Composite Written Report

Pamantasan ng Lungsod ng MaynilaCollege of Engineering and TechnologyDepartment of Chemical Engineering

What are Composite Materials?

Composite is a shortened word for composition of material these are materialsthat are made from two or more constituent materials with different physical andchemical properties, this material is considered to be any multiphase material thatexhibits a significant proportion of the properties of both constituents phases. Accordingto the Principle of Combined Action, better property combinations are made by thecareful combination of two or more distinct materials.

Example:

This is a good illustration to describe what composite materials are, the pictureabove shows a Composite Rotor blade, this blade is made up of ultrasonic profiling ofpartially reinforced cured fiber, reinforced plastics or also termed as pre-preg. By usingdifferent types of materials in this blade, desirable characteristics and properties areachieved, for example in this material good strength-to-density ratios, which are four tosix times greater than those of the aluminum and steel is achieved, other than that thiscomposite rotor blade is 45% lighter than those blades that are made up by puremetals. Another illustration is that of natural fiber which occurs in nature like wood, referto figure below.

COMPOSITES: GROUP IV Page 1


Wood consists of flexible cellulose fibers surrounded and held together by a stiffermaterial which is lignin. Lignin is a polymer that holds together the cellulose andhemicelluloses components of woody biomass. Lignin constitutes about 15 to 25percent of the weight of woody biomass. This reflects the chemical complexity of lignin.

Another example is the bone:

Human compact bone is a composite of the strong but consists of soft proteincollagen and the hard, brittle material apatite. Thus Composite in its context is amultiphase material which is artificially made, as compared to those that occursnaturally. Composites being a multiphase should take the consideration that thesephases should be chemically dissimilar and must be separated by different interface.

Examples of engineering use of composites date back to the use of straw in clayas a construction material by the Egyptians. Modern Composites using fiber-reinforcedmatrices of various types have created a revolution in high performance structures inrecent years. Now we have Advanced composite materials in which it offers important



advantages in strength and stiffness coupled with light weight, relative to conventionalmetallic materials. The cost competitiveness of composites depends on how importantthe weight reduction or environmental resistance provided by the composites is to theoverall function of the particular application.

Composites are classified into three main divisions: Particle-reinforced, Fiber-reinforced, and Structural composites and each classification have at least twosubdivisions.

Particle Reinforced Composites

First Particle Reinforced Composites, this type is the cheapest and the mostwidely used, composites of this type have a large volume fraction of particle dispersedin the matrix and the load is shared by the particles and the matrix. Most commercialceramics and many filled polymers are under this classification. Large Particle andDispersion-strengthened composites are the two sub classifications of this type; thedifference between the two is based on reinforcement or strengthening mechanism.



Large-Particle Composites

When we say Large, this term is used to indicate that the particle-matrixcannot be treated on the atomic or molecular level; most of these composites,the particulate is harder and stiffer than the matrix that tends to restrainmovement of the matrix phase in the surrounding area of each particle, meaningthe matrix transfers some of the applied stress to the particles, which bear afraction of the load. The strong bonding at the matrix-particle interfacedetermines the degree or improvement of mechanical behavior. PolymericMaterials at which fillers are added are considered to be under this type for thefillers modify or improve the properties of the material and/ or replace some ofthe polymer volume with a less expensive material.

Particles have a variety of geometries, but they have the same dimensionsin all directions or which is termed as equiaxed. Effective reinforcement is said tobe small and evenly distributed throughout the matrix. The increase inparticulate content enhanced the mechanical properties, also the volume fractionof a certain particle influence the behavior of the two phases. Elastic Modulus isdependent on the volume fraction of the constituent phases for a two-phasecomposite which introduce the mathematical expression or the rule of mixturesequations which predicts that the elastic modulus should fall between an upperbound represented by:

Where:

Ec: elastic modulus of composite, Ep: elastic

modulus of particle, Em: elastic modulus of matrix, Vm: volume fraction of matrix, Vp:

volume fraction of particle



Figure 16.3 (Callister)Modulus of elasticity VERSUS volume percent tungsten for a composite of tungsten particles dispersed within a copper matrix. Upper and lower bounds are according to the the mixture equation

In this graph Tungsten is the particulate phase; experimental; data points fallbetween two curves, going back to the rule of mixture that states, that the elasticmodulus should fall between an upper bound and the lower bound, thus makingTungsten within a copper matrix a good mixture.

Large-Particle composites areused with all three material types (metals,polymers, and ceramics). Cermets areexamples of ceramic-metal composites. Themost common of it is the Cemented Carbide,which is usually made up of hard particles ofa refractory carbide ceramic such asTungsten carbide (WC) or Titanium Carbide(TiC) embedded in a matrix of a metal likeTungsten and Nickel. This material is used as a cutting tool. Large volume of theparticulate phase may be used like of 90% volume, making the abrasive action of thecomposite is maximized.

Dispersion-Strengthened Composite

For the Dispersion strengthened composites, particles are much smallerwith diameters between 0.01 & 0.1 m (10 & 100nm).Particle-matrix interactionsthat lead to strengthening occur on the atomic or molecular level. In this type of



composite the matrix bears the major portion of an applied load, the smalldispersed particles slows down the motion of dislocations thus it restricts thematerial to plastic deformation. Metals and Metal alloys may be strengthened andhardened by the uniform dispersion of several volume percent of fine particles ofa very hard and inert material. This phase can be of metallic or nonmetallicwherein oxide materials are often used. The strengthening mechanism of thismaterial involves the particles and dislocations within matrix.

FIBER- REINFORCED COMPOSITES

This type is the most important composites in which the dispersed phase is in theform of Fiber, material of this classification is expected to have high strength and ofstiffness of a weight basis. These characteristics may be expressed in terms of SpecificStrength which is the ratio of Tensile strength to specific gravity, specific strength is alsotermed as breaking length or self support length, this is the maximum length of a verticalcolumn of the material with fixed constant cross sectional area that could suspend itsown weight when supported only at the top. The materials with the highest specificstrengths are typically fibers such as carbon fiber, glass fiber and various polymers, andthese are frequently used to make composite materials (e.g. carbon fiber-epoxy). Thesematerials and others such as titanium, aluminum, magnesium and high strength steelalloys are widely used in aerospace and other applications where weight savings areworth the higher material cost.

Specific Strength of Some Materials

MATERIAL TENSILE STRENGTH(MPa)

Specific Gravity(g/cm3)

SpecificStrength(kN*m/kg)

Concrete 12 2.30 5.22

Rubber 15 0.92 16.3

Copper 220 8.92 24.7

Polypropylene 25-40 0.90 28-44



Brass 580 8.55 67.8

Nylon 78 1.13 69.0

Oak 90 0.78-0.69 115-130

Magnesium 275 1.74 158

Aluminum 600 2.80 214

Stainless Steel 2000 7.86 254

Titanium 1300 4.51 288

Carbon-EpoxyComposite

1240 1.58 785

Glass Fiber 3400 2.60 1307

The next characteristic is in terms of Specific Modulus, which is the ratio ofmodulus elasticity to specific gravity, which is also termed as stiffness to weight ratio orspecific stiffness. The use of this characteristic is to find materials that will producestructures with minimum weight.

INFLUENCE OF FIBER LENGTH The performance of a fiber composite is judged by its length, shape,

orientation, composition of the fibers and the mechanical properties of the matrix.The first one is that of the length in which this is the degree in applied load is



transmitted to fibers by the matrix phase. Under an applied stress, this fibermatrix bond stops at the fiber ends that yield a matrix deformation pattern.

Above is the picture of the deformation pattern in the matrix surrounding afiber which is in the case of an applied tensile load.

Meaning there is no load transmittance from the matrix at each fiber thusthe deformation occurs only at the matrix phase because the fiber can resists theapplied tensile load.

Another thing that is important for effective strengthening and stiffening ofthe composite material is that of the Critical Fiber Length which is dependent on thefiber diameter d and its ultimate (or tensile) strength, and on the fiber-matrix bondstrength which is represented by a mathematical formula:

lc= critical length f = tensile strength of the fiber

d = diameter of the fiber tc = shear strength of the bond between the matrix and the fiber

Stress Position Profile



When a stress is equal to tensile strength applied to the fiber it will usually resultto a maximum fiber load which is achieved only at the axial center of the fiber. As shownin figure (a) in which the fiber length is equal to the critical length in figure (b) the fiberlength is greater than the critical length and in figure (c) the fiber length is less than thecritical length for a fiber reinforced composite that is subjected to a tensile stress equalto the fiber tensile strength. In conclusion to this as the fiber length increases the fiberreinforcement becomes more effective.



INFLUENCE OF FIBER ORIENTATION AND CONCENTRATION

Another factor that influence the strength and other properties of fiberreinforced composites is that of its orientation and concentration. When it comes toorientation, two extremes are possible: (1) A parallel alignment of the longitudinal axis ofthe fibers in a single direction and (2) Totally random alignment.

Continuous Fibers are normally aligned as shown in Figure (a) whereindiscontinuous fibers are partially aligned as shown in Figure (b) and in the last Figure(c) it is discontinuous and randomly oriented. Composite is on its better properties whenthe fiber distribution is uniform.

Continuous and Aligned Fiber Composites

Tensile stress-strain behavior-longitudinal loading



Above is the graph of the stress-versus-strain behaviors for fiber and matrixphases. In the first graph, figure (a) we interpret the fiber to be totally brittle and thematrix phase to be ductile. On the second graph there is uniaxial stress-strainbehaviour. In the initial Stage I region, both fibers and matrix deform elastically, normallythe graph is linear. A composite of this type, the matrix yields and deforms plastically,while the fibers continue to stretch elastically thus the tensile strength of the fibers issignificantly higher than the yield strength of the matrix. This is shown in the Stage II.Also the failure of the composite begins as the fibers start to fracture. Composite failureis not catastrophic or in which there is a sudden damage because not all fibers fractureat the same time, because there will always be considerable variations in the fracturestrength of brittle fiber materials. Even after the fiber failure the matrix is still intact withthe particulates.

ELASTIC BEHAVIOR- LONGITUDINAL LOADING

Considering the elastic behavior of a continuous and oriented fibrouscomposite that is loaded in the direction of fiber alignment. First, it is assumed that thefiber-matrix interfacial bond is very good, in which it exhibits an isostrain situation, in thissituation the matrix and fibers is the same. Under these conditions, the total loadsustained by the composite Fc is equal to the sum of the loads carried by the matrixphase Fm and the fiber phase Ff, or

From the definition of stress in which F= A ; and thus expression for Fc, Fm and Ff in

terms of their respective stresses ( c, m and f ) and their respective cross

sectional areas (Ac, Am and Af ) are possible. When we substitute this, it will be:

Then we can divide this equation to cross sectional area:



where Am / Ac and Af /Ac are the area ratio of the matrix and fiber phase. If the composite,matrix, and fiber phase lengths are all equal, Am / Ac is equivalent to the volume fractionof the matrix Vm and also in the Fiber Vf= Af / Ac , then we can have a new equation

In conclusion the isostrain can be expressed as:

then lets divide it by their respective strain:

Furthermore, if composite, matrix and fiber deformations are all elastic, then c c=Ec ,

mm

=Em and ff

=Ef , and the Es being the modulus of Elasticticity for the

respective phases, Again substituting it, we have a new equation for the modulus ofelasticity of a continous and aligned fibrous composite in the direction of alignment (orlongitudinal direction)



because the composite composed of only matrix and fiber phases: that is Vm + Vf = 1.

Thus, the Ecl is equal to the volume-fraction weighted average of the moduli ofelasticity of the fiber and matrix phases. Other properties, including density, also havethis dependence on volume fractions. Thus for longitudinal loading, this is the formula:

Elastic Behavior-Transverse Loading

Continous and oriented fiber composite may be loaded in the transversedirection: in which the load applied is perpendicular to the direction of fiber alignment. In

this situation the stress to which the composite as well as both phases

This is an isostress state. Thus the total strain derformation of the composite c is

but, because = /E



in which Ect is the modulus of elasticity in the transverse direction. Again by dividing

through the stress yields

which simplifies the equation to

LONGITUDINAL TENSILE STRENGTH

For the Longitudinal Tensile Strength we take in to the consideration of thestrength characteristics of continous and aligned fiber reinforced composites which areloaded to longitudinal direction. In these situation the strength is normally taken as themaximum stress on the stress-strain curve.

Typical Longitudinal and Transverse Tensile Strengthsfor Three Unidirectional FiberReinforced Composites.The Fiber Content for Each Is Approximately 50 Vol%

MATERIAL LONGITUDINAL TENSILE Transverse Tensile Strenght



STRENGHT (MPa) (MPa)

Glass-Polyester 700 20

Carbon(high modulus)-epoxy 1000 35

Kevlar- epoxy 1200 20

Source: D. Hull and T.W. Clyne, An Introduction to Composite Materials, 2ndedition, Cambridge University Press, New York, 1996, p. 179.

Failure of this type of strength of a composite material is a relatively complex process,several failures are possible .The failure mode that operates for a specific composite willdepend on the fiber and matrix properties and the nature and strength of the fiber-matrixinterfacial bond. The equation for the longitudinal strength of this type of composite is:

'm is the stress matrix at fiber failure and f is the fiber tensile strength.

Transverse Tensile Strength

The strengths of continous and unidirectional fibrous composite materials arehighly anisotropic. With the condition of having a Transverse Tensile Load it may resultto a premature failure in which the transverse strength is very low usually that it liesbelow the tensile strength of the matrix giving the reinforcing effect of the fiber to benegative. The properties of the fiber and the matrix, the fiber matrix bond strength, andthe presence of the voids have a significant effect on the transverse strength; usuallymodifying the properties of the matrix improves the transverse Length.

DISCONTINOUS AND ALIGNED FIBER



Compared into continuous fiber the reinforcement efficiency of thediscontinuous and aligned fiber is lower but it is now becoming increasingly importantinto the market. Usually this type of material can give a modulus of elasticity to a valueapproaching to 90% and 50% tensile strength, of their continuous-fiber counterparts.

in situations in which l>lc the longitudinal strength is given by the formula:

in cases in which l


and randomly oriented glass fibers and also it will give an idea of the magnitude of thereinforcement that is possible.

Property Unreinforced Fiber reinforcement (vol %)

20 30 40

Specific Gravity 1.19-1.22 1.35 1.43 1.52

Tensile Strength (MPa) 59-62 110 131 159

Modulus of Elasticity (GPa) 2.24-2.345 5.93 8.62 11.6

Elongation (%) 90-115 4-6 3-5 3-5

Impact Strength, notchedIzod (N/cm)

21-28 3.5 3.5 4.4

Source: Adapted from Materials Engineerings Material Selector

It is good to say that fibrous composites are inherently anisotropic in that the maximumstrength and reinforcement are achieved along the alignment (longitudinal) direction. Inthe transverse direction, fiber reinforcement is virtually nonexistent: fracture occurs atrelatively low tensile stresses.

STRUCTURAL COMPOSITES- this type of composite is normally composed of both homogeneous and

composite materials, the properties of which depend not only on the constituents butalso on the geometrical design of the various structural elements. The two subdivisionsfor this type are laminar composites and Sandwich Panels.



LAMINAR COMPOSITES

Source: Figure 16.16 from Callister

This type of composite is composed of two-dimensional sheets or panels they have apreferred high-strength direction like in wood and continuous and aligned fiber-reinforced plastics. The layers are stacked and subsequently cemented together.

SANDWICH PANEL



Source: Figure 16.17 Callister

These types of composites are designed to lightweight beams or panels havingrelatively high stiffness and strengths.

Sandwich Panels are composed of two outer sheets, or faces, which areseparated by and adhesively bonded to a thicker core. The outer sheets are usuallymade up of aluminium alloys, fiber-reinforced plastics, titanium, steel or plywood thesematerials are relatively stiff and strong and also thick enough to withstand tensile andcompressive stresses result from loading. The core material is lightweight and has a



low modulus of elasticity normally these have three categories: rigid polymeric foams( phenolics, epoxy, polyurethanes), wood (balsa wood) and honey combs. These corematerial functions as a continuous support for the faces so it should have enough shearstrength to resist transverse shear stresses and also be thick to offer high shearstiffness. The figure above shows a honeycomb structure- these are thin foils that havebeen formed form interlocking hexagonal cells, with axes oriented perpendicular to theface planes. The honeycomb is usually made from an aluminium alloy or aramidpolymer. The strength and stiffness is dependent on cell size, cell wall thickness, andthe material from which the honeycomb is made.

PROCESSING OF COMPOSITES

Introduction

Since we all know what the composites are: Composite materials that arematerials made from two or more constituent materials with significantly differentphysical or chemical properties, that when combined, produce a material withcharacteristics different from the individual components.

We are sure that you are very curious about how is it being processed. Nowcomposites are being process through different methods, just like in the processing ofmetals, ceramics and polymers which we have discussed earlier.

A wide range of different processes have developed for moulding of compositesparts ranging from very simple manual processes such as hand lay to verysophisticated highly industrialised processes such as SMC moulding. Each process hasits own particular benefits and limitations making it applicable for particular applications.The choice of process is important in order to achieve the required technicalperformance at an economic cost.

The main technical factors that govern the choice of process are the size andshape of the part, the mechanical and environmental performance and aesthetics. Themain economic factor is the number of identical parts required or run length. This isbecause composite parts do not generally come as standard components but are



custom designed for a particular application. Pultrusion and continuous sheeting areexceptions but most processes will have an initial investment or set up cost that must beamortised over the length of the project. This is a major factor in the choice of processand is one of the reasons for the proliferation in processing.

methods.

Open Moulding - Hand and Spray Lamination

Open moulding is by far themost common process used tofabricate composites partsaccounting for over 40% ofcomposites processed world-wide.It is a relatively simple processwith low investment cost but ahigh degree of manual handling.Virtually all types ofreinforcement can been used inopen moulding which together

with the use of core materials to create sandwich structures enables access to thewidest range of mechanical and structural performance of any compositesprocess. Unsaturated polyester resins dominate in this area but epoxy and vinyl esterresins are also common. Open moulding can be used for a very wide range ofmouldiqngs from caravan parts and cladding panels to boat hulls and radomes. Typicaleconomic run lengths range from 2 or 3 individual parts up to several hundred.


Process Composites

mechanical and

environmental performance

Aesthetics

Shape

size


Hand Lamination

The process starts with the construction of a mould. The mould is most commonlyconstructed from composites material using a model made from wood, plaster or anyother suitable modelling material. Only one, normally female, mould half is neededwhich defines the exterior surface of the part. Most often the first stage of moulding is tobrush or spray a polymer coating or gel coat onto the mould surface. Gel coats areavailable in a wide range of different colours and effects and are selected to protect thepart from environmental degradation or chemical attack and provide the desiredaesthetics.

After the gel coat has been allowed to cure fibre reinforcement in sheet form is laid inplace in the mould. Glass fibre is the most common reinforcement in the form ofchopped strand mat but other fibres such as carbon or aramid may be used. A very widerange of reinforcement types are available and can be positioned and oriented in themould giving the ability to vary the mechanical performance across the part. The designof the glass fibre pack is a key to the performance of the composite part and the abilityto change it at will gives great flexibility to the hand lay process.

The next stage of the process is to pour liquid catalysed resin over the reinforcementand to work it into the reinforcement using rollers. This process is very labour intensivebut extremely important as it ensures even distribution of the resin, full impregnationand wetting out of the reinforcement and removal of air. Unless the process is carriedout effectively the composite part will not perform correctly. Further layers ofreinforcement and resin are applied according to the requirements of the part and corematerials such as rigid foam, balsa wood or honeycomb may be included to createsandwich structures.

When the lay up is complete the moulding is left to cure. This normally occurs atambient temperature and can take anything up to 10 hours. The moulding is thenreleased from the mould and trimmed toremove excess material from the edges ofthe moulding. Sometimes mouldings arecured at slightly elevated temperatures toimprove speed and productivity and mayalso be post cured at even highertemperatures to achieve the maximumperformance.



Spray Lamination

Spray lamination is very similar to hand lay-up apart from the method of placing thereinforcement and the resin into the mould. Rather than using reinforcement in the formof mat, resin and reinforcement are co-sprayed into the mould using a combined resinspray and chopper gun. This process is much easier and faster than hand lay-up andcan be automated using robotised spray guns. This does eliminate much of the manuallabour involved in positioning the reinforcement in the mould and will result in higherproductivity although rolling is still necessary to ensure proper consolidation of the part.Spray lamination can be used in combination with hand lay where different types of fibreor constructions are required to achieve specific properties.

Vacuum Infusion

Vacuum infusion (VI) dates back tothe 1950's when patents for a broadly

similar process were published in theUSA. The process however has come toprominence in recent years due toincreasing pressure on the control ofVOC emissions from the open mould

process. The basic principal of VI is that reinforcing fibres are placed in a mould, whichis sealed using a plastic film, or vacuum bag and resin is drawn into the mould undervacuum. Moulds for VI are fitted with a peripheral channel to enable vacuum to beapplied and catalysed resin is fed in at the centre of the part and allowed to diffusethrough the reinforcement to the edge of the mould.

Design of the reinforcement and setting up of the plastic film or vacuum bag, whichnormally incorporates tubes or channels to help even distribution of the resin, isabsolutely critical. However once optimised a major advantage of the VI process is thatit can be reproduced exactly each time without the need for the use of skilledlaminators. The mould is also fully enclosed during the moulding process virtuallyeliminating VOC emissions. A further advantage of the use of vacuum is that parts areextremely well consolidated, even at high fibre content, with very low air content givingvery good structural performance.

One disadvantage of VI is that the excellent consolidation favours thin high fibre contentparts, which may not have sufficient stiffness. To combat this effect core materials of



various types can be used to bulk the moulding out and give the required thickness. Afurther complication with VI is that whilst the lamination process itself is very quick thesetting up of the mould with the bag and resin distribution network is time consumingand costly as these materials are thrown away.

Typically VI is used for parts where high performance and quality are the main criteriaand where the extra cost associated with the disposable elements can be supported.The process has been used particularly in the marine industry for boat hulls and mastsbut is still very much in its infancy.

Resin Injection

Resin injectionmoulding accountsfor some 5% ofcompositesprocessingworldwide and isgrowing due againto increasingregulation of VOCemissions but alsothe drive for moreautomated higherproductivityprocesses. Thereare a number of

different versions of the Resin Injection Moulding of which the most widely known isResin Transfer Moulding (RTM); although all are based essentially on similar principals.RTM uses two matched mould halves to create a cavity that defines the shape of thepart. The mould is opened and a gel coat applied to one or both mould halves ifrequired. Dry reinforcement is placed in the mould and resin is injected into the cavitywetting out the fibre. The part is then allowed to cure, themould opened and the finished mouldingremoved.

Resin Transfer Moulding



The basic set up for RTM moulding is a set of matched moulds and a resininjection machine. Moulds can be constructed from composites materials but other moredurable materials such as aluminium, electroformed nickel and steel have also beenused. Mould accuracy and strength are important in RTM as the two mould halves needto be well matched to ensure consistent part thickness and be sufficiently rigid towithstand the pressure generated during the injection process.

Moulds can also become hot during processing and may be fittedwith heating elements to speed up moulding so mould materials need tobe heat resistant. A range of injection machines is available for the RTMprocess ranging from simple pumps that inject pre-catalysed resin tosophisticated mixing and metering systems which incorporate automaticinjection sealing and cleaning.

The RTM process is much faster with a far lower manual labour content than openmoulding and has the advantage that parts have two smooth sides and can beproduced under controlled conditions with very little VOC emission. Core materials caneasily be incorporated as part of the RTM process and the development of low profilesystems enable very smooth good quality finishes to be achieved.

The disadvantage of RTM is that compared to open moulding the initialinvestment cost is much higher requiring a more expensive mould and a resin injectionmachine. The higher investment cost of RTM means that the minimum economic runlength is probably of the order of 500 parts ranging up to about 3000 parts/annum.

Typical parts manufactured by RTM include lorry, bus and car parts where thetwo finished sides, tighter tolerances and higher volume manufacture are important.Some large mouldings have been produced in RTM but generally as the mouldsincrease in size beyond about 6 square metres the costs of making the mouldsufficiently rigid and the size of injection machine needed make the costs prohibitive.Simpler versions of RTM have been developed using lighter weight moulds and vacuumeither on its own or with some positive pumping in order to limit costs and enable largermouldings to be produced economically at lower volume. Some of these processeshave been given names such as RTM Lite, VARI and Vacflo but are all based on similarconcepts.



Prepregs -Vacuum Bag And Press Moulding

Prepregs are very often associated with high performance aerospaceor sporting applications although they have

more recently found application in architectural mouldings andinfrastructure repair. Several types of prepreg are

available but most commonly they arebased on epoxy resin and carbon fibres

although other resin fibre combinations arenot uncommon. A prepreg is basically areinforcement material that has been pre-

impregnated with resin using a specially adaptedimpregnation machine. The material is supplied as a sheet by the prepregmanufacturer ready for use by the moulder. There are two basic methods for mouldingprepregs namely Vacuum Bag or Press moulding.

Vacuum Bag Moulding

There are two main versions of the vacuum bag process for prepregs both ofwhich start in a similar way. Both use a single sided mould onto which sheets or strips ofthe prepreg are laid according to a specified pattern. This pattern can be complex and isdesigned to meet the mechanical requirements of the finished part. Precise positioningof the prepreg is required and can either be achieved by manually or, as frequentlyemployed in the Aerospace industry, by using a tape laying machines. A bleeder fabric,normally a felt of synthetic fibres, and a vacuum bag are fitted and sealed onto themould. Vacuum is then applied which fully consolidates the prepreg, squeezing excessresin out into the bleeder fabric.

In one version of this process the sealed mould is placed in a heated,pressurised chamber (Autoclave) where the part is fully consolidated and cured. In theother version of this process an autoclave is not used and the part is cured byemploying a heated mould. The advantage of an autoclave is that relatively simpletooling can be used to produce high performance parts reliably which, if the run length isshort, is an advantage. This does not of course take account of the autoclave itselfwhich is expensive but will normally be used for many different parts. The advantage of



the other process is that it is faster and simpler and whilst moulds may be moreexpensive and there is no need for an expensive autoclave.

Both processes are well adapted to producing high performance parts andbecause tooling is relatively simple they can be used effectively for even very largeparts. The cost of materials, disposables and labour is however high so that parts basedon vacuum bagged prepregs can be expensive.

Press Moulding

Where larger numbers of high performance parts are required such as in thesports market press moulding of prepregs has been developed. The basic prepregmaterial remains broadly similar but in this version of the process heated matched metalmoulds and hydraulic presses are used to stamp out parts. The process consists ofcutting and preparing a prepreg pack and then placing this pack in a heated matchedmetal mould fitted to a hydraulic press. Again the pack will be designed to meet therequirements of the part and may include several different types of prepreg as well asother materials. The press is then closed onto the prepreg forming into the shape of themould and pressurising it in the mould cavity.

The moulds are heated to a temperature of between 150 an 180C which issufficient to cure the part in from 10 to 30 minutes depending on the type of prepregused. The press moulded process is much quicker and less labour intensive than thevacuum bag process but the need for metal moulds and hydraulic presses means thatonly small to medium sized parts are viable and run lengths need to be of the order of atleast 3 - 4000. Typical applications for this type of process include skis and leaf springsfor cars and trucks.

Compression Moulding Of SMC

Compression moulding of Sheet Moulding Compound (SMC) and the closelyrelated Bulk Moulding Compound (BMC) represent the second most widely used type ofcomposites process used world-wide, accounting for more than 25% of all compositesuse in Europe and the USA. These processes are highly automated and designed tomeet the requirements of industrial users where run lengths can exceed 50,000 parts.

SMC has some similarities to prepregs in that it comes as a sheet containingboth resin and fibre; however in this case the resin is predominantly unsaturated



polyester and the fibre almost exclusively chopped glass fibre (Usually 25mm ). SMCnormally contains high levels of mineral filler and is available from the compoundsupplier in a very wide range of different formulations including different fibre contents,different surface qualities and different colours.

Similarly to prepregs SMC is moulded in heated matched metal moulds mountedin a hydraulic press but the SMC pack is prepared at the correct weight to fill the mouldcavity and cut so that it covers only 50 - 70% of the mould surface. The pack is placedin the mould which is heated to 140 - 160C and the press closed. The combination ofheat and pressure as the press closes causes the SMC to flow and completely fill themould cavity. Curing is very rapid and parts can be demoulded in as little as one-minutealthough 3-4 minutes is more typical.

Moulds for SMC are designed with a small gap or shear edge at the periphery ofthe mould that allows air to escape as the compound flows. However at 0.3 - 0.6 mmthis is so small that the compound cures as it enters the gap effectively sealing themould. This means that SMC parts require very little finishing and trimming afterdemoulding making for a very rapid automated process. A further advantage of SMC isthat because it is a flow moulding process complex features such as ribs, bosses andfixings can be moulded in rather than needing to be bonded on later.

The disadvantage of SMC is that the initial investment cost is very high whichmeans that normally a minimum of 10,000 mouldings is required to make the processviable. Also as with press moulded prepregs there is a limit on the size of mouldings dueto the high cost and difficulty in manufacture of large metal moulds and hydraulicpresses. In recent years the development of SMC type compounds which mould atlower pressures (Low Pressure Moulding Compound or LPMC) has allowed largersmaller series parts to be produced economically but SMC is essentially a medium tohigh volume process.

SMC is used in a wide variety of different applications including car and truck parts,electrical cabinets and switchgear, sectional water storage panels and modularbuildings. SMC's offer increased stiffness, and higher dimensional stability compared tomany other composites materials but generally have more variable mechanicalproperties.

BMC is similar to SMC except that it contains less glass fibre cut to a shorter fibrelength than SMC and is delivered in the form of a dough rather than a sheet. BMC is



more often used wheremechanical performance is lesscritical and for smaller morecomplex mouldings.

Pultrusion Pultrusion is aprocess whereby continuousfibres in the form of roving, tapeor fabric are impregnated withresin, pulled through a shapeddie and cured to create acontinuous profile. The process isthe equivalent to extrusion ofthermoplastic polymers or metals.Typical sections include I beams,T sections circular and squaretubing as well as more complexgeometries.

Unlike most other composites processes pultrusions are available in standardsections as well as custom made profiles and so it is relatively easy to specify aparticular standard profile from a maker's catalogue that will have a defined set ofproperties.

Pultrusions have excellent mechanical properties and are often used inaggressive or corrosive environments that would present problems for certain metals.Typical applications include access ladders and walkways on oilrigs, concretereinforcement bars, roof trusses and space frames. Like all composites they haveexcellent strength to weight ratios and can be designed to meet a variety of differentloading conditions by utilising a combination of on axis, off axis fibres and specialityfabrics.

For many applications standard pultrusions are available but it is relatively easyto create custom profiles for large projects as the pultrusion dies are not overlyexpensive. Pultrusion is a highly automated process but it is still relatively slowcompared to extrusion and the raw material cost is quite high. Pultrusions findapplication where their lightweight and corrosion resistance are key. Most recentlypultrusions have found use in bridge structures and infrastructure repair particularly in



the USA. Pultrusion accounts for about 5% of the composites market worldwide but itsuse is growing rapidly particularly in building applications.

Filament Winding

Filament winding consists of impregnating continuous roving tape or fabric with resinand applying it to a rotating mandrel. In order to achieve the required mechanicalproperties fibres are oriented at different angles resulting in complex winding patternsthat are normally computer controlled. The process is not entirely limited to cylindricalshapes and both conical tanks and box beams can be produced. When finished theresin is allowed to cure and the moulding removed from the mandrel. Filament windingcan produce very large tanks and as the process can be very precisely controlled thefibre content and orientation is very uniform giving very reliable structural performance.

More recently in an adaptation of this technology bridge piers have been strengthenedby wrapping with composite material using a modified portable version of a filamentwinder.

Centrifugal Casting

Centrifugal casting is primarily used for pipes and tanksand consists of co-spraying resin and chopped fibreonto the inner face of a rapidly rotating circular mould.The centrifugal force created by the rapid rotation of themould causes the resin to impregnate though the glassfibre forming the moulding and also results in arelatively smooth inner surface on the pipe or tank. Theprocess can also used to produce slightly tapered partssuch as masts or poles. There are some very large companies in the Middle Eastproducing large quantities of pipe by this method.

Continuous Sheeting

Continuous sheeting is produced in a factory on a dedicated machine in either flat orcorrugated profile and can be coloured or translucent. The process consists ofdepositing a layer of resin onto a moving plastic film followed by chopped glass fibre.The fibre is then impregnated with resin by rolling and the sheet passed through anCOMPOSITES: GROUP IV Page 30


oven where it is cured, usually by heating,although in some cases UV curing is employed.

In the case of corrugated sheet the corrugationsare introduced using rolls or formers during

the curing stage. The finished sheet is then eitherrolled or cut into individual lengths for packaging

and transportation Typical applications includecladding, roofing and refrigerated truck bodies

Procurement

There are certain composite parts such aspultrusions, pipes and continuous sheeting that are available in standard sizes so thatonce manufacturers have been located specification and procurement is relativelystraightforward. The vast majority of composite parts are however custom designed andmoulded to meet the needs of a particular application. As described in this paper thereis a very wide range of different composites processes and very often companies willspecialise in one or two processes. It is however rare to find a company that proposesevery process and material combination and with more than 2000 composites mouldersin the UK alone procurement can be a challenge. A useful source of information andadvice can be the raw material suppliers particularly the resin and reinforcementcompanies who will often be willing to advise on the choice of process and sometimesrecommend suitable moulders.

CLASSIFICATION AND APPLICATION OF COMPOSITES

PARTICLE REINFORCED

I. Large Particle Composites

A. Concrete

The most common large-particle composite is concrete, made of acement matrix that bonds particles of different size (gravel and sand.)

In its general from, cement is a fine mixture of lime, alumina, silica,and water. Portland cement is a fine powder of chalk, clay and lime-bearing minerals fired to 1500o C (calcinated). It forms a paste whendissolved in water. It sets into a solid in minutes and hardens slowly (takes4 months for full strength). Properties depend on how well it is mixed, and



the amount of water: too little - incomplete bonding, too much - excessiveporosity.

Reinforced concrete is obtained by addingsteel rods, wires, mesh. Steel has the advantage ofa similar thermal expansion coefficient, so there isreduced danger of cracking due to thermalstresses. Pre-stressed concrete is obtained byapplying tensile stress to the steel rods while thecement is setting and hardening. When the tensilestress is removed, the concrete is left undercompressive stress, enabling it to sustain tensile loads without fracturing.Pre-stressed concrete shapes are usually prefabricated. A common use isin railroad or highway bridges.

Advantages of cement is that it can be poured in place, it hardensat room temperature and even under water, and it is very cheap.

Disadvantages are that it is weak and brittle, and that water in thepores can produce crack when it freezes in cold weather.

Concrete is improved by making the pores smaller using finer powder, adding polymeric lubricants, and applying pressure during hardening.

B. Fillers

Fillers are less expensive materials than polymer that modify orimprove the properties of material and/or replace some of the polymervolume.

These are also called extender in which they are used to cheapenend products. Among the 21 most important fillers, calciumcarbonate holds the largest market volume and is mainly used in theplastics sector.While the plastic industry mostly consumes ground calciumcarbonate the paper industry primarilyuses precipitated calcium carbonatethat is derived from natural minerals.

Moreover, fillers also enhanceproperties of the products. In suchcases, a beneficial chemical


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interaction develops between the host material and the filler. As a result, anumber of optimized types of fillers, nano-fillers or surface treated goodshave been developed.

C. CermetsCermets are ceramic-metal composites. The most common is

cemented carbide, which is composed of extremely hard particles of arefractory carbide ceramic such as tungsten carbide (WC) or titaniumcarbide (TiC), embedded in a matrix of metal such as cobalt or nickel.

Why would you want to combine a metal and a ceramic? Metals,though versatile, aren't capable of withstanding the incredibly hightemperatures you typically encounter in airplane jet engines or spacerockets. Ceramics are brilliant at high temperatures and able to resistattack by chemicals and things.

Electrical components are one obvious application. Because theycan get extremely hot, they need to behave like ceramics but, since theyalso need to conduct electricity, it helps if they work like metals. Cermetsoffer a perfect solution in components such as resistors and vacuum tubes(valves).



Machine tools areanother increasingly commonuse forcermets. Tungsten carbide,from which many cuttingand drilling tools are made, iseffectively a cermet. Cermetsbased on titanium are anotherpopular choice for tools usedin milling, turning and boring,and for making threads andgrooves. Generally, cermets

provide higher cutting-tool speeds, better surface finish, and last muchlonger than traditional tool parts. Unlike tools coated in carbide, cermet-coated tools do not wear in the same way but effectively regeneratethemselves.

II. Dispersion-Strengthened Composites

A. Thoria-Dispersed Nickel

The Thoria- dispersed nickel alloys (TD Nickel) contain thoriumoxide additions (~2 wt %) for increased elevated temperature strength upto 1200 C. Dispersion strengthened nickel was one of the firstcommercially available dispersion strengthened materials.

In these powdermetallurgy alloys, either or nickelor a 78Ni-20Cr alloy is mixedwith a fine dispersoid of Thoria(ThO2). Thoria contents rangefrom 1.80 to 2.60% ThO2, withthe normal content being 2.0%ThO2.

Commonly referred to asTD Nickel or TD NiCr, thesealloys were developed for use incomponents in combustionsystems of advanced gas turbine engines, fixtures for high-temperature


Cutting tools made from cermets last longer and produce abetter surface finish than traditional carbide tools. Photo by Eduardo Zaragoza courtesy of US Navy.


tensile testing, and specialized furnace components and heatingelements.

There are several disadvantages associated with TD Nickel that have limited its commercial viability.

This material has poor oxidation resistance and should be coated for long term high temperature surface stability.

It is difficult to process and cannot be hot worked, and

It is mildly radioactive (thorium is an actinide metal).

As a result of the problems, production of TD Nickel has ceased. Today Yttria (Y2O3) dispersion strengthened nickel-base superalloys are being used for elevated temperature applications.

B. Sintered-Aluminum Powder (SAP)

Sintered aluminum powder alloys haveproperties quite different from those ofmaterial fabricated by conventionaltechniques. The oxide that formsimmediately on the surface of aluminum isnot reduced back to metal during sinteringand the resulting powder product containsa substantial amount of oxide. This oxideprevents grain growth and movement ofdislocations at the boundaries or throughthem and produces high strength, highcreep resistance and insensitivity to high-temperature exposure.

The material properties depend on theamount of naturally formed oxide. Heatingpowder to increase the thickness of theoxide film or addition of Al2O3 powder,however, does not increase strength andonly reduces ductility.

The oxide is present as finely dispersed particles, which interactwith vacancies and dislocations and prevent their easy movement. Theoxide effect persists even above the melting point of aluminum.


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Another important characteristic of sintered aluminum powders istheir insensitivity to high temperature: exposure for several years attemperatures up to 800 K produces practically no change in structureor properties, especially in the higher-oxide-content alloys thats why itcan be applied in gears.

C. Modern Rubbers

Carbon black has been used as a reinforcing agent in tires. Today,the uses of carbon black have expanded to include acting as apigmenting, UV stabilizing and conductive agent in a variety ofcommon and specialty products, including tire innerliners, carcasses,sidewalls and treads, as well as in industrial rubber products, like belts,hoses and gaskets.

FIBER-REINFORCED COMPOSITES

I. Fiber Phase

A. Whiskers

Whiskers are very thin single crystals that have extremely largelength-to-diameter ratios. As a consequence of their small size, they havea high degree of crystalline perfection and are virtually flaw-free, whichaccounts for their exceptionally high strengths; they are among thestrongest known materials. In spite of these high strengths, whiskers arenot used extensively as a reinforcement medium because they areextremely expensive. Moreover, it is difficult and often impractical toincorporate whiskers into a matrix. Whisker materials include graphite,silicon carbide, silicon nitride, and aluminum oxide; some mechanicalcharacteristics of these materials are given in Table 16.4.


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B. Fibers

Fibresconstitute the mainbulk ofreinforcements thatare used in makingstructuralcomposites. A fibreis defined as amaterial that has theminimum 1/d ratioequal to 10:1, where1 is the length of thefibre and d is itsminimum lateraldimension. Thelateral dimension d(which is thediameter in the case of a circular fibre) is assumed to be less than 254m. The diameter of fibres used in structural composites normally variesfrom 5 m to 140 m. A filament is a continuous fibre with the l/d ratioequal to infinity.

From the micro-structure point of view, fibres can be amorphous(glass), polycrystalline (carbon, boron, alumina, etc.) or single crystals(silicon carbide, alumina, beryllium and other whiskers). The strength andstiffness properties of a fibre are significantly higher compared to those ofthe bulk material from which the fibre is formed. Most of the commonfibres are brittle in nature. The tensile strength of bulk brittle material isconsiderably lower than the theoretical strength, as it is controlled by theshape and size of a flaw that the bulk material may contain. As thediameter of a fibre is very small, a flaw, it may contain, must be smallerthan the fibre diameter. The smaller flaw size, in turn, reduces thecriticality of the flaw and thereby the tensile strength is enhanced. Forexample, the tensile strength of an ordinary glass (bulk) may be as low as100-200 MPa, but that of a S-glass fibre may be as high as 5000 MPa.However, the tensile strength of a perfect glass fibre, based on



intermolecular forces, is 10350 MPa. Further, the orientation of crystallitesalong the fibre direction also helps considerably in improving the strengthproperties.

Both inorganic and organic fibres are used in making structuralcomposites. Inorganic fibres (including ceramic fibres) such as glass,boron, carbon, silicon carbide, silica, alumina, etc. are most commonlyused. The structural grade organic fibres are comparatively very few innumber. Aramid fibres are the most popular organic fibres. Another recentaddition is a high strength polyethylene fibre (Spectra 900) which has avery low density and excellent impact resistant properties. The carbonfibres may also be grouped with organic fibres, although they are moreoften considered as ceramic (inorganic) fibres. Inorganic fibres in generalare strong, stiff, thermally stable and insensitive to moisture. They exhibitgood fatigue resistant properties, but low energy absorptioncharacteristics. Organic fibres, on the other hand, are cheaper, lighter andmore flexible. They possess high strength and better impact resistantproperties.

Other Fiber Reinforcement Materials

Other fiber materials that are used to much lesser degrees areboron, silicon carbide, and aluminum oxide; tensile moduli, tensilestrengths, specific strengths, and specific moduli of these materials in fiberform are contained in Table 16.4. Boron fiberreinforced polymercomposites have been used in military aircraft components, helicopterrotor blades, and some sporting goods. Silicon carbide and aluminumoxide fibers are used in tennis rackets, circuit boards, military armor, androcket nose cones.

C. Wires

A wire is a single, usually cylindrical, flexible strand or rod of metal.Wires are used to bearmechanical loads or electricity andtelecommunications signals. Wire iscommonly formed by drawing the metal through a hole in a die or drawplate. Wire gauges come in various standard sizes, as expressed in termsof a gauge number. The term wire is also used more loosely to refer to a



bundle of such strands, as in 'multistranded wire', which is more correctlytermed a wire rope in mechanics, or a cable in electricity.

Fine wires have relatively large diameters; typical materials includesteel, molybdenum, and tungsten. Wires are used as a radial steelreinforcement in automobile tires, in filament-wound rocket casings, andin wire-wound high-pressure hoses. The high pressure steel wirebraided hose is mainly used for hydraulic power transmission or fordelivery of water, gas, oil and other high pressure media at operatingtemperature of -40- +100.

Wire comes in solid core,stranded, or braided forms.Although usually circular incross-section, wire can be madein square, hexagonal, flattenedrectangular, or other cross-sections, either for decorativepurposes, or for technicalpurposes such as high-efficiency voicecoilsin loudspeakers. Edge-wound[1] coil springs, such asthe Slinky toy, are made ofspecial flattened wire.

II. Matrix Phase


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A. Polymer-Matrix Composites (PMCs)

Polymer-matrix composites consist of a polymer resin as the matrix,with fibers as the reinforcement medium.

Two main kinds of polymers are thermosets and thermoplastics.Thermosets have qualities such as a well-bonded three-dimensionalmolecular structure after curing. They decompose instead of melting onhardening. Merely changing the basic composition of the resin is enoughto alter the conditions suitably for curing and determine its othercharacteristics. They can be retained in a partially cured condition too overprolonged periods of time, rendering Thermosets very flexible. Thus, theyare most suited as matrix bases for advanced conditions fiber reinforcedcomposites. Thermosets find wide ranging applications in the choppedfiber composites form particularly when a premixed or mouldingcompound with fibers of specific quality and aspect ratio happens to bestarting material as in epoxy, polymer and phenolic polyamide resins.

Thermoplastics have one- or two-dimensional molecular structureand they tend to at an elevated temperature and show exaggeratedmelting point. Another advantage is that the process of softening atelevated temperatures can reversed to regain its properties during cooling,facilitating applications of conventional compress techniques to mould the compounds

Polyester resins on the other hand are quite easily accessible,cheap and find use in a wide range of fields. Liquid polyesters are storedat room temperature for months, sometimes for years and the mereaddition of a catalyst can cure the matrix material within a short time. Theyare used in automobile and structural applications. The cured polyester isusually rigid or flexible as the case may be and transparent. Polyesterswithstand he variations of environment and stable against chemicals.Depending on the formulation of the resin or service requirement ofapplication, they can be used up to about 75C or higher. Otheradvantages of polyesters include easy compatibility with few glass fibersand can be used with verify of reinforced plastic accountrey. AromaticPolyamides are the most sought after candidates as the matrices of



advanced fiber composites for structural applications demanding longduration exposure for continuous service at around 200-250C .

-Classification of PMCs According to Reinforcement Type-

1. Glass Fiber-Reinforced Polymer (GFRP) Composites

Over 95% of the fibers used in reinforced plastics are glass fibers, as they are inexpensive, easy to manufacture and possess high strength and stiffness with respect to the plastics with which they are reinforced. Asa fiber it is relatively strong, and when embedded in a plastic matrix, it produces a composite having a very high specific strength.

As a fiber it is relatively strong, and when embedded in a plasticmatrix, it produces a composite having a very high specific strength.Newly drawn fibers are normally coated during drawing with a size, a thinlayer of a substance that protects the fiber surface from damage andundesirable environmental interactions. This size is ordinarily removedprior to composite fabrication and re- placed with a coupling agent or finishthat produces a chemical bond between the fiber and matrix. There areseveral limitations to this group of materials. In spite of having highstrengths, they are not very stiff and do not display the rigidity that isnecessary for some applications (e.g., as structural members for airplanesand bridges). Most fiberglass materials are limited to service temperaturesbelow 200C (400F); at higher temperatures, most polymers begin to flowor to deteriorate. Service temperatures may be extended to approximately300C (575F) by using high-purity fused silica for the fibers and high-temperature polymers such as the polyimide resins. Many fiberglassapplications are familiar: automotive and marine bodies, plastic pipes,storage containers, and industrial floorings. The transportation industriesare using increasing amounts of glass fiberreinforced plastics in an effortto de- crease vehicle weight and boost fuel efficiencies. A host of newapplications are being used or currently investigated by the automotiveindustry

2. Carbon Fiber-Reinforced Polymer (CFRP) Composites



Carbon is a high-performance fiber material that is the mostcommonly used reinforcement in advanced (i.e., nonfiberglass) polymer-matrix composites. This is because carbon fibers have the highest specificmodulus, high tensile modulus at elevated temperatures and specificstrength of all reinforcing fiber materials.

These fibers exhibit a diversity of physical and mechanicalcharacteristics, allowing composites incorporating these fibers to havespecific engineered properties. Fiber and composite manufacturingprocesses have been developed that are relatively inexpensive and costeffective.

Use of the term carbon fiber may seem confusing because carbonis an element, and its stable form at ambient condition is graphite.

One classification scheme for carbon fibers is by tensile modulus; on thisbasis the four classes are standard, intermediate, high, and ultrahighmoduli. Furthermore, fiber diameters normally range between 4 and 10m; both continuous and chopped forms are available. Protective epoxysize is normally coated to fibers that improve its adhesion with polymermatrix.

Carbon-reinforced polymer composites are currently being usedextensively in sports and recreational equipment (fishing rods, golf clubs),filament-wound rocket motor cases, pressure vessels, and aircraftstructural componentsboth military and commercial, fixed-wing andhelicopters (e.g., as wing, body, stabilizer, and rudder components).



3.Aramid Fiber-Reinforced Polymer Composites

Aramid fibers are high-strength, high-modulus materials that wereintroduced in the early 1970s which are especially desirable for theiroutstanding strength-to-weight ratios, which are superior to those ofmetals.

Chemically, this group of materials is known as poly(paraphenyleneterephthalamide). There are a number of aramid materials; trade namesfor two of the most common are Kevlar and Nomex.

Kevlar has several grades (Kevlar 29, 49, and 149) that havedifferent mechanical behaviors. During synthesis, the rigid molecules arealigned in the direction of the fiber axis, as liquid crystal domainsMechanically, these fibers have longitudinal tensile strengths and tensilemoduli that are higher than other polymeric fiber materials; however, theyare relatively weak in compression. In addition, this material is known forits toughness, impact resistance, and resistance to creep and fatiguefailure.

Even though the aramids are thermoplastics, they are,nevertheless, resistant to combustion and stable to relatively hightemperatures; the temperature range over which they retain their highmechanical properties is between 200 and 200C (-330 and 390F).Chemically, they are susceptible to degradation by strong acids andbases, but they are relatively inert in other solvents and chemicals.

The aramid fibers are most often used in composites havingpolymer matrices; common matrix materials are the epoxies andpolyesters. Because the fibers are relatively flexible and somewhat ductile,they may be processed by most common textile operations. Typicalapplications of these aramid composites are in ballistic products


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(bulletproof vests and armor), sporting goods, tires, ropes, missile cases,and pressure vessels and as a replacement for asbestos in automotivebrake and clutch linings and gaskets.

B. Metal-matrix Composites (MMCs)

As the name implies, for metal-matrix composites (MMCs) the matrixis a ductile metal. These materials may be used at higher servicetemperatures than their basemetal counterparts; furthermore, thereinforcement may improve specific stiffness, specific strength, abrasionresistance, creep resistance, thermal conductivity, and dimensionalstability. Some of the advantages of these materials over the polymermatrix composites include higher operating temperatures, nonflammability,and greater resistance to degradation by organic fluids. Metal-matrixcomposites are much more expensive than PMCs, and, therefore, MMCuse is somewhat restricted.

The superalloys, as well as alloys of aluminum, magnesium,titanium, and copper, are employed as matrix materials. The reinforcementmay be in the form of particulates, both continuous and discontinuousfibers, and whiskers; concentrations normally range between 10 and 60vol%. Continuous-fiber materials include carbon, silicon carbide, boron,



aluminum oxide, and the refractory metals. On the other hand,discontinuous reinforcements consist primarily of silicon carbide whiskers,chopped fibers of aluminum oxide and carbon, and particulates of siliconcarbide and aluminum oxide. Table 16.9 presents the properties of severalcommon metal-matrix, continuous and aligned fiberreinforcedcomposites.

Some matrixreinforcement combinations are highly reactive atelevated temperatures. Consequently, composite degradation may becaused by high-temperature processing or by subjecting the MMC toelevated temperatures during service. This problem is commonly resolvedeither by applying a protective surface coating to the reinforcement or bymodifying the matrix alloy composition.

Automobile manufacturers have recently begun to use MMCs intheir products. For example, some engine components have beenintroduced consisting of an aluminum-alloy matrix that is reinforced withaluminum oxide and carbon fibers; this MMC is light in weight and resistswear and thermal distortion. Metal-matrix composites are also employed indriveshafts (that have higher rotational speeds and reduced vibrationalnoise levels), extruded stabilizer bars, and forged suspension andtransmission components.



The aerospace industry alsouses MMCs. Structuralapplications include advancedaluminum-alloy metal-matrixcomposites; boron fibers areused as the reinforcement forthe space shuttle orbiter, andcontinuous graphite fibers forthe Hubble Space Telescope.

C. Ceramic-matrix Composites

The fracturetoughnesses of ceramicshave been improved significantly by the development of a new generationof ceramic-matrix composites (CMCs)particulates, fibers, or whiskersof one ceramic material that have been embedded into a matrix of anotherceramic. Ceramic-matrix composite materials have extended fracturetoughnesses to between about 6 and 20 MPa.

In essence, this improvement in the fracture properties results frominteractions between advancing cracks and dispersed phase particles.Crack initiation normally occurs with the matrix phase, whereas crackpropagation is impeded or hindered by the particles, fibers, or whiskers.

One particularly interesting and promising toughening techniqueemploys a phase transformation to arrest the propagation of cracks and isaptly termed transformation toughening. Small particles of partiallystabilized zirconia are dispersed within the matrix material, often Al2O3 orZrO2 itself.Typically, CaO, MgO, Y2O3, and CeO are used as stabilizers.Partial stabilization allows retention of the metastable tetragonal phase atambient conditions rather than the stable monoclinic phase. The stressfield in front of a propagating crack causes these metastably retainedtetragonal particles to undergo transformation to the stable monoclinicphase. Accompanying this transformation is a slight particle volumeincrease, and the net result is that compressive stresses are establishedon the crack surfaces near the crack tip that tend to pinch the crack shut,thereby arresting its growth.

In general, increasing fiber content improves strength and fracture toughness. Furthermore, there is a considerable reduction in the scatter offracture strengths for whisker-reinforced ceramics relative to their unreinforced counterparts. In addition, these CMCs exhibit improved high-


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temperature creep behavior and resistance to thermal shock (i.e., failure resulting from sudden changes in temperature).

Relative to applications, SiC whisker-reinforced aluminas are being used as cutting-tool inserts for machining hard metal alloys; tool lives for these materials are greater than for cemented carbides.

D. Carbon-carbon Composites

One of the most advanced and promising engineering materials isthe carbon fiberreinforced carbon-matrix composite, often termed acarboncarbon composite; as the name implies, both reinforcement andmatrix are carbon. These materials are relatively new and expensive and,therefore, are not currently being used extensively. Their desirableproperties include high-tensile moduli and tensile strengths that areretained to temperatures in excess of 2000C (3630F), resistance to creep,and relatively large fracture toughness values. Furthermore, carboncarbon composites have low coefficients of thermal expansion andrelatively high thermal conductivities; these characteristics, coupled withhigh strengths, give rise to a relatively low susceptibility to thermal shock.Their major drawback is a strong natural tendency to high temperatureoxidation.

The carboncarbon composites are employed in rocket motors, asfriction materials in aircraft and high-performance automobiles, for hot-pressing molds, in components for advanced turbine engines, and asablative shields for re-entry vehicles.

The primary reason that these composite materials are soexpensive is the relatively complex processing techniques that areemployed. Preliminary procedures are similar to those used for carbon-fiber, polymer-matrix composites. That is, the continuous carbon fibers arelaid down having the desired two- or three-dimensional pattern; thesefibers are then impregnated with a liquid polymer resin, often a phenolic;the workpiece is next formed into the final shape, and the resin is allowedto cure.

E. Hybrid Composites



A relatively new fiber-reinforced composite is the hybrid, which isobtained by using two or more different kinds of fibers in a single matrix;hybrids have a better all-around combination of properties thancomposites containing only a single fiber type. A variety of fibercombinations and matrix materials are used, but in the most commonsystem, both carbon and glass fibers are incorporated into a polymericresin. The carbon fibers are strong and relatively stiff and provide a low-density reinforcement; however, they are expensive. Glass fibers areinexpensive and lack the stiffness of carbon. The glasscarbon hybrid isstronger and tougher, has a higher impact resistance, and may beproduced at a lower cost than either of the comparable all-carbon or all-glass reinforced plastics.

The two different fibers may be combined in a number of ways,which will ultimately affect the overall properties. For example, the fibersmay all be aligned and intimately mixed with one another, or laminationsmay be constructed consisting of layers, each of which consists of a singlefiber type, alternating one with another. In virtually all hybrids theproperties are anisotropic.

When hybrid composites are stressed in tension, failure is usuallynoncatastrophic (i.e., does not occur suddenly). The carbon fibers are thefirst to fail, at which time the load is transferred to the glass fibers. Uponfailure of the glass fibers, the matrix phase must sustain the applied load.Eventual composite failure concurs with that of the matrix phase.

Principal applications for hybrid composites are lightweight land,water, and air transport structural components, sporting goods, andlightweight orthopedic components.

ENGINEERING APPLICATION OF COMPOSITEMATERIALS IN DIFFERENT INDUSTRIES



Composites are one of the most in demand materials used in major industries because of their adaptability to different situations. Its relative combination with other materials exhibits desirable properties and serves specific purposes.

An important consideration in the use of composites is lightweight. Researchstudies of specific components have shown that using all composite structures saves 20to 45% while selectively reinforced metal structures offer about 10 to 25% only. Thisweight reduction is required to maintain the center of gravity of the system.

Application of Composites in Aircraft Industry

The first structural composite aircraft components, which were introduced during1950-60, were made from glass fibre reinforced plastics. These components includedthe fin and the rudder of Grumman E-2A, helicopter canopies, frames, radomes,fairings, rotor blades, etc. Due to high strength and stiffness combined with low density,composites like Boron Fibre Reinforced Plastics (BFRP) and Carbon Fibre ReinforcedPlastics (CFRP) were preferred instead of aluminum for high performance aircraftstructures. For lightly loaded structures, Aramid Fibre Reinforced Plastics (AFRP) whichpossess low density, have been used. The use of AFRP continues to be restricted to thelightly loaded structures due to the fact that although these fibres possess high tensilestrength, they have very low compressive strength. For light aircraft and lightly loadedstructural components, Glass Fibre Reinforced Plastics (GFRP) has become one of thestandard materials.

COMPOSITES: GROUP IV Page 49Composite used for different part of B- 737 aircraf


Composite materials arebeing used for differenthelicopter components aswell. Use of advancedcomposites in helicopterapplication started wayback in 1959 with thedevelopment of OptimumPitch Blade for the XCH-47 twin rotor helicopter ofVertol AircraftCorporation. There-after,use of composites inhelicopter application hasbeen progressivelyextended to various parts,which include main & tailrotor blades, stabilizersand fuselage portions. Experience has shown that GFRP main rotor blades have aservice life of around 10,000 hours as compared to blades with steel/titanium spars,which have a life of around 1000- 2000 hours.

Application of Composites in Construction Industry

Composites have long been used in the construction industry. Their benefits ofcorrosion resistance and low weight have proven attractive in many low stressapplications. An extension to the use of high performance FRP in primary structuralapplications, however, has been slower to gain acceptance although there is muchdevelopment activity. Composites present immense opportunities to play increasing roleas an alternate material to replace timber, steel, aluminum and concrete in buildings.

Construction


Composites used in Cockpit Helicopter


Construction holds priority for the adaptation of composites in place of conventionalmaterials being used like doors and windows, paneling, furniture, non-structuralgratings, long span roof structures, tanks, bridge components and complete bridgesystems and other interiors. Components made of composite materials find extensiveapplications in shuttering supports, special architectural structures imparting aestheticappearance, large signages etc. with the advantages like corrosion resistance, longerlife, low maintenance, ease in workability, fire retardancy etc. Some of the compositestructural applications are listed in the Table M11.10

Road Bridges

Bridges account for a major sector of the construction industry and haveattracted strong interest for the utilization of high performance FRP(Fiber ReinforcedPlastics). FRP has been found quite suitable for repair, seismic retrofitting andupgrading of concrete bridges as a way to extend the service life of existing structures.FRP is also being considered as an economic solution for new bridge structures.Polymer composites are seen to offer advantages that are lacking in the traditionalmaterials, particularly for their resistance to corrosive attack in those areas that rely onthe application of de-icing salts to maintain road access. Design approaches andmanufacturing efficiencies developed for road bridge applications will benefit theirintroduction into a broader range of civil construction fields.

Decks for both pedestrian and vehicle bridges across waterways, railways androadways are now a commercial reality; with some pedestrian bridges being builtentirely from composites. The lightweight of composites is especially valuable for the



construction of waterwaybridges incorporating alift-up section to permitthe passage of boats, andfor ease of transportationand erection in remoteareas without access toheavy lifting equipment.The composite deck hassix to seven times theload capacity of areinforced concrete deckwith only 20 percent ofthe weight.Power Transmission

High voltageelectrical transmissiontowers are now beingconstructed from

composite sections using a "snap and build" assemblyprocedure, which eliminates the use of fasteners andadhesives. Weighing less than one third of conventionalsteel equivalent structures, the composites towercomponents can be readily airlifted into remote areas andassembled by small teams, thus eliminating the need toconstruct access roads. In addition, the inherent insulatingcharacteristics of composite permits closer placement ofattached insulators, thus enabling overall tower size andenvironmental obtrusiveness to be reduced.

FRP Doors and Door Frames

With the scarcity of wood for building products,the alternative, which merits attention, is to promote themanufacturing of low cost FRP building materials to meetthe demands of the housing and building sectors. Thedoors made of FRP skins, sandwiched with core materials such as rigid polyurethanefoam, expanded polystyrene, paper honeycomb; jute/coir felt etc. can have potentialusage in residential buildings, offices, schools, hospitals, laboratories etc. As structuralsandwich construction has attained broad acceptance and usage for primary loadbearing structures, the FRP doors can be manufactured in various sizes and designsusing this technology.



Plumbing Components

Lightweight fibre glass composite components for toilet are easy to install andthey are corrosion resistant. Due to poor thermal conductivity, the composite surface iswarm to the touch unlike porcelain and steel. Ease in moulding technique for compositeallows more aesthetic shapes and excellent surface finishes.

Piping System

Glass Reinforced Epoxy (GRE) piping systemoffers complete solution for offshore environmentagainst highly corrosive fluids at various pressures,temperatures, adverse soil and weather conditions(especially in oil exploration, desalination, chemicalplants, fire mains, dredging, portable water etc.) GREpipes are commonly used in oil transpo

Composite Written Report

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