Composite Materials for Aerospace Application

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Composite Materials for Aerospace Application
Composite materials are created and applied to
take advantage of the high strength and/or
stiffness of fibers.
Fibers are combined with a matrix material in
order to create a useful structure. The matrix

• Binds the fibers together
• Transfers loads to the fibers
• Protects fibers from damage

Specific matrix materials are usually selected
according to the manufacturing processes to be
used.
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Fiber and Matrix Material Selection
Selection of materials for a composite structure
requires consideration of much more than
properties in a table.
Selection of the fiber the form in which the
fiber is to be used and the matrix material
involves many factors.

• Structural performance
• Strength
• Stiffness
• Shape
• Environmental conditions e.g. fatigue
  temperature humidity corrosives
• Number to be produced

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Fiber Materials
For many materials much higher strength can be
achieved in fiber form than in bulk form.
Several of these materials are suitable for use
in fiber composites.

• Glass
• Carbon
• Boron
• Aramid e.g. Kevlar (DuPont)
• Other materials such as SiC and Al2O3 used in
  metal or ceramic matrix

Lets compare some properties of these materials.
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Representative Fiber Properties
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Fiber (Pre-Preg) Forms
Tape Unidirectional fibers
Fabric Simple and complex weaves that are
selected depending on the application.
Plain
5-harness satin
8-harness satin
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Matrix Materials

• Although some fiber composite structures have
  been produced with thermoplastic matrix most to
  date have utilized thermosets.
• Many lower cost composite structures have used
  polyester resins.
• For higher performance requirements such as in
  aerospace applications epoxies are most commonly
  used.
• Epoxies have reasonably high modulus and
  strength.
• Epoxies exhibit low shrinkage on cure (good for
  adhesives).
• Epoxy service temperature to 125-175ºC.

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Potential Benefits of Thermoplastic Composites

• Thermoplastics bring to fiber composites some
  advantages over thermosets
• Fracture toughness
• Environmental resistance
• Potentially simplified joining and repair
• Faster turn over time in production
• Thermoplastics also offer the potential for lower
  cost fabrication.
• However the manufacturing processes for
  thermoplastic composites are different from those
  for thermoset composites and require different
  equipment.

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Representative Matrix Material Properties
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Unidirectional Composite Material Coordinates
The basic element of a unidirectional composite
is a thin sheet (ply). Material axes are defined
as follows
Longitudinal direction (1) parallel to
fibers Transverse direction (2) perpendicular
to fibers in plane Normal direction (3) out of
plane
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Modeling Unidirectional Composite Behavior

• Fibers are assumed to be all the same (perhaps
  with circular cross section) and uniformly
  distributed throughout the matrix.
• The quantity of fibers present is expressed as
  the fiber volume fraction Vf.
• The matrix volume fraction is given by Vm 1 -
  Vf.
• Composite density rc rfVf rmVm.

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Composite Modulus using Rule of Mixtures
Longitudinal Modulus
E1 Vf Ef Vm Em
Transverse Modulus
Shear Modulus
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Example Composite Modulus
Calculate the density and longitudinal modulus
for a graphite/epoxy composite material with a
fiber volume fraction of 70. The properties for
the carbon fibers and epoxy matrix is as
follows Carbon Fibers Density 1.77
gm/cm3 Modulus 241 GPa Epoxy Matrix Density
1.2 gm/cm3 Modulus 3.12 GPa
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Unidirectional Composite Properties
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Composite Laminates
A useful structure will require that fibers be
oriented in more than one direction. A common
approach to creating such a structure is by
stacking layers or plies to form a laminate.
Ply orientations are defined using the angle
(1-2 axis relative to the x-y axis).
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Laminate Stacking Sequence Definition

• Sequence of plies (laminae) within lay-up
• Ply orientations are defined using the angle q
  (1-axis relative to the x-axis).
• Individual ply angles are separated by slashes
  e.g 45/0/-45
• Plies are ordered top to bottom (positive
  z-direction)
• The total stacking sequence is enclosed in square
  brackets
• Subscripts
• s symmetric with respect to middle surface
• n repeated n times
• Center layer uses overbar in symmetric lay-up
  with odd number of plies.

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Stacking Sequence Examples
(0/-30/30)2
0/90/45S
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Stacking Sequence Examples
45/02/90S
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Composite Structure Manufacturing

• An engineer must understand the capabilities and
  limitations of manufacturing processes to create
  cost-effective designs regardless of the
  materials to be used.
• In order to design any composite structure it is
  essential that the processes be understood.
• Selection of processes depends on
• the type and form of material
• the shape of the parts to be made (some shapes
  cannot be made)
• the quantity to be produced
• the quality e.g. tolerances required
• the allowable cost of manufacturing

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Manufacturing Processes

• Manual lay-up
• Wet lay-up
• Prepreg lay-up
• Bag Molding
• Autoclave processing
• Filament winding
• Resin transfer molding
• Thermoplastic composites

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Wet Lay-up
Wet lay-up is the simplest and most widely used
process.
Layers of dry fabric are placed on a mold and
resin is brushed or sprayed on.
Advantages Large parts Low tooling cost
Disadvantages Labor intensive Uniformity
difficult to maintain
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Prepreg Lay-up
Prepreg is preimpregnated fiber-reinforced
material.
Fiber can be unidirectional (tape) or woven
fabric.
Advantages High fiber volume fractions Unidirecti
onal material not possible with wet
lay-up Uniform fiber distribution -- higher
quality Simplified manufacturing
Disadvantages Labor intensive (like wet
lay-up) More expensive curing equipment Added
cost of prepreg Thermoset prepregs have limited
shelf life
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Vacuum Bag Molding
Application of uniform pressure before curing
improves consolidation and facilitates removal of
excess resin and volatiles.
The stack of prepreg plies built up on the tool
surface is covered with a film of flexible
heat-resistant material.
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Vacuum Bag Molding
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Autoclave Processing
An autoclave is a pressure vessel that permits
application of a higher pressure than a vacuum
bag alone.
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Autoclave Systems
Aircraft parts stacked in an autoclave
Laboratory-size autoclave
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Typical Cure Cycle for Carbon/Epoxy Composite
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Filament Winding
Filament winding wraps a continuous reinforcement
of resin-impregnated fibers onto a mandrel.
The combination of mandrel rotation and axial
motion of fiber source produce a helical pattern.
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Filament Winding
Advantages Automated process low labor
cost Capable of making large parts
Disadvantages Limited to axisymmetric parts Void
content may be high without the use of an
autoclave.
In winding thick parts the process may have to
be stopped to allow partial cure of initial
layers as the pressure of additional layers may
squeeze out resin.
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Filament-Wound Products
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Resin Transfer Molding (RTM)
In RTM resin injected into a mold that contains
a fiber preform. The mold is heated to cure the
resin.
Advantages High quality Good surface
finish Large parts
Disadvantages Expensive mold Limited Vf
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Vacuum Assisted Resin Transfer Molding (VARTM)
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Manufacturing Thermoplastic Composites

• Processes adapted from thermoset processes
• Pultrusion
• Filament winding
• Provision must be made for melting the matrix
  material prior to creating shape.
• Equipment must allow for higher temperature
• Matched die forming as used for sheet metal is
  also used for thermoplastic composites.

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Rubber Forming
With external heating the dies can be maintained
at a lower temperature.
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Diaphragm Forming
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AV-8B HarrierComposite Wing Box
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V-22 OspreyComposite Primary Structure (Carbon
Glass)
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B-2 SpiritAll Composite Skin and Much of Other
Structure
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Use of Composites in Military Aircraft
R. Martin D. Evans JOM 52 2000 pp. 24-28
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Commercial Transport Aircraft
The Boeing 787 has 50 of its primary structure
made of composites.