Research on the Structural Characteristics and the Fracture Resistance Mechanism of the Primary Flight Feather Shaft in Vulture

The primary flight feather shaft (PFF) plays a crucial role in adjusting avian's flight posture and flight speed while bearing intense aerodynamic force, and it demonstrates exceptional mechanical properties and fracture resistance. The influence of the structure of a PFF on its fracture resistance mechanism (including crack initiation resistance and crack propagation resistance) remains unclear. This study focused on the research subject of the PFF of Aegypius monachus (cinereous vulture) and the impact of the macro/microstructural characteristics and components on the fracture resistance mechanism. The results showed that the variation in cross-sectional shape (from circular to square) enhanced the bending stiffness of the PFF. The PFF exhibited a maximum compressive stress of 28.85 ± 0.91 MPa and a maximum tensile strength of 51.59 ± 4.43 MPa. Layered fibers and the bridging structure between fibers inhibited crack initiation and propagation. The medullary foam was composed of closed-cell porous units, which reduced the overall weight of the PFF and absorbed external mechanical energy. The cuticle layer contained 5.97 wt% sulfur (S) and abundant cysteine disulfide bonds, which endowed the PFF surface with high rigidity and inhibited crack generation. This study reveals the influence of structural characteristics of vulture PFF on its fracture resistance mechanism and provides inspiration for the design of bioinspired lightweight composite structures.