Principle

Fibre reinforced polymers are composites that consist of fibres which are dispersed in a continuous matrix phase. Due to a number of characteristic advantages such as lightness, resistance to corrosion and resilience, fibre reinforced polymers are predicted to have a great potential in future transportation technologies, from cars and railways to aerospace.

Composition and basic materials

The two physical phases of fibre reinforced polymers are the fibres themselves and the resin matrix. A wide range of materials can be used for fibre reinforced materials. The most common fibre materials are: carbon, glass and aramid. Carbon and aramid give better stiffness properties to the composite material than glass fibre. Resins used for matrices include polyester, epoxy, vinyl ester and phenolic types. Selecting the appropriate resin type determines the service temperature capabilities, chemical resistance properties, electrical and adhesive characteristics of the composite. Furthermore a number of additives are applied such as fire retardants etc.

The overall strength of fibre-reinforced polymers is owed to the plastic flow of the polymeric material under stress transferring the load to the high strength fibre.

Fields of Application

Fibre-reinforced polymers are an attractive substitute for steel (or other metals) in a wide range of applications in aerospace, automotive and construction industries.

Mair 1999 gives the following cost and performance emphasis of different applications of fibre-reinforced polymers.

Source: Mair 1999

• Weight saving
• Corrosion resistance
• Taylored performance (cf. General criteria - Benefits)

Fibre-reinforced polymers offer a number of advantages over steel (and other metals). Mair 1999 gives the following list:

• High specific strength properties (20-40% weight savings)
• Ability to fabricate directional mechanical properties
• Outstanding corrosion resistance
• Excellent fatigue and fracture resistance
• Lower tooling cost alternatives
• Lower thermal expansion properties
• Simplification of manufacturing by parts integration
• Potential for rapid process cycles
• Ability to meet stringent dimensional stability requirements

The weight reductions in railway vehicles attainable through the use of fibre-reinforced plastics obviously depend on the share of the vehicle mass replaced by these materials and thus on the application context.

According to estimates of the Rocky Mountain Institute, weight reductions through the use of fibre-reinforced plastics may be up to 65% in the automotive industry (Pehnt 2001). This is obviously very optimistic and not as such transferable to railway conditions.

The energy efficiency through weight reductions could be compensated by the high energy demand in the production phase of fibre-reinforced polymers.

Estimates made by DLR (Deutsches Zentrum für Luft- und Raumfahrt) for automotive applications indicate that the life cycle net result of higher energy demand in production but higher energy efficiency during use may be close to zero. The concrete outcome depends on many factors such as product life, recycling strategy etc. (Pehnt 2001).

• Recycling properties are generally worse than for metals due to heterogeneity of material.
• Fibre reinforced materials often contain toxic or at least problematic chemicals especially those composites equipped with fire retardants.