MATERIAL SELECTIONPART 1: CHOOSING MATERIAL PER LOADING CASE (MATERIAL INDICES)

Adam Witthauer (AKA adambomb)Iowa State Univeristy Formula SAE Team Sensei

10-6-11, revised 2-18-13

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)

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

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

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

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

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

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

Common material properties

Which is lighter? Which is stiffer? Which is stronger?

Material Density (lb/in3)

Young’s Modulus (Msi)

Yield Strength (ksi)

6061 Aluminum (heat treated)

0.0975 10 37

1020 Steel

0.284 29.7 50.8

Nylon 0.045 0.5 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

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

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=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

F

Ashby’s Materials Selection Charts!

How can I find materials with large Sy/r?

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

CARBON FIBER

YO

Example 2: Bending 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

F

M

M=F*L

Ashby’s Materials Selection Charts!

How can I find materials with large Sy2/3/r?

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