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 th e actual carcass of bias aircraft tires. Under uniaxial tension, the a ngle-plied carcass composite specimens were subjected to a considerabl y 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 del amination eventually leading to gross failure of the composite. The pr ocess 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 co mposite were strongly influenced by cyclic frequency. The use of highe r cyclic frequency resulted in shorter fatigue lifetime at a given str ess amplitude and lower fatigue endurance limit. The extent of dynamic creep at gross failure, which is defined as the increase of cyclic st rain beyond initial elastic deformation, was roughly independent of st ress amplitude under the frequency of 1 Hz, but decreased with higher stress amplitude when the frequency was raised to 10 Hz. Obviously a c ritical level of dynamic creep exists for gross failure of the composi te 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 situ ation, the critical level of dynamic creep for gross failure may be re duced by the loss of matrix flexibility as well as fiber-matrix bondin g strength. The study also examined the effect of load sequence on the damage accumulation process of composite based on dual stress level f atigue testing. Palmgren-Miner's linear damage accumulation theory ten ded to overestimate the cumulative damage of angle-plied carcass compo sites. As an alternative to Palmgren-Miner'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.