1. Adam Witthauer (AKA adambomb) Iowa State Univeristy Formula SAE Team Sensei 10-6-11, revised 2-18-13

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 (Ashbys 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, dont weld… FSAE uses: Engine covers Wheel centers

7. Titanium AKA the most hyped material EVER Very few people (including engineers) understand what its 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? MaterialDensity (lb/in 3 ) Youngs Modulus (Msi) Yield Strength (ksi) 6061 Aluminum (heat treated) 0.09751037 1020 Steel0.28429.750.8 Nylon0.0450.510

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 no material 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 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= hA Substitute: w= h*F/Sy or w=Fh* /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 thats along for the ride. May as well call it ß. That leaves us with a more pretty looking: w=ß* /Sy So…if we want small w, then we want small /Sy!!! (or alternately, large Sy/ Sy/ =Material index F F

13. Ashbys Materials Selection Charts! How can I find materials with large Sy/? Guide Lines: -Lines with a constant Sy/ -All materials along one guide line have equal Sy/ -Going up and left leads to larger Sy/ -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 CARBON FIBER YO

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

15. Ashbys Materials Selection Charts! How can I find materials with large Sy 2/3 /? Guide Lines: -Lines with a constant Sy 2/3 / -All materials along one guide line have equal Sy 2/3 / -Going up and left leads to larger Sy 2/3 /