TUTORIAL

Using Material Selection Charts

Here is a materials selection chart for 2 common properties: Young's modulus (which describes how stiff a material is) and density.

On these charts, materials of each class (e.g. metals, polymers) form 'clusters' or 'bubbles' that are marked by the shaded regions. We can see immediately that:

metals are the heaviest materials,
foams are the lightest materials,
ceramics are the stiffest materials.

But we could have found that out from tables given a bit of time, although by covering many materials at a glance, competing materials can be quickly identified.

Where selection charts are really useful is in showing the trade-off between 2 properties, because the charts plot combinations of properties. For instance if we want a light and stiff material we need to choose materials near the top left corner of the chart - so composites look good.

Note that the chart has logarithmic scales - each division is a multiple of 10; material properties often cover such huge ranges that log scales are essential.

There are a selection charts for many combinations of material properties, e.g. 'strength - toughness' and 'electrical resitivity - cost'. The next section shows how we cantake selection charts further.

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). The Young's modulus - density chart helps us to find the best materials - they lie towards the top left. 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 on another selection chart. So, what else does this tell us about suitable materials classes?

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

We should now be able to identify a promising class of materials, but how do we decide which members of this class are the best. For instance metals look promising, which particular metal should we select?

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.