From nature to engineering: A comprehensive mechanical response analysis of novel bio-inspired CFRP adhesive joints

• Introduces a bio-inspired gradual helicoidal CFRP design for adhesive joints. • Achieves comparable strength with UD-type joints for bio-inspired gradual joints with substantially improved toughness and energy absorption. • Reveals crack-twisting as a key mechanism for enhanced bending fatigue resistance. • Twofold fatigue life and ∼80% residual strength in bio-inspired joints. For effective adhesive joint design, both strength and toughness must be considered. This work investigates bio-inspired carbon fiber reinforced polymer (CFRP) substrates, replicating the natural architecture of the dactyl club, to enhance the mechanical performance of bonded joints. The CFRP substrates were stacked in gradually increasing ply angles to mimic the dactyl club microstructure. Three additional stacking sequences were considered: conventional helicoidal (constant ply angle change), unidirectional (UD), and quasi-isotropic (QI). To evaluate the proposed novel gradual helicoidal concept, composite single-lap joints (SLJs) with all stacking sequences were tested under quasi-static four-point bending. The best-performing configurations (gradual helicoidal and UD) along with QI joints, were subsequently tested under four-point bending fatigue. Cohesive element-based numerical simulations were also employed to predict the influence of stacking sequence on joint strength and toughness. Results showed that maximum load was similar for gradual helicoidal and UD joints and higher than for conventional helicoidal and QI SLJs. The energy absorption capacity of gradual helicoidal joints was significantly improved, with 46% and 115% higher values compared to UD and QI joints, respectively. Under fatigue loading, gradual helicoidal (G-type) single-lap joints exhibited enhanced damage tolerance. At load levels of 43% and 50%, the fatigue tests for the G-type joints were terminated without failure. At these load levels, the G-type joints sustained 140% and 207% more cycles, respectively, than the UD-type joints, which failed. After fatigue loading, the G-type joints retained high residual strength, corresponding to 82% and 70% of their static strength.