Design Progression and Materials Selection

Clearly, new product design is not a single stage, and usually goes through several steps and iterations before the end design is reached as laid out in the diagram. As these stages are progressed through, the number of valid material selections reduces until the choice comes down to a small number, or perhaps even only one valid choice.

During the concept design stage, possible designs are sketched out as rough ideas. At this point a wide array of materials may be suitable.

Next is the embodiment stage. Here the concept design(s) are refined, overall dimensions and shape are emerging at this stage though they are far from finalised. Here we have narrowed material choice down somewhat to generic classes of materials and processes.  Materials selection will involve ensuring they perform under a certain stress, temperature, environment etc.

Finally we reach the detailed design stage.  At this stage the preferred layout design is fully dimensioned, and materials selection is completed and is limited to one or a few choices.

Figure 3 The design process. © Gareth Bradley

This description makes the design process seem neat, ordered, and straightforward. Almost even boring! In practice of course it is not this simple and design problems are frequently open-ended, there is often no unique solution – but some solutions will be better than others. As discussed over this unit, there is no such thing as the “perfect” material, and some level of compromise will be needed, and choices often conflict, e.g., minimising weight whilst minimising cost (aluminium is more expensive than steel). There is a three-way pull, based on the materials available, the design of the product, and the manufacturing process itself – there is no point in designing a product that cannot be built! It is possible to make a selection by hand based on experience, but often computer software can be utilised to aid the choice of material(s).

Figure 4 The 3-way pull of factors on design. © Gareth Bradley

Materials Selection Example

Constraints are the boundaries that we place on our choices, and they are usually split into two sections. Primary and secondary constraints.

Primary constraints are the requirements or properties that the material must have such as:

• Certain mechanical properties such as strength, hardness, stiffness, fatigue resistance
• Certain behaviour in specific environments, e.g., high temperature, corrosive
• Electrical and/or thermal properties
• Ability to be formed or fabricated to meet the requirements of the object’s shape
• Dimensions
• Cost factor

Secondary constraints on the other hand are the requirements or properties that it is desirable for a property to have. This list would include:

• The quantity of items required and the required production rate
• The dimensional accuracy required
• The surface finish required
• Aesthetics, this might include form, feel, colour, texture etc.
• The size of the items, i.e., both overall size and section thickness
• The requirements of holes, inserts and undercuts
• Environmental considerations in both the manufacture, usage, and disposal of a product
• A cost factor

Obviously the ideal material would meet all the primary and secondary requirements, and possibly even some tertiary ones, but of course the ideal material never exists. For this reason, compromise will be needed, and combinations of properties are important when evaluating a material’s usefulness in solving a problem. Sometimes it can be useful to plot properties against each other, such as:

• Strength v density
• Stiffness v density
• Strength v fracture toughness
• Fatigue strength v cost
• Tensile strength v cost as shown on the plot below

Graph 1 Tensile strength of materials plotted against cost per/kg. © Gareth Bradley

As you would expect, within the design process the usual approach is to select a group of materials that meet the primary constraints. As discussed above, materials selection begins in conceptual and embodiment design stages by using primary constraints, which are imposed by the design or environment – they tend to be given in a high-level manner at this stage such as:

•  e.g. “must be water proof”,
• “the density must be less than 7800 kg/m3”,
• "must not corrode in seawater“

As the design progresses, we would then look to select those materials which maximise the performance of the component from this subset of materials that satisfy the primary constraints. The design of a component often requires the consideration of a combination of properties. A performance index is a grouping of properties which when maximised give the maximum performance of a material. e.g., strength to weight ratio, σ_{tensile strength}/ρ, or stiffness to weight ratio E/ρ.

Let us consider the design of a portable device that requires the stiffness (E) of the material to be between 1 and 10 GPa  and the density (r) to be less than 2000 Kgm^-3

One way we could choose suitable materials is using an elastic modulus/density chart such as that shown. The first shows all materials, the second shows those materials that meet our requirements.

From this subset of materials secondary constraints such as manufacturing processes, dimensional requirements, surface finish etc. are now considered. Often several materials will meet the requirements and the final choice may be further enhanced through the use of a decision matrix.

Consider The Nissan Xterra™ Luggage Rack System (LRS). This was a tubular design  incorporating aluminium side rails and roof supports allowing the attachment of a thermoplastic storage bin and fixed wind deflector.

• The LRS had to meet Nissan’s G64 (dark grey) colour specification.
• LRS typically employ glass and mineral filled polyamide (PA-6) plastics because of their good structural and wear resistant properties.
• Concerns over the weatherability of PA-6 in the G64 colour meant that other plastics were considered.

The decision matrix allowed comparison of all materials that meant the constraints, as you can see.

Table 1 Decision Matrix for the Nissan Xterra(TM) Luggage Rack System.

As you can see from the decision matrix, PA-6 would have been the overall choice, but poor weather-ability meant that it would require painting and increase the cost considerably.  Therefore ASA/PC (acrylonitrile-styrene-acrylate and polycarbonate) was chosen.

Summary

In summary then, materials selection is important because it results in the most suitable materials being chosen for a specific application, it is closely related to the design process, primary and secondary constraints aid the engineer/designer in reducing the number of potential materials to a manageable level and a decision matrix can be used to determine the most suitable material.

Always remember there is no perfect material, and remember some constraints are more important than others.