Page 1 530.352 Materials Selection Lecture #11 Materials Selection Software Tuesday October 4 th,...
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Transcript of Page 1 530.352 Materials Selection Lecture #11 Materials Selection Software Tuesday October 4 th,...
Page 1
530.352 Materials Selection530.352 Materials Selection
Lecture #11 Materials Selection SoftwareTuesday October 4th, 2005
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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
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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.
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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
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Material indices:Material indices:
Performance = f [F,G,M]
p = f [(Functional requirements),
(Geometric constraints),
(Material properties)]
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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?
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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.
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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).
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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
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Table leg design:
Design goals• minimize weight• maximize slenderness
Constraint• resistance to buckling
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Modeling a table leg:
Mass
• m = r2 l
Buckling load
• Pcrit = 2 EI = 3Er 4
l2 4l2
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Minimizing weight :
Mass of legs:
• m = [4P / ]1/4 [l]2 [ / E1/2]
• M1 = E1/2 /
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Criterion for slenderness:
Minimum leg radius
• Pcrit = 3Er 4
4l2
• r = [4P /3 ]1/4 [l]1/2 [1 / E ]1/4
• M2 = E
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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”
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Table leg materials:
Good :• light weight:
– woods ; composites ; ceramics
• slender (stiff)– CFRP ; ceramics
Not good :• polymers (too compliant) ; • metals (too heavy - except Be)
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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
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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”
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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
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Stored energy / mass :
Energy / mass is :
• U/m = 1/4 R2 2
Same for all materials ???
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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]
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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
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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 =