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FiberReinforced CompositesFiberreinforced composites are
composed of axial particulatesembedded in a matrixmaterial.The
objective of fiberreinforced composites it to obtain a material
withhighspecific strength and high specific modulus. (i.e. high
strength and high elastic modulus forits weight.) The strength is
obtained by having the applied loadtransmitted from the matrixto
the fibers. Hence, interfacial bonding is important.
Classic examples of fiberreinforced composites include
fiberglass and wood.
Fiber GeometrySome common geometries forfiberreinforced
composites:
AlignedThe properties of aligned fiberreinforced composite
materials are highly anisotropic.The longitudinal tensile strength
will be high whereas the transverse tensile strengthcan be much
less than even the matrix tensile strength. It will depend on
theproperties of the fibers and the matrix, the interfacial bond
between them, and thepresence of voids.
There are 2 differentgeometries for aligned fibers:
1. Continuous & AlignedThe fibers are longer thana critical
length whichisthe minimum lengthnecessary such that theentire load
is transmittedfrom the matrix to thefibers. If they are shorterthan
this critical length,only some of the load istransmitted. Fiber
lengthsgreater that 15 times thecritical length areconsidered
optimal.Aligned and continuousfibersgive the mosteffective
strengthening forfiber composites.
2. Discontinuous & AlignedThe fibers are shorter thanthe
critical length. Hencediscontinuous fibers areless effective in
strengthening the material, however, their composite modulus
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and tensile strengths can approach 5090% of their continuous and
alignedcounterparts. Andthey are cheaper, faster and easier to
fabricate intocomplicated shapes.
RandomThis is also called discrete,(or chopped) fibers. The
strength will not be as high as withaligned fibers, however, the
advantage is that the material will beistropic andcheaper. WovenThe
fibers are woven into a fabric which is layered with the matrix
material to make alaminated structure.
Fiber Cross SectionOf course, the fibercross sectional shape and
size is also important.Here are some examples of the crosssectional
areas and shapes for a wide variety ofreinforing fibers:
Some general catagories of fibers based on cross section:
Whiskers
very small diameter (~1 micron) single crystalsstrong because
they are virtually flaw free
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expensivedifficult to put in a matrixexamples include graphite
(C), SiN, Al2O3, SiC
Fibers
small diameters (~10 microns)can be polycrystalline or
amorphous
Wires
large diameters (~25 microns)made from metals such as steel, Mo,
W
Fiber Materials for FiberglassAnd of course, the fiber material
is important too.A commonly used glass fiber composition for
structural composites is Eglass, in which Estands for "electrical
type". It is a limealuminumborosilicate glass with zero or low
sodiumand potassium levels. It is popular because it has chemical
durability. A more advanced and expensive
fiberisSglass,amagnesiaaluminasilicate glass that isusedfor
highstrength applications. The composition of these and other
common glass fiber materials are listed here:
Designation CharacteristicComposition
SiO2 Al2O3+Fe2O3
CaO MgO Na2O K2O B2O3 TiO2 ZrO2
Aglass commonsodalimesilica
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MatrixMaterials for FiberglassSome common thermosetting
polymeric matrix materials for fiberglass
includeepoxies,polyesters, phenolics and silicones. Some of the
common thermoplastic polymeric matrix materials for fiberglass
include nylon66, polycarbonate and polystyrene.Advanced
FiberReinforced Composite Systems Other Than FiberglassAdvanced
composites include those systems in which reinforcing fibers have
moduli higherthan that of Eglass. Here is a list of a variety of
advanced composite systems.
Class Fiber Matrix
Polymermatrix Paraaramid(Kevlar)1 epoxy
Paraaramid(Kevlar) polyester
C(graphite)2 epoxy
C(graphite) polyester
C(graphite) polyetheretherketone(PEEK)
C(graphite) polyphenylenesulfide(PPS)
Metalmatrix B Al
C Al
Al2O3 Al
Al2O3 Mg
SiC Al
SiC Ti(alloys)
Ceramicmatrix Nb MoSi2
C C
C SiC
SiC Al2O3
SiC SiC
SiC Si3N4
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SiC LiAlsilicate(glassceramic)
1. Kevlar is a Du Pont trade name for poly
pphenyleneterephthalamide (PPDT). It is an aramid, i.e. an aromatic
(benzene ring type) polyamide polymerfiber with a very rigid
molecular structure. It is used for highperformancecomposite
applications where light weight, high strength and stiffness,damage
resistance, and resistance to fatigue, creep, and stress rupture
areimportant.
Kevlar 29 is a lowdensity highstrength aramid fiber designed for
suchapplications as ballistic protection, ropes, and cables.Kevlar
49 is characterized by a low density and high strength andmodulus.
It is used as reinforcement for plastics in composites
foraerospace, marine, automotive, and other industrial
applications.
2. Carbon fibers are made of graphitic and noncrystalline
regions. It has thehighest specific strength and specific modulus
of all fiber materials.Itretains tensile strength at high
temperatures andis not affected bymoisture, solvents, acids or
bases at room temperatures. However, at hightemperatures it is
subject to oxidation.
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