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Structure and mechanical behavior of bird beaks


The structure and mechanical behavior of Toco toucan (Ramphastos toco) and Wreathed hornbill (Rhyticeros undulatus) beaks were examined. The structure of Toco toucan and Wreathed hornbill beak was found to be a sandwich composite with an exterior of keratin and a fibrous bony network of closed cells made of trabeculae. A distinctive feature of the hornbill beak is its casque formed from cornified keratin layers. The casque is believed to have an acoustic function due to the complex internal structure. The toucan and hornbill beaks have a hollow region that extends from proximal to mid-section. The rhamphotheca is comprised of superposed polygonal scales (45 \[mu\]m diameter and 1\[mu\]m thickness) fixed by some organic adhesive. The branched intermediate filaments embedded in keratin matrix were discovered by transmission electron microscopy (TEM). The diameter of intermediate filaments was ̃10 nm. The orientation of intermediate filaments was examined with TEM tomography and the branched filaments were homogeneously distributed. The closed-cell foam is comprised of the fibrous structure of bony struts with an edge connectivity of three or four and the cells are sealed by the thin membranes. The volumetric structure of bird beak foam was reproduced by computed tomography for finite element modeling. The mechanical behavior of bird beaks was evaluated by tensile and compression testing. Micro and nanoindentation hardness measurements were used to corroborate these values. The mechanical response of toucan rhamphotheca exhibits isotropy whereas hornbill rhamphotheca may behave anisotropically in tension. The Young's moduli of toucan and hornbill rhamphotheca were found to be ̃1.0 GPa. The Young's modulus of rhamphotheca at high humidity condition dropped by a order of magnitude. The rhamphotheca exhibits a strain-rate sensitivity with a transition from slippage of the scales due to release of the organic glue, at a low strain rate, to fracture of the scales at a higher strain rate. The compressive strength of beak foam is dependent on the density. The higher apparent density of hornbill foam provides a four-fold higher strength than toucan foam. The mechanical behavior of beak skulls were modeled by the Gibson-Ashby constitutive equation. The compressive response of the beak revealed that there is a synergistic effect between foam and shell evidenced by the separate response of shell, foam, and foam + shell. The sandwich design of bird beaks were evaluated by the Karam-Gibson and Dawson-Gibson models. The models proved that the design of bird beaks was optimized for effectively achieving high resistance to buckling when they are subjected to bending rather than uniaxial loads. The synergistic interaction between foam and shell is also successfully confirmed by a finite element analysis (LS- DYNA). The foam stabilizes the deformation and prevents local buckling of the keratin shell by providing an internal support which increases its buckling load under compressive loading. The bending behavior of bird beak in finite element analysis was successfully compared with experimental results. The volumetric structure of bird beak foam was reconstructed in 3D by a visualization technique and this rendering was successfully applied to finite element calculations which predict compressive strength in agreement with experimental results.

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