CompositesCompositesIntroduction:

•Modern technology require materials with combinations of properties that can’t be met by conventional materials

•Composite : any multiphase material that exhibits a significant proportion of properties of both constituent phases such that a better combination of properties is realized.

•This can be made with two or more distinct materials

•It is a multiphase material that is artificial made. The constituent phases must be chemically dissimilar & separated by a distinct interface

• Many composites are composed of two phases, the matrix which is continuous & surrounds the other phase, the dispersed phase

Classification for various types of composites

► Particle Reinforced CompositesParticle Reinforced Composites Sub classified as large particle & dispersion strengthened Sub classified as large particle & dispersion strengthened

compositescomposites

► Large Particle CompositesLarge Particle Composites The term large indicate that particle-matrix interactions can’t be The term large indicate that particle-matrix interactions can’t be

treated on the atomic or molecular leveltreated on the atomic or molecular level The particulate phase is stiffer & harder than the matrixThe particulate phase is stiffer & harder than the matrix These particles tend to restrain movement of the matrix phase in These particles tend to restrain movement of the matrix phase in

the vicinity of each particle due to its small size & even distributionthe vicinity of each particle due to its small size & even distribution The volume fraction of the two phases influences the behavior; The volume fraction of the two phases influences the behavior;

mechanical properties are enhanced with particulate content mechanical properties are enhanced with particulate content incrementincrement

The rules of mixtures equations predict that elastic modulus should The rules of mixtures equations predict that elastic modulus should fall between upper bound :fall between upper bound :

► EEcc (u)=E (u)=EmmVVmm + E + EppVVpp

► & lower bound,& lower bound,

► E= elastic modulus, V= volume fraction, c=composite, m=matrix, E= elastic modulus, V= volume fraction, c=composite, m=matrix, p=particulate phasesp=particulate phases

mppm

pmc

EVEV

EElE

)(

► ConcreteConcrete A common example of large particle composite in which A common example of large particle composite in which

both matrix & dispersed phases are ceramic materialsboth matrix & dispersed phases are ceramic materials Concrete is a composite material consisting an aggregate of Concrete is a composite material consisting an aggregate of

particles that are bound together in a solid body.particles that are bound together in a solid body. The binding medium is the cementThe binding medium is the cement Example: Portland cement concreteExample: Portland cement concrete

► The ingredients-portland cement, fine aggregate (sand), coarse The ingredients-portland cement, fine aggregate (sand), coarse aggregate (gravel) & wateraggregate (gravel) & water

► The aggregate particles act as filler materialThe aggregate particles act as filler material► The aggregates comprise 60%-80% of total volumeThe aggregates comprise 60%-80% of total volume► The amount of cement-water paste should be sufficient to coat The amount of cement-water paste should be sufficient to coat

sand & gravel particles, if not cementious bond will be sand & gravel particles, if not cementious bond will be incompleteincomplete

► Addition of correct quantity of water will ensure a complete Addition of correct quantity of water will ensure a complete bonding & sufficient porositybonding & sufficient porosity

Another example :reinforced concreteAnother example :reinforced concrete► The concrete is reinforced with steel rods, wires or meshThe concrete is reinforced with steel rods, wires or mesh► Capable to support greater tensile, compressive & shear Capable to support greater tensile, compressive & shear

stressesstresses

► Dispersion Strengthened CompositesDispersion Strengthened Composites Particles normally smaller. Diameters between 0.01 & 0.1 Particles normally smaller. Diameters between 0.01 & 0.1

mm Particle matrix interactions that lead to strengthening Particle matrix interactions that lead to strengthening

occur on the atomic or molecular leveloccur on the atomic or molecular level Whereas, the matrix bears the major portion of an applied Whereas, the matrix bears the major portion of an applied

load, the small dispersed particles hinder or impede the load, the small dispersed particles hinder or impede the motion of dislocations.motion of dislocations.

Plastic deformation is restricted. Yield, tensile strength, Plastic deformation is restricted. Yield, tensile strength, hardness improved hardness improved

Metal & metal alloy can be strengthened by this way by Metal & metal alloy can be strengthened by this way by involving interactions between particles & dislocations involving interactions between particles & dislocations within the matrixwithin the matrix

The strengthening is retained at elevated temperatures & The strengthening is retained at elevated temperatures & for extended time periods due to the dispersed particles for extended time periods due to the dispersed particles chose to be unreactive with the matrix phasechose to be unreactive with the matrix phase

► Fiber Reinforced CompositesFiber Reinforced Composites The dispersed phase in fiber formThe dispersed phase in fiber form Designed to have high strength &/or stiffness on weight basis. Designed to have high strength &/or stiffness on weight basis.

Expressed in specific strength & modulusExpressed in specific strength & modulus These correspond to the ratios of tensile strength to specific These correspond to the ratios of tensile strength to specific

gravity & modulus of elasticity to specific gravitygravity & modulus of elasticity to specific gravity Short fiber don’t produce significant improvement Short fiber don’t produce significant improvement

► Influence of fiber lengthInfluence of fiber length Some critical fiber length is necessary for effective

strengthening & stiffening

lc , critical length, dependent on

d , fiber diameter & tensile strength, & c, fiber –matrix bond strength

c

fl

2

d *c

f

Fibers which lFibers which l>> l>> lcc (normally l> 15 l (normally l> 15 lcc) are termed continuous. ) are termed continuous.

Shorter than this are discontinuous or short fibersShorter than this are discontinuous or short fibers For discontinuous fibers of lengths significantly less than lc, the For discontinuous fibers of lengths significantly less than lc, the

matrix deforms around fiber. This results in no stress transference matrix deforms around fiber. This results in no stress transference & little reinforcement by the fiber.& little reinforcement by the fiber.

Thus, for strength improvement of composites, the fibers must be Thus, for strength improvement of composites, the fibers must be continuouscontinuous

► Influence of Fiber Concentration and OrientationInfluence of Fiber Concentration and Orientation

•Continuous fibers are normally aligned as in figure (a) !

In this figure, fracture strength in tension for fiber & matrix & ,

& their corresponding fracture strains, & ; It is assume that

However, a fiber reinforced composite consist of these fiber & matrix materials will exhibit the uniaxial stress strain response

m

f

m

f

f

m

f

Continuous & Aligned Fiber CompositesContinuous & Aligned Fiber Composites

•Tensile Stress-Strain Behavior-Longitudinal LoadingTensile Stress-Strain Behavior-Longitudinal Loading

► In the initial Stage I region, both fibers & matrix deform elastically; In the initial Stage I region, both fibers & matrix deform elastically; normally this portion of the curve is linear.normally this portion of the curve is linear.

► Typically, for a composite of this type, the matrix yields & deforms Typically, for a composite of this type, the matrix yields & deforms plastically at while the fibers continue to stretch elastically, in plastically at while the fibers continue to stretch elastically, in as much as the tensile strength of the fibers is significantly higher as much as the tensile strength of the fibers is significantly higher than the yield strength of the matrix.than the yield strength of the matrix.

► This process constitutes Stage II which stage is ordinarily very This process constitutes Stage II which stage is ordinarily very nearly linear, but of diminished slope relative to Stage II. nearly linear, but of diminished slope relative to Stage II.

► In passing from Stage I to Stage II, the proportion of the applied In passing from Stage I to Stage II, the proportion of the applied load that is borne by the fibers increases.load that is borne by the fibers increases.

► The onset of composite failure begins as the fibers starts to The onset of composite failure begins as the fibers starts to fracture, which corresponds to a strain approximately . fracture, which corresponds to a strain approximately .

► Composite failure is not catastrophic. The reason: Composite failure is not catastrophic. The reason: Not all fibers fracture at the same timeNot all fibers fracture at the same time These fractured fibers are still embedded within the intact These fractured fibers are still embedded within the intact

matrix, thus capable of sustaining diminishing load as the matrix, thus capable of sustaining diminishing load as the matrix continues to plastically deformmatrix continues to plastically deform

ym

f

• Elastic behavior-Longitudinal LoadingElastic behavior-Longitudinal Loading• It is assumed that the fiber-matrix interfacial bond is very good, It is assumed that the fiber-matrix interfacial bond is very good,

such that deformation of both matrix & fibers is the samesuch that deformation of both matrix & fibers is the same• The total load sustained by the composite FThe total load sustained by the composite Fcc is equal to the loads is equal to the loads

carried by the matrix phase Fcarried by the matrix phase Fmm & the fiber phase F & the fiber phase Fff or or

• From the stress formula, F=From the stress formula, F=A, the substitution of these into A, the substitution of these into above formula yields:above formula yields:

• Then, the equation is divided by AThen, the equation is divided by Acc , where A , where Amm/A/Acc & A & Aff/A/Acc are the are the area fractions of the matrix & fiber phases. If the composite, area fractions of the matrix & fiber phases. If the composite, matrix, & fiber phase lengths are all equal, Amatrix, & fiber phase lengths are all equal, Amm/A/Acc is equivalent to is equivalent to the volume fraction of the matrix Vthe volume fraction of the matrix Vmm, & for the fibers, V, & for the fibers, Vff=A=Aff/A/Acc

• If an isostrain state means: If an isostrain state means: cc==mm==ff, by dividing, by dividing the above the above equation with its respective strain:equation with its respective strain:

fmc FFF

ffmmcc AAA

ffmmc VV

ff

fm

m

m

c

cVV

• If composite, matrix, & fiber deformations are all elastic, then If composite, matrix, & fiber deformations are all elastic, then cc//cc=E=Ecc, , mm//mm=E=Emm, , ff//ff=E=Eff, , the E’s being the moduli of the E’s being the moduli of elasticityfor respective phases. elasticityfor respective phases.

• Then we substitute these into the previous equation which Then we substitute these into the previous equation which yields an expression for the modulus of elasticity of a yields an expression for the modulus of elasticity of a continuous & aligned fibrous composite in the direction of continuous & aligned fibrous composite in the direction of alignment, Ealignment, Eclcl, as, as

• Since the composite consists of only matrix & fiber phases; Since the composite consists of only matrix & fiber phases; that is Vthat is Vmm+V+Vff=1=1

• Ecl is equal to the volume-fraction weighted average of the Ecl is equal to the volume-fraction weighted average of the moduli of elasticity of the fiber and matrix phases.moduli of elasticity of the fiber and matrix phases.

• Other properties including density, also have this dependence Other properties including density, also have this dependence on volume fractions.on volume fractions.

ffmmcl VEVEE

ffmcl VEfEE )1(

• Longitudinal Tensile StrengthLongitudinal Tensile Strength

• Failure of this type of composite material is a relatively complex Failure of this type of composite material is a relatively complex process. Several different failure modes are possibleprocess. Several different failure modes are possible

• The mode that operates for a specific composite will depend on The mode that operates for a specific composite will depend on fiber & matrix properties, & the nature & strength of the fiber-fiber & matrix properties, & the nature & strength of the fiber-matrix interfacial bondmatrix interfacial bond

• If , then fibers will fail before the matrixIf , then fibers will fail before the matrix• Once the fibers have fractured, the majority of the load that Once the fibers have fractured, the majority of the load that

was borne by the fibers is now transferred to the matrixwas borne by the fibers is now transferred to the matrix• From equation: , we obtain the following From equation: , we obtain the following

expression for the longitudinal strength of composite, :expression for the longitudinal strength of composite, :

f

ffmmc VV

cl

fffm VVcl

*)1('*

• Transverse Tensile Strength Transverse Tensile Strength • Transverse tensile load might be present during in service Transverse tensile load might be present during in service

applicationsapplications• Under these circumstances, premature failure may result Under these circumstances, premature failure may result

inasmuch as transverse strength is usually extremely lowinasmuch as transverse strength is usually extremely low• Whereas, longitudinal strength is dominated by fiber Whereas, longitudinal strength is dominated by fiber

strength, a variety of factors will influence on the strength, a variety of factors will influence on the transverse strength:transverse strength:• Properties of both fiber & matrixProperties of both fiber & matrix• Fiber-matrix bond strength Fiber-matrix bond strength • Presence of voidsPresence of voids

► Discontinuous & Aligned Fiber CompositesDiscontinuous & Aligned Fiber Composites Advantage : short fiber composites can be produced Advantage : short fiber composites can be produced

having moduli of elasticity & tensile strength that approach having moduli of elasticity & tensile strength that approach 90% & 50% of continuous fiber counterparts.90% & 50% of continuous fiber counterparts.

For a discontinuous & aligned fiber composite having a For a discontinuous & aligned fiber composite having a uniform distribution of fibers & in which l>lc.uniform distribution of fibers & in which l>lc.

The longitudinal strength ( ) is given by the The longitudinal strength ( ) is given by the relationship:relationship:

Where, & represents, the fracture strength of the Where, & represents, the fracture strength of the fiber & the stress in the matrix when the composite fails.fiber & the stress in the matrix when the composite fails.

If the fiber length is less than critical (l<lc), then the If the fiber length is less than critical (l<lc), then the longitudinal strength is given by:longitudinal strength is given by:

cd

)1()2

1( '**fm

cff V

llV

cd

f

cd

'cd

)1('*

'fmf

c VVdl

cd

d=fiber diameter

c=the smaller of either fiber matrix bond strength or matrix shear yield strength

► Discontinuous & Randomly Oriented Fiber CompositesDiscontinuous & Randomly Oriented Fiber Composites

For these kinds of fiber orientation, an expression for the For these kinds of fiber orientation, an expression for the elastic modulus similar to rule of mixture may be utilized:elastic modulus similar to rule of mixture may be utilized:

K=fiber efficiency parameter that depends on VK=fiber efficiency parameter that depends on Vff & E & Eff/E/Emm ratio ratio

ffmmcd VKEVEE

► The Fiber PhaseThe Fiber Phase An important characteristic of most materials, i.e. brittle one, An important characteristic of most materials, i.e. brittle one,

small diameter fiber is much stronger than the bulk materialsmall diameter fiber is much stronger than the bulk material On the basis of diameter & character, fibers are classified to:On the basis of diameter & character, fibers are classified to:

► WhiskersWhiskers► FibersFibers► WiresWires

Whiskers-very thin single crystals that have extremely large Whiskers-very thin single crystals that have extremely large length-to-diameter ratiolength-to-diameter ratio

Due to their small size, they have high degree of crystalline Due to their small size, they have high degree of crystalline perfection, flaw free, high strengths, known as strongest perfection, flaw free, high strengths, known as strongest materialmaterial

Not utilized as reinforcement medium because they are Not utilized as reinforcement medium because they are extremely expensive. Impractical to incorporate into matrixextremely expensive. Impractical to incorporate into matrix

Whisker materials include graphite, silicon carbide, silicon Whisker materials include graphite, silicon carbide, silicon nitride & aluminum oxidenitride & aluminum oxide

Fibers-are either polycrystalline or amorphous & have small Fibers-are either polycrystalline or amorphous & have small diameter. Generally polymer or ceramicsdiameter. Generally polymer or ceramics

Fine wires-relatively large diameters. Typical materials include Fine wires-relatively large diameters. Typical materials include steel,molybdenum & tungstensteel,molybdenum & tungsten

► The Matrix PhaseThe Matrix Phase The matrix phase of fibrous composites can be a metal, The matrix phase of fibrous composites can be a metal,

polymer or ceramic polymer or ceramic Metal & polymers are used as matrix materials due to their Metal & polymers are used as matrix materials due to their

ductility ductility For ceramic-matrix composites, reinforcing component is For ceramic-matrix composites, reinforcing component is

added to improve fracture toughnessadded to improve fracture toughness For fiber reinforced composites, matrix phase serves as:For fiber reinforced composites, matrix phase serves as:

► To bind the fibers together & acts as the medium by which an To bind the fibers together & acts as the medium by which an externally applied stress is transmitted & distributed to the externally applied stress is transmitted & distributed to the fibers; only a very small proportion of an applied load is fibers; only a very small proportion of an applied load is sustained by the matrixsustained by the matrix

► To protect individual fibers from surface damage as a result To protect individual fibers from surface damage as a result of mechanical abrasion or chemical reactions with the of mechanical abrasion or chemical reactions with the environmentenvironment

Matrix phase serves as barrier to crack propagation. Matrix phase serves as barrier to crack propagation. Although some individual fibers fail, total composite Although some individual fibers fail, total composite fracture will not occur until large numbers of adjacent fracture will not occur until large numbers of adjacent fibers have failedfibers have failed

► Polymer-Matrix CompositesPolymer-Matrix Composites Consist of a polymer resin as the matrix, with fibers as the Consist of a polymer resin as the matrix, with fibers as the

reinforcement mediumreinforcement medium

► Glass Fiber Reinforced Polymer CompositesGlass Fiber Reinforced Polymer Composites Fiberglass is a composite consisting of glass fibers, either Fiberglass is a composite consisting of glass fibers, either

continuous or discontinuous, contained within a polymer continuous or discontinuous, contained within a polymer matrixmatrix

Produced in largest quantities.Fiber diameters range Produced in largest quantities.Fiber diameters range between 3 & 20 between 3 & 20 mm

Glass is popular because:Glass is popular because:► Easily drawn into high strength fibers from molten stateEasily drawn into high strength fibers from molten state► Readily available & maybe fabricated into glass-reinforced Readily available & maybe fabricated into glass-reinforced

plastic economically using wide variety of composite plastic economically using wide variety of composite manufacturing techniquesmanufacturing techniques

► As a fiber, it is relatively strong & when embedded in a plastic As a fiber, it is relatively strong & when embedded in a plastic matrix, it produces a composite having a very high specific matrix, it produces a composite having a very high specific strengthstrength

► When coupled with various plastics, it possesses a chemical When coupled with various plastics, it possesses a chemical inertness that renders the composite useful in a variety of inertness that renders the composite useful in a variety of corrosive environmentscorrosive environments

Newly drawn fibers are coated during drawing. This is Newly drawn fibers are coated during drawing. This is made from a thin layer of substance that protects the fiber made from a thin layer of substance that protects the fiber surface from damage & undesirable environmental surface from damage & undesirable environmental interactionsinteractions

Coatings are desirable to avoid surface flaws which are Coatings are desirable to avoid surface flaws which are easily introduced by rubbing the surface with harder easily introduced by rubbing the surface with harder materialsmaterials

Limitations:Limitations:► Not very stiff, do not display rigidity that is necessary for Not very stiff, do not display rigidity that is necessary for

some applicationssome applications► Limited to applications below 200Limited to applications below 200ooCC

► Carbon Fiber Reinforced Polymer CompositesCarbon Fiber Reinforced Polymer Composites Commonly used for reinforced in advanced Commonly used for reinforced in advanced

(i.e.nonfiberglass) polymer-matrix composites(i.e.nonfiberglass) polymer-matrix composites Reasons of application:Reasons of application:

► Carbon fibers have the highest specific modulus & specific Carbon fibers have the highest specific modulus & specific strength of all reinforcing fiber materialsstrength of all reinforcing fiber materials

► Retain high tensile modulus & high strength at elevated Retain high tensile modulus & high strength at elevated temperatures; high temperature oxidation temperatures; high temperature oxidation

► At room temperature; not affected by moisture & wide variety At room temperature; not affected by moisture & wide variety of chemicalsof chemicals

► Relatively inexpensive & cost effectiveRelatively inexpensive & cost effective

The stable form of crystalline carbon at ambient The stable form of crystalline carbon at ambient temperature is graphitetemperature is graphite

Carbon fibers are not totally crystalline but are composed Carbon fibers are not totally crystalline but are composed of graphitic & non crystalline region of graphitic & non crystalline region

The classification scheme is based on tensile modulus: The classification scheme is based on tensile modulus: standard, intermediate, high & ultrahigh modulistandard, intermediate, high & ultrahigh moduli

Range of fiber diameters between 4-10 Range of fiber diameters between 4-10 m. Available in m. Available in continuous & chopped formcontinuous & chopped form

Usually coated with protective epoxy to improve adhesion Usually coated with protective epoxy to improve adhesion with polymer matrixwith polymer matrix

► Aramid Fiber Reinforced Polymer CompositesAramid Fiber Reinforced Polymer Composites Known chemically as poly paraphenylene terephtalamideKnown chemically as poly paraphenylene terephtalamide High strength, high modulus, tough; resistance to impact, High strength, high modulus, tough; resistance to impact,

creep & fatigue failure. High longitudinal tensile strength & creep & fatigue failure. High longitudinal tensile strength & tensile modulitensile moduli

Desirable for their outstanding strength to weight ratios Desirable for their outstanding strength to weight ratios (superior than metal)(superior than metal)

Aramid are thermoplastic, but combustion resistant & stable to relatively high temperature. They can retain their high mechanical properties between -200oC & 200oC.

Chemically susceptible to degradation by strong acids & bases, but relatively inert in other solvents & chemicals

Most often used in composites have polymer matrices; common matrix materials are epoxies, & polyesters.

The fibers are relatively flexible & somewhat ductile, they maybe processed by common textile operations

Schematic representation of mer & chain structures for aramid. Chain alignment with the fiber direction & hydrogen bonds between adjacent chains are shown

► Other Fiber Reinforcement MaterialsOther Fiber Reinforcement Materials Common fiber reinforcements incorporated in polymer Common fiber reinforcements incorporated in polymer

matrices:matrices:► Glass, carbon & aramidsGlass, carbon & aramids► Others (but not that common): boron, silicon carbide, Others (but not that common): boron, silicon carbide,

aluminum oxidealuminum oxide

► Polymer Matrix MaterialsPolymer Matrix Materials The matrix determines the maximum service temperature, The matrix determines the maximum service temperature,

since it normally softens, melts, or degrades at much lower since it normally softens, melts, or degrades at much lower temperature than the fiber reinforcementtemperature than the fiber reinforcement

Polyesters & vinyl esters are widely utilized due to Polyesters & vinyl esters are widely utilized due to inexpensivenessinexpensiveness

Epoxies are more expensive but have better mechanical Epoxies are more expensive but have better mechanical properties & resistance to moistureproperties & resistance to moisture

Polyimide resins can be used for high temperature Polyimide resins can be used for high temperature application i.e. up to 230application i.e. up to 230ooC.C.

Other materials-polyetheretherketone (PEEK), Other materials-polyetheretherketone (PEEK), polyphenylene sulfide (PPS), & poly etherimide (PEI).polyphenylene sulfide (PPS), & poly etherimide (PEI).

► Metal Matrix CompositesMetal Matrix Composites The matrix is a ductile metalThe matrix is a ductile metal Maybe utilized at higher service temperatures than their base Maybe utilized at higher service temperatures than their base

metal counterparts. metal counterparts. The reinforcement may improve specific stiffness, strength, The reinforcement may improve specific stiffness, strength,

abrasion resistance, creep resistance, thermal conductivity & abrasion resistance, creep resistance, thermal conductivity & dimensional stabilitydimensional stability

Advantage over polymer-matrix composites: higher operating Advantage over polymer-matrix composites: higher operating temperature, non flammability, greater resistance to temperature, non flammability, greater resistance to degradation by organic fluids. However, more expensivedegradation by organic fluids. However, more expensive

Superalloy, as well as alloys of aluminum, magnesium, titanium, Superalloy, as well as alloys of aluminum, magnesium, titanium, & copper, are employed as matrix materials& copper, are employed as matrix materials

Reinforcement might be in particulates, continuous & Reinforcement might be in particulates, continuous & discontinuous fibers, whiskers. Concentrations normally range discontinuous fibers, whiskers. Concentrations normally range between 10 & 60 vol%.between 10 & 60 vol%.

Continuous fiber materials: carbon, silicon carbide, boron, Continuous fiber materials: carbon, silicon carbide, boron, alumina & refractory metals. Discontinuous-silicon carbide alumina & refractory metals. Discontinuous-silicon carbide whiskers, chopped fibers of alumina & carbon, & particulates of whiskers, chopped fibers of alumina & carbon, & particulates of silicon carbide & aluminasilicon carbide & alumina

Some matrix-reinforcement combinations are highly Some matrix-reinforcement combinations are highly reactive at elevated temperaturesreactive at elevated temperatures

Composite degradation maybe caused by high Composite degradation maybe caused by high temperature processing or by subjecting the metal-matrix temperature processing or by subjecting the metal-matrix composites to elevated temperature during servicecomposites to elevated temperature during service

This is resolved by applying protective surface coating to This is resolved by applying protective surface coating to the reinforcement or by modifying the matrix alloy the reinforcement or by modifying the matrix alloy compositioncomposition

► Ceramic Matrix CompositesCeramic Matrix Composites The fracture toughness of ceramics have been The fracture toughness of ceramics have been

improved significantly by these composites improved significantly by these composites where particulates, fibers, or whiskers of one where particulates, fibers, or whiskers of one ceramic materials have been embedded into a ceramic materials have been embedded into a matrix of another ceramicsmatrix of another ceramics

Ceramic matrix composite materials have Ceramic matrix composite materials have extended fracture toughness between 6 & 20 extended fracture toughness between 6 & 20 MPaMPamm

The improvement in the fracture properties The improvement in the fracture properties results from interactions between advancing results from interactions between advancing cracks & dispersed phase particlescracks & dispersed phase particles

Crack initiation normally occurs with the matrix Crack initiation normally occurs with the matrix phase whereas crack propagation is impeded or phase whereas crack propagation is impeded or hindered by the particles, fibers or whiskershindered by the particles, fibers or whiskers

In general, increasing fiber content improves In general, increasing fiber content improves strength & fracture toughnessstrength & fracture toughness

There is also a considerable reduction in the There is also a considerable reduction in the scatter of fracture strengths for whisker-scatter of fracture strengths for whisker-reinforced ceramics relative to their unreinforced reinforced ceramics relative to their unreinforced counterpartcounterpart

In addition, these ceramic matrix composites In addition, these ceramic matrix composites exhibit improved high temperature creep exhibit improved high temperature creep behavior & resistance to thermal shockbehavior & resistance to thermal shock

► Carbon-Carbon CompositesCarbon-Carbon Composites Both reinforcement & matrix are carbonBoth reinforcement & matrix are carbon Newly developed & expensive. Not extensively utilizedNewly developed & expensive. Not extensively utilized Desirable properties include:Desirable properties include:

► High tensile moduli & tensile strength that are retained up to High tensile moduli & tensile strength that are retained up to 20002000ooCC

► Resistance to creepResistance to creep► Large fracture toughness valueLarge fracture toughness value► Low coefficients of thermal expansion Low coefficients of thermal expansion ► High thermal conductivitiesHigh thermal conductivities► Low susceptibility to thermal shockLow susceptibility to thermal shock

► Hybrid CompositeHybrid Composite Obtained by using two or more different kind of fibers in a Obtained by using two or more different kind of fibers in a

single matrixsingle matrix Hybrids have better all around combination of properties Hybrids have better all around combination of properties

than composites containing only single fiber typethan composites containing only single fiber type A variety of fiber combinations & matrix materials are used, A variety of fiber combinations & matrix materials are used,

usually are from carbon & glass fibers which incorporated usually are from carbon & glass fibers which incorporated into polymeric resininto polymeric resin

The glass-carbon hybrid is stronger & tougher, has higher The glass-carbon hybrid is stronger & tougher, has higher impact resistance, maybe produced at lower costimpact resistance, maybe produced at lower cost

► When hybrid composites are stressed in tension, failure is When hybrid composites are stressed in tension, failure is usually non catastrophicusually non catastrophic

► The carbon fibers are the first to fail, at which time the load is The carbon fibers are the first to fail, at which time the load is transferred to the glass fiberstransferred to the glass fibers

► Upon failure of the glass fibers, the matrix phase must sustain Upon failure of the glass fibers, the matrix phase must sustain the applied loadthe applied load

► Eventual composite failure concurs with that of the matrix phaseEventual composite failure concurs with that of the matrix phase

► Structural CompositesStructural Composites Laminar CompositesLaminar Composites

► Composed of 2 dimensional sheets or panels that have a Composed of 2 dimensional sheets or panels that have a preferred high strength directions as in wood & preferred high strength directions as in wood & continuous & aligned fiber reinforced plasticscontinuous & aligned fiber reinforced plastics

► The layers are stacked & subsequently cemented together The layers are stacked & subsequently cemented together such that the orientation of the high strength direction such that the orientation of the high strength direction varies with each successive layervaries with each successive layer

► Laminar composite has relatively high strength in a Laminar composite has relatively high strength in a number of directions in 2D plane, but the strength in number of directions in 2D plane, but the strength in any given direction is lower than it would be if all the any given direction is lower than it would be if all the fibers were oriented in that directionfibers were oriented in that direction

Sandwich PanelSandwich Panel► Considered to be a class of structural compositesConsidered to be a class of structural composites► Consist of 2 strong outer sheets or faces, separated by a Consist of 2 strong outer sheets or faces, separated by a

layer of less dense material, core which has lower stiffness & layer of less dense material, core which has lower stiffness & lower strength lower strength

► The faces bear most of the in-plane loading & also any The faces bear most of the in-plane loading & also any transverse bending stressestransverse bending stresses

► Typical face materials- aluminum alloys, fiber reinforced Typical face materials- aluminum alloys, fiber reinforced plastics, titanium, steel, plywoodplastics, titanium, steel, plywood

► Structurally, the core serves 2 functions.Structurally, the core serves 2 functions. Separates the faces & resists deformations perpendicular Separates the faces & resists deformations perpendicular

to the face planeto the face plane Provides a certain degree of shear rigidity along planes Provides a certain degree of shear rigidity along planes

that are perpendicular to the facesthat are perpendicular to the faces Core materials-foamed polymers, synthetic rubbers, Core materials-foamed polymers, synthetic rubbers,

inorganic cementsinorganic cements Another popular core consists of honeycomb structure-Another popular core consists of honeycomb structure-

thin foils that have been formed into interlocking thin foils that have been formed into interlocking hexagonal cellshexagonal cells

The material maybe the same as face materialThe material maybe the same as face material