1. Chapter 5 Materials Selection The Basics
Materials Selection in Mechanical Design, 4th Edition, © 2010 Michael Ashby

2. Figure 5.1
This chapter sets out the basic procedure for selection, establishing the link between material and function
Materials Selection in Mechanical Design, 4th Edition, © 2010 Michael Ashby

3. Figure 5.2
The universe of materials is divided into families, classes, subclasses, and members; each member is characterized by a set of attributes: its properties
Materials Selection in Mechanical Design, 4th Edition, © 2010 Michael Ashby

4. Selection Strategies
Figure 5.3
Required features are constraints; they are used to screen out unsuitable cars. The survivors are ranked by cost of ownership
Materials Selection in Mechanical Design, 4th Edition, © 2010 Michael Ashby

5. Choosing a Material
Figure 5.14
Design requirements are first expressed as constraints and objectives. The constraints are used for screening. The survivors are ranked by the objective, expressed as a material index.
Materials Selection in Mechanical Design, 4th Edition, © 2010 Michael Ashby

6. Strategy for Materials Selection
Figure 5.5
The four main steps:
Translation
Screening
Ranking
Documentation
Materials Selection in Mechanical Design, 4th Edition, © 2010 Michael Ashby

7. Translating Design Requirements
Materials Selection in Mechanical Design, 4th Edition, © 2010 Michael Ashby

8. Figure 5.14
Materials Selection in Mechanical Design, 4th Edition, © 2010 Michael Ashby

9. Materials Selection in Mechanical Design, 4th Edition, © 2010 Michael Ashby

10. Material Indices
Constraints set property limits. Objectives define material indices, for which we seek extreme values.
Materials Selection in Mechanical Design, 4th Edition, © 2010 Michael Ashby

11. Materials Selection in Mechanical Design, 4th Edition, © 2010 Michael Ashby

12. Minimizing Mass: A light, strong tie
Objective Function: equation describing the quantity to be maximized or minimized.
Materials Selection in Mechanical Design, 4th Edition, © 2010 Michael Ashby

13. Eliminating A between these two equations gives:
We can reduce the mass by reducing the cross-section, but there is a constraint: A must be sufficient to carry F*, requiring that:
Eliminating A between these two equations gives:
Material indices are generally expressed so that a
maximum value is sought, so the material index
for a light, strong tie is:
Materials Selection in Mechanical Design, 4th Edition, © 2010 Michael Ashby

14. Minimizing Mass A light, stiff panel
Objective Function
Constraint on Stiffness
Second Moment of Area
Materials Selection in Mechanical Design, 4th Edition, © 2010 Michael Ashby

15. Material index for a light, stiff panel
Material index with a constraint of strength rather than stiffness
Materials Selection in Mechanical Design, 4th Edition, © 2010 Michael Ashby

16. Minimizing Mass A light, stiff beam
Objective Function
Constraint on Stiffness
Second Moment of Area
Materials Selection in Mechanical Design, 4th Edition, © 2010 Michael Ashby

17. Material index for a light, stiff beam
Material index with a constraint of strength rather than stiffness
Materials Selection in Mechanical Design, 4th Edition, © 2010 Michael Ashby

18. Performance Equation
The performance of a structural element is determined by three things: the functional requirements, the geometry, and the properties of the material of which it is made.
Materials Selection in Mechanical Design, 4th Edition, © 2010 Michael Ashby

19. Figure 5.7
The specification of function, objective, and constraint leads to a materials index. The combination in the highlighted boxes leads to the index E1/2/ρ.
Materials Selection in Mechanical Design, 4th Edition, © 2010 Michael Ashby

20. Materials Selection in Mechanical Design, 4th Edition, © 2010 Michael Ashby

21. Translating and Deriving the Index
Materials Selection in Mechanical Design, 4th Edition, © 2010 Michael Ashby

22. Figure 5.8
A schematic E-ρ chart showing a lower limit for E and an upper limit for ρ.
Materials Selection in Mechanical Design, 4th Edition, © 2010 Michael Ashby

23. Ranking: Indices on Charts
Figure 5.9
A schematic E-ρ chart showing guide lines for the three material indices for stiff, lightweight design.
Materials Selection in Mechanical Design, 4th Edition, © 2010 Michael Ashby

24. Figure 5.10
A schematic E-ρ chart showing a grid of lines for the material index M=E1/3/ρ.
Materials Selection in Mechanical Design, 4th Edition, © 2010 Michael Ashby

25. Figure 5.11
A selection based on the index M=E1/3/ρ > 2(GPa)1/3 (Mg/m3) together with the property limit E > 50 GPa. The materials contained in the search region become the candidates for the next stage of the selection process
Materials Selection in Mechanical Design, 4th Edition, © 2010 Michael Ashby

26. Figure 5.12
Computer-aided selection using the CES software. The schematic shows the three types of selection window. They can be used in any order and any combination. The selection engine isolates the subset of materials that passes all the selection stages.
Materials Selection in Mechanical Design, 4th Edition, © 2010 Michael Ashby