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 Am diameter and 1 Am 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 � 10 � 5 s � 1 ) to fracture of the scales at a higher strain rate (1.5 � 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. D 2005 Elsevier B.V. All rights reserved.