Effect of pre-existing microstructural damage and residual stresses on the failure response of carbon fiber reinforced polymers

A comprehensive numerical study, relying on high-fidelity finite element (FE) failure simulations, is presented to quantitatively understand the effect of pre-existing microstructural damage and residual stresses caused by instantaneous load/unload cycles on the failure response of a carbon fiber reinforced polymer (CFRP). A representative volume element (RVE) of this composite is virtually reconstructed/meshed and realistic pre-existing damage/stresses are induced into its microstructure via loading/unloading simulations with different types (tensile or compressive), directions, intensities, and sequences. After applying a final load causing failure, we study the impact of these parameters on the CFRP strength calculated via computational homogenization relying on FE simulation results. The study shows that the strength of a pre-damaged RVE is highly unpredictable, meaning despite causing notable microstructural damage, in some cases the effects of initial load/unload cycles on strength are either negligible and may even slightly increase that, while in other cases it could lead to a massive drop in strength. The lack of a meaningful trend and in particular no direct correlation between strength and the pre-loading parameters (intensity, type, direction, and number/sequence of cycles) and the extent of pre-existing microstructural damage could be attributed to the effect of residual stresses, causing a high level of uncertainty in the failure response of CFRP.