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lecture12 - 4.3 Mimicking Natural Composites(Biological...

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Unformatted text preview: 4.3 Mimicking Natural Composites (Biological Materials) Through million years of evolution, most natural composites, i.e. biological materials, have optimized themselves to survive various types of environmental impact. Understanding the way the natural composites work can help to design better engineering composites. . A. Natural Composites - In terms of microstructure, natural composites can be divided into cellular solids (with open or closed cells), fibrous materials, and sandwich constructions. The special characteristics of natural composites are (1) the orientation dependency due to fibrous cells, (2) the weight reduction (also for nutrition transportation) due to porous cells, and (3) the high stiffness due to sandwich construction. a. natural cellular materials ~ cork, balsa, sponge, cancellous bone, coral, cuttlefish bone, iris leave, stalk of a plant, wood, etc. F. natural fibrous materials - felt, paper, cotton wool, tissue, bamboo, etc. G. natural sandwich materials — human bone, bird wing, etc. B. Biomaterials, Biomimetics and Biological Growth a. biomimetics — The study of mimicking the biological systems is called biomimetics. The geometry, the material construction, and the mechanisms of sensoring and controlling mechanisms of biological systems can be lent to enhance engineering designs, e.g. aerofoil (bird wings), submarine (fish), architecture (water lily). b. hypothesis of biological growth — Certain laws concluded from observing the way the biological systems work can be applied to mechanical optimization for reducing weight, increasing fatigue resistance, etc. For example, the growth of tree butt and branch joint, the healing of tree wound, the varieties of size and shape of deer antler, antelope horn, tiger claw, and camel thorn, and the diversity of leaves among ivy, maple, and grass, are believed to be attributed to response of the biological system to various environmental impacts. C. Engineering Materials a. engineering cellular solids -— They have high energy absorption capability, e.g. the foamed materials based on nickel, copper, zirconia, mullite, glass and polyurethane. Some foods are also made in foam style to improve tasting, e.g. bread, meringue, chocolate bar, junk food crisp, malteser and Jaffa cake. b. engineering fibrous materials ~ Examples are space shuttle tile and curved fiber composites. 0. engineering honeycomb materials ~— With large section moduli, these materials are good for bending but bad for transverse shearing. llerlzo , Buses 8. THEORY OF ANISOTROPY 8.1 Anisotropy of Crystal Solids Some tensors, like stress and strain, are kinematic quantities whose properties are not constrained by the symmetry class of the material to which they are attached On the other hand tensors representing physical properties (piezoelectric coefficients diyk and stiffness coefficients Cot!) are constrained by the symmetry class of the material to which they are attached. A. Neumann’s Principle The key to the question that how the geometrical symmetry of a crystal is related to the symmetry of its physical properties is based on the fundamental postulate of crystal physics that known as Neumann s Principle — The symmetry elements of any physical property of a crystal must include the symmetry elements 01‘ the point group of the crystal. The symmetry group of a given material must be included in the symmetry group of any tensor function in any constitutive laws of the material (The symmetry elements of any physical property of a crystal must include the symmetry elements of the geometry of the crystal Consequently, the more the geometrical symmetry of the crystal, the higher the number of zero terms in stiffness (or compliance) matrix.) For example, cubic crystals are optically isotropic and hexagonal crystals are mechanically transversely isotropic. B. Types of Crystal ‘ Based on the symmetry of geometry, there are 32 types of crystals. The most popular crystals are of the following groups: triclinic, monoclinic, orthohombic, tetragonal, trigonal, hexagonal, cubic and isotropic. In a three dimensional crystal structure the six organizing parameters shown in Figure 2 1 can affect the elements of geometrical symmetry and result in various anisotropy of the crystal: a b, 0, 0t, [3 and V For example, triclinic (a i b i c (it 95 900, B¢ 900, and y i 900 ).has 21 independent material constants monoclinic (clinotropic) (a at b i c (it = B = 900, and y i 900‘) has 13 independent material constants orthotropic (orthorhombic) (a e b i c, 0t = B = Y = 900) has 9 independent constants tetragonal (a : be 6, 0t 2 B = y 2: 900)has7 independent material constants transversely isotropic (hexagonal) (a = b i c, 0t : B = 900 and y ¢ 900) has 5 constants cubic (a— — b = c, and 0!. = B = y: 900 )has 3 independent matenal constants isotropic has 2 independent material constants Details of the individual crystals are shown in Figure? -2 along with the number of independent elastic constants ...
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