1. 530.352 Materials Selection Lecture #11 Materials Selection Software
Tuesday October 4th, 2005

2. Material Selection - the basics:
All materials Screening: apply property limits / eliminate those who cannot do the job Ranking: apply material indices / find best candidates Subset of materials Supporting info: Handbooks, software, WWW, etc. Prime candidates Local conditions: in-house expertise or equipment Final Material Choice

3. Deriving property limits:
Simple limits on material properties can be used to eliminate possible materials e.g.
Toperating = 250o C
Electrically insulating
must be available in wire form
etc.

4. Deriving material indices:
Combination of material properties
Used when component characteristics can be achieved in more than one way: e.g. high stiffness
high modulus
increasing the cross-section
changing the shape

5. Material indices: Performance = f [F,G,M]
p = f [(Functional requirements), (Geometric constraints), (Material properties)]

6. Function, objective, constraint:
what does component do?
Objective:
what is to be maximized -or- minimized?
Constraints:
what non-negotiable conditions must be met?
what other conditions are desired?

7. Function, object, constraint ...
Tie
Beam
Shaft
Column
Objective
Minimum cost
Minimum weight
Maximum energy storage
etc.
Constraint
Stiffness
Strength
Geometry
Corrosion

8. Procedure for deriving material indices:
Define design requirements
Develop an equation for the objective in terms of functional requirements, geometry and material properties.
Identify the free (unspecified) variables.
Develop constraint equations.
Substitute for the free variables.
Group the variables into three groups and determine: p = f1(F),f2(G),f3(M)
Identify the Material Index (M1).

9. Table legs: Goal: light weight coffee table of
daring simplicity: a flat sheet of glass
with slender light weight legs.
Legs must:
be solid
be light as possible
support a load P without buckling

10. Table leg design: Design goals Constraint minimize weight
maximize slenderness
Constraint
resistance to buckling

11. Modeling a table leg: Mass Buckling load m = p r2 l r
Pcrit = p 2 EI = p 3Er l l2

12. Minimizing weight : Mass of legs: m = [4P / p ]1/4 [l]2 [r / E1/2]
M1 = E1/2 /r

13. Criterion for slenderness:
Minimum leg radius
Pcrit = p 3Er l2
r = [4P /p3 ]1/4 [l]1/2 [1 / E ]1/4
M2 = E

14. CES Software:
CES software available in the HITS Computing Lab (Krieger 160) or Senior Design Computer Lab.
Access it the following way:
1. Click “Start” menu
2. Go to “Programs”
->”Engineering Applications”
->“CES”
-> “CES Selector”
Why Hawaii ??
thousands of miles of thermally stable ocean.
no nearby mountain ranges to roil the upper atmosphere
or throw light-reflecting dust into the sky
Few city lights
Clear, calm and dry 300 nights per year.

15. Table leg materials: Good : Not good : light weight: slender (stiff)
woods ; composites ; ceramics
slender (stiff)
CFRP ; ceramics
Not good :
polymers (too compliant) ;
metals (too heavy - except Be)

16. Table leg materials M1 = E1/2 ; M2 = E r Make Modulus-density chart
Materials M M2 Comment
wood cheap, reliable
steel poor M1
CFRP very good, expensive
Ceramics excellent but brittle

17. Materials for Flywheels :
Flywheels store energy
Current flywheels are made out of :
children’s toys
lead
steam engines
cast iron
modern electric vehicles
HSLA steels and composites
Efficiency measured in “stored energy per unit weight”

18. Stored energy :
For a disc of radius (R) and thickness (t) rotating with angular velocity (w), the energy (U) stored in the flywheel is :
U = 1/2 J w2 = 1/4 p r R4 t w2
The mass of the disk is :
m= p R2 t r

19. Stored energy / mass : Energy / mass is : Same for all materials ???
U/m = 1/4 R2 w2
Same for all materials ???

20. Centrifugal stress :
Maximum principal stress in a spinning disk of uniform thickness : smax = [(3+ n)/8] r R2 w2
This sets the upper limit of w ; U/m = [2/(3+n)] [sf / r] M = sf / r [kJ / kg]

21. Materials for flywheels :
Material M [kJ/kg] Comments
Ceramics ,000 Brittle in tension.
CFRP best performance good choice.
GFRP cheaper than CFRP
excellent choice.
Steel, Al, Ti, Mg Steel cheapest
Cast iron high density
Lead alloys 3 high density

22. Why use lead and cast iron ??
Children’s toys use these -- why ??
Cannot accelerate to the burst velocity
If angular velocity is limited by the drive mechanism (pull string) then :
U = 1/4 p r R4 t w2
M2 = r