Material Selection in Mechanical Design Indiana FIRST / Purdue FIRST Forums October 24, 2015 Matthew...
-
Upload
giles-murphy -
Category
Documents
-
view
217 -
download
0
Transcript of Material Selection in Mechanical Design Indiana FIRST / Purdue FIRST Forums October 24, 2015 Matthew...
Material Selection in Mechanical Design
Indiana FIRST / Purdue FIRST ForumsOctober 24, 2015
Matthew PrallSchool of Mechanical Engineering, Purdue University
Overview
2
• Product Analysis• Key Concepts• Stress and Strain• Bending and Torsion• Material Selection
Product Analysis
3
For 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?
Product Analysis
4
What is important?• Functionality• Material properties– Mechanical, physical, electrical, ect…
• Geometry• Manufacturability• Cost
Product Analysis
5
3 main factors:Loads Material Geometry
Know 2, solve for the 3rd!(usually an iterative approach)
• How strong is a material?– Toughness– Strength– Stiffness– Resilience
• All are different, important, and RELATED terms!
Most important for robot design:Strength Stiffness
Key Concepts
6
Key Concepts
7
Strength versus toughness• 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
Key Concepts
8
Important Material Properties:• Strength– Yield (Tensile/Compressive)– Ultimate– Fatigue
• Flexural Modulus• Young’s Modulus• Poisson's Ratio
Material properties are independent of geometry!
Key Concepts
9
Material Terminology:• Alloy
– 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
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
Stress and Strain
11
• 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
Stress versus Strain Diagram
Stress and Strain
12
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]
Bending and Torsion
13Image: Wikimedia Commons
*Sign convention: tension is positive, compression is negative
Torsion
• A veritable alphabet soup– 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]
Bending and Torsion
14Image: Wikimedia Commons
Hooke’s Law
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:
Bending and Torsion
15
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)
Mechanical Loads
16
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)
Mechanical Loads
17
Material Selection
18
Common FIRST Materials• Aluminium• Steel• Plastics• Composites
Material Selection
19
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 Element1xxx 99.000% Minimum Aluminum2xxx Copper3xxx Manganese4xxx Silicon5xxx Magnesium6xxx Magnesium and Silicon7xxx Zinc8xxx Other Elements
Material Selection
20
Aluminum comparison
2024-T4 5052-H32 6063-T6 6105-T5 7075-T6Density (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 68.9 95 159
Poisson’s Ratio 0.33 0.33 0.33 0.33 0.33
Material Selection
21
Steel• Categories
– Carbon Steel (low, medium, high)– Alloy Steel– Stainless Steel– Tool Steel
• Commonly used for gears, fasteners
Material Selection
22
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
Material Selection
23
Plastics• Common plastics
– ABS– Polycarbonate– Acrylic
• Much lighter than metals• Lower strength• Non-conductive
Material Selection
24
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
Material Selection
25
Where to find information:• Textbooks• Databooks• Manufacturer’s literature• Internet Sites
Material Selection
26
Summary:1. Determine functionality and design requirements2. Calculate loads and geometry3. Choose suitable materials4. Iterate 2 and 3 if necessary
Don’t forget the big picture:
cost versus performance
Concluding Thoughts
27
• Balance of cost and performance
• Loads, material, geometry
• Mechanical properties are independent of geometry
• Each material has advantages and disadvantages
28
Thank you!
Questions?
References
29
[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 2015. https://www.nde-ed.org/EducationResources/CommunityCollege/Materials/Mechanical/Toughness.htm[3] Campbell, Flake C. Elements of Metallurgy and Engineering Alloys. ASM International. p. 206. ISBN 9780871708670.[4] Engineering and Materials Education Research Group. “Materials Selection for Engineering Design”. University of Liverpool. www.materials.ac.uk/resources/fe/materialsselection.ppt