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530.352 Materials Selection. Lecture #11 Materials Selection Software Tuesday October 4 th , 2005. Material Selection - the basics:.

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530 352 materials selection
530.352 Materials Selection

Lecture #11 Materials Selection Software

Tuesday October 4th, 2005

material selection the basics
Material Selection - the basics:
  • All materialsScreening: apply property limits /eliminate those who cannot do the jobRanking: apply material indices / find best candidatesSubset of materialsSupporting info:Handbooks, software, WWW, etc.Prime candidatesLocal conditions:in-house expertise or equipmentFinal Material Choice
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.
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
material indices
Material indices:
  • Performance = f [F,G,M]
  • p = f [(Functional requirements), (Geometric constraints),(Material properties)]
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?
function object constraint
Function

Tie

Beam

Shaft

Column

Constraint

Stiffness

Strength

Geometry

Corrosion

Function, object, constraint ...
  • Objective
    • Minimum cost
    • Minimum weight
    • Maximum energy storage
    • etc.
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).
table legs
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
table leg design
Table leg design:
  • Design goals
    • minimize weight
    • maximize slenderness
  • Constraint
    • resistance to buckling
modeling a table leg
Modeling a table leg:
  • Mass
    • m = p r2 l r
  • Buckling load
    • Pcrit = p2 EI = p3Er 4 l2 4l2
minimizing weight
Minimizing weight :
  • Mass of legs:
    • m = [4P / p ]1/4 [l]2 [r / E1/2]
    • M1 = E1/2 /r
criterion for slenderness
Criterion for slenderness:
  • Minimum leg radius
    • Pcrit = p3Er 4 4l2
    • r = [4P /p3 ]1/4 [l]1/2 [1 / E ]1/4
    • M2 = E
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”

table leg materials
Table leg materials:
  • Good :
    • light weight:
      • woods ; composites ; ceramics
    • slender (stiff)
      • CFRP ; ceramics
  • Not good :
    • polymers (too compliant) ;
    • metals (too heavy - except Be)
table leg materials16
Table leg materials
  • M1= E1/2 ; M2 = E r
  • Make Modulus-density chart

Materials M1M2 Comment

wood 5-8 4-20 cheap, reliable

steel 1.8 210 poor M1

CFRP 4-8 30-200 very good, expensive

Ceramics 4-8 150-1000 excellent but brittle

materials for flywheels
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”
stored energy
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 pr R4 t w2
  • The mass of the disk is :
    • m= p R2 t r
stored energy mass
Stored energy / mass :
  • Energy / mass is :
    • U/m = 1/4 R2w2
  • Same for all materials ???
centrifugal stress
Centrifugal stress :
  • Maximum principal stress in a spinning disk of uniform thickness :smax = [(3+ n)/8] r R2w2
  • This sets the upper limit of w ;U/m = [2/(3+n)] [sf / r]M = sf / r[kJ / kg]
materials for flywheels21
Materials for flywheels :

MaterialM [kJ/kg]Comments

Ceramics 200-2,000 Brittle in tension.

CFRP 200-500 best performance good choice.

GFRP 100-400 cheaper than CFRP

excellent choice.

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

Cast iron 8-10 high density

Lead alloys 3 high density

why use lead and cast iron
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 pr R4 t w2
      • M2 = r