Introduction to Composites

Question 1

What is a composite material?

Answer 1

A composite is a combination of 2 or more constituent materials to give synergy
* This synergy produces properties unachieveable from each constituent alone
* They are stronger, lighter or less expensive than traditional materials
* Usually, high strength stiff fibres embedded in a polymer matrix

Question 2

What are the function of the fibres and matrix in a composite?

Answer 2

Fibres provide high mechanical properties for the composite, along the length of the fibre.
The matrix holds the fibres in position, and prohibits them from buckling and damage. It also enables stress transfer from fibre to fibre.

Question 3

What is the Voigt model?

Answer 3

The Voigt model is a simplification of how a composite with uni-directional reinforcement responds to an axial load. If a load is applied axially (in the direction of the fibres) the composite can be simplified to two slabs, each with properties of the matrix and fibres at identical volumes. This is analogous to springs in parallel, allowing mechanical properties to be calculated using Hooke’s law.
E1=(1-Vf)Em+VfEf
Where f =fibres and m=matrix

Question 4

What is the Reuss model?

Answer 4

The Reuss model is a simplification of how a composite with uni-directional reinforcement responds to a transverse load. A transverse load (applied perpendicular to the direction of the fibres) the composite can be simplified to two slabs, each with properties of the matrix and fibres at identical volumes. This is analogous to springs in series, allowing mechanical properties to be calculated using Hooke’s law.
1/E2=(Vf/Ef+(1-Vf)/Em)
Where f=fibres, m=matrix

Question 5

How do composites moduli perform in comparison to metals?

Answer 5

Comparing the modulus (stiffness), unidirectional carbon fibres has a marginally higher modulus than steel in the fibre direction
E_carbon = 235 GPa
E_steel = 209 GPa
Carbon-fibre reinforced plastic (CFRP) has an even higher specific modulus, and GRP possesses only moderate stiffness.

Question 6

How do composites strength perform in comparison to metals?

Answer 6

Fibre reinforced materials are very strong in the directions of the fibres, but weak in the transverse direction.
σ1 = 2000 MPa, σ2 = 60 MPa
For steel σ = 400 MPa
If the strength is divided by density then they are stronger per unit mass than steel or aluminium alloy.

Question 7

What are typical applications of composites?

Answer 7

• Aerospace: Typically 50% of modern planes are made from composites. CFRP is used in the wings, centre wing box, tail cone, skin panels, frames, doors and more. Used in military applications and helicopter components like rotor blades, hub, glazing bars, seats, fuselage.
• Automotive: Used in motorsports to improve driver safety. CF/epoxy structures absorb 3-4 times the specific energy of steel. Increasing number of carbon fibre chassis for mid price range cars.
• Marine: Boat hulls and masts. GRP used for environmental resistance, can be made into seamless complex shaped structures, excellent strength to weight characteristics
• Civil engineering: Advantages include time saving, durability, in-situ strengthening/ repair, tailor-made properties, radio transparent.
• Energy: Used in wind turbines, tidal turbines, fuel tanks, power transmission. For wind turbines, larger and higher structures improve energy capture because they are subject to stronger winds. CF increases stiffness and reduces mass over GF blades, reducing loads on the tower.
• Sport: Bicycle frames, racquets in sports, skis, watersports etc.
• Medical: Prosthetic limbs and joints, fracture plates in bone repair.

Question 8

What are the main challenges with composites?

Answer 8

Materials
* High material costs
* Anisotropic properties
* Extensive testing required to characterise materials
Manufacturing
* Long cycle times which can’t compete with metals
* Many manufacturing routes to choose from
* Difficult to automate which makes processing labour intensive
* Material wastage
Design
* Design tools not as developed as those for metals
* Known size and scale effects
Recycling
* Difficult to extract high value fibres
* Difficult to process fibres that are recovered
* Difficult to identify end of life materialsl due to different fibre/resin combinations