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Metals Pros • electrically, thermally conductive • good strength & ductility • high toughness • magnetic Cons • dense • low creep resistance • low/moderate corrosion resistance Ceramics Pros • electrically, thermally insulating • wear and corrosion resistant • high strength & stiffness • creep resistant • low density Cons • difficult to form/machine • very low toughness Polymers Pros • very ductile • easy to form • corrosion resistant • high strength-to-weight ratio Cons • low stiffness & strength • poor high temperature properties Composites • Metal Matrix • Ceramic Matrix • Polymer Matrix Motivation Many of our modern technologies require materials with unusual combinations of properties that cannot be met by the conventional metal alloys, ceramics, and polymeric materials. For example, aerospace, underwater, transportation applications. For aircraft engineers, structural materials – low-density, strong and stiff, abrasion and impact resistant, not easily corroded. It’s a rather formidable combination of characteristics for conventional materials. Purpose • With a knowledge of the various types of composites, as well as an understanding of the dependence of their behaviors on the characteristics, relative amounts, geometry/distribution, and properties of the constituent phases, it is possible to design materials with property combinations that are better than those found in the metal alloys, ceramics, and polymer materials. Chapter 14 Composite Materials Definition • A multiphase material that is artificially made and • The constituent phases must be chemically dissimilar and separated by a distinct interface. • Pearlitic steels: • Wood: strong and flexible cellulose fibers surrounded and held by lignin. A classification scheme for the various composite types discussed in this chapter. Fiber reinforced composites continuous and aligned discontinous and aligned discontinuous and randomly oriented No stress transference Little reinforcement by fiber For a significant improvement in strength of the composite, the fibers must be continuous. Example 1 A continuous and aligned glass fiber-reinforced composite consists of 40% of glass fibers having a modulus of elasticity of 69GPa and 60% of a polyester resin that, when hardened, displays a modulus of 3.4 GPa. (a) Compute the modulus of elasticity of this composite in the longitudinal direction. (b) If the cross-sectional area is 250 mm2 and a stress of 50 MPa is applied in this longitudinal direction, compute the magnitude of the load carried by each of the fiber and matrix phases. (c) Determine the strain that is sustained by each phase when the stress in part (b) is applied. Fiber reinforced composites continuous and aligned discontinous and aligned discontinuous and randomly oriented Loading perpendicular to fibers : isostress (transverse loading) c m f V V c m m f f Vm V f 1 ECT Em E f Em Ef Springs in series ECT Em E f Vm E f V f Em Example 2 A continuous and aligned glass fiber-reinforced composite consists of 40% of glass fibers having a modulus of elasticity of 69GPa and 60% of a polyester resin that, when hardened, displays a modulus of 3.4 GPa. Compute the modulus of elasticity of this composite in the transverse direction. ECT 3.4 69 5.5(GPa) Vm E f V f Em 0.6 69 0.4 3.4 Em E f ECL EmVm E f V f 3.4 0.6 69 0.4 30(GPa) Degree of anisotropy of continuous and oriented fiber composites. EXAMPLE 3 A continuous and aligned fiber-reinforced composite having across-sectional area of 1130mm2 is subjected to an external tensile load. If the stresses sustained by the fiber and matrix phases are 156MPa and 2.75MPa, respectively, the force sustained by the fiber phase is 74,000N and the total longitudinal strain is 1.2510-3, determine (a) the force sustained by the matrix phase, (b) the modulus of elasticity of the composite material in the longitudinal direction, and (c) the moduli of elasticity for the fiber and matrix phases.