The fruits of the pomelo, the coconut and the bark of the giant sequoia have one thing in common: they are very likely to face high energy impacts. The fruits of the pomelo and the coconut reach high kinetic energies when dropping onto the ground from great heights after being shed. Thereby, the pomelo peel is characterized by a density gradient within the whitish albedo – a thick layer of living parenchymatic cells constituting the major part of the peel. The albedo can be regarded as an open pored graded foam with an embedded network of vascular bundles. Inspired by this biological role model, technical metal foams were developed with a density gradient that was implemented by thickening the foam struts in selected areas. With that a selective stiffening of certain parts of the metal foam structure could be achieved. The introduction of stiff ceramic fiber bundles parallel to the loading axis lead to a further increase of the stiffness.
Contrary to the albedo of the pomelo, the tough coconut endocarp (inner shell of the fruit wall) dissipates impact energy by crack path deviation and crack path elongation. It is composed of a stiff matrix of highly interconnected cells which are reinforced by thick cell walls and embedded bundles of hollow fibers. The fiber bundles represent weak spots that are arranged more or less in parallel to the outer surface of the endocarp. Occuring cracks will thus propagate along the fiber bundles during impact and consume high amounts of energy during this process without penetrating the entire endocarp wall.
The bark of the giant sequoia has to withstand high impact energies during rockfall events. A sophisticated multi-layered composition and a high amount of fibers in the bark result in an increase of impact duration (leading to a marked decrease of transmitted peak forces).
In order to improve impact and earthquake protection of technical construction elements consisting of functionally graded concrete, two contrary approaches are envisaged: (a) fibers that are weaker than the concrete matrix or have weaker connecting zones to the concrete may serve as weak spots to deviate cracks into uncritical zones within the concrete structures, inspired by the fiber bundles of the coconut endocarp, or (b) fibers that are stronger than the concrete matrix may serve as reinforcement in order to develop fiber-reinforced functionally graded lightweight concretes with the fiber arrangement and composition inspired by the bark of the giant sequoia.

---

Bio

Dr. Georg Bauer studied biology and received his PhD at the University of Freiburg, Germany. After a post-doc position at the department of Biomaterials at the Max Planck Institute of Colloids and Interfaces in Potsdam, Germany, he is currently a group-leader in the Plant Biomechanics Group Freiburg. His research interests comprise plant biomechanics and biomimetics, with a focus on energy dissipation mechanisms of plants.