Page 1

530.352 Materials Selection530.352 Materials Selection

Lecture #11 Materials Selection SoftwareTuesday October 4th, 2005

Page 2

Material Selection - the basics:Material Selection - the basics:

All materialsScreening: apply property limits /

eliminate those who cannot do the job

Ranking: apply material indices / find best candidates

Subset of materialsSupporting info: Handbooks, software, WWW, etc.

Prime candidatesLocal conditions: in-house expertise or equipment

Final Material Choice

Page 3

Deriving property limits: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.

Page 4

Deriving material indices: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

Page 5

Material indices:Material indices:

Performance = f [F,G,M]

p = f [(Functional requirements),

(Geometric constraints),

(Material properties)]

Page 6

Function, objective, constraint:Function, objective, constraint:

Function: • 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?

Page 7

Function, object, constraint ...Function, object, constraint ...

Function• Tie• Beam• Shaft• Column

Constraint• Stiffness• Strength• Geometry• Corrosion

Objective• Minimum cost• Minimum weight• Maximum energy

storage• etc.

Page 8

Procedure for deriving material indices: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).

Page 9

Table legs:

Goal: light weight coffee table of daring simplicity: a flat sheet of glasswith slender light weight legs.

Legs must:• be solid• be light as possible• support a load P without buckling

Page 10

Table leg design:

Design goals• minimize weight• maximize slenderness

Constraint• resistance to buckling

Page 11

Modeling a table leg:

Mass

• m = r2 l

Buckling load

• Pcrit = 2 EI = 3Er 4

l2 4l2

Page 12

Minimizing weight :

Mass of legs:

• m = [4P / ]1/4 [l]2 [ / E1/2]

• M1 = E1/2 /

Page 13

Criterion for slenderness:

Minimum leg radius

• Pcrit = 3Er 4

4l2

• r = [4P /3 ]1/4 [l]1/2 [1 / E ]1/4

• M2 = E

Page 14

CES Software: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”

Page 15

Table leg materials:

Good :• light weight:

– woods ; composites ; ceramics

• slender (stiff)– CFRP ; ceramics

Not good :• polymers (too compliant) ; • metals (too heavy - except Be)

Page 16

Table leg materials

= 1/2 ; M2 = E

Make Modulus-density chart

Materials M1 M2 Commentwood 5-8 4-20 cheap, reliablesteel 1.8 210 poor M1 CFRP 4-8 30-200 very good, expensiveCeramics 4-8 150-1000 excellent but brittle

Page 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”

Page 18

Stored energy :

For a disc of radius (R) and thickness (t) rotating with angular velocity (), the energy (U) stored in the flywheel is :

• U = 1/2 J 2 = 1/4 R4 t 2

The mass of the disk is :

• m= R2 t

Page 19

Stored energy / mass :

Energy / mass is :

• U/m = 1/4 R2 2

Same for all materials ???

Page 20

Centrifugal stress :

Maximum principal stress in a spinning disk of uniform thickness :

max = [(3+ )/8] R2 2

This sets the upper limit of ;

U/m = [2/(3+)] [f / ]

M = f / kJ / kg]

Page 21

Materials for flywheels :

Material M [kJ/kg] Comments

Ceramics 200-2,000 Brittle in tension.

CFRP 200-500 best performancegood choice.

GFRP 100-400 cheaper than CFRPexcellent choice.

Steel, Al, Ti, Mg 100-200 Steel cheapest

Cast iron 8-10 high density

Lead alloys 3 high density

Page 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 R4 t 2

– M2 =