Architectured Structural Materials: A Parallel Between Nature and Engineering
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Architectured Structural Materials: A Parallel Between Nature and Engineering John W. C. Dunlop1, Yves.J.M.Brechet2 1 Department of Biomaterials, Max Planck Institute of Colloids and Interfaces, Am Mühlenberg 1, 14424, Potsdam Germany. 2 SIMAP, INP Grenoble/CNRS, 101 Rue de la Physique, BP46, 38402 St Martin d’Hères cedex, France ABSTRACT Nature builds materials like an architect to obtain a variety of properties with a limited number of building blocks. In contrast, engineers have access to a wide range of constituent materials to fulfil a variety of requirements. The classical degrees of freedom for controlling the properties of man-made materials are the microstructure, or the macroscopic shape. Only recently, the architecture at the millimetre scale was perceived as an efficient way of expanding the range of properties offered by bulk materials. The aim of this paper is to compare the different strategies and to outline some observations on natural materials which may serve as inspiration to develop engineering architectured materials. Keywords: Architectured materials, microstructure, biomimetics, design INTRODUCTION Facing a set of requirements, even the simplest ones (for instance the structural requirements which amount to bear a load with limited damage) is always a dilemma. Clearly one has to make the best use of matter to fulfil often conflicting requests, such as strength and damage tolerance. It is interesting to see that biological and engineering materials may represent quite different solutions of a similar problem [1].
Figure 1: A parallel between biological and engineering materials. Reproduced from [1, 2] While the engineer has access to the whole set of elements in the Mendeleev table and with all their variety of chemical bonds (metallic, ionic or covalent), Nature has had to develop relatively low temperature processes, and therefore is limited either to organic
chemistry, or to ionic solutions. As a result, only ceramics and polymers are used as major constituents for structural materials in living bodies. The different strategies are also reflected in the processes by which materials are made [1, 2]. Nature “grows” its structural materials in an approximate design, while the engineer “shapes” the structural material according to a predefined form. This difference also reflects a different treatment of the information necessary to build a component or an organ: Nature follows a “rule” (the genetic code) while the Engineer follows a very stiff set of requirements (the design). Last but not least, the strategy to deal with damage is also very different. The engineer typically tries to dimension the components so that the damage during the expected lifetime is as limited as possible, while nature has developed a whole range of adaptive and healing processes. Not surprisingly, these very different limiting conditions have led to different strategies in meeting the mechanical requirements with an optimal use of matter. For example, one major strategy observed in natural materials i
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