Some laws of plastics friction

The dependence of the actual contact area S of plastics on temperature, specific load, and sliding speed has been investigated. The value of S increases exponentially with the specific load, the maximum value at large specific loads being less than the nominal contact area. The temperature dependence of S under static conditions between 20 and 130° C is attributable to the decrease in the static modulus of elasticity of the plastic near the glass transition point and to the development of high-elastic and plastic deformations at elevated temperatures. There is practically no change in S as the sliding speed varies from 10^–3 to 10 cm/min; at the same time the force of friction increases slightly.Mekhanika Polimerov, Vol. 3, No. 6, pp. 1078–1081, 1967     

Effect of sliding velocity on area of real contact and on force of friction in high-elastic materials

The velocity dependence of the force of friction and the area of real contact of SKN-18, SKN-26, and SKN-40 rubbers has been simultaneously determined on an optical instrument. In the region of small sliding velocities the area of real contact is virtually independent of the sliding velocity, while the force of friction increases in proportion to the logarithm of the sliding velocity, in accordance with the molecular-kinetic theory of friction. At high sliding velocities a deviation in the velocity dependences of both quantities is observed. The constant of friction has been determined over the entire velocity range and the resulting velocity dependence is examined in molecular terms.Mekhanika Polimerov, Vol. 3, No. 2, pp. 309–311, 1967     

Temperature dependence of the actual contact area and friction force of high-elasticity materials

An investigation of the temperature dependence of the actual contact area (under static conditions and during friction at a given sliding speed) of materials based on butadienenitrile rubber in a wide temperature interval is described. Measurements of the friction force were also made. It was established that the temperature dependence of the contact area is associated with corresponding changes in the elasticity modulus (nonequilibrium static modulus for contacts at rest and dynamic modulus for nonstationary contacts). The friction constant c decreases linearly with rising temperature and does not depend on the applied load.Mekhanika Polimerov, Vol. 3, No. 4, pp. 726–729, 1967     

Antifriction basalt-plastics based on polypropylene

A study is made of the dependence of the mechanical and friction-engineering properties of polypropylene reinforced with basalt fibers on the viscosity of the polymer matrix. It is established that the main factors that determine the mechanical properties of the plastics are the quality of impregnation of the fibers by the binder and the residual length of the reinforcing filler in the composite after extrusion and injection molding. The material that was developed has a low friction coefficient and low rate of wear within a relatively brood range of friction conditions. The basalt-plastics can be used in the rubbing parts of machines and mechanisms subjected to dry friction.Ukrainian State Chemicotechnical University, Dnepropetrovsk, Ukraine. Translated from Mekhanika Kompozitnykh Materialov,33, No. 3, pp. 417–421, May–June, 1997.     

Bifurcation and chaos in friction-induced vibration

Friction-induced vibration is a phenomenon that has received extensive study by the dynamics community. This is because of the important industrial relevance and the ever-evolving development of new friction models. In this paper, we report the result of bifurcation study of a single-degree-of-freedom mechanical oscillator sliding over a surface. The friction model we use is that developed by Canudas de Wit et al., a model that is receiving increasing acceptance from the mechanics community. Using this model, we find a stable limit cycle at intermediate sliding speed for a single-degree-of-freedom mechanical oscillator. Moreover, the mechanical oscillator can exhibit chaotic motions. For certain parameters, numerical simulation suggests the existence of a Silnikov homoclinic orbit. This is not expected in a single-degree-of-freedom system. The occurrence of chaos becomes possible because the friction model contains one internal variable. This demonstrates a unique characteristic of the friction model. Unlike most friction models, the present model is capable of simultaneously modeling self-excitation and predicting stick–slip at very low sliding speed as well.     

Friction properties of highly elastic materials at high contact pressures

A newly designed high-pressure tribometer has been employed to investigate the pressure dependence of the friction force of SKN-40 crosslinked butadiene-nitrile rubber in contact with a steel surface on the pressure range to 1200 kgf/cm² (20°C). Over the entire range of contact pressures the friction process is molecular-kinetic in nature and characterized by a linear dependence of the friction force on the logarithm of the sliding velocity. In the region of normal pressures up to 200 kgf/cm², where the effect of pressure on the friction force reduces to the formation of the actual contact area, the friction constant (proportionality factor relating the friction force and the actual contact area) is practically independent of the pressure. At pressures above 200–300 kgf/cm² the increase in the friction force at fixed actual contact area is attributable to the effect of pressure on the friction constant. The nature of this effect is related not with an increase in the chain-surface interaction energy (the activation energy does not increase), but with an increase in the forces of adhesion owing to the greater number of polymer chain-steel surface contacts on the actual contact area (increase in contact density).Lenin Moscow State Pedagogical Institute, Laboratory for Problems of Polymer Physics. Translated from Mekhanika Polimerov, No. 1, pp. 140–146, January–February, 1971.     

Deformation of tape-wound cylindrical shells under internal pressure

The state of stress of a cylindrical shell would from glass tape at arbitrary angles to the generator is examined. The pressure dependence of the strains and stresses is obtained for uniform internal pressure, and the question of rationalization of the structure of the shell material is investigated. To describe the deformation characteristics associated with failure of the resin under load, a perfectly plastic model is introduced. The results of an experimental determination of the relative elongations are presented for the case of active loading and unloading.Mekhanika Polimerov, Vol. 3, No. 5, pp. 915–920, 1967     

Investigation of the friction of rubber under constant compressive strain conditions at low pressure

A vacuum tribometer was developed which was used to investigate the maximum friction force occurring at the initial instant of slipping in rubber-metal friction pairs under conditions of a given (from 5 to 40%) compressive strain at a low pressure in the temperature range from +100 to –100°C. Filled rubbers on a base of nitrile-butadiene rubbers were studied. Up to the glass transition temperature T_g the vacuum had practically no effect on the maximum friction frocef _m; at temperatures T_g and lower the values off _m obtained in a vacuum were 10–15% higher than those obtained in the atmosphere. It is shown that with a decrease of temperature from 20°C to the glass transition temperature T_g the slope of the dependence of the maximum friction force on the degree of deformation increases, and below T_g decreases. The effect of the slipping speed v on the maximum friction forcef _m was also studied.Laboratory of Polymer Physics, V. I. Lenin Moscow State Pedagogical Institute. Leningrad Branch, Scientific Research Institute of the Rubber Industry. Translated from Mekhanika Polimerov, No. 3, pp. 486–492, May–June, 1970.     

Temperature deformation of laminated composite rings

The results of dilatometric measurements on wound circular glass-reinforced plastic rings composed of glass tape and epoxy-phenolic resin are presented. The changes in inside and outside diameter were measured on the temperature interval from 20 to 100°C at various diameter ratios. The theoretical conclusion [1] that anisotropy has a strong influence on the temperature deformation of circular cylinders and rings of composite material is confirmed.Moscow Power Engineering Institute. Translated from Mekhanika Polimerov, No. 6, pp. 1131–1132, November–December, 1969.     

Investigation of polytetrafluorethylene surface texture

Results are given of an X-ray investigation of the formation of a textured surface layer on polytetrafluorethylene (PTFE) film and its dependence on external mechanical factors — unit load and sliding speed. It is established that the thickness of the surface texture of PTFE is determined by the relation between the processes of texture formation under the influence of frictional forces and its destruction by wear. These processes, in turn, are determined by the working parameters of the moving joint. Data on the kinetics of polymer surface texture formation in the presence of friction are extremely useful in relation to the design of efficient antifriction materials.Mekhanika Polimerov, Vol. 1, No. 2, pp. 129–132, 1965     

Effect of liquid media on the deformation of the surface layers of a polymer-metal pair in sliding friction

The relation between the high-elastic component of polyurethane deformation and the deformation of the surface layers of metals has been investigated for sliding friction in various media. The surface layers of polymer and metal are plastically deformed. There is a certain correlation between the coefficient of friction and the amount of deformation.Kiev Institute of Civil Aviation Engineers. Translated from Mekhanika Polimerov, No. 1, pp. 147–149, January–February, 1970.     

Experimental study of the effect of technological parameters on the residual stresses in thin-walled coiled parts

The effect of the parameters of heat treatment and programmed winding under tension on the residual stresses in thin-walled coiled parts made from glass plastic by the "dry" winding of a heated glass strip on an unheated mandrel is studied experimentally. The effect of the thickness of the parts on the maximum radial residual stresses is considered. A method is proposed for regulating the residual stresses in parts with very thick walls.Translated from Mekhanika Polimerov, No. 1, pp. 75–80, January–February, 1972.     

Programmed winding of glass-reinforced plastics

It is proposed to employ a variable winding tension designed to give a required initial stress distribution. The program is constructed on the basis of solutions describing the loss of tension associated with winding and the softening of the resin during heat treatment. The problems are solved in the elastic formulation. Programs are obtained for three cases: constant tension in a ring sill on the mandrel, compensation of the stresses that develop after removal from the mandrel, and compensation of the thermoelastic stresses that develop during cooling.Institute of Polymer Mechanics, Academy of Sciences of the Latvian SSR, Riga. Translated from Mekhanika Polimerov, No. 1, pp. 48–53, January–February, 1970.     

Maximum force of friction in rubber-metal friction pairs under conditions of constant compressive deformation of the rubber

The maximum force of friction at the initial moment of slip has been investigated on rubber-metal friction pairs under conditions of constant compressive deformation of the rubber during transition from the high-elastic to the glassy state. Filled butadiene-nitrile rubber compounds were studied in the temperature range from +20 to –50°C. The temperature dependence of the maximum force of friction has a sharply expressed maximum near the glass transition temperature. As the temperature falls, the force of friction at first increases, in accordance with the molecular-kinetic theory. As the temperature continues to fall, in the transition region the maximum force of friction begins to rise more sharply owing to a sharp increase in the volume-mechanical friction component. The fall in the maximum force of friction below the glass transition point associated with a decrease in the deformed volume of rubber due to shrinkage and with the reduced mechanical loss factor.Mekhanika Polimerov, Vol. 3, No. 3, pp. 533–538, 1967     

Cold flow and melting of plastics under severe friction conditions

Dry friction of a series of crystalline polymers (polypropylene, nylon, PTFE) against steel over a wide range of sliding velocities (4.4–4.4×10^–4 cm/sec) and loads (P=7.5–360 kg) with almost complete mutual overlap of the friction surfaces is studied. It is shown that friction of polymer materials against steel may be accompanied by melting of the crystalline polymer (at high velocities) or cold flow (at low velocities) in the surface layers associated with an orientation effect and the appearance of anisotropy.Mekhanika Polimerov, Vol. 1, No. 5, pp. 95–100, 1965     

Continuity condition for orthogonally reinforced plastics in tension

The proposed continuity condition is based on the hypothesis that the time dependence of the strength and limiting strain of the resin is determined by the limiting value of the specific work done by the stresses. The case when the reinforcement is elastic and the rheological properties of the resin are described by a linear differential equation is considered.Institute of Polymer Mechanics, Academy of Sciences of the Latvian SSR, Riga. Translated from Mekhanika Polimerov, Vol. 4, No. 5, pp. 844–852, September–October, 1968.     

Polypropylene reinforced with aramid and glass fibers

The effect of the chemical nature of the aramid fibers Phenylone, Terlon, Armos, and SVM on the mechanical, thermophysical, and antifriction properties of reinforced polypropylene was investigated. It was found that the composite filled with SVM fibers based on a stiff-chain polymer has high tensile strength and bending modulus. Reinforcement of polypropylene with Phenylone stiff-chain fibers produces a composite with a high impact viscosity. Organoplastics based on polypropylene and aramid fibers have a low density and friction coefficient and high durability. Reinforcement of polypropylene with aramid (SVM) and glass fibers increases the technological properties of the composites. The glass-filled organoplastics developed can be used in instrument making, radio engineering, and machine building as antifriction and construction materials.Ukrainian State Chemical Technological University, Dnepropetrovsk, Ukraine. Translated from Mekhanika Kompozitnykh Materialov, No. 1, pp. 106–110, January–February, 1996.     

Polymer friction at low velocities and high loads

The results of an investigation of the dependence of the coefficient of polymer friction on the normal load and the chemical purity and roughness of the contact surfaces are described. It is shown that in dry friction for a series of polymers the surface deformation is predominantly forced-elastic. The nature of the dry friction mechanism as it relates to polymers is discussed.Mekhanika Polimerov, Vol. 4, No. 3, pp. 494–498, 1968     

Investigation of the temperature residual stresses and strains in thick-walled wound reinforced plastic products

The kinetics of the temperature residual stresses and strains in thick-walled reinforced-plastic cylinders and rings have been experimentally investigated employing a special procedure for continuously monitoring the internal strains and stresses in the various zones of the semifinished product during the heat treatment stage. The effect of the resin polymerization (polycondensation) temperature and the mandrel material on the magnitude and distribution of the temperature stresses is considered. The total residual stresses have been determined by a nondestructive strain-gauge method with differentiation of the components.Translated from Mekhanika Polimerov, No. 6, pp. 1040–1046, November–December, 1971.