The role of through-thickness gradation of graphene nanoplatelet on load transfer and fracture morphology in FRP composites

The development of advanced engineering applications requires materials with specific properties, yet epoxy-based composites' inherent brittleness is still a major limitation. Although graphene nanoplatelets (GNPs) can improve properties, their homogenous dispersion often results in restricted performance at high concentrations. This study fills a research gap in understanding functional gradation (FG) of nanoparticles in fiber-reinforced polymer (FRP) structures, focusing on localized damage resistance and fracture morphology correlations. To address these challenges, glass-fiber/epoxy laminates were fabricated with a controlled, layer-wise variation of GNP concentration through the laminate thickness, producing continuous (FG1), symmetric (FG2, FG3), and asymmetric (FG4) gradation profiles, which were evaluated against a neat laminate (N-G) and a laminate with uniform GNP dispersion (NFG) under quasi-static indentation (QSI) and flexural loading. Results revealed that the performance of composites was significantly influenced by the through-thickness distribution of GNPs. In terms of flexural energy and stiffness, the asymmetric FG4 was the most effective, with gains of 54.17% and 46.49%, respectively. QSI resistance was sensitive to both indenter geometry and gradation pattern, with FG structures showing improved penetration resistance and load-bearing capabilities. Morphological examination verified that these alterations arise from a combination of toughening mechanisms, such as matrix pinning and crack deflection. The outcomes reveal that spatial GNP dispersion exceeds uniform filling under specific loading conditions particularly hemispherical-indentation resistance, flexural stiffness, and damage tolerance while NFG remains preferable for flexural strength and flat-indentation load capacity, indicating that these strategies should be optimized based on specific loading conditions.