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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

72

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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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