Collective behavior of viscoelastic asperities as a model for static and kinetic friction

We propose a statistical model for static and sliding friction between rough surfaces. Approximating the contact between rough surfaces by the contact of an ensemble of one-dimensional viscoelastic elements with a rough rigid surface, we study the collective behavior of the elements. We find that collective response of the contacts can lead to macroscopic behavior very different from the microscopic behavior. Specifically, various observed features of friction emerge as collective phenomena, without postulating them directly at the microscale. We discuss how parameters in our model can be related to material and surface properties of the contacting surfaces. We compare our results to commonly used rate and state phenomenological models, and propose a new interpretation of the state variable.     

Planar oscillations and dissipative heating of viscoelastic plates with a piezoeffect

Planar oscillations of thin piezoplates are important within the context of using this type of piezoelements as resonator frequency filters, frequency stabilizers, elements of piezotransformers, and other technological devices. In the publications currently known one usually considers piezoplates with elastic material behavior and linear governing equations. By their mechanical nature, however, a number of piezoelements, particularly piezoceramics, are viscoelastic, which, depending on the loading conditions, can lead to substantial dissipative heating of the piezoelement and confine its operation [3]. The use of piezopolymers and their composites raises particularly important issues of dissipative heating. At the present time the behavior of a piezoelement including heating can be described by the theory of thermoelectroviscoelasticity (TEVE) [2, 3], including the interaction between electromechanical and thermal fields. The complexity of TEVE problems leads to the necessity of using numerical methods to solve them, with the finite element method (FEM) being widely used in recent years. The present study is devoted to stating and solving TEVE problems concerning thin piezoceramic plates by the FEM. We treat a thin piezoceramic plate, confined by an arbitrary contour L and polarized across its thickness. A harmonic potential difference e^it is supplied to electrodes located on the smooth boundaries of the plate. Convective heat exchange with the surrounding media of temperatures T _k ^s and T^s is implemented at the contour surfaces and boundaries free of electrodes. The heat transfer coefficients equal, respectively, _k ^T and ^T. The initial plate temperature is T₀. The smooth boundary are free of mechanical loading. The mechanical forces at the contour surfaces are distributed symmetrically with respect to the mean plane of the plate.S. P. Timoshenko Institute of Mechanics, Ukrainian Academy of Sciences, Kiev (Ukraine). Translated from Prikladnaya Mekhanika, Vol. 30, No. 2, pp. 69–76, February 1994.     

Rolling resistance of a rigid sphere with viscoelastic coatings

We present a novel three-dimensional boundary-element formulation that fully characterizes the mechanical behavior of the external boundary of a multi-layered viscoelastic coating attached to a hard rotating spherical core. The proposed formulation incorporates both, the viscoelastic, and the inertial effects of the steady-state rolling motion of the sphere, including the Coriolis effect. The proposed formulation is based on Fourier-domain expressions of all mechanical governing equations. It relates two-dimensional Fourier series expansions of surface displacements and stresses, which results in the formation of a compliance matrix for the outer boundary of the deformable coating, discretized into nodes. The computational cost of building such a compliance matrix is optimized, based on configurational similarities and symmetry. The proposed formulation is applied, in combination with a rolling contact solving strategy, to evaluate the viscoelastic rolling friction of a coated sphere on a rigid plane. Steady-state results generated by the proposed model are verified by comparison to those obtained from running dynamic simulations on a three-dimensional finite element model, beyond the transient. A detailed application example includes a verification of convergence and illustrates the dependence of rolling resistance on the applied load, the thickness of the coating, and the rolling velocity.     

Vibration and dissipative heating of a layered rectangular viscoelastic prism under harmonic shear loading

The vibration and vibrational heating of a rectangular prism with copper and polyethylene layers is studied by solving numerically a coupled problem of thermoelasticity. The cases of kinematic and mechanical harmonic shear loads on a section of the prism surface are examined. Local heating regions are revealed. They are due to the stress fields in the neighborhood of the points at which the type of boundary conditions changes. The temperature–time curves have preresonance, resonant, and postresonance sections. The heating process reaches a steady thermal state under kinematic loading and may become avalanche-like (which is typical of thermal instability) under mechanical loading Translated from Prikladnaya Mekhanika, Vol. 45, No. 2, pp. 70–78, February 2009.     

The axisymmetric viscoelastic contact problem with rotational friction

The boundary value problem that arises when a mechanically rough rigid punch of arbitrary axisymmetric profile is pressed against the surface of a linear aging viscoelastic half space and is also made to rotate about its axis, so that there is total slip between the contacting surfaces is analysed and solved. The moment required to make the punch rotate, on the assumption that the coefficient of friction obeys a power law or is a constant, and the total normal pressure acting on the punch may each be evaluated in terms of the history of the radius of the contact area. Application is made to the special cases where the punch has the form of (i) a cone, (ii) a paraboloid of revolution and (iii) a flat ended cylinder. Apart from case (iii) where the contract area is constant we can only find an explicit expression for the moment in terms of total pressure so long as the contact area is increasing. The case of constant total pressure and Maxwell viscoelastic material is examined in more detail.     

Fracture propagation in brittle materials as a standard dissipative process: General theorems and crack tracking algorithms

The present work frames the problem of three-dimensional quasi-static crack propagation in brittle materials into the theory of standard dissipative processes. Variational formulations are stated. They characterize the three dimensional crack front “quasi-static velocity” as minimizer of constrained quadratic functionals. An implicit in time crack tracking algorithm that computationally handles the constraint via the penalty method algorithm is introduced and proof of concept is provided.     

On the skin friction coefficient in viscoelastic wall-bounded flows

Analysis of the skin friction coefficient for wall bounded viscoelastic flows is performed by utilizing available direct numerical simulation (DNS) results for viscoelastic turbulent channel flow. The Oldroyd-B, FENE-P and Giesekus constitutive models are used. First, we analyze the friction coefficient in viscous, viscoelastic and inertial stress contributions, as these arise from suitable momentum balances, for the flow in channels and pipes. Following Fukagata et al. (Phys. Fluids, 14, p. L73, 2002) and Yu et al. (Int. J. Heat. Fluid Flow, 25, p. 961, 2004) these three contributions are evaluated averaging available numerical results, and presented for selected values of flow and rheological parameters. Second, based on DNS results, we develop a universal function for the relative drag reduction as a function of the friction Weissenberg number. This leads to a closed-form approximate expression for the inverse of the square root of the skin friction coefficient for viscoelastic turbulent pipe flow as a function of the friction Reynolds number involving two primary material parameters, and a secondary one which also depends on the flow. The primary parameters are the zero shear-rate elasticity number, El₀, and the limiting value for the drag reduction at high Weissenberg number, LDR, while the secondary one is the relative wall viscosity, μ_w. The predictions reproduce both types A and B of drag reduction, as first introduced by Virk (Nature, 253, p. 109, 1975), corresponding to partially and fully extended polymer molecules, respectively. Comparison of the results for the skin friction coefficient against experimental data shows good agreement for low and moderate drag reduction which is the region covered by the simulations.     

Sliding of two smooth indenters on a viscoelastic foundation in the presence of friction

The axisymmetric contact problem of sliding of two solid parabolic indenters on a viscoelastic half-space with constant velocity is considered. Shear stresses modeling the adhesive component of the friction force act in the contact area. The model of the foundation material is described by an integral operator with an exponential kernel characterized by one relaxation time. The problem is solved by the boundary element method. The dependences of the contact characteristics on the sliding velocity, the normal load, and the distance between the centers of the indenters is analyzed. The results can be used to study the effect of the roughness elements modeled by two indenters on the contact characteristics and the deformation component of the friction force.     

Analysis of a quasistatic contact problem with adhesion and nonlocal friction for viscoelastic materials

A mathematical model is established to describe a contact problem between a deformable body and a foundation. The contact is bilateral and modelled with a nonlocal friction law, in which adhesion is taken into account. Evolution of the bonding field is described by a first-order differential equation. The materials behavior is modelled with a nonlinear viscoelastic constitutive law. A variational formulation of the mechanical problem is derived, and the existence and uniqueness of the weak solution can be proven if the coefficient of friction is sufficiently small. The proof is based on arguments of time-dependent variational inequalities, differential equations, and the Banach fixed-point theorem.     

Energy dissipative characteristic schemes for the diffusive Oldroyd‐B viscoelastic fluid

In this paper, we propose new energy dissipative characteristic numerical methods for the approximation of diffusive Oldroyd‐B equations that are based either on the finite element or finite difference discretization. We prove energy stability of both schemes and illustrate their behavior on a series of numerical experiments. Using both the diffusive model and the logarithmic transformation of the elastic stress, we are able to obtain methods that converge as mesh parameter is refined. Copyright © 2015 John Wiley & Sons, Ltd.     

Forced harmonic vibrations and dissipative heating-up of viscoelastic thin-walled elements (Review)

The paper presents a review of scientific studies on development, of the classical and refined models of the thermomechanical behavior of thin-walled single- and multilayer viscoelastic elements. Allowance is made for the temperature dependence of the properties of the material and physical and geometrical nonlinearities in the case of monoharmonic strain as one of the most typical types of deformation. Methods of solution of nonlinear connected problems of thermoviscoelasticity and results of solution of some specific problems on vibrations and heating-up of thin-walled rods, plates, and shells in quasistatic and dynamic formulations are discussed. A number of thermomechanical effects are noted. They are due to the coupling of mechanical and thermal fields and physical and geometrical nonlinearities, taken into account either separately or jointly. S. P. Timoshenko Institute of Mechanics, National Academy of Sciences of Ukraine, Kiev. Translated, from Prikladnaya Mekhanika, Vol. 36, No. 2, pp. 39–62, February, 2000.     

Calculation of the stiffness and dissipative heating of nonlinearly viscoelastic vibration dampers during cyclic deformation

Institute of Mechanics of the Ukrainian Academy of Sciences, Kiev. Translated from Prikladnaya Mekhanika, Vol. 24, No. 10, pp. 68–75, October, 1988.