For composites the comparison is made between Normalized residual strength and Normalized Number of cycles., Endurance limit can be taken at 0.80 Normalized cycles. Failure modes in composites is different than metals .
Reference : Pg 257-285 , Fracture Mechanics , T L Anderson, Taylor and Francis.
The method has been used for composites having thermoset matrix with carbon fiber reinforcement . For fatigue failure of a composite you will have to consider the behavior ( viscoleastic, elastic-plastic , rubbery , rubber toughening ) of the matrix and reinforcement ( buckling / delamination/ microbuckling )under fatigue loading.