Fruit walls and nut shells as an inspiration for the design of bio-inspired impact-resistant hierarchically structured materials

• Published in: International journal of design & nature and ecodynamics Vol. 8; no. 2; pp. 172 - 179
• Main Authors: Seidel, R., Thielen, M., Schmitt, C., Bührig-Polaczek, A., Fleck, C., Speck, T.
• Format: Journal Article
• Language: English
• Published: Southampton W I T Press 30.06.2013
• Subjects: Biological; Biological materials; Biomimetics; Citrus maxima; Cocos nucifera; Compression; Compression tests; Construction materials; Deceleration; Deformation; Deformation mechanisms; Energy dissipation; Free fall; Fruits; High speed cameras; Impact resistance; Inspiration; Macadamia nuts; Maxima; Packaging; Pericarp; Pore size; Porosity; Viscoelasticity; Walls
• ISSN: 1755-7437; 1755-7445
• DOI: 10.2495/DNE-V8-N2-172-179

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 fibrous 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 first loading cycle. During the first 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.