Fatigue of Cord—Rubber Composites: III. Minimum Stress Effect

The dependence of fatigue lifetime on the minimum cyclic stress was assessed for the rubber matrix composite of bias aircraft tire carcass reinforced by nylon cords as well as two model rubber composites reinforced by steel wire cables. At a given stress range, the use of higher minimum stress up to a certain level led to longer fatigue life of the composite laminate. When the minimum stress exceeded this critical level, an opposite trend of shorter fatigue life occurred with a higher level of minimum stress. The initial trend of longer fatigue life of angle-plied, cord—rubber composite laminates with a higher level of minimum stress stems from a nonlinear stress—strain relationship with a tendency of strain stiffening. Under stress-controlled fatigue loading, a shorter strain range was experienced for the same stress range when the level of minimum stress was raised. However, the increase of minimum stress raises the potential for damage initiation and accumulation. This effect of increased damage potential became dominant above the critical level of minimum stress. When the fatigue life data were plotted against the values of strain range to clarify the role of damage potential, a general trend of shorter fatigue life with a higher level of minimum stress was observed at a given strain range, in striking contrast to the trend of the stress range vs the number of cycles to failure (S-N) curve in a conventional form. The result again confirms that the damage initiation and eventual fatigue failure of angle-plied, cord—rubber composite laminates are “strain-controlled” processes, as demonstrated in our preceding paper. A modified form of the Goodman equation appears to be valid in predicting the relationship between the strain range (instead of stress amplitude) and minimum strain (instead of mean stress) for a given lifetime of a cord—rubber composite laminate.