is a serious challenge, the proposed test protocol applied a uniaxial monotonically increasing tensile stress to the test specimen. The axial and radial deformations of the specimen were recorded and used to calculate the axial and radial strains. The uniaxial tensile loading rate and time were carefully controlled to assure that the strain of the specimen was limited to the small-strain condition (< 100 [mu][epsilon]) under which the specimen was assumed not to be damaged during the test. The applied stress and measured strain were fitted with functions of time. Applying the Laplace transform to the stress and strain functions, the relaxation modulus as a function of time was determined using the Boltzmann superposition principle and the convolution theorem. The relation between the relaxation modulus and the complex modulus was used to determine the complex modulus as a complex function of frequency. Then the magnitude and phase angle of the complex modulus were obtained from the real part and imaginary part of the complex modulus. The proposed test and data analysis protocol were performed on the same specimen at three temperatures, 10, 20, and 30[degrees]C, so master curves of the magnitude and phase angle of the complex modulus were constructed using the time-temperature superposition principle. The master curves of the magnitude and phase angle of the complex modulus were fitted with mathematical functions that were reported in the literature and were modified to be more comprehensive. The parameters in the mathematical functions were searched simultaneously using the Solver Function built into the Microsoft Excel. Compared to the traditional relaxation modulus and the dynamic modulus test protocols, the newly developed test protocol was more efficient to characterize the viscoelastic properties of asphalt mixtures. The newly developed test method did not introduce any damage to the specimen so the same specimen may be retested for its fatigue, healing, and other properties. DOI: 10.1061/(ASCE)TE.1943-5436.0000083 CE Database subject headings: Asphalts; Mixtures; Tensile strength: Viscoelasticity; Material properties. Author keywords: Asphalt mixture; Tensile properties: Complex modulus: Phase angle; Master curve; Viscoelasticity; Laplace transform; Boltzmann superposition; Correspondence principle.

The dynamic viscoelastic properties of uncrosslinked and dynamically crosslinked blends of polypropylene (PP) and ethylene octene copolymer (EOC) were investigated in the melt state to study the mechanism of reinforcement, influence of particle size, and kinetics of modulus recovery. Dynamic vulcanization was performed by coagent assisted peroxide cross-linking system. Addition of peroxide in PP/EOC blend involves two major competing reactions: crosslinking in EOC and degradation of PP by β chain-scission. In this article, morphological and melt rheological properties of the TPVs were studied with special reference to the effect of mixing protocol. Three different mixing techniques were investigated. They are: (i) conventional or preblending method--melt mixing of PP and EOC followed by dynamic vulcanization (ii) phase mixing method--curative master batch of EOC added on the molten PP (iii) split addition of PP--preblending method followed by addition of half part of PP (dilution procedure). The type of mixing protocol has a significant influence on the rheological behavior. Rheological properties have been evaluated at the processing temperature (180°C) in a Rubber Process Analyzer (RPA 2000). A variety of rheological observations such as Payne effect, modulus recovery and shear rate sensitivity were studied by carrying out frequency and strain amplitude sweeps. POLYM. ENG. SCI., 50:455-467, 2010. [C] 2009 Society of Plastics Engineers, INTRODUCTION Thermoplastic vulcanizates (TPVs) are prepared by the dynamic vulcanization process, where crosslinking of an elastomer takes place during its melt mixing with a thermoplastic polymer under high shear. The [...]

In order to study the dynamic response of an asphalt road, a dynamic model of the road under a moving load is proposed, in which the viscoelastic characteristics of the base and pavement are all considered, the pavement is regarded as an infinite beam on a Kelvin viscoelastic base. By using Green's functions, Laplace transforms, and Fourier transforms, the analytical solution in transient is deduced. As the viscosity of the pavement is included in the model, the analytical solution can be used to investigate more of the factors that affect the dynamic response, such as vehicle speed, temperature, and road material properties. Using this analytical solution, some numerical calculations are given to illustrate the effects of vehicles' speeds and different damping on the deflection with the displacement. DOI: 10.1061/(ASCE)0733-9399(2010)136:2(168) CE Database subject headings: Moving loads; Viscoelasticity; Pavements; Transient response. Author keywords: Moving load; Viscouselastic base; Viscouselastic pavement; Transient response; Analysis solution.

Steady-state periodical response is investigated for planar vibration of axially moving viscoelastic beams subjected external transverse loads. A model of the coupled planar vibration is established by introducing a coordinate transform. The model can reduce to two nonlinear models of transverse vibration. The finite difference scheme is developed to calculate steady-state response numerically. Numerical results demonstrate there are steady-state periodic responses in transverse vibration, and resonance occurs if the external load frequency approaches the linear natural frequencies. The effect of material parameters and excitation parameters on the amplitude of the steady-state responses are examined. Numerical results also indicate that the model of coupled vibration and two models of transverse vibration predict qualitatively the same tendencies with the changing parameters, and the two models of transverse vibration yield satisfactory results. [DOI: 10.1115/1.4000468] Keywords: vibration, nonlinearity, axially moving beam, viscoelasticity, finite difference

Transport phenomena in three-dimensional branching channel are important because of their relevance in polymer processing. In this article, an experimental study on viscoelastic flow in a three-dimensional cylindrical branching channel is carried out to investigate variations of flow pattern. Flow visualization in representative symmetric planes is made both for the viscoelastic fluid and Newtonian flow. From the results of the present investigation, the flow field in the three-dimensional cylindrical branching channel is clarified within the range of laminar flow. It is confirmed that corner vortex, shedding vortex, and secondary vortex flow are all obviously changed with the fluid concentration and the Reynolds number, which are much more three-dimensional and complex than the Newtonian fluid, and the flow pattern of the viscoelastic fliud flow largely depends on the Reynolds number and fluid concentration. Even for the viscoelastic flow at the low Reynolds number, shedding vortex and secondary vortex and complex three-dimensional flow occur in the cylinder. The flow field is not symmetric space for the viscoelastic flow and however is fairly symmetric for the Newtonian fluid. The above reasons explain why the flow deflection happens even at the low Reynolds number flow. POLYM. ENG. SCI., 50:32-42, 2010. [C]2009 Society of Plastics Engineers, INTRODUCTION Internal laminar flows in an abrupt change in the area of cross-section are widely studied and mechanisms and basic hydrodynamic characteristics are fairly well understood for simple channel geometries. [...]

The low viscosity of asphalt concrete at T> 135 [degrees]C is necessary for the construction and compaction process of high-quality asphalt concrete layers. Whereas the continuous increase of viscosity with decreasing temperature is desirable for the reduction of permanent deformations during warm periods, so-called top-down cracking may occur in the course of temperature drops during cold periods. In order to explain the complex viscoelastic properties of asphalt concrete, a multiscale model is proposed. Hereby, the viscoelastic behavior of bitumen serves as input and the effect of air voids and aggregates is investigated. The viscous properties of bitumen are identified, using the bending-beam rheometer and the dynamic-shear rheometer, providing access to the viscoelastic material response for different temperature and loading regimes. With the rheological properties of bitumen at hand, the viscoelastic properties of mastic, mortar, and asphalt concrete are determined using continuum micromechanics, employing the elastic-viscoelastic 'correspondence principle.' The creep properties of asphalt concrete obtained from the presented mode of upscaling from the bitumen to the asphalt scale is validated by means of cyclic tests and static uniaxial creep tests. DOI: 10.1061/(ASCE)0899-1561 (2009)21:12(771) CE Database subject headings: Bitumen; Asphalts; Concrete; Material properties; Micromechanics; Creep; Viscoelasticity; Predictions.

There is still no agreement on the nature of tissues' viscoelasticity and on its reliable modeling. We speculate that disagreements between previous observations stem from difficulties of separating between viscoelastic and preconditioning effects, since both are manifested by similar response features. Here, this and related issues were studied in the tendon as a prototype for other soft tissues. Sheep digital tendons were preconditioned under strain that was higher by 1% than the one used in subsequent testing. Each specimen was then subjected to stress relaxation, and quick release or creep. A stochastic microstructural viscoelastic theory was developed based on the collagen fibers' properties and on their gradual recruitment with stretch. Model parameters were estimated from stress relaxation data and predictions were compared with the creep data. Following its validation, the new recruitment viscoelasticity (RVE) model was compared, both theoretically and experimentally, with the quasilinear viscoelastic (QLV) theory. The applied preconditioning protocol produced subsequent pure viscoelastic response. The proposed RVE model provided excellent fit to both stress relaxation and creep data. Both analytical and numerical comparisons showed that the new RVE theory and the popular QLV one are equivalent under deformation schemes at which no fibers buckle. Otherwise, the equivalence breaks down; QLV may predict negative stress, in contrast to data of the quick release tests, while RVE predicts no such negative stress. The results are consistent with the following conclusions: (1) fully preconditioned tendon exhibits pure viscoelastic response, (2) nonlinearity of the tendon viscoelasticity is induced by gradual recruitment of its fibers, (3) a new structure-based RVE theory is a reliable representation of the tendon viscoelastic properties under both stress relaxation and creep tests, and (4) the QLV theory is equivalent to the RVE one (and valid) only under deformations in which no fibers buckle. The results also suggest that the collagen fibers themselves are linear viscoelastic. Keywords: tendon, viscoelastic properties, preconditioning, fibers recruitment [DOI: 10.1115/1.3212107]

The dynamic behavior of sandwich beams with a viscoelastic soft core is analytically studied. The analysis combines the concepts of viscoelasticity with those of the high-order sandwich beam theory and accounts for the shear and transverse (through the thickness) deformability and the viscoelastic effects through the constitutive laws of the core material. The beam model assumes no variation through the width. The viscoelastic response is introduced through the Kelvin--Voigt and Maxwell models, which characterize different materials and provide some of the fundamental building blocks for more refined viscoelastic theories. The governing equations that correspond to the two models and the solution procedures that combine time integration through Newmark's method with a numerical solution in space are derived. The formulation of a refined viscoelastic model based on the building blocks developed in the paper is analytically demonstrated. The capabilities of the model are studied numerically with emphasis on the time variation of the deflections and the interfacial shear and vertical normal stresses under step, impulse, and harmonic loads. A comparison between the theoretical results and experimental results available in the literature is also presented. The analytical models and the numerical study highlight some unique aspects of the dynamic response of sandwich beams with viscoelastic soft cores and provide fundamental analytical tools for their modeling. DOI: 10.2514/1.41840

Airway smooth muscle (ASM) is cyclically stretched during breathing, even in the active state, yet the factors determining its dynamic force-length behavior remain incompletely understood. We developed a model of the activated ASM strip and compared its behavior to that observed in strips of rat trachealis muscle stimulated with methacholine. The model consists of a nonlinear viscoelastic element (Kelvin body) in series with a force generator obeying the Hill force-velocity relationship. Isometric force in the model is proportional to the number of bound crossbridges, the attachment of which follows first-order kinetics. Crossbridges detach at a rate proportional to the rate of change of muscle length. The model accurately accounts for the experimentally observed transient and steady-state oscillatory force-length behavior of both passive and activated ASM. However, the model does not predict the sustained decrement in isometric force seen when activated strips of ASM are subjected briefly to large stretches. We speculate that this force decrement reflects some mechanism unrelated to the cycling of crossbridges, and which may be involved in the reversal of bronchoconstriction induced by a deep inflation of the lungs in vivo. crossbridge kinetics; viscoelasticity; deep lung inflation; computational model; rat trachealis; deep inspiration

An analytical solution of steady-state dynamic response of a multilayered viscoelastic medium to a moving distributed load is obtained using a novel approach that combines transfer matrix method with Sun's convolution representation integrated over impulse response function of the layered medium. The layered media under consideration include elastic and viscoelastic media with four different viscoelastic constitutive models, while the moving load is allowed to have a circular spatial distribution, which is more realistic for mimicking tire footprint than a commonly used point load. Efficient numerical algorithms based on fast evaluation of various integral transformations and their inversions are developed and validated through numerical example. [DOI: 10.1115/1.3086586] Keywords: multilayered medium, viscoelastic, steady-state response, wave equation, integral transform, convolution

Conformational changes of calmodulin (CAM) by additions of [Ca.sup.2+] ions and bindings of CaM-binding peptides to [Ca.sup.2+]/CaM followed by conformational changes were monitored by a CaM-immobilized 27 MHz quartz-crystal microbalance (QCM) with an admittance analysis. Both the binding and the conformational change events could be detected from the time-dependence of frequency decreases (mass increases) and energy dissipation decreases (elasticity increases), respectively. When [Ca.sup.2+] ions were injected to a QCM cell on which biotinylated CaM was immobilized with avidin--biotin interactions, a frequency increase (a mass decrease) and an energy dissipation decrease (an elasticity increase) were observed because of the dehydration and the elasticity increase caused by conformational changes from the flexible CaM to the rigid [Ca.sup.2+]/CaM exposing the hydrophobic surface. In the case of the addition of CaMKII-peptide in the [Ca.sup.2+]/ CaM-immobilized QCM, the immediate frequency decrease (the mass increase) due to the binding of the peptide to [Ca.sup.2+]/CaM and the following energy dissipation decrease (the elasticity increase) with a time lag were observed. This suggests that the interaction of the CaMKII-peptide to [Ca.sup.2+]/CaM follows an allosteric binding mode. Binding kinetics of the peptide to [Ca.sup.2+]/ CaM ([k.sub.1] and [k.sub.-1]) and kinetics of the following conformational change of [Ca.sup.2+]/CaM ([k.sub.2] and [k.sub.-2]) could be obtained. This technique is useful to investigate biomolecular interactions involving the conformational and/or viscoelastic property changes that are biologically important.

In this paper, we analyze how elastic and viscoelastic properties differ across seven locations along the large arteries in 11 sheep. We employ a two-parameter elastic model and a four-parameter Kelvin viscoelastic model to analyze experimental measurements of vessel diameter and blood pressure obtained in vitro at conditions mimicking in vivo dynamics. Elastic and viscoelastic wall properties were assessed via solutions to the associated inverse problem. We use sensitivity analysis to rank the model parameters from the most to the least sensitive, as well as to compute standard errors and confidence intervals. Results reveal that elastic properties in both models (including Young's modulus and the viscoelastic relaxation parameters) vary across locations (smaller arteries are stiffer than larger arteries). We also show that for all locations, the inclusion of viscoelastic behavior is important to capture pressure-area dynamics. Index Terms--Arterial wall properties, data analysis, mathematical modeling, mechanics, parameter estimation, viscoelasticity.

A quartz crystal microbalance (QCM) consists of a resonator, which measures the resonance frequency of the quartz slab. When coupled with a network analyzer or coupled with impulse excitation technology, QCM gives additional impedance or dissipation information, respectively. This report provides a set of equations that bring the QCM community a convergence of the dissipation and impedance analysis. Equations derived from the complex frequency shift were applied to quantitatively analyze the dissipation data of polymer brushes obtained from QCMD. The obtained viscoelastic properties of polymer brushes were then compared with those obtained by the Voigt model method. We believe that these equations will be useful in quantitative studies of interfacial phenomena accompanied with mass or viscoelasticity changes.

In the present paper we first present a derivation based on the time-dependent perturbation theory to develop the dynamical equations which can be applied to model the response of a droplet quartz crystal microbalance (QCM) in contact with a single viscoelastic media. Moreover, the no-slip boundary condition across the device-viscoelastic media interface has been relaxed in the present model by using the Ellis-Hayward slip length approach. The model is then used to illustrate the characteristic changes in the frequency and attenuation of the QCM with and without the boundary slippage due to the changes in viscoelasticity as the coated media varies from Newtonian liquid to solid. To complement the theory, experiments have been conducted with microliter droplets of aqueous glycerol solutions and silicone oils with a viscosity in the range of 50~10 000 cS. The results have confirmed the Newtonian characteristics of the glycerol solutions. In contrast, the acoustic properties of the silicones oils as reflected in the impedance analysis are different from the glycerol solutions. More importantly, it was found that for the silicone oils the frequency steadily increased for several hours and even exceeded the initial value of the unloaded crystal as reflected in the positive frequency shift. Collaborative effects of interfacial slippage and viscoelasticity have been introduced to qualitatively interpret the measured frequency up-shifts for the silicone oils. The present work shows the potential importance of the combined effects of viscoelasticity and interracial slippage when using the droplet QCM to investigate the rheological behavior of more complex fluids.

This investigation addresses in detail a circular inhomogeneity with a viscoelastic interface subjected to remote uniform antiplane shear stresses. Both the inhomogeneity and the surrounding matrix are assumed to be elastic and quasi static, and the interface is viscoelastic, modeled by a linear spring and dashpot. Exact closed-form solutions for both the Kelvin-and Maxwell-type viscoelastic interfaces are obtained by means of the complex variable method. It is observed that when the matrix is subjected to remote uniform shear stresses, the stress field inside the inhomogeneity, although time-dependent, is still uniform. The derived solutions are then used to predict the time-dependent effective shear modulus of the composite based on the Mori--Tanaka mean-field approximation.

Abstract: Many biological, geological and synthetic bodies are anisotropic. In particular, some of these bodies reflect the anisotropy due to fiber reinforcement along a direction or more than one direction. [...]

An exact periodic solution for the time dependent flow of a viscoelastic fluid in the presence of transverse magnetic field is derived. It is assumed that on one plate the fluid is injected with certain velocity and that it is sucked off at the other plate with the same velocity. Both plates are oscillating with a known velocity in their own plane. A perturbation method has been employed by treating the viscoelastic parameter to be small. Effects of viscoelastic parameter, cross-flow Reynolds number, frequency parameter, and Hartmann number on the velocity as well as wall shear stress of the flow are discussed here with graphs. [DOI: 10.1115/1.2062828]

In nature, a soil profile generally consists of several heterogeneous layers. This study is aimed at discussing the interactive problem of oscillatory water waves and flow passing over multilayered soils. The soil behavior is considered as viscoelastic in the present mathematical model modified from Biot's poroelastic theory. Employing this model, the dynamic response including the profiles of pore water pressure and effective stress in the multilayered soils is discussed. The results reveal that the perturbed pore pressure is different from that inside a single-layered soil where the thickness of the first soil layer is less than the water wavelength. The discrepancy of the vertical effective stresses between multilayered and single-layered soils is even much more apparent under the same conditions. Moreover, seepage force is examined and is found to be larger near the bed surface and the bottom of the first soil layer where soils are easily disturbed by external disturbance. The locations where soil failure might happen are found near the troughs of surface water waves. DOI: 10.1061/(ASCE)0733-9399(2007)133:3(357) CE Database subject headings: Water waves; Viscoelasticity; Seepage; Multimedia.

Granular materials exhibit unusual kinds of behavior, including pattern formations during the shaking of the granular materials; the characteristics of these various patterns are not well understood. Vertically shaken granular materials undergo a transition to convective motion that can result in the formation of bubbles. A detailed overview is presented of collective processes in gas-particle flows that are useful for developing a simplified model for molecular dynamic type simulations of dense gas-particle flows. The governing equations of the gas phase are solved using large eddy simulation technique. The particle motion is predicted by a Lagrangian method. Particles are assumed to behave as viscoelastic solids during interactions with their neighboring particles. Interparticle normal and tangential contact forces are calculated using a generalized Hertzian model The other forces that are taken into account are gravitational and drag force resulting from velocity difference with the surrounding gas. A simulation of gas-particle flow is performed for predicting the flow dynamics of dense mixtures of gas and particles in a vertical, pentagonal, prism shaped, cylindrical container. The base wall of the container is subjected to sinusoidal oscillation in the vertical direction that spans to the bottom of the container. The model predicts the formation of oscillon type structures on the free surface. In addition, the incomplete structures are observed. Interpretations are proposed for the formation of the structures, which highlights the role played by the surrounding gas in dynamics of the shaken particles. [DOI: 10.1115/1.2409358] Keywords: localized structures, dense granular flows, vibrations, large eddy simulations, pattern formations, viscoelastic solids

A viscoplastic cap model of the Perzyna type was developed for simulating high strain rate behaviors of soils. An associative viscous flow rule was used to represent time-dependent soil behaviors. The viscoplastic cap model was validated against experimental data from static and dynamic soil tests. The model was also compared with soil behaviors under creep and stress relaxation with good agreement. However, the model was unable to represent tertiary creep where strain softening became significant. The model was subsequently integrated into LS-DYNA for finite-element simulations of high strain rate behaviors of sandy and clayey soils in explosive tests. The significance of strain rate effect on the soil responses is presented herein. It is concluded that the viscoplastic cap model is adequate for simulations of soil behaviors tinder high strain rate loading, creep, and stress relaxation, covering a wide range of time-dependent problems. DOI: 10.1061/(ASCE) 1090-0241(2007)133:2(206) CE Database subject headings: Viscoplasticity; Soil properties; Strain rate; Finite elements; Simulation.

Abstract: We consider the influence of a thin elastic film on a screw dislocation in an elastic semi-infinite substrate. In contrast to the usual assumption of a perfectly bonded interface [...]

A robust scheme for generating unstructured viscous meshes, using elliptic partial differential smoothing, is presented. Linear-elasticity relationships, borrowed from the structural mechanics discipline, provide the governing partial differential equations for smoothly displacing the existing mesh, making room for new viscous layers. These viscous layers are allowed to advance up adjacent, nonviscous boundaries. A process of inserting layers on alternating boundary groupings enables the creation of high-quality hexahedral elements in juncture regions. Viscous layer height is allowed to vary, depending on the size of the local inviscid mesh. Three-dimensional results are included to illustrate the use of the method to produce high-quality viscous meshes.