Composites

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					 Mech 285 Lectures

Professor Rodney Herring
                       Objectives
At the end of these lectures you should be able to:
• Define what a ceramic is.
• List several properties of ceramics.
• Explain the types of chemical bonding found in ceramics
• Discuss several types and applications of ceramics
• Discuss some methods of preparation of ceramics
• Know about non-crystalline ceramics, i.e., glass
• Know how heat transfers through metals, ceramics, glass &
  polymers
• Know how ceramics, metals & polymers make composites
• Know the Rule of Mixtures of composites
• Know some examples of composites and their applications
                          Composites
Composites are composed of two or more different materials to
  obtain a combination of properties not available in either of the
  materials.
Usually one material forms a continuous matrix while the other
  provides the reinforcement.
The two materials must be chemically inert with respect to each
  other so no interaction occurs on heating until one of the
  components melts.
An exception to this condition is a small degree of interdiffusion at
  the reinforcement-matrix interface to increase bonding.
      Composite Reinforcements




The properties of a composite depend on the form of the
reinforcement.
             Composites -Examples


Concrete




Fiberglass




Plywood
MANUFACTURING COMPOSITES
Fuel Cell – Hydrogen Storage
                Composite Combinations
Composite can be classified into three basic types. They are:
PMC - Polymer Matrix Composites
• By far the most common type of composite material.
• Matrix is relatively soft and flexible.
• Reinforcement must have high strength and stiffness
• As the load must be transferred from matrix to reinforcement,
  the reinforcement-matrix bond must be strong.

CMC – Ceramic Matrix Composites
• Matrix is relatively hard and brittle
• Reinforcement must have high tensile strength to arrest crack
  growth
• Reinforcement must be free to pull out as a crack extends, so the
  reinforcement-matrix bond must be relatively weak.
            Composite Combinations
MMC - Metal Matrix Composites
• Matrix is relatively soft and flexible.
• Reinforcement must have high strength and stiffness
• As the load must be transferred from matrix to
  reinforcement, the reinforcement-matrix bond must be
  strong.
            Composite Combinations
There are three basic types of composite
  strengtheners:

    1) Dispersion strengthened alloys

    2) Regular particulate composites

    3) Long fiber reinforcements
           Dispersion Strengthened MMC’s

These composites have little or no interaction between the two
  components and the particulate reinforcement is not soluble in the
  metal matrix.
The dispersoids are usually 10-250 nm diameter oxide particles and
  are introduced by physical means rather than chemical
  precipitation.     What does dispersion mean?
They are located within the grains and at grain boundaries but are
  not coherent with the matrix as in precipitation hardening
The dispersed particles are sufficiently small in size to impede
  dislocation movement and thus improve yield strength, as well as,
  stiffness.
Examples of Dispersioned Strengthened MMC’s




    PbO
            Cemented Carbides (Cermets)
Cemented carbides are an example of regular particulate MMC’s.
Co-WC cermets are produced by pressing Co and W powders into
     compacts, which are heated above the melting point of Co.
On cooling the WC particles become embedded in the solidified
     Co, which act as a tough matrix for the WC particles.
In addition to its strength and toughness, Co is also selected
     because it wets the carbide particles to give a strong bond.


Other ceramics such as TaC
    and TiC are also used to
    make Cermets.

Microstructure of WC-20%
    Co cermet (x1000)
              CEMENTED CARBIDES
                  (CERMETS)
Cemented carbides are commonly used as inserts for cutting
                        tool inserts




  Cutting tool inserts   milling                   lathe tool
                         tool



      I’m sure you’ve seen these in the machine shop.
This hard ceramic is very brittle so can crack or chip under
                     excess impact loads.
                   CAST METAL
                PARTICULATE MMC’S

Aluminum alloys for automotive connecting rods and pistons can
   be strengthened and hardened by the addition of SiC (silicon
                           carbide) particles.
 The SiC particles are introduced at a temperature at which the
    alloy is in the solid plus liquid state, ie., by “compocasting”.
             Cast Metal Particulate MMC’s
Compocasting of Al-SiC
a) Partially solidified alloy is stirred to break up dendrites
b) Particles of SiC are added at this temperature
c) In a pressure die casting machine, the solid mixture becomes
   thixotropic to form a high density casting.




  Why does SiC need to be added when Al is partially solidified?
  Professor Dost has a Canadian Space Agency contract to grow Al/SiC
  composite on the International Space Station.
            Cast Metal Particulate MMC’s




Microstructure of cast Al Alloy reinforced with particles of SiC.
The SiC accumulate around the grain boundaries.
   Why would the SiC collect around the grain boundaries?
         Composite - Rule of Mixtures
For particulate composites, the rule of mixtures predicts the
    density, modulus of elasticity, electrical and thermal
    conductivity of fiber-reinforced composites.

Density, r, is given as a fraction, f, as:

     rc  f m rm  f f r f             Note that f m  1  f f


Where the subscripts m and f refer to the matrix and fiber.
                   Rule of Mixtures
For thermal conductivity:

                    Kc  fm Km  f f K f

Where K is the thermal conductivity.
For electrical conductivity:

                    c  f m m  f f  f
Thermal and electrical energy can be transferred through the
composite at a rate that is proportional to the volume fraction of
conductive material.
                     Rule of Mixtures


In a composite material with a metal matrix and ceramic fibers,
     the bulk of the energy would be transferred through the
     matrix.
In a composite consisting of a polymer matrix containing metallic
     fibers, the energy would be transferred through the fibers.

When the fibers are not continuous or unidirectional, the simple
   rule of mixtures may not apply.
                       Rule of Mixtures
Modulus of Elasticity:
The rule of mixtures is used to predict the modulus of elasticity when
    the fibers are continuous and unidirectional.
Parallel to the fibers, the modulus of elasticity may be as high as:
                     Ec  f m E m  f f E f
However, when the applied load is very large, the matrix begins to
   deform and the stress-strain curve is no longer linear. Since the
   matrix now contributes little to the stiffness, the modulus is
   approximated by:

                 Ec  f f E f
        Load is transferred to fiber.
                     Rule of Mixtures
Modulus of Elasticity: (cont’d)
When the load is applied perpendicular to the fibers, each
     component of the composite acts independently of the other.
E is given by:
                      1   ff   fm
                            
                      Ec E f Em
There are many good examples provided in your text by Askland,
    Fulay and Wright in the chapter, “Composites: Teamwork
    and Synergy in Materials”.
          Failure of Fiber Reinforced MMC’s
In fiber reinforced metal matrix composites, the matrix material
     transmits the force to the fibers, which carry most of the
     applied load.
The strength of the composite thus depends on the bond between
     the fibers and the matrix.
If this bond is weak, the fibers will pull out causing a reduction in
     strength and a premature fracture.

 Fracture surface of Ag-Cu alloy
     reinforced with carbon fibers.
     Due to poor bonding the
     fracture surface follows the
     fiber/matrix interface.
Characteristics of Fiber-Reinforced Composites


As can be seen from
     this plot, the
    strength of the
 composite increases
  as the fiber length
  increases (this is a
   chopped E-glass-
   epoxy composite)
             Effect of fiber Orientation


Maximum strength is
   obtained when long
   fibers are oriented
   parallel to the applied
   load.




                       Effect of fiber orientation on tensile
                           strength of E-glass fiber-reinforced
                           epoxy composite.
              Effect of fiber Orientation


The properties of
  fiber composites
 can be tailored to
    meet different
        loading
    requirements.
     By using
  combinations of
    different fiber
 orientation quasi-
isotropic materials
 may be produced      Figure (a) shows a unidirectional
                      arrangement
                      Figure (b) shows a quasi-isotropic
                      arrangement
           Effect of Fiber Orientation




A three-dimensional weave for fiber-reinforced composites.
This could be found when fabrics are knitted or weaved
    together
              Specific Fiber Properties

In most fiber-reinforced composites, the fibers are strong, stiff
     and lightweight.
If the composite is to used at elevated temperatures, the fiber
     should also have a high melting temperature.
The specific strength and specific modulus of fibers are
     important characteristics given by:
                                           TS
                     Specific Strength 
                                           r

                                            E
                     Specific modulus 
                                            r

Where TS is the tensile strength, E is the elastic modulus and
   r is the density.
              Specific Fiber Properties




On the right is a
   graph showing
specific strength vs.
  specific modulus
 for different types
      of fibers.
COMMERCIALLY AVAILABLE FORMS
     OF REINFORCEMENT




     Random mat and woven fabric
            (glass fibers)
    COMMERCIALLY AVAILABLE FORMS
         OF REINFORCEMENT




Carbon fiber woven fabric
                    Sandwich Structures




a) A hexagonal cell honeycomb core, b) can be joined to two face
     sheets by means of adhesive sheets, c) producing an
     exceptionally lightweight yet stiff and strong honeycomb
     sandwich structure. Use a lot for aerospace applications.
                    Sandwich Structures




In the corrugation method for producing a honeycomb core, the
     material such as Al is corrugated between two rolls, which are
     joined together with adhesive and then cut to the desired
     thickness.
The plastic deformation will work harden the Al.
          The End

(Any questions or comments?)

				
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posted:10/5/2011
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