Material Selection

Matrix Selection (Thermosets vs. Thermoplastics)

There are many factors to be considered when choosing the material to make up the matrix of the composite. There are two types of polymers that are used in composite materials. The first is thermoplastics (TP), which soften on heating. This is because they are often described as linear polymers as there aren’t any cross-links between the chains, which is why when it is heated, it softens as the secondary bonds that bind the chains together melt and it flows like a highly viscous liquid. This can be done reversibly time and time again. The other type of polymer matrices are thermosets (TS) which are a combination of two components, a resin and a hardener, which, when heated together and harden due to a chemical cross-linking reaction causing the liquid to solidify to form an infusible mass. On reheating, the secondary bonds melt, but thermosets aren’t reversible as the cross-links prevent true melting so the polymer cannot be hot-worked as further heating just causes is to decompose.

As confirmed in Principles of Polymer Engineering ^[10], thermosets are more beneficial for motorsport as there is quite a high volume fraction of reinforcing fibres in the material. The low viscosity of the precursor liquid, before it reacts with the hardener, allows the resin to pass through the closely packed array for thorough wetting of the reinforcing fibres. Also, thermosetting resins have greater toughness, and with high temperatures possible close to the engine, it would be impractical to use a reversible material for the matrix if it just ends up melting and returning to a viscous liquid during performance.

Table 5.2 Comparison of properties of polymers ^[11][12]

TS/TP Density,

Epoxy TS 1.2-1.4 2.1-5.5 40-85 100-200 2-5

Polyesters TS 1.1-1.4 2.0-4.5 40-90 120 2

Polypropylene, PP TP 1.2-1.7 1.0-1.4 50-70 40 40-80

Nylons TP 2.0-3.5 1.4-2.8 50-100 60-110 300

The table clearly shows thermosets to be stiffer, both in the Young’s Modulus and the failure strain columns. For motorsport, an epoxy resin is the most popular to be impregnated to the reinforcing fibres, shown by the very high stiffness.

and Packaging Fibre Selection

The major reinforcement fibres consist of Glass fibres, Carbon fibres and Aramid fibres. These are all desired due to their high stiffness and relatively low density. As the main priority for reinforcing fibres is to be light, usually the specific properties are compared, which is the property of interest divided by the density of the material, which are included in the Table 5.3.

Table 5.3 Comparison of the properties of reinforcing fibres [12] [13] [14]

Diameter

Carbon (High Strength) 7 1.75 143 1.83 1.1

Carbon (High Modulus) 7 1.95 200 1.35 0.6

Aramid (Kevlar49) 12 1.45 90 2.07 2.3

It is all very well comparing the strength and stiffness of a fibre on its own, but what is actually relevant is the strength and stiffness of the fibres when they are in an epoxy resin – the composite material behaviour. In the table below are some estimates of the densities, stiffness and tensile strength of UD laminates made up of 50% fibre volume fraction using the Rule of Mixtures equations 5.1, 5.2 and 5.10 from earlier.

Table 5.4 Comparison of UD lamina of 50% fibre/epoxy ratio Density,  (Mg/m³) Lamina Specific

Stiffness, E/ Lamina Specific buckling. The compressive strength can be estimated through the Euler bucking of an individual fibre, as it can be seen as a very long cylindrical rod.

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As the fibres all have very small diameters with respect to the length, buckling is the most likely method of failure in compression, confirmation of the formation of kink bands. The compressive strengths of the fibres in a composite material are very comparable to the tensile strengths, shown in Table 5.4, due to the fibres having some lateral support from the surrounding matrix. However,

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this isn’t the case for Aramid fibres, which posses a compressive strength of around 20% that of its tensile strength. This is due to the structural formation of the fibres, as it is made up of weak van der Waals bonds, which are overcome in compression resulting in fibrillation of the fibre and buckling. This weakness in compression rules out Aramid fibres from selection, as a car will experience a combination of tensile and compressive forces due multiple combinations of loads exerted whilst being driven. Carbon fibres are more expensive than the rest, however they posses the best all round performance for their purpose. High Modulus Carbon fibres have been singled out as the choice for this design, as the main priority is for good torsional stiffness of the vehicle, so the slightly lower tensile strength shouldn’t be an issue as it is still strong enough to survive from the forces exerted on the vehicle under the extreme racing conditions.

Figure 5.10 Processing Acrylonitrile to PAN for manufacture of carbon fibres ^[15]

Looking into some background of how carbon fibres are made, the most popular method is outlined in Figure 5.10, which is from organic precursor fibres called polyacrylonitrile (PAN) ^[16]

which is formed by polymerisation of acrylonitrile. This is obtained by reacting propylene with ammonia and oxygen with the use of catalysts (1). Bulk PAN fibres are isolated and then stretched to align the molecular chains. These stretched fibres are heated where the active nitrile groups react resulting in a ladder polymer (2). Sustaining the tension on the fibres and heating it in an oxygen-containing environment causes further chemical reactions to form cross-links (3).

The numbers in the text represent the stages in the

diagram.

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Further heat treatment of the carbon ring structure at a suitable temperature then produces the desired characteristics of the fibre, high modulus or high strength, as shown in Figure 5.11 below.

Figure 5.11 Relationship between heat treatment and properties of PAN based carbon fibres ^[17]

From the graph it can be seen that high strength carbon fibres need to be heated between 1450-1600˚C, whereas a temperature of more than 2500˚C for high modulus carbon fibres to be made, which are the ones desired for this design.