1. Material Selection in Mechanical Design
Indiana FIRST / Purdue FIRST Forums
October 24, 2015
Matthew Prall
School of Mechanical Engineering, Purdue University

2. Overview Product Analysis Key Concepts Stress and Strain
Bending and Torsion
Material Selection

3. Product Analysis For a system: For each part of a system:
What does it do?
How does it do it?
Where does it do it?
Who uses it?
What should it cost?
For each part of a system:
What is the function?
What does the geometry look like?
How many are going to be made?
How is it going to be manufactured?
Before making any design decisions, determine the function of the product and each part

4. Product Analysis What is important? Functionality Material properties
Mechanical, physical, electrical, ect…
Geometry
Manufacturability
Cost
Determine the function of the product

5. Product Analysis 3 main factors: Loads Material Geometry
Know 2, solve for the 3rd!
(usually an iterative approach)
Determine the function of the product

6. Most important for robot design:
Key Concepts
How strong is a material?
Toughness
Strength
Stiffness
Resilience
All are different, important, and RELATED terms!
Most important for robot design:
Strength Stiffness

7. Key Concepts Strength versus toughness Stiffness versus resilience
Strength - the resistance of a material to failure due to an applied stress
Toughness - a material’s ability to absorb energy and plastically deform without fracturing
Stiffness versus resilience
Stiffness - the resistance of a material to deflection or deformation due to an applied force
Resilience - a material’s ability to absorb energy when it is deformed elastically and release that energy upon unloading
Strength -
Stiffness -
Toughness – a material’s ability to absorb energy and plastically deform without fracturing [2]
Resilience – a material’s ability to absorb energy when it is deformed elastically, and release that energy upon unloading [3]

8. Material properties are independent of geometry!
Key Concepts
Important Material Properties:
Strength
Yield (Tensile/Compressive)
Ultimate
Fatigue
Flexural Modulus
Young’s Modulus
Poisson's Ratio
Material properties are independent of geometry!
Flexural modulus – tendency for a material to bend
Young’s modulus – linear strain of a material
Poisson’s Ratio – negative ratio of transverse to axial strain

9. Key Concepts Material Terminology: Alloy Brittle versus ductile
A metallic material consisting of two or more elements that cannot be readily separated
e.g. Steel (iron and carbon)
Brittle versus ductile
Brittle materials – little plastic deformation and low energy absorption before fracture
Ductile materials – extensive plastic deformation and energy absorption before fracture
Alloy:
Brittle vs Ductile:

10. Stress and Strain Stress: force, or load, per unit area (Pa, psi)
Can come from: compression (pushing), tension (pulling), shear, bending, torsion (twisting), etc.
Image: Wikimedia Commons

11. Stress and Strain Strain: how much a material deforms
Variation: normal strain (tension/compression)
But what are all these letters?
Force (P), length (L), area (A)
Stress (σ), strain (є), deformation (δ)
Young’s modulus (E) [material dependent]
Hooke’s Law

12. Stress and Strain
Stress versus Strain Diagram

13. Bending and Torsion Bending The letters never stop
Moment / torque (M), height in beam (y)
Curvature (C), radius of bend (R)
Young’s Modulus (E) [material]
Moment of Inertia (I) [geometry]
*Sign convention: tension is positive, compression is negative
Image: Wikimedia Commons

14. Bending and Torsion Torsion A veritable alphabet soup Hooke’s Law
Radius (r), angle (θ), length (L), torque / moment (T)
Shearing strain (γ), shearing stress (τ)
Modulus of rigidity (G) [material]
Polar moment of inertia (J) [geometry]
Hooke’s Law
Image: Wikimedia Commons

15. Bending and Torsion Moments of inertia (I and J)
Essentially, resistance to rotation / bending about a given axis
A cautionary note: There are two types of moment of inertia: area and mass [we’re concerned with area]
For a Rectangle:

16. Mechanical Loads Tension: Structures stretch under tension
Dependent on material and area
It doesn’t matter what shape
Commonly use cables
Compression: Squeeze under compression
Dependent on material, length, and geometry
Shape matters! Want large value for I
Short sections for compression loads (buckling)

17. Mechanical Loads Bending: Torques / moments can also bend
Dependent on geometry, material
Shape still matters, want large I
Max stress near top (tension) and bottom (compression)
Torsion: Torques / moments can twist
Dependent on length, geometry, material
Again, shape matters! Want large J
Shorter members better resist twisting
Stress concentrated near outside (shear)

18. Material Selection Common FIRST Materials Aluminium Steel Plastics
Composites

19. Principal Alloying Element
Material Selection
Aluminium
Common alloys
2024
5052
6061/6063
7068
7075
Can be heat treated or annealed
Strength and price vary
Common structural components
Alloy Series
Principal Alloying Element
1xxx
99.000% Minimum Aluminum
2xxx
Copper
3xxx
Manganese
4xxx
Silicon
5xxx
Magnesium
6xxx
Magnesium and Silicon
7xxx
Zinc
8xxx
Other Elements
Table:
Al info:

20. Material Selection Aluminum comparison 2024-T4 5052-H32 6063-T6
Density (g/cc)
2.78
2.68
2.70
2.69
2.81
Modulus of Elasticity (GPa)
73.1
70.3
68.9
69.0
71.7
Tensile Yield Strength (MPa)
324
193
214
260
503
Fatigue Strength (MPa)
138
117 95
159
Poisson’s Ratio
0.33
Properties: ASM
Square tubing: 6061/6063

21. Material Selection Steel Categories Commonly used for gears, fasteners
Carbon Steel (low, medium, high)
Alloy Steel
Stainless Steel
Tool Steel
Commonly used for gears, fasteners

22. Material Selection Aluminium versus steel Cost
Steel generally cheaper per pound (prices vary)
Strength
Steel is much stronger, but cannot be formed or machined as easily
Weight
Aluminium is much lighter
Strength to weight ratio
Aluminum has a higher ratio

23. Material Selection Plastics Common plastics Much lighter than metals
ABS
Polycarbonate
Acrylic
Much lighter than metals
Lower strength
Non-conductive

24. Material Selection Composites
A combination of two (or more) different materials that produces a material of superior performance than the components
Fiber and matrix components
High strength to weight ratio
Tailor strength in specific directions

25. Material Selection Where to find information: Textbooks Databooks
Manufacturer’s literature
Internet Sites

26. Material Selection cost versus performance Summary:
1. Determine functionality and design requirements
2. Calculate loads and geometry
3. Choose suitable materials
4. Iterate 2 and 3 if necessary
Don’t forget the big picture:
cost versus performance

27. Concluding Thoughts Balance of cost and performance
Loads, material, geometry
Mechanical properties are independent of geometry
Each material has advantages and disadvantages

28. Thank you!
Questions?

29. References
[1] Stuart, Jeff. “Designing for Strength and Durability”. School of Aeronautics and Astronautics, Purdue University, West Lafayette, IN. October 2015.
[2] NDT Resource Center. “Toughness”. The Collaboration for NDT Education, Iowa State University. October
[3] Campbell, Flake C. Elements of Metallurgy and Engineering Alloys. ASM International. p ISBN
[4] Engineering and Materials Education Research Group. “Materials Selection for Engineering Design”. University of Liverpool.