Marc Thielen

Structure-function relationship of the foam-like pomelo peel (Citrus maxima)-an inspiration for the development of biomimetic damping materials with high energy dissipation.

The mechanical properties of artificial foams are mainly determined by the choice of bulk materials and relative density. In natural foams, in contrast, variation to optimize properties is achieved by structural optimization rather than by conscious substitution of bulk materials. Pomelos (Citrus maxima) have a thick foam-like peel which is capable of dissipating considerable amounts of kinetic energy and thus this fruit represents an ideal role model for the development of biomimetic impact damping structures. This paper focuses on the analysis of the biomechanics of the pomelo peel and on its structure-function relationship. It deals with the determination of the onset strain of densification of this foam-like tissue and on how this property is influenced by the arrangement of vascular bundles. It was found here that the vascular bundles branch in a very regular manner-every 16.5% of the radial peel thickness-and that the surrounding peel tissue (pericarp) attains its exceptional thickness mainly by the expansion of existing interconnected cells causing an increasing volume of the intercellular space, rather than by cell division. These findings lead to the discussion of the pomelo peel as an inspiration for fibre-reinforced cast metallic foams with the capacity for excellent energy dissipation.

Biomimetic cellular metals—using hierarchical structuring for energy absorption

Fruit walls as well as nut and seed shells typically perform a multitude of functions. One of the biologically most important functions consists in the direct or indirect protection of the seeds from mechanical damage or other negative environmental influences. This qualifies such biological structures as role models for the development of new materials and components that protect commodities and/or persons from damage caused for example by impacts due to rough handling or crashes. We were able to show how the mechanical properties of metal foam based components can be improved by altering their structure on various hierarchical levels inspired by features and principles important for the impact and/or puncture resistance of the biological role models, rather than by tuning the properties of the bulk material. For this various investigation methods have been established which combine mechanical testing with different imaging methods, as well as with in situ and ex situ mechanical testing methods. Different structural hierarchies especially important for the mechanical deformation and failure behaviour of the biological role models, pomelo fruit (Citrus maxima) and Macadamia integrifolia, were identified. They were abstracted and transferred into corresponding structural principles and thus hierarchically structured bio-inspired metal foams have been designed. A production route for metal based bio-inspired structures by investment casting was successfully established. This allows the production of complex and reliable structures, by implementing and combining different hierarchical structural elements found in the biological concept generators, such as strut design and integration of fibres, as well as by minimising casting defects. To evaluate the structural effects, similar investigation methods and mechanical tests were applied to both the biological role models and the metallic foams. As a result an even deeper quantitative understanding of the form-structure-function relationship of the biological concept generators as well as the bio-inspired metal foams was achieved, on deeper hierarchical levels and overarching different levels.

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

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 impactand 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.

Impact behaviour of freeze-dried and fresh pomelo (Citrus maxima) peel: influence of the hydration state

Pomelos (Citrus maxima) are known for their thick peel which—inter alia—serves as energy dissipator when fruits impact on the ground after being shed. It protects the fruit from splitting open and thus enables the contained seeds to stay germinable and to potentially be dispersed by animal vectors. The main part of the peel consists of a parenchymatous tissue that can be interpreted from a materials point of view as open pored foam whose struts are pressurized and filled with liquid. In order to investigate the influence of the water content on the energy dissipation capacity, drop weight tests were conducted with fresh and with freeze-dried peel samples. Based on the coefficient of restitution it was found that freeze-drying markedly reduces the relative energy dissipation capacity of the peel. Measuring the transmitted force during impact furthermore indicated a transition from a uniform collapse of the foam-like tissue to a progressive collapse due to water extraction. Representing the peel by a Maxwell model illustrates that freeze-drying not only drastically reduces the damping function of the dashpots but also stiffens the springs of the model.

Developing the Experimental Basis for an Evaluation of Scaling Properties of Brittle and ‘Quasi-Brittle’ Biological Materials

The development of lightweight structures exhibiting a high energy dissipation capacity and a locally adapted puncture resistance is of increasing interest in building construction. As discussed in Chap. 7, inspiration can be found in biology, as numerous examples exist that have evolved one or even several of these properties. Major challenges in this interdisciplinary approach, i.e. the transfer of biological principles to building constructional elements, are scaling (different dimensions) and (at least for the botanic examples) the fact that different material classes constitute the structural basis for the functions of interest. Therefore, a mathematical description of the mechanical properties and the scalability is required that is applicable for both biological and technical materials. A basic requisite for the establishment of mathematical descriptions are well-defined test setups rendering a reliable data basis. In the following, two biological role models from the animal and plant kingdoms are presented, namely, sea urchin spines and coconut endocarp, and two experimental setups for quasi-static and dynamic testing of biological and bio-inspired technical materials are discussed.

Fruit Walls And Nut Shells As An Inspiration For The Design Of Bio-inspired Impact-resistant Hierarchically Structured Materials

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.

Biomechanics and Functional Morphology of Plants—Inspiration for Biomimetic Materials and Structures

During the last decades, biomimetics has attracted increasing attention as well from basic and applied research as from various fields of industry. Biomimetics has a high innovation potential and offers the possibility for the development of sustainable technical products and production chains. Novel sophisticated methods for quantitatively analyzing and simulating the form–structure–function relationship on various hierarchical levels allow new fascinating insights into multi-scale mechanics and other functional parameter spaces of biological materials systems. On the other hand, new production methods enable for the first time the transfer of many outstanding properties of the biological role models into innovative biomimetic products at reasonable costs. Presented examples of biomimetic developments and products inspired by plants include branched and unbranched fiber-reinforced lightweight composite materials, structural materials with a high energy dissipation capacity as fiber-reinforced graded foams and compound materials, solutions for elastic architecture as the biomimetic facade-shading systems Flectofin® and Flectofold inspired by the bird of paradise flower and the waterwheel plant, respectively. Finally, a short overview of bioinspired self-repairing materials is given and a short discussion of the potential of biomimetic products to contribute to sustainable material development is presented.

What Can Be Learnt from Ageing in Biology and Damage-Tolerant Biological Structures for Long-Lasting Biomimetic Materials?

Ageing in biology and the principles of how living beings deal with shortcomings occurring during ageing on the structural and functional level are exemplified. The presented examples mainly focus on damage repair and damage tolerance in biological materials and structures, and on what can be learnt for ageing man-made materials and structures by using bio-inspired approaches. The potential of such an approach is specified by three examples comprising biomimetic self-repairing foam coatings for pneumatic structures, bio-inspired self-healing elastomers and biomimetic damage-tolerant fibre-reinforced gradient foams with high-energy dissipation.