Modern vehicles must be reliable, durable, low fuel-consumption with low operating costs and with a price/performance ratio in-line with customer requirements.Roving – Parallel strands are orientated to make a long narrow bundle of fibre with no twist.
Yarn – Parallel strands are twisted together to make a thicker Rope.
Mat – Roving is cut up casually to make a felt.
Cloth – Roving and/or Yarn are woven together to make an ordered grid or flat cloth sheet.
For these reasons, the automotive industry invests in researching innovative materials and construction techniques to obtain the best in class vehicles.
In particular, the fuel-consumption depends on the aerodynamic resistance, the rolling resistance of the complete drivetrain, the engine efficiency and the the vehicle weight.
The vehicle weight can be reduced using accurate calculation of the vehicle structure, using CAE methodologies, but also by using improved materials.
These materials are based on synthetic fibres bonded with thermally activated resins, and/or sandwich structures with composites of diverse materials (thin sections of aluminium, card, stainless steel, nomex/aramid paper impregnated with phenolic resins) covered with skins of metallic or fibre covering.
Metallic composite sandwiches have attractive mechanical strength characteristics and lower costs than non-metallic composites, but they do not offer the weight advantages of the non-metallic composites, furthermore the use of metal requires additional after-treatments against corrosion, the classical problem of metal structures.
Therefore, composite non-metallic structures bear further investigation.
The fibres that are commonly used are Glass, Carbon and Kevlar, and marginally Boron.
.:: GLASSFIBRE ::.
Glass fibres are the most commonly used due to their low-cost and material characteristics; They are elastic up until almost their break-point. Two main types; E-glass (alumino-borosilicate glass with less than 1 wt% alkali oxides, mainly used for glass-reinforced plastics), whose characteristics change depending on the ambient conditions and thermal treatment, and R-glass (alumino silicate glass without MgO and CaO with high mechanical requirements), which maintains a stable response up to 400°C.:
Several orientations of the fibre-fibres can be used:
.:: KEVLAR ::.
An organic fibre produced by DuPont since 1972 has the trade-name KEVLAR. Of note are variants 29 and 49. The second of these has stronger mechanical properties. Kevlar is more constant resistance when the ratio of length to diameter is varied, but has poor compressibility. Kevlar is available as roving or more commonly as cloth.
.:: CARBON ::.
Is available in different forms depending on the manufacturing method; it is possible to obtain elastic or higher resistance fibres, which anyway maintain a linear elastic characteristic until break-point.
The fibres can be orientated as per glass-fibre.
.:: RESINS ::.
Three types of Resins are commonly used to bond the above fibres together :
Thermoset Resins reach structural hardness levels after a chemical reaction is activated, or heat is applied, to "cure" the fibre/resin combination. They do not liquify when additional temperature is again applied (the curing is irreversible) but they can scorch and carbonize at critical temperatures. Common thermosetting resins are Polyester resin, Vinylester and Epoxy resin. In the aerospace industry, epoxy is used as a structural matrix material or as a structural glue.
Thermoplastic Resins have structural characteristics which are temperature dependent; when heat is applied they soften and melt, and then upon cooling they harden again, without permanent curing.
These resins are polyamides, polypropylene, and PEEK.
Expanding or Foaming resins are porous, due to their nature to absorb air, vapours or gases during their polymerisation,; almost all the resin materials named herein have the property to become foams following a suitable process, producing a finished material with a density of 3-5% of the original liquid. Their mechanical properties decrease proportionally with this reduced density.