Demystifying flash temperatures II. First-order approximation for plastic contact spots

Abstract

In the accompanying paper, Part I, expressions for the flash temperatures ΔT e at elliptical contact spots were derived. They were uniformly found to be proportional to q, the rate of heat evolution per unit area of contact. For frictional heat, q is proportional to the velocity v of sliding and also depends on the total force P exerted between the two relatively sliding objects and the number N of contact spots at which they touch. For plastically deformed contact spots (which is by far the most frequent case) the following expressions are found. The characteristic velocity is given by ▪ The flash temperature in general is given by ▪ The flash temperature at very low speeds for circular contact spots is given by ▪ the flash temperature at very high speeds for circular contact spots by ▪ and the flash temperature of circular contact spots at v r = 1 by ▪ Lastly, when the contact spot moves relative to both sides, with speeds v 1 and v 2, i.e. relatie velocities v r1 = κ 1/ r and v r2 = κ 2/ r where r = (F/π) 1 2 , the flash temperature is, in general, given by ▪ The model is expanded in an effort to take account of the fact that the heat is evolved in a narrow zone in the softer material just below the contact spot rather than in the interface. For this case a maximum attainable temperature (if below the melting point) is found of roughly ▪ attained at a speed of the order of 100v 0. In order to assist in the practical application of the above equations, tables are presented of approximate values of the parameters p, j, g, b and f for a number of metals and four non-metals. Pin and disks asymmetries arise because at high speeds the heat flow is almost independent of the pin material but is determined by the substrate, unless it is a poor heat conductor compared with the pin.