Composites

What is a composite?

A material made from 2+ phases to achieve better properties than either phase alone.

What are the two main parts of a composite?

Matrix and reinforcement.

What is the matrix?

The continuous phase that transfers load and protects the reinforcement.

What is the reinforcement?

The dispersed phase that improves properties like stiffness, strength or hardness.

Why are composites used?

To combine useful properties from different material classes.

Example of a polymer matrix composite?

Carbon fibre reinforced epoxy.

Why use carbon fibre reinforced epoxy?

Lightweight with high stiffness and strength.

Example of a metal matrix composite?

Tungsten carbide particles in cobalt matrix.

What is WC/Co composite called?

Cemented carbide.

Why is WC/Co used for cutting tools?

WC gives hardness; cobalt adds toughness/ductility.

What is a cermet?

A ceramic-metal composite.

What are the 3 matrix-based composite classes?

MMC, CMC, PMC.

Typical MMC matrix materials?

Al, Mg, Cu, Co, Ti

Typical MMC reinforcements?

Carbon, Al₂O₃, SiC, boron, WC.

Main advantage of MMCs over PMCs?

Higher modulus, toughness, ductility and high-temperature performance

Main disadvantage of MMCs?

Higher density and harder/more expensive processing.

Typical CMC matrix materials?

SiC, SiN, Al₂O₃.

Main advantage of CMCs?

Excellent high-temperature and corrosion resistance.

Main disadvantage of CMCs?

Expensive and brittle.

Typical PMC matrix materials?

Nylon, polypropylene, epoxy, phenolic, polyester.

Typical PMC reinforcements?

Glass, carbon, boron, Kevlar.

Main advantage of PMCs?

Low density, easy processing, high specific properties.

Main disadvantage of PMCs?

Poor high-temperature resistance.

How can composites be classified by reinforcement?

Particulate

fibre-reinforced

structural composites

What are particulate composites?

Composites reinforced with particles/flakes.

What are fibre-reinforced composites?

Composites reinforced with short, intermediate or continuous fibres.

What are structural composites?

Composites using layers, honeycombs or sheets.

What is fibre volume fraction, Vf?

The fraction of composite volume occupied by fibres.

What is Vm?

Matrix volume fraction.

Relationship between Vf and Vm?

Vf + Vm = 1.

For continuous constant-diameter fibres, what equals Vf?

Fibre area fraction, Af.

Typical max Vf for aligned fibres?

About 0.6–0.65.

Why can’t Vf usually be 1?

Some matrix is needed to bond fibres together.

What is critical fibre length?

The minimum fibre length needed for the matrix to fully transfer its load to the fibres

What happens if fibres are too short?

The fibre fails and pulls out before it breaks.

What happens if fibres are too long

There is more than enough force to break the adhesive bond without causing the fibre to fail

Typical lc for carbon fibre in epoxy?

About 0.2 mm.

Typical lc for glass fibre in polyester?

About 0.5 mm.

Typical lc for glass fibre in polypropylene?

About 1.8 mm.

Fibres parallel to applied stress means what loading case?

Iso-strain.

What does iso-strain mean?

Fibre and matrix experience the same strain

Fibres perpendicular to applied stress means what loading case?

iso-stress

What does iso-stress mean?

Fibre and matrix experience the same stress

Why are aligned fibres most effective?

They carry load best when aligned with tensile stress.

What happens in Stage I of composite stress-strain behaviour?

Linear response, mostly controlled by fibre stiffness.

What happens in Stage II?

Matrix yields, so composite stiffness decreases.

When does the composite fail?

When fibres reach their critical failure strain.

Why is composite strength harder to predict than modulus?

Failure depends on fibre, matrix, interface and damage mechanisms.

What is a laminate?

Layers of composite with fibres in different directions.

Why use different fibre directions in laminates?

To give strength/stiffness in multiple loading directions.

Main limitation of fibre composites in design?

Fibres cannot just be cut around holes; load paths must be managed.

Why are structural composites expensive?

Often require careful fibre lay-up and curing.