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

Abstract

Until today the structuring of different types of fruit walls has only been used as an inspiration for packaging when seen from a biomimetic perspective. However, by detailed investigation of the Macadamia nut with its tough testa, Citrus maxima, which possesses a large spongy mesocarp and Cocos nucifera, which has 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 impact from heights of more than 10 metres. By conducting high speed camera controlled free fall experiments of Citrus maxima from six metres high we could demonstrate a deceleration of the fruits of 3100m/s², which corresponds to 316g, without any visible damage to the fruit. An analysis using cyclic quasi static compression tests of the pericarp of Citrus maxima revealed that the material behaves constantly 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 one minute 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 gradual increase in pore size. Understanding the principles of how combining 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.

The Structure of the Barley Husk Influences Its Resistance to Mechanical Stress

“…The relationship between parenchyma and material properties is not straightforward, however. Despite its reduced strength and stiffness, thick parenchymal layers in the pericarp are key for impact resistance in coconuts and pomelos (Seidel et al, 2010 ; Masselter and Speck, 2011 ). Due to its structure, parenchyma can absorb a considerable amount of energy (Gibson, 2005 , 2012 ).…”

“…Parenchyma cells crush together to form a fragmentary layer (Evers and Millar, 2002 ; see also micrographs in Olkku et al, 2005 ). As the husk parenchyma are crushed together, the tissue lacks the cellular structure key to impact resistance in other fruit (Seidel et al, 2010 ; Masselter and Speck, 2011 ).…”

Hierarchical levels of organization of the Brazil nut mesocarp

“…For example, the endocarp of the cocoyol palm tree (Acrocomia mexicana) 24 , has a harder external layer to protect seeds against predators trying to penetrate the shell and an inner layer with mechanisms to enhance shell toughness. Nature employs a variety of strategies to achieve such properties gradients, like adjusting the degree of lignification, as in the stem of Washingtonia robusta 28 , differential cell wall thickening, as in Phyllostachys pubescens 29 , changes in cell geometry, as seen in the endocarp of the cocoyol 24 , or by introducing density gradients by different pore sizes, as in Citrus maxima 30,31 . Whether other mechanisms are involved in the mesocarp density gradient, specifically different degrees of lignification, have to be investigated in the future.…”

Analysis of the dynamic mechanical property of multiple direction impact protection structure inspired by C60 molecule

“…Seidel et al. 17 studied the free fall experiments of Citrus maxima by conducting high-speed camera controlled from 6 m high. They could demonstrate a deceleration of the fruits of 3100 m/s 2 , which corresponds to 316 g , without any visible damage to the fruit.…”