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Nature has developed ways of achieving structural efficiency - shapes that are stiff and strong, and at the same time use as little material as possible. Natural selection has seen to that - the least efficient tree gets the least sunlight, and the least efficient animal is the first to get devoured. So we find efficient structures in nature - and among the most common are the sandwich beam and sandwich panel - a dense skin supported by a low density, foam-like core. To work well, the core must meet certain constraints: it must be light, have adequate shear stiffness and strength, and - if anyone is going to use it - it must be affordable. Cellular solids - foams - go some way to meeting these requirements. But foams made by conventional processing have the characteristic that their deformation is bending-dominated, making them less stiff and strong that a stretch-dominated structure of the same weight. The talk elaborates on these ideas, demonstrating criteria for bend and stretch domination, and strategies for design stretch-dominated structures.

Professor Mike Ashby is a Professor in the Engineering Department at Cambridge University and a Visiting Professor at the Royal College of Art in London. He is a member of the Royal Society, the Royal Academy of Engineering and the U.S. National Academy of Engineering. He is past editor of Acta Metallurgica and present Editor of Progress in Materials Science. His interests include materials selection in design, process modeling and the study of the properties of composites and foams. He has written and co-written textbooks in these fields, as well as over 200 papers on mechanisms of plasticity and fracture, powder compaction, mechanisms of weat, methodologies for materials selection, and the modeling of material shaping processes, among other topics.