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530.352 Materials Selection

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

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

  2. 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

  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: • 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?

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

  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 • minimize weight • maximize slenderness • Constraint • resistance to buckling

  11. Modeling a table leg: • Mass • m = p r2 l r • Buckling load • Pcrit = p2 EI = p3Er 4 l2 4l2

  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 = p3Er 4 4l2 • 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”

  15. Table leg materials: • Good : • light weight: • 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 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

  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 pr R4 t w2 • The mass of the disk is : • m= p R2 t r

  19. Stored energy / mass : • Energy / mass is : • U/m = 1/4 R2w2 • Same for all materials ???

  20. 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]

  21. 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

  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 pr R4 t w2 • M2 = r

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