The Validity of Simulating Tests in Evaluating Lubricants for Deformation Processes

Abstract

One of the most significant stumbling blocks in establishing correlation between theory and application in deformation processes has been the uncertainty attached to the magnitude of interface friction. Even the simplest theory must account for the effects of friction on forces, power requirements and material flow, and the magnitude of this friction must be known-together with the correct value of the flow stress-if the validity of a theory is to be checked. While there is still a great scarcity of data on flow stresses at relevant strain rates and temperatures, a beginning has, nevertheless, been made by determining flow stresses in plastometers. No comparable development occurred with regards to friction. There is no universal, basic method of determining friction under conditions applicable to deformation processing; instead, a numerical value representing friction (in the form of a coefficient of friction, or an interface shear strength) is usually derived from experimental data through the utilization of a theory, often at the same time when proof of the validity of the very same theory is sought. Under these circumstances, there is a danger of friction becoming an adjustable, variable “constant” chosen at convenience. It would be highly desireable, therefore, that friction should be determined in simple tests that are readily evaluated, rely on a minimum of theoretical or simplifying assumptions, yet simulate actual deformation processing conditions sufficiently to make the results relevant, and to allow lubricant evaluation with a measure of confidence. This paper aims at clarifying to some degree the suitability of some existing test techniques for these purposes.