Seagull feather shaft: Correlation between structure and mechanical response

Flight feathers are unique among a variety of keratinous appendages in that they are lightweight, stiff and strong. They are designed to withstand aerodynamic forces, but their morphology and structure have been oversimplified and thus understudied historically. Here we present an investigation of the shaft from seagull primary feathers, elucidate the hierarchical fibrous and porous structure along the shaft length, and correlate the tensile and nanomechanical properties to the fiber orientation. An analysis of the compressive behavior of the rachis based on a square-section model shows a good fit with experimental results, and demonstrates the synergy between the cortex and medulla. Flexural properties of the shaft along the shaft length, analyzed as a sandwich composite, reveal that although all flexural parameters decrease towards the distal shaft, the specific equivalent flexural modulus and strength increase by factors of 2 and 3, respectively. The failure mode in flexure for all specimens is buckling on the compressive surface, whereas the foamy medulla prevents destructive axial cracking and introduces important toughening mechanisms: crack deflection, fiber bridging, and microcracking. Statement of Significance Using mechanics principles, we analyze the feather shaft as a composite beam and demonstrate that the flexural strength is extraordinary, considering its weight and tailored along the length. The cross section changes from circular in the proximal base to square/rectangular in the distal end. We also discovered that the composite design, a solid shell enclosing a foam core, produces synergistic strengthening and toughening to the feather at a minimum of weight.