The toucan beak: Structure and mechanical response Yasuaki Seki * , Bimal Kad, D. Benson, Marc A. Meyers Department of Mechanical and Aerospace Engineering, University of California, San Diego, La Jolla, CA 92093-0411, USA Available online 3 October 2005 Abstract 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. Keywords: Toucan; Beak; Keratin; Foam; Viscoplastic; FEM 1. Introduction The study of biological materials is inspiring new proces- sing methods for materials. Examples such as silk, shell, spider web, and sea sponge spicule, abound. A fascinating class of biological materials is sandwich structures consisting of a solid shell and a cellular core; the cellular core increases the resistance of the shell to buckling, leading to a synergism between the two constituents. Plant stems and porcupine quills fall under this category. Bird beaks are light-weight structures that need to possess significant specific strength and structure. The toucan has a long beak that is also thick, a necessity for food gathering in tall trees. This biological material can serve as a useful source for research and as an inspiration for structural design in engineering. 2. Experimental techniques Toucan (Ramphastos toco ) beaks (both the upper and lower parts), obtained after the natural death of animals from a local breeder, were used for mechanical tests and structural analysis. The black color region of the exterior beak was avoided because coloration has an effect on its hardness [1]. Humidity and temperature were measured because it is known that the mechanical properties are dependent on them. Specimen preparation for nanoindentation and microinden- tation was the same. The toucan beak shell and foam were cut into small pieces by knife. Both samples were mounted in epoxy and glued on a glass plate. The experimental set up was the same as the one used earlier for hardness measure- ments of the starling beak [1]. A LECO M-400-H1 hardness testing machine with a load 100 gf was used. The indenter was applied for 15 s, and a subsequent 45 s was allowed to elapse before the diagonals of the indentation were measured. Since nanoindentation is highly sensitive to the roughness of the sample, specimens were polished to 0.05 Am. A Hysistron Triboindenter was used to determine the reduced Young’s modulus and hardness of the exterior and interior. Loads of 500 and 1000 AN (Berkovich-type indenter) were applied to specimens. The outer shell of the toucan beak was cut into rectangles with a knife to prepare tensile specimens. The rectangles were then inserted into a laser cutting machine and the dog bone shaped specimens were cut out (shape programmed into the 0928-4931/$ - see front matter D 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.msec.2005.08.025 * Corresponding author. Tel.: +1 858 543 6091. E-mail address: yaseki@ucsd.edu (Y. Seki). Materials Science and Engineering C 26 (2006) 1412 – 1420 www.elsevier.com/locate/msec