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Materials. Composites. Introduction. Introduction. The major problem in the application of polymers to engineering is their low stiffness and strength Moduli are 100 times lower Strengths are 5 times lower. Introduction. Two methods are used to overcome these deficiencies

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  • The major problem in the application of polymers to engineering is their low stiffness and strength
    • Moduliare 100 times lower
    • Strengths are 5 times lower
  • Two methods are used to overcome these deficiencies
    • Use of shape (moment of inertia)
      • Ribs
      • Gussets
    • The addition of reinforcing fibers to form a composite material
  • A good reinforcing additive has the following properties
    • It is stiffer and stronger than the polymer matrix
    • It has good particle size, shape, and surface character for effective mechanical coupling to the matrix
    • It preserves the desirable qualities of the polymer matrix
  • The best reinforcement in any application is the one that achieves the designers objective at the lowest cost
mechanism of fiber reinforcement9
Mechanism of Fiber Reinforcement
  • We have a single reinforcing fiber embedded in a polymer matrix and perfectly bonded to it.
  • The particle is stiffer than the matrix and deform less, causing the matrix strain to be reduce overall
    • The strain is much less at the interface
mechanism of fiber reinforcement10
Mechanism of Fiber Reinforcement
  • The reinforcing fiber achieves its restraining effect on the matrix entirely through the fiber-matrix interface
  • The strength of the composite depends on the strength of bond between fiber and matrix, and the area of the bond.
mechanism of fiber reinforcement11
Mechanism of Fiber Reinforcement
  • A useful parameter for characterizing the effectiveness of the reinforcement is the ratio of surface area of the reinforcement to the volume of reinforcement.
  • We want the area to volume ratio to be as high as possible.
  • We define the aspect ratio (a) as the ratio of length to diameter
mechanism of fiber reinforcement12
Mechanism of Fiber Reinforcement
  • The figure on the next slide show a plot of aspect ratio(a) vs area to volume ratio.
  • It show the optimum shapes for a cylindrical reinforcement to be:
    • a>>1, a fiber
    • a<<1, a platelet
mechanism of fiber reinforcement14
Mechanism of Fiber Reinforcement
  • Two main classes of reinforcement are fibers and platelets.
  • Examples of fibers:
    • Glass fibers
    • Carbon fibers
    • Carbon nanotubes
  • Examples of platelets
    • Mica
    • Talc
forming reinforced plastics16
Forming Reinforced Plastics
  • Reinforced thermoplastics are usually formed using extrusion or injection molding.
  • Alignment of the fibers is caused by drag on the particle by the flowing viscous polymer.
    • Usually aligned in the direction of flow.
    • But the flow field varies greatly and we end up with random fiber alignment.
  • The damage done to the fiber must also be taken into account.
forming reinforced plastics18
Forming Reinforced Plastics
  • Thermoset resins can be formed by compression molding.
  • The fiber and resin are premixed before being loaded into a heated mold which causes the resin to crosslink.
  • Many forms of premix are available, making a variety of fiber arrangements possible.
forming reinforced plastics19
Forming Reinforced Plastics
  • Many other forming processes:
  • Pultrusion
    • Continuous fibers are pulled through a bath of resin, then through a shaping die.
    • The resin is then crosslinked.
    • Produces a long fiber with uniaxial alignment.
forming reinforced plastics20
Forming Reinforced Plastics
  • Filament winding
    • Continuous fibers are pulled through a bath of resin, then wound onto a driven mandrel.
    • Then the resin is crosslinked.
    • This method is used for making pipe and other shapes
forming reinforced plastics21
Forming Reinforced Plastics
  • Pultrusion and Filament winding
forming reinforced plastics22
Forming Reinforced Plastics
  • Hand Layup
    • The fiber is laid down by hand in the required arrangement and shape, then resin is applied with a brush.
    • The resin then crosslinks.
  • Hand Spray Layup
    • Fibers are fed to a spray gun which chops and sprays the fibers at a panel where the reinforcement is needed.
    • Resin is then applied with a brush.
    • The resin then crosslinks.
physical properties25
Physical Properties
  • Density
  • The density of the composite differs from that of the polymer
  • A mass (m) of composite occupies a volume (V)
    • mf of fibers occupies Vf
    • mm of matrix (polymer) occupies Vm
    • m = mf + mm
    • V = Vf +Vm
physical properties26
Physical Properties
  • The proportion of fibers and matrix in the composite are expressed as fractions of the total volume they occupy.
physical properties27
Physical Properties
  • The density(ρ) of the composite with no voids is:
physical properties28
Physical Properties
  • In practice, composite materials contain voids.
    • A void is a source of weakness
  • Over 2% voids indicates poor fabrication.
  • Less than 0.5% voids indicates “aircraft quality” fabrication.
mechanics of fiber reinforcement30
Mechanics of Fiber Reinforcement
  • Accurately predicting the mechanical properties of a composite material is not easy
  • The differences between properties of the reinforcing particle and the polymer matrix cause complex distributions of stress and strain at the microscopic level, when loads are applied.
  • By using simplified assumptions about stress and strain, reasonably accurate predictions can be made
mechanics of fiber reinforcement31
Mechanics of Fiber Reinforcement
  • Consider the case of the fibers that are so long that the effects of their ends can be ignored.
mechanics of fiber reinforcement32
Mechanics of Fiber Reinforcement
  • The equation for the Composite Modulus (E) in the 1 direction is:
  • The equation for the Composite Modulus (E) in the 2 direction is:
mechanics of fiber reinforcement33
Mechanics of Fiber Reinforcement
  • Poisson’s ratio (ν), the elastic constant of the composite in the 1,2 direction is:
  • Poisson’s ratio (ν), the elastic constant of the composite in the 2,1 direction is:
mechanics of fiber reinforcement34
Mechanics of Fiber Reinforcement
  • When a shear stress acts parallel to the fibers, the composite deforms as if the fibers and matrix are coupled is series.
  • The shear Modulus (G12) is: