7 composite-new.pdf

8/11/2019 7 COMPOSITE-new.pdf

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composites


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introduction

Many modern tech. & application need unusual combinationof material properties aerospace, submarine, train,bioengineering etc.

increasingly searching for structural materials that have low

densities, strong stiff, light, abrasion-impact-resistant, andare not easily corroded.

Frequently, strong materials are relatively dense heavy;

also, increasing the strength or stiffness generally results ina decrease in impact strength rigid

omposite! multiphase material that e"hibit betterproperties of the constituents and are insoluble in eachother

omposite materials may be selected to give unusualcombinations of stiffness, strength, weight,

high-temperature performance, corrosion resistance,hardness or conductivit .


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$atural composite! wood, bone

%ood fibers strong & fle"ible cellulose' surrounded & heldby lignen stiffer material'

bone is collagen a composite of the strong yet soft protein'

and mineral apatite the hard, brittle' (composite) artificially made

*equirements! the constituent phase must be chemicallydissimilar & separated by a distinct interface


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Most composites have been created to improve

combinations of mechanical characteristics such asstiffness, toughness, and ambient and hightemperature strength.

Many composite + phases!

. matri"! continuous & surrounds other phase+. dispersed phasereinforcement

he interface is the area of contact between thereinforcement and the matri" materials.

/roperties of composite are a function of theproperties of the prop. of constituent phase, theamounts, geometry of dispersed phase

0ispersed phase geometry) in this conte"t meansthe shape of the particles and the particle si1e,

distribution, and orientation)


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2eneral characteristics

Matri" serves!

. 3inding reinforcement phases in place

+. 4haping & deforming to distribute the stress

among the constituent reinforcement materialsunder an applied force. 4uperior to all other mechanical prop. of

individual materials specific strength, stiffness,high strength etc'

. omposites cannot be made from constituentswith divergent linear e"pansion characteristics.

. 5ne of the prime considerations in the selectionand fabrication of composites is that the

constituents should be chemically inert-


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omposites with combine matri" multi-matri"6layer composite

Filler6dispersed comp. are uniformly distributedin matri"; posses high strength, hardness, elasticmodulus higher than the matri"7s

omposite contains 8+ fillercomple"-reinforced composite


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Matri" materials

9lthough the high strength of composites islargely due to the fibre reinforcement, theimportance of matri" material cannot beunderestimated as it provides support for the

fibres and assists the fibres in carrying theloads.

:t also provides stability to the compositematerial.

:t is essential that adhesive bonding forcesbetween ber and matri" be high to minimi1eber pull-out

he ultimate strength of the composite depends

to a large degree on the magnitude of this bond;


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Functions of a Matrix

:n a composite material, the matri" material serves thefollowing functions!

< =olds the fibres together and act as medium by whiche"ternally applied stress is transmited & distributed

the fiber< protect individual fiber from surface damage as a result

of mechanical abrasion6chemical reaction

< 0istributes the loads evenly between fibres so that all

fibres are sub>ected to the same amount of strain.< ?nhances transverse properties of a laminate.

< :mproves impact and fracture resistance of acomponent.

< =elps to avoid propagation of crac# growth through thefibres by providing alternate failure path along the


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Properties of a Matrix

he needs or desired properties of the matri" which areimportant for a composite structure are as follows!

< distribute stress to the fiber *educed moisture absorption.

< @ow shrin#age.< @ow coefficient of thermal e"pansion.< should not crac#< 4trength at elevated temperature depending on

application'.

< @ow temperature capability depending onapplication'.< ?"cellent chemical resistance depending on

application'.< 4hould be easily processable into the final composite

shape.< 0imensional stabilit maintains its sha e .


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Matri" classification

he matri" phase may be a metal, polymer, orceramic.


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/olymer matri" materials

/olymers ma#e ideal materials as they can beprocessed easily, possess lightweight, and desirablemechanical properties.

wo main #inds of polymers are thermosets and

thermoplastics. hermosets undergo irreversible cross-lin#ing reaction

& decompose instead of melting on heat application.

?.g! ?po"y , phenolic , polyester , polyimide,

polyurethane , silicone resins. hermoplastics change in viscosity, melt & tend to

loose their strength at an elevated temperature

9nother advantage of thermoplasatics is that theprocess of softening at elevated temperatures canreversed to regain its properties during cooling


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/olymer-matri"

composites/M' /olymer matri" composites /Ms' are comprised of

a variety of short or continuous fibers bound togetherby an organic polymer matri".

onsist of high-M%-reinforced plastic resin' polymeras the matri" with fibers as reinforcement medium

2reatest diversity & quantity

@ight, easy to fabricated & cost

lassify according to reinforcement type! glass,carbon, aramid


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Glass fiber-reinforced polymer (GFRP'

composite fiberglass'

continuous6discontinuous within polymer matri"

glass is popular as fiber reinforcement material!

easily drawn into high-strength fibers from themolten state

+ readily available, may fabricated economically many technique

A fiber is strong, embedded in plastic matri" highspecific strength composite

B %hen coupled with plastics possess chemicalinertness that renders composite useful in variety

corrosive environment


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4urface flaw affect composite stretching; can be introduced byrubbing6friction with other hard materials or e"posed to normal

environment for long time @imitation of 2F*/!

limited service C +DD , higher polymer deteriorate

4ervice temperatures may be e"tended to appro"imatelyADDE by using high-purity fused silica for the bers andhigh-temperature polymers such as the polyimide resins

+ not very stiff & not display rigidity for some application

9pplication! automotive & marine bodies, plastic pipe, storagecontainer

he transportation industries are utili1ing increasing amounts

of glass ber-reinforced plastics in an effort to decreasevehicle weight and boost fuel efciencies


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Carbon fiber-reinforced polymer(CFRP) composite

*easons for using carbon fibers

have highest specific strength

+ retain high tensile strength in elevated

A are not affected by moisture & various solvents in r

B e"hibit diversity of physical & mechanical characteristic allowing various properties & application

relatively ine"pensive

9pplications! in sports and recreational equipment

shing rods, golf clubs', lament-wound roc#et motorcases, pressure vessels, and aircraft structuralcomponentsGboth military and commercial, "ed wingand helicopters e.g., as wing, body, stabili1er, andrudder components'.


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ramid Fiber-Reinforced PolymerComposites

hemically, this group of materials is #nown aspolyparaphenylene terephthalamide'

Mechanically, these bers have longitudinal tensilestrengths and tensile moduli but are relatively wea# incompression.

:n addition, this material is #nown for its toughness, impactresistance, and resistance to creep and fatigue failure.

?ven though the aramids are thermoplastics, they are,nevertheless, resistant to combustion and stable torelatively high temperatures H+DD and +DD '

hemically, they are susceptible to degradation by strongacids and bases, but they are relatively inert in othersolvents and chemicals.

common matri" materials are the epo"ies and polyesters

9pplications!min ballistic products bullet-proof vests andarmor', sporting goods, tires, ropes, missile cases, pressure


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Metal matri" material

9dvantages! high strength, fracture toughness andstiffness, withstand elevated temperature in corrosiveenvironment than polymer composites.

Most metals and alloys could be used as matrices and

they require reinforcement materials which need to bestable over a range of temperature and non-reactivetoo.

Most metals and alloys ma#e good matrices.

he melting point, physical and mechanical propertiesof the composite at various temperatures determinethe service temperature of composites. Most metals,ceramics and compounds can be used with matricesof low melting point alloys.

itanium, 9luminium and magnesium useful for


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eramic matri" materials

=igh melting points, good corrosion resistance,stability at elevated temperatures and highcompressive strength, render eramic-based matri"materials a favourite for applications requiring a

structural material that doesn7t give way attemperatures above DDI.

$aturally, ceramic matrices are the obvious choice for

high temperature applications.


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arbon matrices

arbon and graphite have a special place in compositematerials options, both being highly superior, hightemperature materials with strengths and rigidity thatare not affected by temperature up to +ADDI.

arbon-carbon composites are not be applied inelevated temperatures, as many composites haveproved to be far superior at these temperatures.=owever, their capacity to retain their properties atroom temperature as well as at temperature in the

range of +BDDI and their dimensional stability ma#ethem the oblivious choice in applications related toaeronautics, military, industry and space.


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2lass matrices

: n comparison to ceramics and even considered ontheir own merit, glass matrices are found to be morereinforcement-friendly.

he various manufacturing methods of polymers can

be used for glass matrices. 2lass matri" composite with high strength and

modulus can be obtained and they can be maintainedupto temperature of the order of JDI.

omposites with glass matrices are consideredsuperior in dimensions to polymer or metal system,due to the low thermal e"pansion behaviour.


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=ybrid composite

5btained by using two or more different #inds of bers in asingle matri";

hybrids have a better all around combination of propertiesthan composites containing only a single ber.

the most common system, both and glass bers areincorporated into a polymeric resin. he bers are strongand relatively stiff and provide a low-density reinforcement;however, they are e"pensive. 2lass bers are ine"pensiveand lac# the stiffness of carbon.

he glassHcarbon hybrid is stronger and tougher, has ahigher impact resistance, and may be produced at a lowercost

applications! lightweight land, water, and air transportstructural components, sporting goods, and lightweight

orthopedic components.


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*einforcements incomposites


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*einforcing constituents in composites provide thestrength heat resistance or conduction, resistance tocorrosion and provide rigidity.

*einforcement can be made to perform all or one of

these functions as per the requirements. 9 reinforcement that embellishes the matri" strength

must be stronger and stiffer than the matri" andcapable of changing failure mechanism to theadvantage of the composite the ductility should beminimum or even nil the composite must behave asbrittle as possible.


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reinforcement classification


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' /article-reinforced composites

he dispersed phase is equia"ed particle dimensionsare appro"imately the same in all directions'

Filler improve properties, replace with cheapermaterial

/articulate phase is harder &stiffer than matri"

he particles tend to restrain movement of the matri" he matri" transfers the stress to particles which bear

of load :mprovement depends on bonding at matri"-particle

interface

/articles may variety in geometries ?ffective reinforcement! small & distributed

throughout the matri"

@arge-particle and dispersion-strengthenedcomposites are the two subclassications ofparticle-reinforced composites.


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oncretelarge particle composites'

matri" & dispersed material are ceramic

cement matri"' & sand & gravel particles'

:n a broad sense, concrete implies a composite material consistingof an aggregate of particles that are bound together in a solid bodyby some type of binding medium, that is, a cement

he two most familiar concretes are those made with portland andasphaltic cements, where the aggregate is gravel and sand correct proportion to achieve opt. strength

9sphaltic concrete is widely used primarily as a paving material,whereas portland cement concrete is employed e"tensively as astructural building material.

0ense pac#ing & good interfacial contact + aggregate si1es

4and fill void space between gravel

4ufficient cement to coat all the aggregates; too little water incomplete, too much e"cessive porosity


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*einforced concrete

:ncrease the strength of concrete steel rods wire bars;embedded into fresh & uncured concrete

hus, the reinforcement renders the hardened structurecapable of supporting greater tensile, compressive, andshear stresses. ?ven if crac#s develop in the concrete,considerable reinforcement is maintained.

4teel coef. hermal e"pansion similar to concrete, is notrapidly corroded in cement environment, strong adhesivebond with concrete

s teel is not rapidly corroded in the cement environment,and a relatively strong adhesive bond is formed between itand the cured concrete. his adhesion may be enhanced bythe incorporation of contours into the surface of the steelmember


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+' Fiber-reinforced composites he mechanical characteristics of a ber-reinforced

composite depend not only on the properties of the ber,but also on the degree to which an applied load istransmitted to the bers by the matri" phase

4mall diameter fiber is better with high tensile strength

he matri" transmit the load to fibres which absorb the

stress 9n important characteristic of most materials, especially

brittle ones, is that a small diameter ber is much strongerthan the bul# material.

the probability of the presence of a critical surface Kaw thatcan lead to fracture diminishes with decreasing specimenvolumev advantage in the ber-reinforced composites.

?ffective strength! f :88:c continue',orientation of fiber,fiber concentration and distribution'

brous materials are generally either polymers or ceramics

e.g., the polymer aramids, glass, carbon, boron, aluminumo"ide, and silicon carbide'


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%ith respect to orientation, two possible orientation of fiber!

. parallel alignment of the longitudinal a"is of the fibres in thesingle direction

+. totally random allighment ontinuous bers are normally aligned a), whereas

discontinuous fibers may be aligned (b), randomlyoriented c), or partially oriented.

Better overall composite properties are reali1ed when the

ber distribution is uniform.


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A'4tructural composites

normally composed of both homogeneousand composite materials,

depend not only on the properties of theconstituent materials but also on thegeometrical design of the variousstructural

@aminar & sandwich panel

@9M:$9* 5M/54:?4

- composed of +0 sheets6panels

- layers are stac#ed & cemented together ;

designed for high-strength

- cotton,paper, woven glass fiber in plasticmatri"

- application! plywood, formica

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49$0%:= 4*LL*?4 considered to be a class of structural composites, are

designed to be light-weight beams or panels havingrelatively high stiffnesses and strengths.

two outer sheets, or faces, that are separated by andadhesively bonded to a thic#er core

5uter sheet thic# enough, stiff & strong material; impart

high stiffness and strength to the structure, and must bethic# enough to withstand tensile and compressivestresses that result from loading! aluminium alloy,titanium, steel, plywood

ore lightweight, lower elasticity, high stiffnesses &

strength


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ore type! polymeric foams, wood, honeycombs

ensile & compressive stresses of core C the faces

ore functions!

provide continuous support for the faces

+ transverse stress strength

A provide high stiffness thic#

9pplication! roofs, floors, wall of building,aerospace

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