ASHBY PLOTS

WHAT ARE ASHBY PLOTS?

• Also known as Material Selection Charts

• It plots two properties of various materials, e.g. density

vs Young’s Modulus or tensile strength vs cost/unit of

tensile strength

• Such plots are used for design decisions that require

maximizing the two properties

• Named for Michael Ashby (1935-), Cambridge Materials

Science Professor

EXAMPLES OF ASHBY PLOTS

HOW DO WE USE ASHBY PLOTS?

• First we determine the criteria we want to apply

• For example: Finding the best material for the rims/spokes in motorcycle

tyres.

• Specifically for members under tension.

• Start with theory: 𝐴 =𝑃

𝜎, 𝑚 = 𝜌𝐿𝐴 =

𝜌𝐿𝑃

𝜎

• Say we want to make the tyre as light as possible

• So to minimize mass, we want to maximize yield stress(failure

strength) to density.

• This assumes that L, P and A are non-negotiable constraints.

HOW DO WE USE ASHBY PLOTS?

• Then we draw a line (or curve) that matches the criteria we want

to apply.

• In this case, it’s a line with a slope of strength over density.

• Then we shift this line up until we reach the best type of material

to apply.

• In this case, it is CFRP or Carbon Fiber Reinforced Polymer

• Should the material prove to be unsuitable (i.e. too expensive),

we shift the line downwards up until we find a more satisfactory

choice.

• In this case, Magnesium alloys which are widely used in

bicycle tyres.

• You can also use Ashby plots to maximize two

requirements, such as beams under axial loads and

beams under a moment.

• To do this, you plot lines that correspond to the

equations you are using, in this case maximizing 𝜎

𝜌 𝑟𝑒𝑑 𝑙𝑖𝑛𝑒𝑠 and

𝜎

𝜌 (𝑏𝑙𝑢𝑒 𝑙𝑖𝑛𝑒𝑠)

• Your ideal choice would be at the intersection of the

two lines.

RESOURCES

• http://en.wikipedia.org/wiki/Material_selection

• http://www.rose-hulman.edu/~stienstr/em203/Ashby