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Lecture 11


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  • 1. Introduction to Composites
    • What is the matrix in a composite and what materials are commonly used as a matrix?
    • What are the possible strengthening mechanisms for particle reinforced composites (there are 2)?
    • Be able to calculate upper and lower bounds for the Young’s modulus of a large particle composite.
    • Know the equation for the critical length (Lc) of a fiber.
    • Know the stress distribution on fibers of various lengths w/r Lc in a composite.
    Reading: Chapter 16
  • 2. Composites in Action
  • 3. Composite Structures Particle, fiber, and structural composite.
  • 4. TERMINOLOGY/CLASSIFICATION • Composites : --Multiphase material w/significant proportions of ea. phase. • Matrix : --The continuous phase --Purpose is to: transfer stress to other phases protect phases from environment --Classification: MMC, CMC, PMC • Dispersed phase : --Purpose: enhance matrix properties. MMC: increase  y , TS, creep resist. CMC: increase Kc PMC: increase E,  y , TS, creep resist. --Possible Classifications: Particle , fiber , structural metal ceramic polymer Reprinted with permission from D. Hull and T.W. Clyne, An Introduction to Composite Materials , 2nd ed., Cambridge University Press, New York, 1996, Fig. 3.6, p. 47.
  • 5. Types of composites (MMC, PMC, CMC) Wood (cellulose fibers with stiffer lignin matrix) Bone (soft collagen and brittle apatite) Clay (particles and glass naturally form when fired) We will focus on artificial composites. Natural composites include:
  • 6. Particle and Fiber variables
    • For any composite, regardless of the selection of matrix and disperse phase (material and type), there are many options that will affect properties:
    Each option will impart different benefits to the final part. Also surface coatings on the dispersed phase
  • 7. Particle Reinforced Composites Particle-reinforced • Examples: Adapted from Fig. 10.10, Callister 6e . (Fig. 10.10 is copyright United States Steel Corporation, 1971.) Adapted from Fig. 16.4, Callister 6e . (Fig. 16.4 is courtesy Carboloy Systems, Department, General Electric Company.) Adapted from Fig. 16.5, Callister 6e . (Fig. 16.5 is courtesy Goodyear Tire and Rubber Company.)
  • 8. Large particle composites
    • Large particle composites
      • Involves large particles that are harder or stiffer than matrix.
      • The matrix transfers applied stress to the particles, which thus bear a fraction of the load.
      • Bonding at the interface is necessarily important.
    • Particles should be:
      • Equiaxed
      • Uniformly distributed
      • Properties generally determined by the rules of mixtures.
    Upper bound: Lower bound:
  • 9. COMPOSITE SURVEY: Particle-II • Elastic modulus, E c , of composites: -- two approaches. Particle-reinforced Adapted from Fig. 16.3, Callister 6e . (Fig. 16.3 is from R.H. Krock, ASTM Proc , Vol. 63, 1963.)
  • 10. Large Particle Composite Examples
    • Cermets (not cements ) are ceramic-metal composites
      • Cermented Carbide—cutting tools
        • WC or TiC particles (incredibly hard)
        • Metal matrix (Co or Ni)
      • The particles will crack under the high stresses in cutting applications, so the matrix prevents crack propagation between particles by separating them.
      • Up to 90 volume percent of particles.
    • Polymer/Carbon composites include
      • Tires
        • Elastomer matrix with carbon black particles (15-30 vol%).
        • Improved tensile strength, tear and abrasion resistance, and toughness.
        • Small particles are optimal, <50 nm.
    • Ceramic-ceramic composites include
      • Concrete is:
        • ~7 0 vol% sand and gravel particles (different sizes promotes better packing).
        • Portland cement is the binder once water is added.
      • Improved tensile, compressive, and shear response by reinforcing with steel rods, bars (rebar), wires, or wire mesh (ceramic-ceramic-metal composite).
        • Steel is selected for thermal expansion coefficient
        • Not corroded during cement hardening
        • Strong composite/matrix bond is possible, especially if the steel surface is contoured
      • Pre stressing
  • 11. Dispersion strengthened (higher tech…)
    • Similar to precipitation hardening
      • Strengthening is not as good as for precipitation hardening at low temperatures
      • At higher temperatures the properties are generally better.
        • Particles are selected to be unreactive (no precipitate growth or dissolution of the precipitate).
    • Dispersion strengthened composites
      • Small particles (10 to 100 nm)
      • Matrix bears most of the applied load
      • Particles hinder or impede motion of dislocations
      • Plastic deformation is restricted
      • Improves yield and tensile strength.
    • Examples
      • Thoria dispersed nickel (Ni with up to 3 vol% ThO 2 particles)
      • Sintered aluminum powder (Al matrix with Al 2 O 3 coated Al flakes)
  • 12. Large-Particle vs. Dispersion-Strengthened Composites Strong Particle >500 nm Strong Particle <100 nm Shear  Dislocation stopped Stress field of dispersion Dislocation shears through the dispersion Dispersion Strengthened Large-Particle
  • 13. Fiber composites
    • Why are we using fibers?
      • Especially for ceramics, due to Weibull statistics the fracture strength of a small part is usually greater than that of a large component (smaller volume=fewer flaws=fewer big flaws).
    • Fibers come in three forms
      • Whiskers (graphite, SiC, Si 3 N 4 , Al 2 O 3 )
        • Single crystals
        • Huge length/diameter
        • Small, so nearly flaw free
        • Strongest known materials
        • expensive
      • Fibers (aramids, glass, carbon, boron, Si 3 N 4 , Al 2 O 3 )
        • Polycrystalline or amorphous
        • Small diameter
      • Wires (usually metals)
        • Large diameter
  • 14. Matrix phase
    • Usually a metal or polymer since some ductility is desirable
    • Serves several functions for fiber composites
      • Bonds with the fibers (Very important).
      • Protect fibers from surface damage due to abrasion or corrosion (i.e., avoid cracks on surfaces of fibers).
      • Separate the fibers.
      • Prevent propagation of brittle cracks between fibers.
  • 15. Fiber Reinforced
    • Most common composite type.
    • Generally applied for improved strength and stiffness with respect to weight
      • Aerospace applications
      • High value sporting goods
      • Since the load cannot be transferred beyond the end of the fiber, there is a critical fiber length (L c ) for effective strengthening and stiffening that will depend on:
        • d, the fiber diameter; σ f * , the fiber ultimate tensile strength; and on tau c , either the matrix/fiber bond strength or the matrix shear yield strength (whichever is smaller) .
    • The bond between the matrix and the fiber dictates whether the fiber will improve the properties of the composite by transferring an applied load to the fiber.
    L c is approximately 1 mm for glass/Carbon fiber/matrix composites (20 to 150 times diameter). d
  • 16. Stress along a fiber
    • For L=L c , the maximum fiber load is achieved at the center of the fiber length.
    • For L>L c , the maximum fiber load is carried by most of the fiber. These are considered to be “Continuous” fibers and are optimal .
    • For L<L c , the maximum fiber load is never reached, so that a weaker, cheaper and longer fiber or even particles could have been used instead.
  • 17. Optimal fiber length
    • So, as fibers get longer and thinner, the overall properties of the composite are improved.
    • Optimal fiber lengths are usually about 30*L c .
    Poorer fiber efficiency Better fiber efficiency fiber diameter shear strength of fiber-matrix interface fiber strength in tension • Ex: L c is 1mm for fiberglass, so the optimal fiberglass length is >=30mm.
  • 18. SUMMARY
    • What is the matrix in a composite and what materials are commonly used as a matrix?
    • What are the possible strengthening mechanisms for particle reinforced composites (there are 2)?
    • Be able to calculate upper and lower bounds for the Young’s modulus of a large particle composite.
    • Know the equation for the critical length (Lc) of a fiber.
    • Know the stress distribution on fibers of various lengths w/r Lc in a composite.
    Reading for next class Composite Applications Chapter sections: The rest of Ch. 16.