TUTORIAL

Using Material Selection Charts

Consider a design problem where the specification is for a component that is both light and stiff (e.g. the frame of a racing bicycle).

To find the best materials we need to use the Young's modulus - density chart from amongst the available charts. The charts can be annotated to help reveal the 'best' materials, by placing a suitable selection box to show only stiff and light materials.

What can we conclude?

The values of Young's modulus for polymers are low, so most polymers are unlikely to be useful for stiffness-limited designs.
Some metals, ceramics and woods could be considered - but composites appear best of all.

This still leaves quite a lot of choices, so what might be considered next to narrow the choice further?

It is unlikely that only 2 material properties matter, so what other properties are important? Let's consider strength and cost - these properties are plotted as another chart.

What can we conclude?

The strength of ceramics is only sufficient for loading in compression - they would not be strong enough in tension, including loading in bending.
Woods may not be strong enough, and composites might be too expensive.
Metals appear to give good overall performance

Once we have identified a promising class of materials, we need to decide which members of this class are the best - for instance which metals?

Selection charts can also be used to select between members of a given class by populating it with the main materials. For instance, we can do this for metals in the stiffness-density chart.

What can we conclude?

Some metals look very good for light, stiff components - e.g. magnesium, aluminium, titanium, while others are clearly eliminated - e.g. lead.
Steels have rather a high density, but are also very stiff. Given their high strength and relatively low cost, they are likely to compete with the other metals.

Let's summarise what we've learnt about materials selection.

Summary:

By considering 2 (or more) charts, the properties needed to satisfy the main design requirements can be quickly assessed.
The charts can be used to identify the best classes of materials, and then to look in more detail within these classes.
There are many other factors still to be considered, particularly manufacturing methods. The selection made from the charts should be left quite broad to keep enough options open. A good way to approach the problem is to use the charts to eliminate materials which will definitely not be good enough, rather than to try and identify the single best material too soon in the design process.

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