Cumulative Damage of Fiber-Reinforced Elastomer Composites under Fatigue Loading

Fracture mechanisms under fatigue loading were assessed in the case of nylon fiber-reinforced elastomer matrix composite which represents the actual carcass of bias aircraft tires. Under uniaxial tension, the angle-plied carcass composite specimens were subjected to a considerably large interply shear strain before failure. The composite specimens exhibited semi-infinite fatigue life when stress amplitude was below a threshold level, i.e., fatigue endurance limit. Under cyclic stresses exceeding the endurance limit, localized damage in the form of fiber-matrix debonding and matrix cracking was formed and developed into delamination eventually leading to gross failure of the composite. The process of damage accumulation was accompanied by heat generation and a continuous increase of cyclic strain (dynamic creep). Fatigue lifetime and the resistance to damage accumulation of aircraft tire carcass composite were strongly influenced by cyclic frequency. The use of higher cyclic frequency resulted in shorter fatigue lifetime at a given stress amplitude and lower fatigue endurance limit. The extent of dynamic creep at gross failure, which is defined as the increase of cyclic strain beyond initial elastic deformation, was roughly independent of stress amplitude under the frequency of 1 Hz, but decreased with higher stress amplitude when the frequency was raised to 10 Hz. Obviously a critical level of dynamic creep exists for gross failure of the composite and this level appears to be independent of the stress amplitude at low frequency. When the frequency is high enough, heat generation due to hysteretic loss is expected to degrade the materials. In this situation, the critical level of dynamic creep for gross failure may be reduced by the loss of matrix flexibility as well as fiber-matrix bonding strength. The study also examined the effect of load sequence on the damage accumulation process of composite based on dual stress level fatigue testing. Palmgren-Miner's linear damage accumulation theory tended to overestimate the cumulative damage of angle-plied carcass composites. As an alternative to PalmgrenMiner's rule, a linear cumulative damage model based on dynamic creep was utilized. The sum of dynamic creep to gross failure was found to be unity indicating that the level of cumulative damage is independent of the load sequence.