How Do Ashby Charts Help Compare Materials?

• Region of one material group
• Region of another group
• The two property axes

What you will learn

• Describe what an Ashby chart shows and why it has many regions.
• Explain why the chart uses logarithmic scales and straight lines.
• Read how sliding a line on the chart picks a material.

Before you start

Know that density is mass per unit volume, while Young's modulus describes a material's resistance to elastic deformation. A whole part's stiffness also depends on shape; the article on properties and performance explains the distinction.

What does an Ashby chart show?

It shows many materials at once on two properties, so they can be compared instead of judged one by one. The Northwestern University Materials Design text says it is generally more useful to plot properties against one another for a better visual picture of the full property space, and it includes an Ashby plot of tensile strength against density.

Comparison is needed because a part asks for several things at once. The University of Cambridge Young's Modulus - Density selection map package says a balance between different properties must be found to choose the most suitable material, and that this is achieved by finding the appropriate merit index and comparing its value across materials.

Why does the chart use logarithmic scales and straight lines?

Because material properties cover a very wide range of values. The Northwestern text says it is almost always desirable to plot such charts on log scales. The Cambridge package adds that most materials-selection maps are plotted on logarithmic scales.

The scale gives one advantage: a given value of the performance index shows as a straight line. Northwestern gives the case that a line with a slope of one on a double logarithmic plot corresponds to materials sharing the same value of strength divided by density.

• Line for one index value
• Parallel lines for other values

How do you read the chart to pick a material?

Slide the index line in the helpful direction until it touches the best materials. The Cambridge package says to move a line of the correct gradient as far as possible to the top and the left, which means raising stiffness and lowering density, to find materials with the best index value.

The index uses only material properties. The rest, such as the size and load of the part, does not change with the material, so it is left out of the index. For a flat sheet in bending, low density matters a great deal, so wood is favoured; aluminium alloys also perform well in bending and are used in aeroplanes. For a flywheel, the best materials have high strength and low density.

• The slid line
• Selected material region
• Direction of sliding

Real-world example

Imagine choosing a material for a bending board that must be light. You plot stiffness against density, place the index line and slide it up and to the left. For the free-thickness panel in the Cambridge example, the index to maximise is the cube root of Young's modulus divided by density. Young's modulus measures the material's elastic stiffness; density is mass per unit volume. The line must have the slope for that particular index. The source identifies wood and aluminium alloys as promising for lightweight bending applications, but that does not establish that they always rank first or are touched first. This hypothetical case illustrates how to read the chart; it is not a selection result for a specific material grade.

Common misunderstandings

The chart names one winner for every job. Each index belongs to one requirement; change the requirement and the line changes.

Log scales distort the result. They let a very wide range of values fit and turn one index value into a straight line.

The index includes the part's size. The package leaves out values fixed by the application and keeps only material properties.

Frequently asked questions

What is an Ashby chart?

It plots properties of many materials against one another so they can be compared in one property space.

Why use logarithmic scales?

Because material properties span a very wide range, and the scale turns one index value into a straight line.

Which way do you slide the line to choose better?

In the stiffness-against-density example, up and to the left.

Summary

• An Ashby chart compares many materials on two properties in one property space.
• Logarithmic scales turn one performance-index value into a straight line.
• Slide the index line in the helpful direction to pick the best material for the requirement.