The simplicity of the Ashby’s maps
With their simplicity, Ashby's maps are an excellent support for a reasoned choice of the material with which to make your parts.
Here is a practical example
Let’s consider a design problem in which you are asked for a light and rigid component at the same time (for example the frame of a normal citybike).
Let’s start with the map “Young’s modulus” / “Density” and draw 2 lines that allow us to highlight only the materials that correspond to our needs:
1. Young’s modulus values for polymers are low (have low rigidity), therefore most polymers seem to be unusable.
2. Some metals, ceramics and woods can be taken into consideration, so composites apparently seem the best solution.
But let’s say right away that it is unlikely that the analysis of only 2 properties is sufficient for the choice of a material, therefore: what other properties is it important to compare before choosing?
So let’s consider a second map that relates “toughness” and “cost”.
What can we observe at this point?
1. The toughness of the ceramic is sufficient only for the compression load – but it is insufficient both in tension and in flexion.
2. Wood is not tough enough while composites may be too expensive.
3. We can safely conclude that the metals seem to give the general performance
Once we have identified a promising class of materials (in this case metals), we have to decide which members of this class are the best.
So we have to ask ourselves: which metal?
Selection tables can be used to select among members of a particular class by populating with the main materials.
For example in this case we can do this for metals in the stiffness-density graph.
What reflections can we make?
Given their high strength and relatively low cost, we can say that steel could represent the best solution.
1. Some metals look very good for lightness and rigidity such as magnesium, aluminum, titanium, while others are clearly eliminated – for example lead.
2. Steels are heavy, but also very rigid. So what metal do we choose?
This of course does not mean that steel is absolutely the best material for the frame of any bike, in fact, depending on the uses, carbon fiber could be considered
nio, aluminum or even wood as you can see in the image alongside; but our “design specification” required “a normal citybike”; do a test by checking the position of these other materials in the maps and you will see that with costs or other parameters you will go outside what can be defined as a normal citybike.
We summarize what we have learned to know about the selection of materials.
• By comparing two (or even more) maps, the properties needed to meet the main design requirements can be quickly assessed.
• 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 yet to be considered, in particular the manufacturing methods, the thickness and shape of the component, the tolerances and more. We say that the selection made by the graphs must be left wide enough to keep a sufficient number of options open.
Ultimately, a good way to deal with the problem is to use the tables to quickly eliminate materials that are certainly not going well and possibly to quickly build a prototype rather than trying to identify the best single material too early in the design process.
Our design specification clearly said “normal city bike” otherwise it would have been easy to be tempted by wood as in this beautiful Selva.
Sources and further reading
Material selection from Wikipedia, the free encyclopedia https://en.wikipedia.org/wiki/Material_selection
Materials Selection for Mechanical Design.
Ashby Map for Stiffness-based and Strength-based Design: youtube video here
What is a Material Property Chart?