Robin Seidel*, Andreas B"uhrig-Polaczek+, Claudia Fleck#, Thomas Speck*

* Plant Biomechanics Group Freiburg, Botanic Garden, University of Freiburg, Germany
+ Foundry-Institute of the RWTH Aachen, Aachen, Germany
# Materials Engineering, Berlin Institute of Technology, Berlin, Germany


As we humans have to protect ourselves from the large amounts of energy freed instantaneously during a car crash so some fruits have to cope with the impact on the ground when being shed. Understanding the principles of how combining structure and material yields a fully functional protection layer will allow us to construct new lightweight bio-inspired materials of high impact and puncture resistance.
As biological role models we have selected the Macadamia nut with its tough pericarp containing highly lignified sclerids, the drupe related Citrus grandis, also known as pumello, with its large spongy mesocarp and the real drupe, Cocos nucifera, having a combination of a fibrous mesocarp and tough endocarp. All fruits are relatively heavy, lack any aerodynamic adaptation and share the same challenge of having to withstand impact from heights of more than 10 metres.
Conducting high speed camera controlled free fall experiments of Citrus grandis from six metres and comparing the potential energy of the fruit before and after impact (n=13) shows that a high proportion of the energy, possibly up to 90 %, is dissipated by the fruit wall and pulp. Further there is no decrease in energy dissipation from one to the next free fall dropping the same specimen up to seven times. These large values of energy dissipation correspond well with the values obtained during quasi static testing of the pumello fruit wall under compression. Compressing the fruit wall to approximately 55% of its original height yields a 50% to 66% energy loss during one hysteretic cycle. Afterwards the samples recover almost completely to 90-95% of their initial height. The Young's modulus of Citrus grandis is surprisingly low with values ranging from 0.14MPa to 0.45MPa and a mean of 0.2MPa. The Youngs's modulus of the immature Coconut mesocarp obtained from specimen loaded in tension nearly parallel to the fibre orientation lies at about 10MPa. In contrast to the fruit wall of Citrus grandis the samples show a benign failure behaviour with a bell shaped stress-strain-curve having its maximum stress at 20% strain and failure only at 40% strain. As discussed by Wang et al. (1995) the Macadamia nut shell is highly optimised for toughness. Their results obtained from compression tests are in the same range as ceramics and glass, but when considering the relative low density of the pericarp of the Macadamia nut shell it even outperforms these two technical materials.
Until today these protective structures have only been used as an inspiration for packaging when seen from a biomimetic perspective. But combining these extraordinary properties of high energy dissipation, benign failure and almost full recovery from large deformations will lead to completely new bio-inspired light weight technical materials. At the same time the translation and abstraction of the natural principles into technical products is bound to produce innovative fabrication techniques (rapid prototyping, casting of hierarchical and fibre reinforced metal foams) to be successful.
Potential applications for biomimetic impact resistant composite materials are for example the transportation of dangerous goods, e.g. explosives or hydrofluoric acid, space station protection against meteoroid impact, helmets and other protection wear, and impact protection of vehicles.

[Wang, C.-H., Zhang, L., Mai, Y.-W., , "Deformation and fracture of Macadamia nuts", International Journal of Fracture 69: 67-85, 1994/1995]