Composites- Intro to Composites

Question 1

How do composites compare to polymers for tensile modulus and density?

Answer 1

Composites have similar densities but a greater YM

Question 2

What is a composite material?

Answer 2

A material made from two or more constituent phases. These phases have different properties and remain separate within the composite

Question 3

The idea behind composites

Answer 3

Take advantage of the properties of both phases. Specifically tailor properties to an application

Question 4

What are the constituent phases in composites?

Answer 4

Reinforcements: typically fibres, add stiffness, strength, creep resistance, thermal stability, could be particulates.
Matrix: typically thermosetting polymer, add toughness, shear and compressive modulus and strength

Question 5

One key advantage to composites

Answer 5

They are a lightweight alternative to traditional materials. Their specific stiffness and strength are better than other materials

Question 6

What is a lamina?

Answer 6

Where many fibres are incorporated into a thin layer of matrix. Only one layer. Aka ply. Typically 0.1-1mm thick

Question 7

Long fibres

Answer 7

Aka continuous fibres. If unidirectional the properties are mainly in that direction. Can be made bidirectional by weaving to reduce the anisotropy. Can be multidirectional so that the properties can be tailored to suit an application

Question 8

Short fibres

Answer 8

Aka discontinuous fibres. If unidirectional the properties are poorer than for continuous fibres. Can be randomly oriented to give equal properties in all directions

Question 9

Laminates

Answer 9

Consolidate several laminar into a laminate. Can be parallel laminate to give high anisotropy or cross-ply laminate to reduce anisotropy.

Question 10

Hybrid laminates

Answer 10

Can be interply where each laminate only contains one type of fibre. Can be intraply where each laminate contains two or more kinds of interwoven fibres. Can make hybrids with other classes of materials like fibre-metal laminates (e.g GLARE which alternates glass fibres with Al)

Question 11

Sandwich panels

Answer 11

A core material (foam or honeycomb) with two thin composite skins. Gives better flexures stiffness (bending) for little extra weight

Question 12

What do fibre reinforcements do?

Answer 12

Bear most of the load in tension

Question 13

General properties of fibre reinforcements

Answer 13

High tensile modulus and tensile strength. Lightweight. Low strain to failure. High dimensional stability. Expensive

Question 14

Examples of fibres

Answer 14

E-glass, high strength carbon, high modulus carbon, Kevlar 49, spectra, flax, boron

Question 15

Glass fibres

Answer 15

White. Most common fibre reinforcement. E-glass is cheapest but S-glass gives very high tensile strength for extra cost. Structure is random 3D network of tetrahedra linked by O atoms. They are amorphous and isotropic

Question 16

Advantages of glass fibres

Answer 16

Low cost, high tensile strength, chemically resistant, electrically insulating, thermally insulating, radar transparent.

Question 17

Disadvantages of glass fibres

Answer 17

Low tensile modulus, relatively high density, easily abraded, low fatigue resistance, high hardness, absorbs water

Question 18

Common matrices and applications for glass fibres

Answer 18

Unsaturated polyesters, vinylesters, epoxies.
Automotive, military, construction, storage tanks and silos, electrical equipment, baths and shower enclosures, sporting goods.
All driven by low cost

Question 19

Carbon fibres

Answer 19

Black. Very high performance and expensive. Interconnected carbon atoms hexagonally arranged in layers. Strong covalent bonds in basal plane but weak van der waals between basal planes meaning fibres are anisotropic.

Question 20

Order within carbon fibres

Answer 20

Mixture of ordered graphitic carbon on the skin and less ordered carbon internally. Less ordered doesn’t take as much advantage of strong covalent bonds within planes

Question 21

Advantages of carbon fibres

Answer 21

High tensile strength and modulus to weight ratios. Low linear thermal expansion. High thermal conductivity. Moisture resistant

Question 22

Disadvantages of carbon fibres

Answer 22

Low strain to failure. Low impact resistance. High electrical conductivity. High cost

Question 23

Why can high electrical conductivity also be an advantage to carbon fibres?

Answer 23

Means they can carry signals across parts

Question 24

Common matrix and applications for carbon fibres

Answer 24

Epoxy
High performance automotive, aerospace, sporting goods.
All driven by low weight

Question 25

Aramid fibres

Answer 25

Yellow. Highly crystalline aromatic polyamides. Famous is Kevlar. Structure is phenyl rings along backbone (enhance stiffness) between amide linkages that H-bond between chains.

Question 26

Advantages of aramid fibres

Answer 26

Very high tensile strength to weight ratio, low linear thermal expansion, good impact resistance (toughness), no sizing needed, good temperature and corrosion resistance.

Question 27

Disadvantages of aramid fibres

Answer 27

Hard to process, poor matrix compatibility, poor in compression, susceptible to moisture (water replaces H-bonds between chains), susceptible to UV radiation, high cost

Question 28

Applications of aramid fibres

Answer 28

Aircraft (impact prone areas like leading edges), sporting goods, pure Kevlar used for ballistic armour.
All driven by impact tolerance

Question 29

Polyethylene fibres

Answer 29

White. Highly crystalline (95-99%) polyolefins. UHMWPE. Highly aligned molecules provide high tensile strength. Used for ballistic and blast protection, sporting goods

Question 30

Advantages of polyethylene fibres

Answer 30

Very high tensile strength to weight ratio. Low density. Low moisture absorption. High abrasion resistance. High impact resistance

Question 31

Disadvantages of polyethylene fibres

Answer 31

Very low service temperature (loss of properties above 80-90C). Poor adhesion with matrices

Question 32

Natural fibres

Answer 32

Most brown. From agricultural plants. E.g hemp, flax, jute. Primarily cellulose fibres in a lignin matrix. Used for automotive interior panels

Question 33

Advantages of natural fibres

Answer 33

Environmentally friendly. Biodegradable. Lower density and higher strength to weight ratio than E-glass. Acoustic damping. Cheap

Question 34

Disadvantages of natural fibres

Answer 34

Low strength, modulus, melting point. Absorbs lots of moisture. Degrade above 200C

Question 35

What are fibre treatments for?

Answer 35

Protection during manufacture and processing. Improve wettability. Improve bonding between fibre and matrix by removing weak layers and contaminants, adding chemically reactive groups and altering topography to promote mechanical interlocking

Question 36

Fibre treatment of glass

Answer 36

Fibres given a sizing (often an organo-silane treatment). Uses Si bonded to R’ (organic) group and 3 OH groups. OH groups interact with surface of glass and organic group has good adhesion to organic matrix

Question 37

Carbon fibre treatments

Answer 37

Oxidative: apply oxygenated functional groups to surface which are easier for a matrix to react with than pure carbon, can use O2, O3, CO2, HNO3.
Non-oxidative with reactive coatings has similar principle

Question 38

Kevlar and UHMWPE fibre treatments

Answer 38

Similar idea to those for carbon fibres. Can oxidise surface. Can plasma treat either to etch surface and add functional groups