Material Selection by using CES software

Introduction:

A vehicle jack is being select as a project to find out what kind of materials is suitable. It should able to withstand 500kg of load which is means it must has high yield strength or elastic limit. The price of the material should be cheap as well.

The material which is going to be consider for the vehicle jack:

·         Steel alloy: high yield strength, low cost
·         Composites: high tensile strength, low density
·         Other metal alloys: lower density

By checking from CES software the designer comes out of a solution to distinguish using other metal alloys as it is low yield strength or elastic limit and distinguish composites as well as the percentage composition material price is higher. Consequently, the materials which is going to compare is low carbon steel (AISI 1020), low alloy steel (AISI 4130 steel) and nodular graphite cast iron.

Theory

Generate Performance Index Formula
Material Weight (Mass)

Constraint: Yield strength

Objective: Mass

Constraint equation, σ = F/A              (1)

Objective equation, m (cylinder) = Cross section area, A x Length, L x Density, ρ (2)

Substitute this expression for A from equation (1) into equation (2), m = FL ρ / σ      (3)

The performance index, M₁ = σ /ρ

 

Material Cost

Constraint: Mass

Objective: Minimum material cost

For a component of mass, m kg, which costs £ C/kg, the material cost is m x C       (4)

Substitute this expression for m from equation (3) into equation (4), cost = FLρC/ σ

Hence to minimise material cost the performance index to be maximised is

M₂ = σ/ρC

A Materials Selection Chart can be made where elastic limit is displayed in function of the volume (figure1) and another displayed in function of the material cost per volume (figure 2). Figure 1 is useful to get a clear view on the material density that can be expected for certain material classes while figure 2 is useful to get a clear view on the material costs that can be expected for certain material classes.

Figure 1 show a graph Elastic Limit versus Density. The straight lines with slope = 1 (gradient) is constant. [The performance index, M₁ = σ /ρ; log M₁ = log σ – log ρ. Rearrange, log σ = log ρ + log M₁ = (y = mx + c). So, m = 1]. It is clear that low alloy steel will be low weight with the highest strength among all the materials selection.

Figure 2 show a graph Elastic Limit versus Density x Price. It is clear that nodular graphite cast iron will be expensive for this case. Carbon steel, AISI 1020 is beneficial from the viewpoint of cost but have low strength properties. This graph also shows that low alloy steel, AISI 4130 is more strength, low cost and very advantageous materials.

Figure 3 show a graph Elastic Limit/ Density which is represent performance index M₁. The best result regarding low weight (index M₁) is achieved with low alloy steel, AISI 4130 which is amongst other used in vehicle jack.

Figure 4 show a graph Elastic Limit/ Density/ Price which is represent performance index M₂. With respect to material cost (index M₂), low alloy steel, AISI 4130 give the best result and the selected carbon steel, AISI 1020 is situated closer to the defined lower limit.

Conclusion:

When all performance indices are taken into account, low alloy steel, AISI 4130 appear to be very attractive material for vehicle jack. This low alloy steel has elastic limit of 483-533.8MPa. For this material M₁ = 0.062-0.068 And M₂ = 0.151-0.248 (table).