Materials

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Materials

• Composites

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Introduction
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Introduction

• The major problem in the application of polymers
  to engineering is their low stiffness and
  strength compared to steel.
• Moduli are 100 times lower
• Strengths are 5 times lower

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Introduction

• 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

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Introduction

• 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

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Introduction

• The best reinforcement in any application is the
  one that achieves the designers objective at the
  lowest cost

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Mechanism of Fiber Reinforcement
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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

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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.

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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

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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
• agtgt1, a fiber
• altlt1, a platelet

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Mechanism of Fiber Reinforcement
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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

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Forming Reinforced Plastics
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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.

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How Molecular Orientation Occurs
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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.

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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.

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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

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Forming Reinforced Plastics

• Pultrusion and Filament winding

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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.

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Physical Properties
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Physical Properties
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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

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Physical Properties

• The proportion of fibers and matrix in the
  composite are expressed as fractions of the total
  volume they occupy.

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Physical Properties

• The density(?) of the composite with no voids is

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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.

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Mechanics of Fiber Reinforcement
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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

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Mechanics of Fiber Reinforcement

• Consider the case of the fibers that are so long
  that the effects of their ends can be ignored.

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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

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Mechanics of Fiber Reinforcement

• Poissons ratio (?), the elastic constant of the
  composite in the 1,2 direction is
• Poissons ratio (?), the elastic constant of the
  composite in the 2,1 direction is

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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