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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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 Spring '06
 LIU
 Composite Materials

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