Based on the theory by Blok and Jaeger, simple equations have been obtained for the theoretical evaluation of flash temperatures, i.e., the temperatures of circular or elliptical contact spots between two homogeneous materials, when friction heat and/or Joule heat is evolved at the interface. The parameters appearing in the equations have been expressed in terms of experimental data, the coefficient of friction, known materials properties, and the ellipticity and number of contact spots. The equations are especially simple in the limiting cases of very high and of very low speeds, and were indeed known in these limits for circular, albeit not for elliptical contact spots. As an example, the flash temperatures of plastic contact spots on account of friction heat have been computed for (i) an electrical brush material sliding on copper, and (ii) a carbon steel sliding on itself. In these examples the dependence of the flash temperature on the velocity of the contact spots relative to either or both of the two sides has been investigated, wherein the effect of sliding rate on the flash temperature via the strain rate dependence of local hardness has been taken into consideration,- it is believed for the first time in any theoretical investigation of flash temperatures. In the high-speed case, in agreement with intuitive expectation, the numerical examples show that under otherwise same conditions and same macroscopic velocity, minimum flash temperature is attained when the spot moves relative to both sides. Typically, that minimum does not occur when the contact spot speed is one half of the relative speed on each of the two sides, however. Moreover, and somewhat unexpectedly, the results show that the previously neglected strain rate dependence of the hardness has a considerable effect.

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