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Prediction of the residual stiffness of the carbon fiber reinforced polymer composite, subjected to fatigue loading, can be performed using some of the phenomenological models. However, it is still a challenge to find the stiffness based on the known microstructural damage state (that was developed irrespective of the load history). In this work, two micromechanics-based models were developed to predict reduction in the stiffness of the damaged composite. Fiber crack density and interface debonding was used to define the microstructural damage state of the composite. These models account for the fiber crack density in the form of change in either geometry (equivalent ellipsoid model) or material property of the fiber (reduced stiffness model). The microstructural damage state in the unidirectional carbon fiber reinforced polymer composite, obtained from the on-axis tension–tension fatigue loading, was used to validate the models. The results from reduced fiber stiffness model were compared against experiment and finite element analysis for the given microstructural damage. The stiffness obtained using reduced fiber stiffness model was in good agreement with that obtained from the experiment. However, reduced fiber stiffness model underestimated reduction in stiffness compared to finite element analysis.

Prediction of the fiber crack density (as one of the microstructural damages) for unidirectional fiber-reinforced polymer composite under monotonic tensile load, using strength models, has been reported in the literature. However, the microstructural damage prediction for a fiber-reinforced polymer subjected to fatigue loading is still a challenge. In this work, a progressive damage initiation model was developed to predict the fiber crack density in carbon fiber-reinforced polymer composite subjected to fatigue loading. A stochastic model was used for modeling the fiber fatigue strength. Reduction in effective life of the fiber was modeled using linear Miner’s rule. Effect of fatigue strength parameters on fiber crack density was found to be considerable compared to the effect of interface shear strength. At a low number of cycles, fiber crack density obtained from the model was in good agreement with the experimentally measured fiber crack density.