Bio-inspired composite design with variable-stiffness curvilinear helicoidal layup for enhanced impact resistance

Natural biological helical structures inspire impact-resistant composites for aerospace applications. However, current strategies are constrained by straight fiber layup paths, limiting performance gains. This work proposes a unique, efficient and facile composite microstructural design strategy: a variable-stiffness curvilinear helicoidal (VSCH) configuration in carbon fiber/epoxy. This design synergistically integrates in-plane curvilinear fiber steering and small-angle helicoidal stacking through the thickness to achieve multi-dimensional architectural features. VSCH laminates were fabricated using automated fiber placement (AFP) and hot-pressing. Comparative experiments with quasi-isotropic (QI), variable-stiffness curvilinear (VSC), and constant-stiffness linear helicoidal (CSLH) laminates revealed the VSCH configuration significantly outperformed the QI benchmark: exhibiting a 29.9 % improvement in impact mechanical performance and 32.1 % increase in damage tolerance. Experimental and numerical simulation results demonstrate this superior impact resistance stems from: 1) More uniform inter-ply angle transitions; 2) Helically interlaced fiber microstructures; 3) Customized curvilinear paths and in-plane fiber orientation gradients. These mechanisms collectively alleviate interlaminar concentration, promote crack redirection/deflection, and inhibit delamination propagation and fiber damage via crack coupling and stress decomposition, enabling exceptional damage tolerance. This synergistic integration of curvilinear fiber steering and helicoidal stacking can transcend conventional performance boundaries in advanced composites.