This research uses an analytical model to examine the impact of the R-curve on interlaminar crack propagation in composite materials under fatigue loading conditions. The R-curve describes the change in a materials critical interlaminar fracture toughness as a crack advances. In fiber reinforced composites a steady state or plateau is typically reached, and this is associated with saturated fiber bridging in mode I loading. The analytical model is based on a family of parallel Paris curves that are a function of the critical fracture toughness. Experimental data of pure mode I unidirectional composite specimens, drawn from existing literature, indicates that the R-curve influences the crack propagation rate, particularly at higher cycle counts. The analytical model described herein is an effective tool for instigating a materials behavior and may facilitate an understanding of the precision required for key material properties. The analytical model is intended to complement more complex Finite Element Analysis (FEA) simulations traditionally used for predicting lifespan and ensuring the durability of structures under cyclic loading.