Periodical contact problems for a half-space involving frictional heating

A periodical system of rigid parabolic punches moving steadily over the surface of a half-space is considered. Frictional shear tractions are developed with the heat generation along the interface. It is assumed that the surfaces of the punches are insulated and that the half-space is a thermal conductor. The problem is formulated in terms of two governing integral equations with unknown pressure and temperature at the interface. These equations are solved numerically. The effect of thermal distortion on the temperature profile under the punches and size of the contact region is investigated.     

Axi-symmetric contact problem with frictional heating for thermally nonlinear sliders

The axi-symmetric contact problem for a sliding elastic sphere on a rigid foundation is considered. It is assumed that the work done against shearing tractions is the source of heat generation and all the heat is absorbed in one moving sphere. The effect of the temperature-dependent properties of the sphere material is investigated. The problem is reduced to the solution of nonlinear Fredholm type integral equations. The approximate solution of these equations is obtained. For a sphere of carbon graphite material it is established that the influence of the nonlinear thermal properties on the contact area, the contact pressure and temperature is essential.     

Dynamic and quasi-static instability of sliding thermoelastic frictional contact

The exact solution of a problem on thermoelastic frictional sliding of a rigid half-plane against the surface of an elastic coating of another half-plane has been obtained both for quasi-static and dynamic formulation of the problem. A solution to the problem has been constructed in contour quadratures form of the inverse Laplace integral transform, after calculating which the solution has been written in form of series over eigenfunctions. Instability area in parameters domain has been determined for both formulations of the problem. Properties of unstable solutions of the problem that can be used for the diagnostics and control of unstable operations of friction pairs have been studied.     

Metal transfer in the frictional contact of a rough hard surface

The frictional behaviour between a hard rough surface and a soft smooth surface was examined under lubricated and unlubricated conditions. Transfer of soft metal to hard asperities in contact with it caused significant changes in the shape, size and height distributions of the asperities. Thus metal transfer reduced the effect of the initial surface roughness of the hard metal on friction.     

Numerical analysis of rolling-sliding contact with the frictional heat in rail

Thermal damage caused by frictional heat of rolling-sliding contact is one of the most important failure forms of wheel and rail. Many studies of wheel-rail frictional heating have been devoted to the temperature field, but few literatures focus on wheel-rail thermal stress caused by frictional heating. However, the wheel-rail creepage is one of important influencing factors of the thermal stress In this paper, a thermo-mechanical coupling model of wheel-rail rolling-sliding contact is developed using thermo-elasto-plastic finite element method. The effect of the wheel-rail elastic creepage on the distribution of heat flux is investigated using the numerical model in which the temperature-dependent material properties are taken into consideration. The moving wheel-rail contact force and the frictional heating are used to simulate the wheel rolling on the rail. The effect of the creepage on the temperature rise, thermal strain, residual stress and residual strain under wheel-rail sliding-rolling contact are investigated. The investigation results show that the thermally affected zone exists mainly in a very thin layer of material near the rail contact surface during the rolling-sliding contact. Both the temperature and thermal strain of rail increase with increasing creepage. The residual stresses induced by the frictional heat in the surface layer of rail appear to be tensile. When the creepage is large, the frictional heat has a significant influence on the residual stresses and residual strains of rail. This paper develops a thermo-meehanical coupling model of wheel-rail rolling-sliding contact, and the obtained results can help to understand the mechanism of wheel/rail frictional thermal fatigue.     

Material transfer of POM in sliding contact

Material transfer is an important factor in the sliding behaviour of polymers, as it governs the development of friction and wear in the course of time. However, little is known about the first stages of material transfer. Therefore, measurements were performed with polymers sliding onto steel. The first stages of material transfer were studied with a slider-on-sheet apparatus. The results of these measurements are compared with the results of pin-on-disc measurements. In the latter case, polymer sliding onto polymer was also measured. Polyoxymethylene (POM)-C was chosen as polymer material.Scanning electron microscopy showed that polymer transfer is initiated by mechanical interlocking of metal asperities into the polymer. The resulting debris particles are smeared out during further passages in slider-on-sheet measurements. Pin-on-disc experiments resulted in a less dense transfer layer. Metal asperities can still plough through the polymer surface in pin-on-disc experiments, while ploughing is hindered in slider-on-sheet experiments.The friction coefficients found during pin-on-disc and slider-on-sheet experiments were comparable within the experimental error. The friction coefficient of POM-C sliding against stainless steel starts at a relatively low friction level of about 0.2 and rises to higher values after 20 h of sliding due to material transfer. Such a relatively high friction value was also found when POM-C was slid against POM-C, confirming that the increase in friction found during the experiments against steel was due to material transfer. Pin-on-disc experiments with POM-C sliding against POM-C showed rather large differences in wear rate of the polymer pin.     

Topological design of heat dissipating structure with forced convective heat transfer

This paper discusses the use of the topology optimization formulation for designing a heat dissipating structure that utilizes forced convective heat transfer. In addition to forced convection, there is also natural convection due to natural buoyancy forces induced by local heating inside fluid. In the present study, the temperature distribution due to forced convection, neglecting buoyancy and viscous dissipation inside fluid, was simulated and optimized. In order to analyze the heat transfer equation with forced convective heat loss and the Navier-Stokes equation, a common sequential computational procedure for this thermo/hydraulic characteristic was implemented. For topology optimization, four material properties were interpolated with respect to spatially defined density design variables: the inverse permeability in the Navier-Stokes equation, the conductivity, density, and the specific heat capacity of the heat transfer equation. From numerical examples, it was found that the balance between the conduction and convection of fluid is of central importance to the design of heat dissipating structures.     

The generation of dislocations in metals under a sliding contact and the dissipation of frictional energy

Observations on static friction behaviour under stick-slip vibration conditions are presented. A static contact-time model of static friction does not adequately describe the observed variation in static friction; the governing variable is the rate of increase of the tangential force coefficient.     

Influence of surface treatment of elastomers on their frictional behaviour in sliding contact

To reduce friction of elastomer parts moving against a metal counter body in dry conditions, two different surface treatment techniques were applied on elastomer parts: laser cladding and plasma treatment at atmospheric pressure.Polyamide 11 (PA 11) based coatings were produced on thermoplastic polyurethane (TPU) substrates by laser cladding. During ball-on-disc tribotesting the effect of a PA 11 coating was identical to that of a PA 11 + 9% MoS₂ coating: friction of the TPU substrate was reduced with 40%. The incorporation of 15 wt% PTFE in the PA 11 coating resulted in a further decrease of the frictional force. A reduction of 80% of the frictional force of the TPU substrate was measured. The surface of the coatings before and after tribotesting was analysed.The plasma treatment of HNBR was done using a Plasmaspot^® to form a plasma polymerised coating based on two different types of siloxanes. A reduction of 74–80% of the initial friction coefficient was measured in two different tribotest rig configurations: ball-on-disc and disc-on-disc. The resulting wear tracks were analyzed by SEM and EDX.     

Flow and convective heat transfer analysis using RANS for a wire-wrapped fuel assembly

This work presents the three-dimensional analysis of flow and heat transfer performed for a wire-wrapped fuel assembly of liquid metal reactor using Reynolds-averaged Navier-Stokes analysis in conjunction with SST model as a turbulence closure. The whole fuel assembly has been analyzed for one period of the wire-spacer using periodic boundary conditions at inlet and outlet of the calculation domain. Three different assemblies, two 7-pin wire-spacer fuel assemblies and one bare rod bundle, apart from the pressure drop calculations for a 19-pin case, have been analyzed. Individual as well as a comparative analysis of the flow field and heat transfer have been discussed. Also, discussed is the position of hot spots observed in the wire-spacer fuel assembly. The flow field in the subchannels of a bare rod bundle and a wire-spacer fuel assembly is found to be different. A directional temperature gradient is found to exist in the subchannels of a wire-spacer fuel assembly. Local Nusselt number in the subchannels of wirespacer fuel assemblies is found to vary according to the wire-wrap position while in case of bare rod bundle, it’s found to be constant.     

Laminar forced convective heat transfer to near-critical water in a tube

Numerical modeling is carried out to investigate forced convective heat transfer to nearcritical water in developing laminar flow through a circular tube. Due to large variations of thermo-physical properties such as density, specific heat, viscosity, and thermal conductivity near thermodynamic critical point, heat transfer characteristics show quite different behavior compared with pure forced convection. With flow acceleration along the tube unusual behavior of heat transfer coefficient and friction factor occurs when the fluid enthalpy passes through pseudocritical point of pressure in the tube. There is also a transition behavior from liquid-like phase to gas-like phase in the developing region. Numerical results with constant heat flux boundary conditions are obtained for reduced pressures from 1.09 to 1.99. Graphical results for velocity, temperature, and heat transfer coefficient with Stanton number are presented and analyzed.     

Wear resistance of composites for sliding electrical contact from reprocessed bearing steel

The electrical contact characteristics are determined of model composites from bearing steel reprocessed from grinding waste. It is established that the tested sliding steel composites have a low contact electrical resistance and wear rate at current densities up to 300 A/cm². An assumption is made that efficient performance of these composites results from the superdispersed structure of the reprocessed bearing steel.     

Evolution of contact pressure during wear of the coating in a thrust sliding bearing

The evolution of contact pressure is analyzed during the wear of a thrust sliding bearing following the law of nonlinear wear. The coating’s deformation properties are described using the nonlinear Winkler model. It is demonstrated that steady wear can exist with a definite contact pressure distribution.     

The frictional sliding contact problems of rigid parabolic and cylindrical stamps on graded coatings

Graded materials, also known as functionally graded materials (FGMs), are generally two-phase composites with continuously varying volume fractions. Used as coatings and interfacial zones they can reduce thermally and mechanically induced stresses resulting from material property mismatch, increase the bonding strength and provide protection against adverse environments. In this paper, the contact problems of parabolic and cylindrical stamps on graded coatings are considered. The objective of this study is to obtain a series of analytical benchmark solutions for examining the influence of such factors as material inhomogeneity constants the coefficient of friction and various length parameters on the critical stresses that may have a bearing on the fatigue and fracture of the components with graded coatings.     

In situ studies of the lubricant chemistry and frictional properties of perfluoropolyalkyl ethers at a sliding contact

In situ analyses of lubricated sliding contacts were performed by interfacing an ultraviolet Raman spectrometer to a ball-on-flat tribotester. The sliding contact was simulated by rotating a sapphire window that is transparent to ultraviolet radiation against a stationary ball. Various loads were transmitted to the contact center through the ball. A branched perfluoropolyaklyl ether (Krytox 479) and two linear perfluoropolyalkyl ethers (Fomblin 491 and Fomblin 497) have been studied under various loads at a 10 cm/s sliding speed. Krytox and a Fomblin of lower viscosity, Fomblin 497, decomposed to amorphous carbon upon sliding on a chrome steel ball but no amorphous carbon was detected from Fomblin 491. The amount of amorphous carbon at the contact area during sliding was a balance of formation and removal rates. It is postulated that surface activity of the chrome steel ball was the main cause for the lubricant degradation. The lubricant degradation at the chrome steel/sapphire interface was found to slightly increase the kinetic coefficient of friction at the contact center. However, catastrophic scuffing was not observed.     

In-tube convective heat transfer characteristics of a CO2-hydrate mixture

Journal of Mechanical Science and Technology - CO2-hydrate has drawn attention as a means to transport captured CO2 as well as to provide a secondary working fluid in refrigeration systems. This...     

On the plane contact problem of a functionally graded elastic layer loaded by a frictional sliding flat punch

This article is concerned with the contact mechanics of a functionally graded layer loaded by a frictional sliding flat punch. The coefficient of friction is assumed to be constant and the lower side of the graded layer is firmly attached to a rigid foundation. The graded, nonhomogeneous property of the medium is represented in terms of an exponential variation of the shear modulus, while Poisson’s ratio is taken to be constant. Based on the use of plane elasticity equations and the Fourier integral transform technique, the formulation of the current contact mechanics problem lends itself to a Cauchy-type singular integral equation of the second kind for the unknown contact pressure, which is solved numerically. As a result, the effects of several parameters, i.e., the material nonhomogeneity, the friction coefficient, the punch width, and Poisson’s ratio, on the distributions of the contact pressure and the in-plane surface stress component are presented.     

Experimental determination of the equivalent stress at the frictional surfaces of a slip bearing in a heavily loaded contact

The significant influence of the frictional forces on the bush deformation in a heavily loaded contact is shown. The results of comparative tests are presented. The coefficients for calculating the equivalent stress at the frictional surfaces of a slip bearing are determined.     

Simulation of combined wearing of the shaft and bush in a heavily loaded sliding bearing

The problem of simultaneous wear of an elastic cylinder and a cylindrical notch surface in an elastic space is discussed (in the planar formulation). The contact problem formulation omits the limitation of the contact area. The contact is solved in steps; the linear wear and contact parameters of the shaft and bush are solved at each step in time. The Multopp-Kalandia method is applied to the solution of the contact problem. The constructed model is used to investigate the evolution of contact pressure and form changes of the shaft and bush in the process of wear.     

Numerical study of fluid flow and convective heat transfer characteristics in a twisted elliptic tube

We investigated the flow and heat transfer characteristics in a Twisted elliptic tube (TET). The effects of geometry parameters such as the aspect ratio and number of rotations in the TET were analyzed comparatively using three-dimensional (3-D) numerical simulation. We also solved numerically the conservation equations of continuity, momentum, and energy in the TET. Fully developed flow in the TET was modeled using the realizable k-ε turbulence model and steady incompressible Reynolds-averaged Navier-Stokes (RANS) equations. The simulation was performed for Reynolds numbers of 100, 1000 and 10000. The pressure drop and the heat transfer of the TET were assessed in terms of the Darcy friction factor and Colburn j-factor, and overall performance was evaluated using the area and volume goodness factors.