The toucan beak: Structure and mechanical response

The structure and mechanical response of a Toco toucan (Ramphastos toco) beak were established. The beak was found to be a sandwich composite with an exterior of keratin scales (50 mu m diameter and 1 mu m thickness) and a core composed of fibrous network of closed-cells made of collagen. The tensile strength of the external shell is about 50 MPa. Micro- and nanoindentation hardness measurements corroborate these values. The keratin shell exhibits a strain-rate sensitive response with a transition from slippage of the scales due to release of the organic glue, at a low strain rate (5 x 10(-5) s(-1)) to fracture of the scales at a higher strain rate (1.5 x 10(-3) s(-1)). The closed-cell foam consists of fibers having a Young's modulus (measured by nanoindentation) of 12.7 GPa. This is twice as high as the keratin shells, which have E = 6.7 GPa. This is attributed to their higher calcium content. The compressive collapse of the foam was modeled by the Gibson-Ashby constitutive equations. There is a synergistic effect between foam and shell evidenced by a finite-element analysis. The foam stabilizes the deformation of the keratin shell by providing an internal support which increases its buckling load under compressive loading. (c) 2005 Elsevier B.V. All rights reserved.