Composite MaterialsFundamental questions

• How do composite materials differ from other engineering materials?

• What are the constituent materials, and how do their properties compare?

• How do the properties of the composite depend on the type, amount and arrangement of the constituents?

• How are composite products made, and why does manufacture affect quality?

Fibres have better stiffness and strength compared to bulk materials

• Atomic or molecular alignment(carbon, aramid)

• Removal of flaws and cracks (glass)

• Strain hardening (metals)

fibre advantages disadvantages

glass high strengthlow cost

low stiffness

aramid high tensile strengthlow density

low compressivestrengthmoisture absorption

boron high stiffnesshigh compressivestrength

high cost

‘HS’ carbon high strengthhigh stiffness

moderately high cost

‘HM’ carbon very high stiffness low strengthhigh cost

ceramic high stiffnesshigh usage ature

low strengthhigh cost

0 100 200 300 400 500 600

E-glass

S-glass

T300 carbon

IM-7 carbon

GY70 graphite

boron

aramid

SiC (Textron)

Saphikon alumina

Fibre Tensile Modulus (GPa)

0 1000 2000 3000 4000 5000 6000

E-glass

S-glass

T300 carbon

IM-7 carbon

GY70 graphite

boron

aramid

SiC (Textron)

Saphikon alumina

Fibre Tensile Strength (MPa)

Most reinforcing fibres are brittle (elastic to failure)

Hollaway (ed), Handbook of Polymer Composites for Engineers

Types of Natural Fibre

• Bast fibres (flax, hemp, jute, kenaf…)- wood core surrounded by stem containing cellulose filaments

• Leaf fibres (sisal, banana, palm)

• Seed fibres (cotton, coconut (coir), kapok)

Density of natural fibres

0

0.5

1

1.5

2

2.5

3

E-glass

flax hemp jute ramie coir sisal abaca cotton

g/c

m3

TNO Centre for Lightweight Structures

Tensile strength

0

500

1000

1500

2000

2500

3000

E-glass

flax hemp jute ramie coir sisal abaca cotton

MP

a

Specific tensile strength

0

200

400

600

800

1000

1200

E-glass

flax hemp jute ramie coir sisal abaca cotton

MP

a /

(g/m

3)

Tensile modulus

0

10

20

30

40

50

60

70

80

90

E-glass

flax hemp jute ramie coir sisal abaca cotton

GP

a

Specific tensile modulus

0

10

20

30

40

50

60

E-glas

sfla

xhe

mpjut

era

mieco

irsis

al

abac

a

cotto

n

GP

a /

(g/m

3)

High specific properties (low density).

A renewable resource; production requires relatively little energy

Crops are sink for CO2, returning oxygen to atmosphere.

Low investment and low cost production.

Low tooling wear.

Better working conditions, no skin irritation.

Thermal recycling possible.

Good thermal and acoustic insulating properties.

Advantages of Natural Fibres

Low strength, especially impact strength.

Variable quality (e.g. weather dependent).

Moisture absorption, which causes swelling of the fibres.

Limited maximum processing temperature.

Lower durability (potential for improvement through fibre treatments).

Poor fire resistance.

Price fluctuation (harvest results or agricultural politics).

Irregular fibre lengths (spinning is required to obtain continuous yarns).

Disadvantages of Natural Fibres

Structures cannot be made from fibres alone - the high properties of fibres are not realisable in practice

A matrix is required to:

• hold reinforcement in correct orientation

• protect fibres from damage

• transfer loads into and between fibres

COMPOSITES - A FORMAL DEFINITION(Hull, 1981)

1. Consist of two or more physically distinct and mechanically separable parts.

Polymer matrix composite combinations

Fibre

E-glassS-glasscarbon (graphitearamid (eg Kevlar)boron

Matrix

epoxypolyimidepolyesterthermoplastics (PA, PS, PEEK…)

Ceramic matrix composite combinations

Fibre

SiCaluminaSiN

Matrix

SiCaluminaglass-ceramicSiN

Metal matrix composite combinations

Fibre

boronBorsiccarbon (graphite)SiCalumina (Al2O3)

Matrix

aluminiummagnesiumtitaniumcopper

0 20 40 60 80

GPa

CSM glass/polyester (Vf 25%)

biaxial woven glass/epoxy (Vf 50%)

UD glass/epoxy (Vf 60%)

E-glass fibres

Tensile Modulus

Composite property might be only 10% of the fibre property:

fibre CSM glass woven glass filament-wound UDglass

wovenaramid

UD aramid UD HS carbon UD HM carbon

resin polyester polyester polyester polyester epoxy epoxy epoxyVf 17% 32% 44% 48% 60% 63% 60%SG 1.46 1.7 1.83 1.3 1.35 1.6 1.6tensile strength(MPa)

110 220 650 390 1380 2280 1260

tensilemodulus (GPa)

8 14 30 24 76 142 200

tensileelongation (%)

1.6 1.7 1.9 1.8 1.5 0.5

compressionstrength (MPa)

150 230 800 86 276 1440 840

shear strength(MPa)

80 90 50 60 71 65

shear modulus(GPa)

3 3.3 4 2.1 7.2 5.5

Some typical polymer composite properties

Examples of particulate composites

• Concrete - hard particles (gravel) + cement (ceramic/ceramic composite). Properties determined by particle size distribution, quantity and matrix formulation

• Additives and fillers in polymers:carbon black (conductivity, wear/heat resistance)aluminium trihydride (fire retardancy)glass or polymer microspheres (density reduction)chalk (cost reduction)

• Cutting tool materials and abrasives (alumina, SiC, BN bonded by glass or polymer matrix; diamond/metal matrix)

• Electrical contacts (silver/tungsten for conductivity and wear resistance)

• Cast aluminium with SiC particles

COMPOSITES - A FORMAL DEFINITION(Hull, 1981)

1. Consist of two or more physically distinct and mechanically separable parts.

2. Constituents can be combined in a controlled way to achieve optimum properties.

Examples of natural composites

COMPOSITES - A FORMAL DEFINITION(Hull, 1981)

1. Consist of two or more physically distinct and mechanically separable parts.

2. Constituents can be combined in a controlled way to achieve optimum properties.

3. Properties are superior, and possibly unique, compared those of the individual components

Addition of properties:

GLASS + POLYESTER = GRP

(strength) (chemical resistance) (strength and chemical resistance)

Unique properties:

GLASS + POLYESTER = GRP

(brittle) (brittle) (tough!)

ADVANCED COMPOSITES vs REINFORCED PLASTICS

• Aerospace, defence, F1…• Highly stressed• Glass, carbon, aramid fibres• Honeycomb cores• Epoxy, bismaleimide…• Prepregs• Vacuum bag/oven/autoclave

• Highly tested and qualified materials

• Marine, building…• Lightly stressed• Glass (random and woven)• Foam cores• Polyester, vinylester…• Wet resins• Hand lay up, room

temperature cure

• Limited range of lower performance materials

Why are composites used in engineering?

• Weight saving (high specific properties)• Corrosion resistance• Fatigue properties• Manufacturing advantages:

- reduced parts count- novel geometries- low cost tooling

• Design freedoms- continuous property spectrum- anisotropic properties

Anisotropic properties - fibres can be aligned in load directions to make the

most efficient use of the material

The ability to vary fibre content and orientation results in a spectrum of available properties

Why aren’t composites used more in engineering?

• High cost of raw materials• Lack of design standards• Few ‘mass production’ processes available• Properties of laminated composites:

- low through-thickness strength- low interlaminar shear strength

• No ‘off the shelf’ properties - performance depends on quality of manufacture

There are no ‘off the shelf’ properties with composites. Both the structure and the material are

made at the same time.

Material quality depends on quality of manufacture.

Poor quality - low fibre content, high void content

Good quality - high fibre content, ‘zero’ void content