Fibres play the most important role as the primary components in fibre-reinforced composite materials. They constitute the predominant volume fraction within a composite laminate and bear the majority of the load applied to the composite structure. The meticulous choice of fibre type, fibre volume fraction, fibre length, and fibre orientation hold significant importance as it directly impacts various characteristics of the composite laminate.
- Density
- Tensile strength and modulus
- Compressive strength and modulus
- Fatigue strength as well as fatigue failure mechanisms
- Electrical and thermal conductivities
- Cost
Carbon fibres
Carbon fibre is obtained by oxidation, carbonisation and graphitisation. The most common precursor is polyacrylonitrile or PAN. Other precursors can be used, but mechanical properties are lower. CFRP or carbon fibre reinforced polymer, are the composite materials obtained when the matrix is embedded with carbon fibres.
Depending on the type of fibres, the temperature of graphitisation process varies. For high strength fibres it is around 2700ºC while for high modulus fibres at 3000ºC. The difference in the cost is due to the energy used for producing them. Finally, a surface treatment is applied to improve matrix bonding.
The filament diameter of carbon fibre is the smallest ones. Carbon fibre has the highest specific stiffness and strength in tension and in compression in comparison with other fibres, however, their impact strength is lower than the aramid.
Glass fibres
Glass fibres are the most used fibres in composite materials. They are created through the fusion of quarry materials like sand, kaolin, or limestone at 1700ºC, resulting in molten glass. This liquid is then extruded through micro-fine bushings while being rapidly cooled, forming filaments of glass fiber. These filaments are gathered into strands or rovings, coated with a sizing agent to enhance cohesion and protect the glass from abrasion.
Various types of glass can be manufactured, with those suitable for structural reinforcement falling into the following categories:
- E-glass: Characterized by lower alkali content and greater strength compared to A-glass. Exhibits good tensile and compressive strength, stiffness, electrical properties, and is relatively cost-effective, though it has poorer impact resistance.
- C-glass: Notable for its superior resistance to chemical attacks. Primarily employed as surface tissue in the outer layer of laminates used for chemical and water pipes and tanks.
- S-glass: Boasting higher tensile strength and modulus in comparison to E-glass. Originally developed for the aerospace and defense industries, it finds applications in certain hard ballistic armor scenarios. However, S-glass is approximately 10 times more expensive than E-glass.
Aramid fibres
Aramid fibre is obtained from the polyamide. The fibre is produced by spinning a solid fibre form a liquid chemical blend. These fibres have a low density and high strength. Its main applications are for ballistic applications due to its resistance to impact. Commercially, the most known trade is Kevlar, produced by Dupont. The texes ranging from about 20 to 800.
Natural fibres
Their origin are fibrous plant materials such as jute, sisal, hemp, flax and remi. The component if natural fibres are cellulose dispersed in an amorphous matrix of lignin and hemicellulose. The final application is not structural as it has low stiffness and strength. In the case of the automotive industry, it has been used for seat back, roof inner panel, door inner panel. In addition, they suffer from the environmental effects, the moisture absorption and the fibres degrade at temperatures higher than 200ºC.
