Thin Viscoelastic Layer Research Articles

The mechanical properties of solidified polyethylene glycol 600, an analog for lava crust

The strength of lava crust and the role of that crust in determining lava flow dynamics and morphology are of fundamental importance in modeling lava flow behavior. Great improvements to flow models have been made by combining theoretical analysis with the results from analog experiments using polyethylene glycol (PEG 600) wax, which solidifies just below room temperature and can be used to simulate solidifying flows. However, a paucity of experimental data on the mechanical properties of solid PEG 600 has limited the quantitative interpretation of the role of crust in these experiments. We have, therefore, conducted experiments to measure the mechanical properties (tensile strength, shear strength, and Young’s modulus) of solidified PEG 600 at temperatures and strain rates similar to those employed in analog experiments. The mechanical properties of PEG 600 can be modeled by a power law function of temperature, with the tensile strength exhibiting an additional dependence on strain rate. At intermediate temperatures, marked changes in the temperature dependence of the Young’s modulus and in the character of failure surfaces indicate a transition from brittle to ductile behavior. We find that the strength of solid PEG 600 at experimental temperatures and strain rates exceeds that inferred for the crust on active laboratory flows by four orders of magnitude. This discrepancy is best explained by the presence of a thin visco-elastic layer separating the solid surface from the fluid interior of PEG 600 flows, a model that has been suggested to explain the behavior of some basaltic lava flows.

A shell finite element for viscoelastically damped sandwich structures

In this paper, a shell finite element is proposed for viscoelastically damped sandwich structures, in which a thin viscoelastic layer is sandwiched between identical elastic layers. The sandwich finite element is obtained by assembling three elements throughout the thickness of the sandwich structure. Using specific assumptions and displacement continuity at the interfaces, one reduces to eight the number of degrees of freedom per node that are the longitudinal displacements of the elastic layers, the deflection and three rotations. The finite element computations have been compared with known analytical, numerical and experimental data concerning the vibrations of sandwich beams, plates and shells.

Rheological properties and adhesive failure of thin viscoelastic layers

Methods have been developed for measuring the linear viscoelastic properties of thin adhesive layers, and for determining the stress intensity factor characterizing the driving force for adhesive failure. Both methods involve bringing a hemispherical indenter in contact with the adhesive layer while simultaneously monitoring the load, displacement, and radius of contact between the indenter and the adhesive. Dynamic moduli for the adhesive layer are obtained by oscillating the indenter, and the adhesive properties are obtained by pulling the indenter completely out of contact with the adhesive layer. Existing theories of viscoelastic contact mechanics were extended to account for the fact that the adhesive layer thickness is not substantially larger than the contact radius, as is generally assumed. A variety of correction factors were introduced that depend on the ratio of the contact radius to the adhesive layer thickness. These methods were applied to a model adhesive based on an acrylic triblock copolymer. Determination of the time-dependent creep and relaxation functions for this material was simplified by the power-law frequency response of the dynamic moduli. The large stress intensity factors observed were related to a Dugdale model of the cohesive zone at the contact edge.

Application of the theory of linear elastic materials with voids to NDT of interfaces of bilaminates

Non-destructive testing (NDT) of interfaces in composite structural elements is mandatory in several industries. This calls for modeling of composite interfaces incorporating real world conditions of imperfect bonding. Theories of materials that modern continuum mechanics offer can be profitably employed to develop these models. For instance, distributed voids are often present in the adhesive zone of a structural element and these voids may lead to a failure of the element due to the imperfect bond. As the theory of linear elastic materials with voids (LEMV) provides a model for solids with properties akin to viscoelastic materials, it is employed in the present work to represent the thin adhesive viscoelastic layer with its minutes pores spread over the interface region of a symmetric bilaminate. The bilaminate is assumed to be insonified in water and the governing leaky Lamb wave (LLW) frequency equation is derived. The dispersion and attenuation of LLWs in an Al/adhesive/Al bilaminate are graphically-presented for various porosity-coupling parameter values and are compared with those in perfectly-bonded Al plates. The results presented in this paper showing the influence of pore-infested adhesive layer on the early leaky Lamb modes promises to be useful in identifying interfacial porosity.

Stability of a two-layer blown film coextrusion

The stability of a bicomponent two-layer blown film coextrusion process has been studied using a simple model in which the two layers consist of a Newtonian and an upper-convected Maxwell fluid (UCM), respectively. These two fluids are chosen to investigate the relative influence of viscous and viscoelastic forces on the stability of the two-layer biaxial extension flow. The results of the linear stability analysis indicate that a critical value of the film thickness exists below which the flow becomes unstable. The critical film thickness decreases with increasing the blow-up ratio (BUR). While a Newtonian single-layer flow exhibits an upper unstable region where the flow is unstable when the BUR is greater than a certain critical value, the presence of a thin viscoelastic layer removes the upper unstable region. It also enhances the flow stability by broadening the range of operating conditions for stable operation. Although the present model is rather simple, many of the predictions appear to be in reasonable agreement with experimental observations qualitatively.

Contact of elastic bodies with thin visco-elastic coatings under conditions of rolling or sliding friction

Analytic methods are developed for solving the contact problem of the rolling of an elastic cylinder along a visco-elastic layer bonded to an elastic base with the aim of studying the effect of the mechanical and geometric characteristics of their surface layers on the contact interaction parameters and the coefficient of rolling friction. A Maxwell model is used to describe the mechanical properties of the visco-elastic layer. The problem is treated assuming that there is partial siding in the contact area which makes it possible to treat the resistance to rolling as the overall result of the manifestation of the imperfect elasticity of the surface layers of the interacting bodies and sliding friction in the contact area. The solution of the problem of the total sliding of a cylinder on an elastic base covered with a thin visco-elastic layer is obtained as a special case.

Un élément fini de poutre amortie par film viscoélastique mince contraint

ABSTRACT In this paper, a new finite element of composite beam damped by means of a thin viscoelastic film is presented. The first part summarizes the new variational formulation of damped beam. The present theory is based upon a piece-wise linear displacement field and a stress field that fulfills continuity conditions at the interfaces between the thin film and the thick skins. Next, it is shown how the finite element was derived. In order to demonstrate the accuracy of the present method, experiments were carried out. The comparison between the experimental results and the computational results shows that the proposed approach is efficient in predicting the dynamic response of beams that are damped with a thin viscoelastic layer.

Damping of toroidal shells by a constrained viscoelastic layer

The analysis of shells damped by a constrained viscoelastic layer is extended to toroidal segments. The absorber consists of thin curved strips bonded to the torus with a thin viscoelastic layer. When applied to the torus, the absorber is most effective on response modes with two or more circumferential waves. In this way it resembles the damped cylinder. However, for a given weight of absorber, the torus is damped more than the cylinder. The reason is that the torus induces and couples those circumstantial harmonics that are damped most effectively by this absorber.

Analysis of boundary conditions for elastic wave interaction with an interface between two solids

The boundary conditions for an interface between two solids are analyzed to model a thin viscoelastic interface layer. Boundary conditions that relate stresses and displacements on both sides of the interface are obtained as an asymptotic representation of three-dimensional solutions for an interface layer in the limit of small wavelength to thickness ratio. The interface boundary conditions obtained include interface stiffnesses and inertia and terms involving coupling between normal and tangential stresses and displacements. The applicability of such boundary conditions is analyzed by comparison with exact solutions for ultrasonic wave reflection. Fundamental boundary conditions are introduced where only one transverse or normal mass or stiffness is included. It is shown that the solution for more exact interface boundary conditions which include two inertia elements and two stiffness elements can be decomposed into a sum of fundamental solutions. The transition between welded and slip boundary conditions on an interface with a thin viscous layer is also analyzed as a function of interface thickness, viscous skin depth, and frequency.

On the transition between slip and rigid boundary conditions between two solid media

In order to describe the transition between slip and rigid boundary conditions, a model with a thin interphasial viscoelastic layer is introduced. By changing the relaxation time (ωτ), the transition from a viscous liquid to a solid layer is observed. When the interface layer is in the liquid state with very small shear nodulus the shear stress components on the boundary vanish, resulting in a slip boundary condition. On the other hand, after the layer is solidified, the two semispaces become welded with the transition of shear and longitudinal stresses to one another. It is shown that the ratio of a hydrodynamic boundary layer thickness to the interface layer thickness plays a major role in this transition. A simple approximate equation for the reflection coefficient from the interface is derived and compared to the exact solutions for different transitional stages of the boundary condition. Theoretical predictions are compared to experimental results.

Damped Response of Shells by a Constrained Viscoelastic Layer

Vibration absorbers are introduced into an asymmetric configuration of thin cylinders and tori enclosing an acoustic medium. The absorbers consist of thin axial strips bonded to the cylinder with a thin viscoelastic layer. The constrained layer dissipates the energy of relative motions between strip and cylinder. The absorber is most effective on response modes with two or more circumferential waves. The use of transfer matrices is extended to the coupled cylinder-absorber system.

Plane and axisymmetric problems of the action of loads on a thin nonuniform viscoelastic layer

Plane and axisymmetric problems of the action of loads on a thin nonuniform viscoelastic layer

Viscoelastic stress relaxation on deep fault sections as a possible source of very long period elastic waves

Observations of very long period elastic waves are attributed to ‘silent’ earthquakes, events which are presumed to occur at depth in a nonbrittle regime. The fault at depth is modeled as a thin viscoelastic gouge layer embedded in an elastic medium. The source of silent earthquakes is envisaged as a prolonged, slow slippage of the fault faces, which yields a radiation spectrum strongly depleted in the high‐frequency components and dominated by the near‐field term. The results of the model are compared with the dominant frequencies, mean duration, and amplitude of a long‐period perturbation recorded at Trieste in the years preceding the 1976 Friuli (Italy) earthquake. The role that silent earthquakes may play in the overall earthquake mechanism (in particular, as precursory phenomena) is discussed.

Vibrations of three layered damped sandwich plate composites

Theoretical and experimental results are presented and discussed for the transverse driving point mechanical impedances, as well as for the transfer impedances, of damped composite plates made up of a thin viscoelastic layer sandwiched between two elastic layers. Analytical results are determined by finite element approximations. Due to the elements used and the system to be modeled, several fundamental assumptions or restrictions usually adopted in analytical investigations are removed. The dependence on frequency and temperature of the dynamic properties of the viscoelastic materials is taken into consideration. A companion experiment was conducted, for comparison purposes, on such damped composite plates suspended in air by lightweight elastic shock cords and driven at the center by an electromechanical vibration shaker. Good correlations between the test data and analytical solutions are obtained over a wide frequency range for two configurations.

Technique for measuring damping properties of thin viscoelastic layers

A method is proposed for performing the difficult task of measuring the complex modulus properties of viscoelastic materials when they are available only in the form of thin layers, such as damping tapes, attached securely to thin sheets of metal. The method involves measuring the response of a simple beam with several layers of the damping tape attached to the entire surface and comparing the results with prior tests on the same configuration with another viscoelastic material having known properties. Results are verified by comparison with data obtained by using a resonance technique. It is shown that the present approach is simple and effective to use, and that the limitations are simple to understand and avoid.