6—an Interpretation of the Mechanical Properties of Covered-Rubber Yarns

The extensibility and other mechanical properties of covered-rubber yarns are considered on the basis of a simple theoretical model in which the covering thread is assumed to be inextensible. It is shown that certain characteristic properties of these yarns are essentially determined by the helix angle during the covering process. The tension in the covering thread and the stresses on the rubber at any state of strain are calculated. The theory is tested by experiments on a series of yarns whose geometrical parameters are systematically varied. The retraction subsequent to the covering process is found to be related to the helix angle of the covering in the manner predicted. The principal features of the force–extension curves are interpreted quantitatively on the same basis. Measurements of the couple about the axis, for singly-covered yarns, provide an indirect check on the theoretical analysis of the tension in the covering thread. It is shown that over a considerable range of extension the coverings are slack, the stress on the yarn being simply the stress on the rubber core. As the breaking point is approached, the fraction of the stress carried by the coverings becomes increasingly important, ultimately reaching a value exceeding 90% of the total tensile force on the yarn in a typical yarn construction. It follows that the rubber core makes no significant contribution to the strength of the yarn, which is determined almost entirely by the strength of the coverings.