Synergistic effects of graphene and carbon nanotubes on the piezoresistive strain sensing of epoxy composite laminates under bending loads

Polymer composites, known for their excellent specific mechanical properties, are increasingly used for structural health monitoring (SHM) to detect hidden damage over time. Despite promising research on single nanomaterials, hybrid mixtures of carbon nanotubes (CNTs) and graphene nanoplatelets (GNPs), capable of synergistically optimizing structural and sensing functions, remain underexplored, especially under fatigue conditions. This study addresses this gap by fabricating glass and carbon fiber composites reinforced with hybrid CNT/GNP combinations and evaluating their static bending strength, fatigue strength, and piezoresistive behavior. A 0.375 wt% GNP/0.375 wt% CNT (0.375 wt% of both nanoparticles) composition improved bending strength by 11% (carbon fiber) and 8% (glass fiber), with stiffness increases of 18% and 55%, respectively. The hybrid nano-reinforced composites also showed improved piezoresistive behavior; in particular, the 0.25 wt% GNP/0.5 wt% CNT composition exhibited the lowest electrical resistance. Gauge factor analysis indicated increased sensitivity during cyclic testing, especially in the carbon fiber composites, suggesting more efficient conductive networks. Fatigue tests revealed a 32.4% increase in bending stress amplitude for 0.5 wt% GNP/0.25 wt% CNT glass fiber composites and a 20.1% improvement for 0.25 wt% GNP/0.5 wt% CNT carbon fiber composites. The results highlight the potential of carbon-based hybrid nanomaterials to enhance both structural performance and sensing capabilities in self-monitoring composites.