Size-dependent Intra-Laminar fracture behavior for Kevlar and Kevlar/Carbon hybrid composites

This study systemically investigates the intra-laminar fracture properties and size effect in 2D Kevlar and Kevlar/Carbon (K-C) hybrid laminates through an integrated approach that combines single-edge notched tension (SENT) tests with three varying in-plane dimensions, Finite Element Analysis (FEA), and Digital Image Correlation (DIC). The research reveals significant differences in fracture performance and size effects among the various reinforcement types, emphasizing the underlying hybridization mechanisms. The experimental results demonstrate that the K-C hybrid composites exhibit superior normal strength, enhanced normalized strength, higher initial fracture energy, and extended effective Fracture Process Zone (FPZ) lengths compared to the Kevlar composites. These improvements are attributed to the synergistic crack-arresting and damage-deflection effects of carbon fibers oriented perpendicular to the notch, which effectively inhibit intra-laminar crack propagation while promoting the transition of damage from intra- to inter-laminar modes. Both material systems follow Bažant’s size effect law (SEL), showing lower nominal strength with increasing in-plane size. This trend reflects a gradual transition from the plastic limit line in small specimens to the Linear Elastic Fracture Mechanics (LEFM) line in large specimens. This phenomenon correlates with the energy release development in an expanding FPZ, which contains a higher probability of critical flaws. The K-C hybrid composites show a significant deviation from LEFM line because of the inhibition of intra-laminar crack propagation. Overall, the findings demonstrate that stiffer fiber hybridization enhances intra-laminar fracture resistance while mitigating size effects, which is critical for the design of large-scale aerospace structures.