An unexpected hardening behaviour of carbon-fiber epoxy composites under flexural fatigue loading and its intensification by graphene nano-fillers

Abstract

The fatigue performance of carbon fiber reinforced polymer (CFRP) composites under cyclic loading remains a significant concern in various engineering applications due to the complex damage mechanisms and uncertain failure modes associated with these materials. Despite their exceptional strength-to-weight ratio, CFRP composites are prone to progressive damage accumulation, leading to premature failure and reduced structural integrity. The present study investigates the flexural fatigue response of pristine CFRPs and CFRPs with added graphene nanoplatelets (GNPs), accompanied by in-situ microscopy for progressive damage analysis. Eight layers of carbon fibers fabric, both pristine and GNP coated were utilized to fabricate the composites using vacuum-assisted resin transfer molding (VARTM) technique. GNP-added composites exhibited significantly improved monotonic flexural strength, strain and fatigue life compared to pristine composites. The dissipated strain energy density (DSED) was also evaluated as a function of cycles and correlated with the damage that happened in the composite using in-situ optical imaging. While cycling at or below three-quarters of the maximum tensile stress, it was observed that the DSED decreased with the number of cycles in both composites. However, at higher loads, the DSED was found to increase progressively, ultimately shooting up during catastrophic failure. These composites that showed a decrease in DSED with cycles, did not fail even at one million cycles and had a higher residual strength than the original when they were forced to break under monotonic loading and the percentage increment was even higher when GNPs were used. Additionally, post-failure fracture surface examination using scanning electron microscopy (SEM) was conducted to understand the influence of GNP addition on damage evolution of CFRPs.