Adding index lines and box selections

Index lines are a way of displaying a performance index on a chart. They can be combined with box selections to graphically select materials based on their attribute values.

Index lines and performance indices are a key element of the materials selection methodology that Granta Selector is based on, pioneered by Prof. Mike Ashby at the University of Cambridge. They enable you to quickly identify optimal materials for an application.

Performance index
A ratio of material parameters to optimize in order to maximize the performance of a component, based on the specific function, limiting constraint, and objective of the design.
Index lines
Can be used on a chart to compare the performance index value of different materials.

Performance index

A performance index is defined by four design factors:
  • The function is the basic geometry and load condition of the design (for example: a panel loaded in bending).
  • The limiting constraint is the main criterion to be met for the component to avoid failure (for example: a stiffness-limited design, where a panel fails if it bends too far under an applied load).
  • The objective is the main variable to optimize (for example: minimize mass or cost).
  • The free variable is the geometry parameter that is free to vary with material choice (for example: the thickness of a panel).
    Note: If there is no free variable (it is a fixed design geometry), then you need to use a different methodology that applies to fixed geometry designs.
Each combination of function, limiting constraint, objective, and free variable has a characteristic performance index. For example, a light (objective), stiff (limiting constraint), panel loaded in bending (function), with thickness as the free variable, has a performance index of E1/3/ρ, where E=Young’s modulus (limiting constraint for stiffness) and ρ=density (objective for minimizing mass).

By convention, materials with the highest value of the performance index will be optimal for the design.

Index lines and display lines

Index lines can be used to compare the performance of different materials, and to find replacement materials. Materials that are on the line will all perform equally well in a given design. Materials above the line have a higher performance index and will therefore perform better. Those below the line have a lower index value.

Display lines are similar to index lines, except that they do not apply any selection. They are useful to illustrate how the function of a component (a change in geometry and load condition) influences the choice of materials. Display lines are created by selecting the ‘Show line for display only’ option from the index line window and plotted in the same way.