Cord—Rubber Tire Composites: Theory and Applications

Abstract

Abstract The stiffness and strength behavior of the constituent cord and rubber components of the pneumatic tire have been studied in detail by investigators in the industry since the early days of tire production. For example, the effect of twist on the stress-strain properties of cotton tire cord and the reinforcing effect of carbon black on the modulus of natural rubber were well known phenomena many decades ago. Similarly, differences between the elastic and viscoelastic behavior of cord and rubber have been well documented since the 1940's. On the other hand, the material properties (specifically, the elastic constants) of the cord-rubber composite system that comprises the tire are not as well known and have only begun to receive serious attention in the last decade. These tire elastic properties, whether they be referred to as stiffnesses, compliances or moduli, are anisotropic—i.e., they vary with direction. It is the properties of the anisotropic cord-rubber composite that primarily control the overall performance characteristics of pneumatic tires. Yet, much of present day tire design has been dominated by the separate influences of rubber compounding and textile technology. This situation has arisen perhaps because the functions of the individual cord and rubber components of the tire are well known, at least qualitatively; the low modulus, high elongation rubber contains the air and provides abrasion resistance and road grip; the high modulus, low elongation cords provide reinforcement for the rubber and carry most of the loads applied to the tire in service. However, it is shown later in this discussion that, in order to optimize a given tire performance parameter, a knowledge of the combined cord-rubber composite material properties is required. For example, the crown angle for maximizing the tread wear resistance for a steel-belted radial tire is not the same as for a rayon-belted radial of the same size. While this fact could be empirically established by “trial-and-error” tire building and test methods, such a program involving the wide variety of available tire cord and rubber compounds would be time consuming and expensive. Thus, composite material mechanics, even though in its early stages of development as far as cord-rubber systems are concerned, can and should be used to economically investigate the technical merits of potential tire designs.