24 composites 2

8/18/2019 24 Composites 2

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CompositeMaterials


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Composite Materials:

• Defnition – A multi-component solid consisting of

two or more dierent materials

– Each of the components retains itsidentity and maintains itscharacteristic structure and properties

–

There are recognizable interfacesbetween the components


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Composite Materials: –

The composite generally exhibits acombination of properties (includingstiness, strength, conductivity, etc!not possible with the individual

components – The components can be metals,

ceramics, polymers, or even naturalmaterials such as wood, stone, or

straw – "ence a wide range of freedom exists

and composite materials can often be

designed to meet a desired set ofro erties and characteristics


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Classifcation by Geometry

• #aminar

• $articulate –%ispersion strengthened

– True particulate

• &iber reinforced


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Laminar (layered) Composites:

• 'ncludes thin coatings, thic protectivesurfaces, claddings, bimetallics,laminates, and sandwiches

•$roperties are anisotropic (dierentwithin the layers and perpendicular tothe layers!

• )sed to impart properties such as* low

cost, corrosion resistance, wearresistance, electrical properties, uni+ueexpansion characteristics, light weight,enhanced strength, or altered

appearance


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Laminar (layered) Composites:•

Examples* – $lywood (laminate!overcomes grainorientation by bonding layers atvarious angles, increases strength

fracture resistance, isotropic withinthe plane


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Laminar (layered)Composites:

• Examples*

– .afety glass (glass and polymer! - 'n this process, twosheets of glass are bonded together with a layer of plastic (theplastic layer goes inside the two glass sheets! The laminationtaes place in an autoclave, a special oven that uses both heatand pressure to form a single, strong unit that is resistant totearing The plastic interlayer is often tinted to act as an ultraviolet /lter 0hen laminated glass is broen, the broen pieces of glass

remain bound to the internal tear-resistant plastic layer, and thebroen sheet remains transparent Thus, visibility remains good

http://www.madehow.com/knowledge/Ultraviolet.htmlhttp://www.madehow.com/knowledge/Ultraviolet.html


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Bi-metallic strips - two metals with different coefficients of thermal expansion, composite offers shape as a function of

temperature (thermostat material)



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• Examples (con’t! – ). 1oins

– 1laddingsoer enhanced surfacewith low cost, can have high strength

or light weight core• Examples*

– Al1lad 2 aluminum clad aluminum

– .tainless claddings on steel


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Honeycomb - Corrugated cardboard, honeycomb, & other sandwiches—thick, low density core with high density surfaces,

strong but light weight



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Particulate Composites:

• %iscrete particles of one material

in a matrix of another


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Dispersion-strengthenedparticulates

–

.mall amounts of hard, brittle, smallparticles dispersed in a softer, ductile matrix

– 3ood bonding between particles and matrix

– .trengthening persists at elevatedtemperatures (dispersant not soluble inmatrix so it doesn’t redissolve, overage, orovertemper!

– 3ood creep resistance

– )sually produced via powder metallurgy

– Examples*

• .A$ (sintered aluminum powder 2 aluminummetal and aluminum oxide ceramic, up to456 ceramic!

• T%-7icel (Thoria-dispersed nicel 2 nicel


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True particulate composites:

– 1oncretesand andgravel in cement

– Asphaltcrushedaggregate in bitumen

(a thermoplastic binder!

Large amounts of rather coarse particles form the bulk of the composite

-usually, properties are isotropic


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True particulate composites:

– 1emented carbidesceramic particles in a metalmatrix which providestoughness

–3rinding wheelsceramicabrasives in a glass orpolymer matrix


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True particulate composites: –

Electrical contactstungsten 9 copper orsilver


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iber-rein!orced composites:

– 3enerally sees enhanced strength,fatigue resistance, stiness orstrength-to-weight ratio by

incorporating strong, sti, butpossibly brittle /bers into a softer,more ductile matrix


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–&ibers• 1arry most of the load

• 'mpart enhanced stiness

– :atrix• .upports /bers

• Transfers load to /bers

• $rotects /bers

• $rovides ductility toughness


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Examples

• 0ood1ellulose /bers inlignin matrix

• $rimitive brics of straw and

mud• Auto tires use nylon, rayon,

;evlar, or steel “/bers” toprovide strength and

durability to rubber

• 3olf club shafts (graphite 9

epoxy!


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– Design !eaturesthe properties of/ber-reinforced composites dependon a number of factors, including*

• $roperties of the /ber material•


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– &iber properties

• 3enerally want strong, sti, andlightweight

– .peci/c strength > ?. @ density

– .peci/c stiness > :odulus @ density

•'f used at high temperatures, may wanthigh melting point /bers

• 3lass – )sually combined with epoxy or polyester

matrix using B-CB6 /ber

–#ow cost

–


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• ;evlar (poly paraphenylene

terephthalamide! – T. -- 5B si

– E -- 4C x 4BC psi

– %ensity F G Aluminum

– 7egative thermal expansion

• 1eramic /bers – Alumina

– .ilicon carbide

• :etal /bers

– Tungsten, beryllium, boron

• 0hisers – :etal, graphite, ceramics


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–


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– =rientation eects

• .hort, randomlyoriented /bers

• )nidirectional /bersgood strengthparallel to /bers, but

poor properties inother direction

• 0oven fabrics ortapes

• -% stitching of

woven plies


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iber orientation


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iber orientation


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– %egree of /ber-matrix bonding• 0ant strong bonding in polymer and

metal matrix composites• 0ant wea bonding in ceramic matrix

materials

– :atrix properties• :atrix materials should be strong, tough,

and ductile so they can transmit theloads to the /bers and prevent cracs

from propagating through the composite• They also provide the maJor component

of electrical properties, chemicalbehavior, and elevated temperaturestability


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• $olymer matrixuseful up to CBBK& –

Thermosetsre+uires curing around /bers – Thermoplasticscan embed /bers in melted

resin

– Examples* sheet molding compound and bulmolding compound

• :etal matrix – $ermits higher operating temp

– "igh stiness and strength at elevated temp

– #ow thermal expansion

– Enhanced resistance to fatigue, abrasion andwear

– 3ood electrical and thermal conductivity

– 1ommon forms include Al, 1u, :g, Ti, 7i orsuperalloy matrix with graphite, boron

carbide, aluminum oxide, or silicon carbide/bers, or even high-temperature metal wires

Types of Composite Matrices


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• 1arbon matrix with graphite /bers – #ow density, useful strength up to CBBBK&

• 1eramic matrix – #ightweight and ade+uate properties at high temp

– Loth glass and crystalline ceramics are used

– &iber reinforcements improve toughness

» %elaminate

» $ull free

»"old cracs together


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• Assets of /ber-reinforced composites – "igh strength and stiness

– #ight weight

– Enhanced fatigue life

– 1orrosion resistant


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• $ossible limitations of /bercomposites

– "igh costmaterial and fabrication – %iDcult to inspect

– "ard to predict properties• Mesponse to impact

• &ailure conditions

• Environmental response

– =ften fails in a brittle manner

–

#imited ability to be repaired ifdamaged

– Mecycling is diDcult

– Assembly (Joining! usually re+uires

adhesives


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actors determining compositeproperties

– The properties of the components

– The relative amounts of thecomponents

– .ize, shape and distribution of thediscontinuous components

– =rientation of the various

components – %egree of bonding between the

components


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• Mule of :ixtures* – $roperty of the composite is simply a

weighted average of the properties ofits components

– )ses volume fractions of thecomponents

–Ncomposite > NA


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– Mule of :ixtures is always good fordensity

– %epending on orientation, Mule of

:ixtures may be good for modulusand conductivity

– Mule of :ixtures should 7=T be reliedupon for mechanical properties such

as* strength, ductility, toughness,fatigue, creep and wear


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actors determining compositeproperties

– The properties of the components

– The relative amounts of thecomponents

– .ize, shape and distribution of thediscontinuous components

– =rientation of the various

components – %egree of bonding between the

components


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"d#anced Composites:

–.peci/cally designed for*• "igh speci/c strength

• "igh speci/c modulus

• 3ood temperature resistance

–$roperty 1omparisons•

.trength and modulus• .trength vs Temperature


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Properties o! some selected composites


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Properties #sTemperature

$igh temperature


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$igh temperatureturbine rotors

.i1 whisers in a silicon nitride matrix


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abrication o! compositeshapes:

– %ie methods

– 1asting

– $ultrusion

– &ilament winding


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Producing composite shapes indies


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Producing compositeshapes by casting


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Producing compositeshapes by pultrusion


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$oneycomb structures


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$oneycomb structures


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Laminar composite

24 composites 2

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