52.1 FIBER REINFORCED PLASTICS (FRP)

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Typical properties that may be desired are,

- light weight

- stiffness

- strength

- heat resistant

- impact resistance

- electrical conductivity

- wear resistance

- corrosion resistance

- low cost

Some notable applications are,

- Automotive - engine blocks, push rods, frames, piston rods, etc.

- Electrical - motor brushes, cable electrical contacts, etc

- Medical - prostheses, wheel chairs, orthofies, etc.

- Sports equipment - tennis racquets, ski poles, skis, fishing rods, golf clubs, bicycle frames, motorcycle frames

- Textile industry - shuttles

- etc.

Some advantages are,

- composites provide a maximum tensile strength to density ration approximately 4 to 6 times greater than steel or aluminum

- can provide a maximum material stiffness to density ratio of 3.5 to 5 times that of aluminum or steel

- high fatigue endurance limits

- absorb higher impact energies

- material properties can be strengthened where required

- corrosion potential is reduced.

- joints and fasteners are eliminated or simplified

Some disadvantages are,

- If either material is susceptible to local solvents, the composite cannot be used

- materials can be expensive

- design and fabrication techniques are not well explored and developed.

fibres are often graphite, glass, aramid, etc.

the fibres are supported in the matrix, quite often a polymer, epoxy, etc.

The polymer matrix is often referred to as the resin

The matrix transfers the load to the reinforcement fibres, and it protects the fibres from environmental effects.

Resins tend to be thermosetting, or thermoplastic

Thermoplastics - melt, and harden with temperature

Thermosets - undergo a chemical change, and cannot be "recast". The setting is often heat activated.

Polyester resins are quite common. The process often begins with molecules like a dialcohol, and diacid. These then cure into a solid polymer.

These reactions create very long chains of polymers in a sort of gel, but the next step involves cross-linking them to make things stiff

Various chemical reactions, and physical properties can be altered by changing the chemicals above. Rates of reaction can be accelerated with higher temperatures.

The initiator is often stored separately from the other reactants to prevent cross linking before use. This may happen spontaneously, and so the chemicals should be discarded if too old.

Epoxies can also be used, they can be expensive and toxic, but they generally have better overall performance than polyesters.

Other general categories are,

- Polymides and Polybenzimidazoles

- Phenolics and Carbon matrices

- Thermoplastics

- Ceramic matrices

- Metal matrices

Polyesters are generally inexpensive, and can be modified for other properties.

Epoxies are used when the matrix must have good adhesion, strength and corrosion resistance in severe environments.

Polyimides are used for high temperature applications (up to 600 F/316 C) but are difficult to process

Phenolics are good for thermal insulation

Ceramics are used for high temperature, low strength applications.

Reinforcements in materials can be

- fibres - long directional filaments

- particles - small non-directional chunks

- whiskers - small directional filaments

Fibres have very long lengths with respect to the surrounding material, and tend to have a significantly higher strength along their length.

Fibres are often drawn to align the molecules along the fibre length

Glass fibres are basically made by,

1. Mixing silica sand, limestone, boric acid, and other minor ingredients

2. The mixture is heated until it melts at about 2300F/1260C

3. Letting the molten glass flow through fine holes (in a platinum plate)

4. The glass strands are cooled, gathered and wound. (protective coatings may be added.)

5. The fibres are drawn to increase the directional strength.

6. The fibres are woven into various forms for use in composites

There are three common glass types used, E, S, C

E - less expensive

S - 40% stronger than E, and more resistant to temperature

C - well suited to corrosive environments

Carbon Fibres are among the highest strength and modulus materials known.

Aramids (Kevlar) fibres are shown below. These do not need to be drawn as they are already in the correct orientation when produced.

aramid fibres come in bundles of 134 to 10,000 filaments

kevlar properties are,

Other popular fibre types are Boron, Silicon, Carbide, Alumina, etc.

The fibres come in a variety of configurations,

Filament - a single fibre

Strand - Could refer to a single fibre, or an untwisted bundle of filaments

Tow - A bundle of untwisted fibres, often a predetermined number.

Yarn - A twisted tow

Roving - A number of yarns pulled together without twisting

Tape - a thin and wide run of parallel fibres

Woven fabric - yarns and tows are interlaced to create flat cloth

Braids - yarns and tows are woven into tubular shapes

Mat - chopped fibres create an undirected pattern in a flat cloth. A binder holds the fibres together.

There are different weave types used, these provide different workabilities, air removal, distortions, etc.

weaves can be made from single fibre types, or combinations

The relative material properties for composites are seen in the figures below

RTW - Rigidity to weight

STW - strength to weight

Strain - failure strain

UTS - ultimate tensile strength

Yield - yield strength

USS - Ultimate shear strength

UCS - Ultimate compressive strength

Composites are sensitive to temperature and humidity during curing.

When dealing with cyclic loading over a million cycles

- Aluminum and steel design with 0.1 of the normal yield strength

- Composites design with 0.5 of normal yield strength

Composites in general are very easy to shape, and form, this is not always possible with other high strength materials

Composites are anisotropic and have good strength along the fibre length, but reduced strength across the fibre axis.

Elongation of composites is typically linear up to fracture at 1% to 2% elongation