Composites 2009

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Definition: Composites are a group of materials from from themixture of metals, ceramics and/or polymers in such a way that

unusual combinations of properties are obtained.

The major classes of composites are:

- fibre reinforced polymer matrix composites (FRP)- fibre reinforced metal matrix composites (MMCs)

- fibre reinforced ceramic matrix composites (CMCs)- hybrid composites

By far, the most commonly used composite is FRP. MMCs areused sparingly and CMCs are used in high-temperature aircraft

applications (eg. brakes, nose cones).


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FRP composites basically consist of fibres embedded in apolymer matrix.The fibres can be in the forms of short particles, whiskers orcontinuous fibres. The fibres are typically 7-15 m in diameter.The fibres provide the composite structures with their importantengineering properties, such as stiffness, strength and fatigue

resistance.

The types of fibres used in aircraft composites are:

- high modulus or high strength carbon (graphite)- Kevlar (Aramid)

- E- and S-glass- Boron


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Continuous Fibres

Short Fibres/Whiskers

Composites consist of a

matrix phase &

reinforcement phase

The matrix phase is the

continuous phase & the

reinforcement phase

contains dispersedparticles.


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The functions are of the resin matrix are to:bind the fibres together into a solid materialact as a load transfer medium between the fibres and resinprotect the fibres from environmental effects such as moisture,

chemical corrosion, oxidation, abrasion and impact, which can

led to embrittlement and failure.

The resin has a strong influence on:

compressive properties transverse properties impact properties service operating temperature.


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Main reasons for using composites: light-weight high specific stiffness & strength good to excellent fatigue resistance good formability into near net shaped complex structures tailored design to maximise structural performance radar absorbing properties (stealth)


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Sound damping Thermal and electricalinsulation

Impact energy absorption Corrosion resistance


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Composites used in leisure & commercial marine craft. Main applications:

- Canoes & kayaks - Jet skis- Lifeboats - Dinghies- Powerboats - Patrol boats- Yachts - Fishing Trawlers- Passenger Ferries - Structures for large ships

Main reasons for using composites are: reduced weight: increased speed & reduced fuel consumption

reduced corrosion improved vibration & noise damping.


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Carbon/graphite fibres:Carbon fibres consist of 93-95% carbon.Graphite fibres consist of more than 95% carbon.Carbon fibres are ~8 m in diameter.Carbon/graphite fibres occur in three forms:

High tensile strength fibres: = 2.41 GPa; E = 207 GPaHigh modulus fibres: = 1.38 GPa; E = 345 GPaUltra high modulus fibres: = 1.03 GPa; E = 483 GPaCarbon fibres are expensive: AUD200-600 per kg.


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2003Brooks/Cole,adivisionofThomsonLearning,Inc.ThomsonLearningisatrademarkusedherein

underlicense.


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Mohr,J.G. & Rowe,W.P. Fiberglass. van Nostrand Reinhold, New York (1978)


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2003 Brooks/Cole, a division of Thomson Learning, Inc. Thomson Learning is a trademark used herein under license.

Kevlar is an organic molecular fibre made by DuPont.

It is an extremely tough, bullet proof fibre that is used for personnel

protection on military and armoured structures.The fibers are joined by van der Waals bonding between oxygen and

hydrogen atoms on adjoining chains.


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Sandwich - A composite material constructed of a lightweight,

low-density material surrounded by dense, solid layers. The

sandwich combines overall light weight with excellent stiffness.


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Metallic laminates have a unique combination of- high specific strength,- high specific stiffness,- corrosion resistance,- fatigue resistance,- fracture toughness and- light-weight.


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Two hybrid composites are ARALL and GLARE:ARALL consists of aluminium alloy sheet bonded to a ply ofKevlar(Aramid) reinforcing with an epoxy matrix.GLARE consists of aluminium alloy sheet bonded to a ply ofglassreinforcing with an epoxy matrix.


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Strong/Stiff

Weak/Compliant

Weak/Compliant


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density of fibres density of matrix phase


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Composites are strong in fibre directionComposites are weak in transverse fibredirectionWhat material properties determine the

strength in the two directions?

Strong/Stiff

Weak/Compliant

Weak/Compliant


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Consider a unidirectional composite (with all the fibres alignedin the same direction) loaded in uniaxial tension:

The Youngs modulus in the fibre direction, E1, can be

estimated using a rule-of-mixtures:

Ef>> E

m

Therefore for maximum stiffness you

need high modulus fibres and a large

amount of fibres.


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Effect of volume fraction of fibres on Youngs modulus of

two glass/polyester composites

The two ways to increase

Youngs modulus are:

-Use high stiffness fibres-Maximise fibre volume

content

Where does the curve

cross the Y-axis?


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The Youngs modulus in the transverse direction, E2, can beestimated using rule-of-mixtures:

Weak/Compliant


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:

:

The in-plane tensile strength,

1, can be estimatedusing rule-of-mixtures:

fis the tensile strengths of fibresmis the tensile stress acting on thematrix when the fibres fail.

The equation shows that maximisingthe fibre strength and fibre contentwill increase composite strength

1


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Other mechanical properties of unidirectional composite loaded inuniaxial tension can also be estimated using rule-of-mixtures:

Poissons Ratio:

In-plane Shear Modulus:


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For a fibre volume fraction of 0.6, determine the density,modulus and transverse modulus and tensile strength of the

boron fiber-aluminium composite.

Comment on the difference in values. Why are

normal and transverse modulus so different?

An example like this will guaranteed be in the exam!


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Rule-of-mixtures can not be used to calculate all properties.

It cannot be used to predict such properties as:

- compressive strength- ductility- impact properties- fatigue properties- creep resistance


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2003Brooks/Cole,adivisionofThomsonLearni

ng,Inc.ThomsonLearningisatrademarkusedhereinunderlicense.

Effect of Fibre Orientation

Stress-strain curves

very different

for different fibre orientations.

Draw the stress-strain curve

for 0 and 90 degree plies.


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Fibres are wound on a frame (unidirectional)

Resin is applied and rolled into fibres to ensure wetting


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Mostly for aerospace

applications, we can buy

carbon/epoxy pre-preg.

The resin is partiallycured, with the final curing

after lay-up undertaken in

the autoclave.


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Fibre breaks accumulate

in composite. Fibres fail at

different times due tovarying strength.

The matrix transfers the

load between neighbouring

fibres and around fibre

breaks.

Figures from Hull, 1981, p.134


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Kinking of brittle fibres

(Carbon)

Kinking of ductile fibres

(Kevlar)

Onset of

fibre buckling


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Cracking along fibre matrixinterface (depends on bonding

strength), crack development

stable if other layers support it.

SEM pictures of crack growth in composite


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Delaminationcommonly occurs

when applying

through-thickness

stress or after impactdamage.

Composites should not

be designed to carry

through-thicknessstress.

S. Feih, PhD thesis, Cambridge University,

UK, 2001


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Further Reading

DR Askeland, The Science & Engineering of Materials, Stanley Thomas

(Publishers) Ltd, Chapter 17.

D Hull, Introduction to Composite Materials, Cambridge Solid State

Science Series, 1981

Internet has lots of good web-sites on engineering polymers.

Composites 2009

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