Material selection analysis for the aerodynamic package

4.2.1 Introduction

In this section the different materials from which the aerodynamic package will be made are considered and analysed. The analysis is mainly considering three parts of the package; the two side pods and a single piece nose cone. The reasons being are that these pieces are for one the

Aerodynamics

largest and therefore contribute the most to the overall mass of the car, as well as requiring the most structural strength because failure will lead to an unusable car. For each material the weight, strength, cost and ease of manufacture are considered. A final decision is made using multi-criteria analysis based on the aforementioned design criteria.

4.2.2 Material properties

The options have been narrowed down to four different materials, one metal alloy, two composites and a polymer.

Table 4.1 Properties of the different materials Materials

CFRP 73.85^[3] 1500-2000^[4] 150-250^[4] 1000-1500^[4]

GFRP 2.62^[3] 1500-2500^[4] 80-100^[4] ~1000^[4]

Acrylonitrile butadiene styrene (ABS) has been selected due to its availability, formability, low cost and low weight. Its weight is even lower than that of carbon fibre reinforced polymer (CFRP).

CFRP is very strong, however this comes at a price as it is by far the most expensive of the four materials. Glass fibre reinforced polymer (GFRP) has comparable strength and is a lot cheaper.

The construction process for both CFRP and GFRP is very expensive and time consuming. The aluminium alloy is the second most economical material and has a simple yet relatively

dimensionally inaccurate manufacturing process.

4.2.3 Manufacture

The construction of the body parts follows a different process depending on the type of material used. It is important to consider each manufacturing method in detail as they vary in the

complexity, price and accuracy. The accuracy is of high importance because the shapes of the

Aerodynamics

aerodynamic body parts designed in this chapter need to be manufactured with high dimensional accuracy to act as the experimental results have predicated.

ABS is a thermoplastic which can be vacuum formed into the desired shapes. This process involves heating a sheet of ABS until it becomes workable and then draping it over a mould. The edges are sealed against the mould; a vacuum pressure is then applied from under the mould, forcing the plastic onto the mould increasing dimensional accuracy. The part then pops off the mould as pressure is released ^[2]. This method is a very cost effective way of producing parts; the moulds themselves are inexpensive and can be constructed from a range of cheap materials such as medium density fiberboard (MDF) or polystyrene foam. The cycle can range from 2-10 minutes per part. The downsides to this method are that a lot of material waste is produced and it cannot guarantee that a constant wall thickness is maintained, which means the strength of the parts may be impaired. Companies such as “Airforme” ^[5] provide the service of producing vacuum formed parts. The total cost of their services for the required body parts is estimated at £200.

High performance parts made out of CFRP are often manufactured using the process of vacuum bagging as it prevents small air bubbles forming in the material which will reduce strength. A polished and waxed fiberglass, carbon fibre or aluminium mould has a release agent applied to it.

The fabric and resin can then be applied; the vacuum is pulled and set aside to allow the piece to harden. There are three different ways to apply the resin to the fabric in a vacuum mould. Dry layup is the preferred method as it has the least amount of resin waste and can produce the lightest constructions ^[6]. The cost of manufacture has been estimated from an online supplier ^[7]; producing the moulds and purchasing the equipment will be around £1000. GFRP has a very similar manufacturing process. The difference between these two composites is the cost of the raw materials. This manufacturing method is expensive in terms of both money and time, each part needs one to two days to cure, but it will produce dimensionally accurate parts. The materials used are also a lot stronger.

Finally aluminium sheet is considered. A specialist is required to work large aluminium sheets into the desired shapes; costing a similar amount to the composite manufacturing procedure.

Aerodynamics

Construction of the parts out of aluminium would take less time than the composites however it is very hard to get a similar level of dimensional accuracy during the metal shaping process.

4.2.4 Multi-criteria decision analysis

Multi-criteria decision analysis is a tool for appraising and ranking alternative options against a given set of weighted objectives and criteria. The relevant decision making criteria (strength, cost, weight, ease of manufacture, dimensional accuracy) are assigned a weight from 1-5, one being unimportant and five important. Each material is then given a rating against these criteria; one is a low score and five high. The ratings and weights are multiplied and totalled for each option. The material with the highest weighted score is selected and used to construct the aerodynamic package.

Table 4.2 Multi-criteria analysis for material selection

Criteria Weight ABS Plastic

Aluminium Alloy

(7075-T6) CFRP GFRP

Rating Score Rating Score Rating Score Rating Score

Strength 4 1 4 3 12 4 16 3 12

The weightings have been set with the aim of winning the Formula Student competition. Therefore the criterion which affects the car’s performance is a priority. The weight and dimensional accuracy of each component are directly related to on track speed and have therefore been weighted the highest. Due to the large budget and time scale set out in this report both cost and ease of

Aerodynamics

manufacture are considered less important criteria and have been weighted to reflect this. CFRP has the highest weighted score, largely due to its low density and dimensionally accurate

manufacturing process, and is therefore chosen as the material used for the parts which make up the aerodynamic package.