AbstractThe materials considered in this investigation were oriented‐hybrid‐glass‐fiber‐reinforced vinyl ester resin sheet‐molding compound (SMC) composites. The composition of one material was 60 percent continuous glass and 5 percent random fibers, while the other was 45 percent continuous glass and 20 percent random fibers. The S‐N curves for the two materials were determined in the continuous fiber direction. The fatigue stress levels ranged from approximately 20 to 80 percent of each material's respective ultimate tensile strength. The residual tensile strengths of the fiber‐reinforced composites were not seriously reduced until near final fracture. The moduli of the composites in the fiber direction are also not reduced in any detectable manner. Damping measurements have shown that the damping increases during fatigue and that it is a sensitive measure of matrix and interface damage, generally debonding and cracking. It is shown by damping measurements that damage accumulates differently for two materials studied. The material with the higher percent of continuous glass appeared to be more cycle‐dependent, while the other was more s
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