Various rubber articles are subjected in ordinary service to repeated stretching, compression and cutting and to combinations of such forces. Repeated flexing is frequently encountered also. All these forces cause repeated changes, either positive or negative, in the dimensions of the outside surface of the rubber article. Among the important rubber products which undergo such stresses are tires, belts and shoe soles. The stresses to which the tread of a tire is subjected are certainly complex and variable. In this particular case, there are, in addition to the initial static tension, forces of compression, flexing and torsion. A rubber belt offers a simpler problem, for in this case it is possible to distinguish the following cycle of changes on the outside surface: a constant elongation in the taut section of the belt; a sudden increase in elongation on passing over the driving pulley, during which time a constant flexing stress (which depends on the curvature of the pulley) must be added to the tension on the belt; a decreased elongation along the surface of the belt in contact with the pulley; and finally a rapid return to the minimum value existing in the belt when under no tension. An analogous cycle of changes takes place at the driven pulley. Each element of the surface of a belt undergoes, therefore, during each complete revolution, two periods of tension, between which the element is alternately in the taut and in the slack section of the belt.