FATIGUE OF CORD-RUBBER COMPOSITES

Fatigue failure mechanisms and their dependence on cyclic loading freq uency were assessed in the case of a nylon fiber-reinforced elastomer matrix composite representing the actual carcass of bias aircraft tire s. Under uniaxial tension, the angle-plied composite specimens were su bjected to a considerably large interply shear strain before failure. The composite specimens exhibited infinite fatigue life when stress am plitude was below a threshold level, i.e., fatigue endurance limit. Un der cyclic stresses exceeding the endurance limit, localized damage in the form of fiber-matrix debonding and matrix cracking was formed and developed into the delamination eventually leading to gross failure o f the composite. The process of damage accumulation was accompanied by a continuous increase of cyclic strain as well as temperature. Fatigu e lifetime and the resistance to damage accumulation of aircraft tire carcass composite were strongly influenced by cyclic frequency. The us e of higher frequency resulted in shorter fatigue lifetimes at a given stress amplitude and lower endurance limit. The extent of dynamic cre ep 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 stre ss amplitude when the frequency was raised to 10 Hz. Obviously a criti cal level of dynamic creep exists for gross failure of the composite a nd this level appears to be independent of the stress amplitude at low frequency. When the frequency is sufficiently high, heat generation d ue to hysteretic loss is expected to degrade the materials. In this si tuation, the critical level of dynamic creep for gross failure seems t o be reduced by the loss of matrix flexibility as well as fiber-matrix bonding strength, with the degree of reduction becoming greater under higher stress amplitude.