Strain-based criteria for fatigue failure of cord-rubber composites

The thesis describes the fatigue behavior of angle-plied cord-rubber composite laminates. Using flat coupon specimens, the dependence of fatigue lifetime on the stress and strain history was examined to establish valid failure criteria. Above a critical level of interply shear strain, composite laminates exhibited localized damage in the forms of cord-matrix debonding, matrix cracking, and delamination. The knee strain for the initiation of cord-matrix debonding and the strain for gross failure were found to be around 10 and 20% respectively, regardless of the variation of matrix properties and strain rate. The result indicates that the damage accumulation and failure of the composite laminate are strain-controlled processes. At a given stress range, the use of higher levels of minimum stress up to 8.9 MPa led to longer fatigue life. When the minimum stress exceeded this critical level, an opposite trend of shorter fatigue life was observed. These two trends were attributed to non-linear stress-strain relationship and the effect of increased damage potential. The process of damage accumulation was accompanied by a steady increase of resultant maximum strain (dynamic creep) and acoustic emission (AE). The gross failure of composite laminate under fatigue loading occurred when the total strain accumulation reached the static tensile strain at failure. As in the case of dynamic creep rate, the accumulation rate of AE activities was linearly proportional to the inverse of the fatigue life. The residual strength and residual failure strain of the composite laminate decreased continuously with the propagation of fatigue damage. Based on the experimental results, an empirical model was proposed to predict fatigue life of cord-rubber composites. The model was based on dynamic creep rate, static tensile strain for gross failure, and static strain in unloading curve corresponding to maximum cyclic stress. Finally, a modified form of Goodman equation was formulated to predict the relationship between the strain range and minimum strain for a given lifetime of composites.