Fatigue Failure in Nylon Reinforced Tires

Abstract

Abstract The principal mechanism leading to fatigue failure of nylon cord tires is progressive loss of cord strength from filament ruptures that occur at an angle of approximately 25 degrees to the fiber axis. Flexing causes cords to become weaker until residual strength is less than tensile forces in the tire, at which time the remaining filaments break with a blunt-ended tensile rupture. Individual cords in tires have stress environments determined by such factors as cord spacing, uniformity, and stiffness of the immediate flexing region, which appear to influence significantly their fatiguing rate. It is probable that these factors also operate in laboratory in-rubber fatigue tests, so that a high degree of precision in sample preparation and loading conditions is essential for such tests to give reproducible data. Measurement of cord strength loss was found to be a valid and useful means for following the progress of fatigue when it was recognized that cords under opposite sidewalls of tires lost strength at markedly different rates, which were not linearly proportional to test mileage. The direction of tire rotation and the cord angle in a given ply determined which half of the ply lost strength fastest. Strength loss was higher for the half of the cord that led into the load bearing region of the tire as it rotated. This presumably happened because of unsymmetrical stress conditions, but no measurement of cord stress was made. Fatigue failures of tires generally occurred at localized points and resulted from failure of a few cords that lost strength faster than the surrounding cords. The failures occurred in the innermost ply at “hinge points” opposite the end of the flipper fabric in the ply splice.