Learning Outcomes

By the end of this unit you should have developed a knowledge and

understanding of:

• the specific names and classifications of composites

• what constitutes a composite material

• the performance characteristics of different types of composites

• why different types of composites are suitable for different applications

• common composite based products

• the benefits of using composite materials

Introduction

Composites can be put in three main categories.

Composite materials

A composite is a mixture of materials combined to enhance their individual properties.

Composites generally fall into one of three groups; fibre based composites, particle based composites and sheet based composites.

The most important of these for your studies are the fibre reinforced composites since they are most commonly used in the manufacture of products.

The term ‘man-made board’ is sometimes used to describe manufactured boards such as MDF and Chipboard where wood has been combined with resin to form a ‘new’ material - These type of boards have increased stability and resistance to warping etc. and have equal strength in all directions unlike natural timber.

Fibre-based composites

These composites have the following characteristics:

• good strength to weight ratio.

• resistance to corrosion.

• good fatigue resistance

• low thermal expansion

Glass is spun to produce a fibre which is then coated to aid bonding to the resin.

The fibres of glass are available in a variety of thicknesses from coarse to very fine.

The Polyester Resin used is quite brittle on its own and the glass fibres are relatively useless without the resin to bind them together.

The composite material produced is called Glass Reinforced Plastic or GRP.

A mould is required for GRP and these can be produced quite cheaply but their surface can often affect the final quality of the GRP shell.

Inserts such are reinforcing struts and mounting plates can be included while the material is being ‘laid up’.

Carbon Fibre Reinforced Polymer (CFRP)

Carbon Fibre Reinforced Polymer (CFRP) such as matting has become popular in the production of F1 car bodies, tennis racquets and other performance equipment.

Carbon fibres are woven into a matt which is cut to shape. This is what gives carbon fibre its distinctive look.

This is then placed in a mould where it is impregnated with resin and forced into shape.

The mould parts are then baked at 170 degrees C for up to 8 hours to promote the rigid cross links in the resin.

Particle-based composites

Particle-based composites have the following characteristics:

• high strength in compression.

• good stability.

• uniform structure ensuring consistent strength.

• generally free from surface defects.

Concrete

Concrete is made up of materials known as aggregates, as well as sand and cement. Concrete is mixed when dry, water is then added and it is mixed thoroughly. Once cast the concrete is left to harden.

Concrete can be further reinforced with glass fibres to make glass reinforced concrete (GRC). This also allows it to be cast into more complex shapes as the glass fibres replace the need for aggregate such as gravel and stone.

Concrete is a relatively modern material although its roots go back to ancient times. Concrete as we know it was patented in the mid 1800’s and became a popular building material in the 20th Century.

It has various properties:

• it can be moulded into complex shapes.

• it has similar properties to stone.

• components are more readily extracted than stone.

• it can be cast in situ (on site) whereas stone has to be quarried and cut to shape.

• it has excellent compressive strength making it particularly good in building and for their foundations.

• however, it is poor in tension making it necessary to reinforce the concrete beams with metal rods when spanning large distances.

Cermets

Cermets are a mixture of both metal and ceramic particles. A good example is Tungsten Carbide; the ceramic Tungsten combined with the metal Cobalt. This material is used extensively for cutting tools as it keeps its edge well.

Another cermet is aluminium oxide and cobalt used in jet engines.

Cermets are resistant to high temperatures and are very tough and shock resistant.

Sheet-based composites

These have been covered in the Woods section as they are generally considered manufactured boards.

Metal / Polymer Composites

Recent development have seen the introduction of metal/polymer composites. An example of this is ALU composite. This material is a laminate of around 0.3 mm Aluminium sheet sandwiching a Polyethylene core. The overall thickness is about 3mm.

This results in a lightweight material that has excellent rigidity along with good impact resistance. It can be drilled, sawn and can be rolled or bent into shape. It is used in vehicle skins for its reduced weight and noise reduction. It is also used for partitions and in boats.

Applications for composites

Summary

• composites are a mixture of materials combined to improve their properties.

• manufactured boards such as Plywood and MDF are composites because they are a combination of wood and resin.

• composites generally fall into one of three groups; fibre based composites, particle based composites and sheet based composites.

• fibre based composites include Carbon Fibre, GRP (glass reinforced plastic) and GRC (glass fibre reinforced concrete)

• particle based composites include concrete and cermet

• sheet based composites include manufactured boards.

• composites are now being used to replace traditional materials such as timber and metal.

Revision notes

A composite is a mixture of materials combined to enhance their properties. Unlike alloys, which are a mixture of metals, composites can be a mixture of different types of materials.

Like many of the other materials studied they can be put into three main categories for easier revision:

1. Particle based such as Concrete

2. Fibre based such as GRP

3. Sheet based such as MDF (these have been covered in the woods section)

Learn the acronyms so you can remember the materials e.g. GRP is Glass Reinforced Plastic.