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Until today the structuring of different types of fruit walls has been used only as an inspiration for packaging when seen from a biomimetic perspective. However, by a detailed investigation of the Macadamia nut with its tough testa, Citrus maxima, possessing a large spongy mesocarp and Cocos nucifera, having a combination of a fi brous mesocarp and a tough endocarp, it becomes evident that those structures also provide excellent biological role models for impact- and puncture-resistant materials. Both Citrus maxima and Cocos nucifera are relatively heavy, lack any aerodynamic adaptation and share the same challenge of having to withstand the impact from heights of >10 m. Conducting high-speed camera-controlled free fall experiments of Citrus maxima from 6 m height, we could demonstrate a deceleration of the fruits of 3100 m/s², which corresponds to 316 g, without any visible damage of the fruit. An analysis using cyclic quasi-static compression tests of the pericarp of Citrus maxima revealed that the material behaves constant in good approximation after the fi rst loading cycle. During the fi rst cycle, almost 75% of the energy is dissipated. The pericarp of Citrus maxima is highly visco-elastic, which causes the samples within 1 min to recover 30% of their initial deformation caused by loading to 40% strain. The mesocarp of Citrus maxima is best described as an open-pore foam with a gradual increase in the pore size. Understanding the principles of as to how combining the structure and material in biological constructions yields a fully functional protection layer will allow us to construct new lightweight bio-inspired materials of high impact and puncture resistance with a combination of high energy dissipation, benign failure and almost complete recovery from large deformations.
Biomimetics, Citrus maxima, Cocos nucifera, energy dissipation, gradual open-pore foam, Macadamia sp.
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