Material selection

The weight of the space frame is heavily influenced by material selection. While making the chassis out of a lightweight material is highly desirable, it must be easy to manufacture and cost-effective. There is a large range of materials from which the chassis could be made from. Space frames are typically manufactured out of either mild steel (AS/NZS1163 C350) or chromoly (AISI 4130). Aluminium 6061-T6 was also investigated to see whether it would be a suitable match for the EV. Aluminium 6061-T6 is one of the strongest grades of aluminium available. The mechanical properties of these three materials are shown in Table 2.33.

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Table 2.33: Mechanical properties of potential chassis materials (Callister, 2007)

Property Unit

The strength to weight ratio is very important for the chassis. Table 2.33 shows that aluminium 6061-T6 has a good strength and a very low density, seemly making it the perfect choice for a lightweight chassis.

However, this grade of aluminium requires T6 heat treatment for it to exhibit its high yield and tensile strengths. This means that after welding, it must be heat treated and aged correctly. This adds significant complexity and time (up to 20 hours to quench and age correctly) to the manufacturing process, making it much more difficult and expensive to manufacture. The additional time and expertise required to age and heat treat aluminium will add significant cost to the chassis fabrication. Aluminium also has to be AC TIG welded which is much slower than MIG welding, and the distortion of aluminium from welding will cause issues with ensuring the suspension geometry and mounting points were kept in the correct position.

Another drawback of using aluminium is that it has a lower elastic modulus (Young’s modulus) than mild steel and chromoly. This will mean additional struts and braces would have to be used to stiffen the chassis to match a mild steel or chromoly one. This adds more manufacturing time, cost and complexity. It would also have to be made out of slightly larger tube than the other two steels due to its lower yield stress, but would still produce the lightest chassis. As the EV needs to have a life of at least 10 years, fatigue should also be considered in the material selection for the chassis. Aluminium does not have an endurance limit, therefore, after a repeated number of load cycles it will eventually fail. How quickly it fails is dependent on the magnitude of the load. In an off-road environment, the terrain will impart a lot of vibration and impact loads to the chassis, so the chassis must be able to endure a large number of load cycles. This grade of aluminium is also very expensive and, consequently, its availability is limited.

As seen in Table 2.33, AS/NZS1163 C350, commonly called mild steel, has a good yield strength and a higher Young’s modulus than aluminium. This will mean that it will produce a stiffer chassis and therefore will require less bracing and a reduced manufacturing time. Of the three materials presented here, it will produce the heaviest chassis due to its high density and lower yield strength than chromoly. However, it is advantageous in that it is inexpensive and readily available. It is also easy to manufacture and weld as it does not require any pre- or post-heat treatment or ageing. This will save a significant amount of manufacturing time and cost, simplifying fabrication. It will also mean that the EV can easily and inexpensively be repaired if it is damaged. Contrastingly, if either of the other two materials were used, they would have to be professionally repaired with correct heat-treatment to ensure their structural integrity

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is maintained. This will be expensive and will result in the EV being unoperational for a longer time. Mild steel can easily and quickly be welded with a MIG welder which will further reduce manufacturing time and complexity. Unlike aluminium, steel does have an endurance limit. This is the stress level steel can withstand for an infinite number of load cycles, producing a more durable and long-lasting chassis. Mild steel has a reasonable plastic region whereby it can be identified that it is failing before it breaks, allowing it to be repaired prior to a major fracture.

The final material proposed for the chassis was AISI 4130 steel, commonly called chromoly. It has very good mechanical properties with the exception of density. However, the higher yield and tensile strengths of chromoly mean that smaller tube sections can be used to achieve the same strength as a mild steel equivalent. This ultimately results in a lighter chassis as desired. The major disadvantages of using chromoly include the complex and time consuming manufacturing process and how expensive it is to purchase. It is also not readily available and generally has to be ordered, resulting in increased lead times.

Chromoly also requires a pre- and post-heat treatment during the welding process. During welding, a heat affect zone (HAZ) is created. If proper pre-heat and post-heat treatment is not completed, cracking will occur in the coarse-grained region of the HAZ. The post-heat treatment is required to relieve stresses in the weld. If these treatments are not completed, the chassis’ structural integrity will be greatly reduced. This also means that the frame cannot easily be repaired as it will require proper heat treatment, resulting in an expensive repair and longer downtimes. Chromoly must be TIG welded which is a lot more time consuming than MIG welding. As with mild steel, chromoly has an endurance limit and so it will produce a durable and long-lasting chassis.

Of the three materials proposed, the chassis will be fabricated from mild steel (AS1163 C350). This is because it is cost-effective, readily available, durable and the easiest to manufacture. It does not require any heat treatment or ageing and can be welded using a MIG welder, significantly reducing manufacturing complexity, time and cost. It can also be easily and quickly repaired, minimizing downtime. It produces the desirable mechanical properties of good yield strength, stiffness and good plastic region. It is also durable and ductile so the steel chassis tubes can be bent to the correct shape and the material will stretch prior to failure instead of cracking. The only major disadvantage of utilizing mild steel is the additional weight it will add to the EV. This additional mass will be offset by the reduced cost to purchase materials and manufacture the frame, producing a more commercially viable product than the other two materials. The final mass of the mild steel chassis was determined to be 20.5kg. If chromoly were used this would have resulted in a weight saving of approximately 3-5kg. Although weight saving is desired, this only equates to 1.5-2.5% of the EV’s mass, so will have a very minor effect of the EV’s range.

As the commercial viability of the EV is based on its range and cost, the weight savings and, consequently, the increased range an aluminium or chromoly chassis would produce, is not enough to offset their high cost and difficulty to manufacture. Using aluminium and chromoly would produce a significantly more

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expensive chassis due to the high material price and long, complex manufacturing process. The low lifespan and durability of aluminium also means that it would not be a suitable option for the EV’s chassis.