Material Selection Part 1: Choosing material per loading case (Material Indices)

1. Adam Witthauer (AKA adambomb) Iowa State Univeristy Formula SAE Team Sensei 10-6-11, revised 2-18-13 Material SelectionPart 1: Choosing material per loading case (Material Indices)

2. Material Selection • Main characters: Low-carbon steel, Chromoly, Al, Mg, Ti, Nylon, CFRP • What is a material index? • Finding material indices for different load cases • Axial load • Bending • Material Selection Charts (Ashby’s Charts)

3. Low-Carbon Steel • AKA plain steel, mild steel, AISI 1008-1020 • Most common industrial material • Lowest cost/strength of all materials • Excellent weldability and formability, toughness • FSAE uses: • Chassis • Brake rotors

4. Chromoly (Chromium-Molybdenum steel alloy) • AISI 4130, 4140, 4340 • 1.5-5x as strong as mild steel (depending on heat treat) • Common uses: • Motorsports • Light aircraft • Strength and stiffness per weight comparable to aircraft Al • FSAE uses: • Suspension components • Axle shafts, drive hubs • Pedals

5. Aluminum • AKA Aluminium. • AISI 11xx, 2xxx, 3xxx, 5xxx, 6xxx, 7xxx, 8xxx • 7000 series strongest commonly available, AKA aircraft aluminum • Good castability, machinability (when heat treated), poor formability, poor welding • FSAE uses: • Suspension uprights & hubs • Final drive housings and supports • Pedals, brake calipers, master cyl. • Engine cases

6. Magnesium • AKA “Mag” • Strength/weight comparable to Al, excellent stiffness/weight • Best castabilitygood for thin castings, good machinability, poor formability, don’t weld… • FSAE uses: • Engine covers • Wheel centers

7. Titanium • AKA the most hyped material EVER • Very few people (including engineers) understand what it’s good for • Strength/weight slightly better than Al, Mg • Stiffness/weight similar to steel, Al • Low hardness, toughness, heat capacity, thermal cond. • Expensive, poor castability, poor machinability, poor formability, have to weld in an inert environment • Niche aerospace and top-end engine applications (valves, connecting rods) Places you can justify spending 5x as much for a 10% improvement • FSAE uses: • Engine valves

8. Plastic • Nylon, UHMW, PTFE, ABS, PVC, Polyeurthane, etc. • Low strength/weight, stiffness/weight • Lowest density (good for size-constrained applications) • Low friction • Excellent toughness • Low cost • Excellent machinability (nylon), injection mold (ABS) • FSAE uses: • ARB bushings • IGUS bushings: • Steering • Shifter, clutch • Pedals • Random spacers • Engine mounts

9. Common material properties • Which is lighter? • Which is stiffer? • Which is stronger?

10. How can we compare these values??? • Aluminum: 1/3 the density of steel, 1/3 the stiffness of steel, 60% of the yield strength of steel • Plastic: ½ the density of Al, 1/20 the stiffness of Al, ¼ the strength of Al

11. How can I select a light material for my design? • Step 1: There is nomaterial • Do basic strength / stiffness / whatever analysis to isolate materials properties before you even think about what material you SHOULD use

12. Example 1: Axial loading: Minimize weight per strength F • Round bar,x-sectional area A, length h, force F, weight w • Will be lightest when: • σ=Sy • or Sy=F/A • so A=F/Sy • Weight equals: • w=rhA • Substitute: • w=rh*F/Sy • or w=Fh*r/Sy • Looking at this equation, two of the terms on the right are materials properties that we care about. F*h is just a constant that’s along for the ride. May as well call it ß. That leaves us with a more pretty looking: • w=ß* r/Sy • So…if we want small w, then we want small r/Sy!!! (or alternately, large Sy/r) • Sy/r =Material index F

13. How can I find materials with large Sy/r? CARBON FIBER YO Ashby’s Materials Selection Charts! Guide Lines: -Lines with a constant Sy/r -All materials along one guide line have equal Sy/r -Going up and left leads to larger Sy/r -So now you just slide that line to a new guide line (parallel line), and all the ones along that line perform similarly -”Optimize” (yes, the o-word) by sliding that bad boy as FAR UP as you can

14. Example 2: Bending F • Square beam in bending, force F, length L, x-sectional area b2, edge length b • Will be lightest when: • σ=Sy=6FL/b3 • b=(6FL/Sy)1/3 • Weight equals: • w=b2rL • Substituting: • w=rL(6FL/Sy)2/3 • w=r/Sy2/3*[other junk] • So to minimize w, maximize: • Sy2/3/r M M=F*L

15. How can I find materials with large Sy2/3/r? Ashby’s Materials Selection Charts! Guide Lines: -Lines with a constant Sy2/3/r -All materials along one guide line have equal Sy2/3/r -Going up and left leads to larger Sy2/3/r