Structural materials: What can we learn from nature?

Abstract

Abstract. The mechanical properties of materials are of vital importance in the natural world. Over millions of years of evolution, Nature has created materials capable of resisting mechanical forces, in our bodies and in those of other animals and plants. Throughout history human beings have created new materials; in particular many materials developed over the last 100 years have greatly surpassed natural materials in their mechanical properties and durability. So is there anything which Nature can still teach us about making and maintaining structural materials? This talk will consider some of the "tricks" which Nature uses, such as bottom-up fabrication processes, functionally graded structures and materials with the capacity for continual self-monitoring and repair. It will be shown that some of these tricks can be used by materials scientists, but some aspects of the behaviour of natural materials are not suitable for copying into manmade structures. NATURE'S MATERIALS ARE NOT VERY GOODFrom a mechanical point of view, the properties of natural, biological materials are quite poor. Any rational comparison on the basis of properties such as strength, toughness, ductility or energy absorption shows that engineering materials developed in the 20 th century -especially metal alloys and fibre composites -far outperform natural materials. So do we really have anything to learn from nature? I believe we do, because (thanks to evolution) nature has been able to make the best possible use of the rather poor-quality materials available. By looking at how nature optimises material use we can learn lessons that we can apply to future materials development. Figure 1 shows a beautiful picture taken by Mueller et al (2006) of a section cut through a tree, revealing the structure near a branch. This image reveals several important aspects of natural materials which are worthy of consideration. Firstly, the material is essentially a fibre composite, in which the fibres have been arranged to alter their orientations in the vicinity of the joint between the main trunk and the side branch to achieve a smooth transition and follow lines of force. This is very difficult (though not impossible) for us to achieve in engineering structures made from fibre composites. This illustrates an important fact which recurs often in natural materials: there is no clear distinction between the material and the structure. Material properties gradually change from place to place within the structure, improving overall performance (e.g. stiffness/weight) and eliminating sudden transitions which are always points of weakness. Similar structure/material interactions can be found in the human body, for example at bone/tendon junctions and in cartilage. Our attempts to replicate this kind of transition, for example using functionally graded materials, is beginning to bear fruit, but we have a long way to go. MATERIAL VERSUS STRUCTUREThe second feature of interest in figure 1 is that the insertion of the branch does not occur only on the surface but ...

Are there severe limitations to the bioinspired approach in machine design?

“…In any case, as Taylor pointed out, 19,20 the absolute mechanical properties are usually inferior if compared with those of the best engineering material, not to speak of those of materials like carbon nanotubes. To use his words, ‘ nature has been able to make the best possible use of the rather poor material available ’, in particular thinking to the limitations in terms of obtaining rare constituents or using extreme manufacturing conditions.…”