Tire Cord Fatigue and Fatigue Testing

Abstract

Abstract Examination of failed tires shows that fatigue failure mostly occurs on the inner plies, i.e., those subjected to compressive bending stress when the sidewall is bent. Continual flexing of the sidewall leads to a reduction of tensile strength of the cords until eventually the sidewall breaks up and the tire fails. This fatigue failure is essentially a function of the complex structure of the tire. When a tire is inflated all the cords are in tension throughout and when it is loaded the sidewall of that part of the tire in contact with the ground is bent so that some of the cords are subjected to relieved tensile stress or, if the deflection is very great, to compressive stress, and the rest of the cords to tensile stress. When the cord goes from tension to compression it tends to collapse but this is complicated by the constraining action of the surrounding rubber. If the tire now rotates the cords will be subjected to alternate tension and quasi-compressional stresses leading to ultimate failure. Fatigue performance is usually assessed by the average mileage to failure of the tire. If the plies of a tire are tested by extracting cords before failure occurs it can be shown that there is a progressive loss of strength of those plies undergoing compression. It has been asserted that this drop is linear with tire life and is greatest on the innermost ply, i.e., at the point of greatest compression. The outer plies undergo a cyclic increase and decrease of tension at constant stress and although there is a small reduction in breaking strength, this does not usually contribute to tire failure.