CHAPTER 22: MATERIALS SELECTION AND DESIGN CONSIDERATIONS

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CHAPTER 22 MATERIALS SELECTION AND DESIGN
CONSIDERATIONS
ISSUES TO ADDRESS...
Price and availability of materials.
How do we select materials based on optimal
performance?
Applications -- shafts under torsion --
bars under tension -- plates under bending
-- materials for a magnetic coil.
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MATERIALS ENGINEERING TETRAHEDRON
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PRICE AND AVAILABILITY
Current Prices on the web e.g.,
http//www.metalprices.com -- Short term
trends fluctuations due to supply/demand.
-- Long term trend prices will increase as rich
deposits are depleted.
Materials require energy to process them
-- Cost of energy used in processing
materials (/MBTU or GJ)
-- Energy to produce materials (GJ/ton)
237 (17) 103 (13) 97 (20) 20 13 9
Al PET Cu steel glass paper
elect resistance propane oil natural gas
25 17 13 11
(BTU British thermal unit1.06 kJ)
Energy prices from http//www.npga.org/i4a/pages/i
ndex.cfm?pageid914
Energy using recycled material indicated in green.
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RELATIVE COST, c, OF MATERIALS
Reference material -- Rolled A36 plain
carbon steel. Relative cost, ,
fluctuates less over time than actual
cost.
Based on data in Appendix C, Callister, 7e. AFRE,
GFRE, CFRE Aramid, Glass, Carbon fiber
reinforced epoxy composites.
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PERFORMANCE INDEX

• The design of a mechanical component is
  specified by three things
• F the specified Functional Requirements
• the functional requirements (the need to carry
  loads, transmit heat, store elastic energy or
  thermal energy, etc)
• 2. G the specified Geometrical Parameters, the
  geometry, and
• 3. M the Material Properties
• the properties of the material of which it is
  made, including its cost.
• p the performance of the element that is to be
  optimised ( its mass, volume, cost or life for
  example )

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STIFF LIGHT TENSION MEMBERS
Bar must not lengthen by more than d under
force F must have initial length L.
-- Stiffness relation
-- Mass of bar
(s Ee)
Eliminate the "free" design parameter, c
minimize for small M
specified by application
Maximize the Performance Index
(stiff, light tension members)
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STRONG LIGHT TENSION MEMBERS
Bar must carry a force F without failing
must have initial length L.
-- Strength relation
-- Mass of bar
Eliminate the "free" design parameter, c
minimize for small M
specified by application
Maximize the Performance Index
(strong, light tension members)
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STRONG LIGHT TORSION MEMBERS
Bar must carry a moment, Mt must have a
length L.
-- Strength relation
-- Mass of bar
Eliminate the "free" design parameter, R
specified by application
minimize for small M
Maximize the Performance Index
(strong, light torsion members)
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DATA STRONG LIGHT TENSION/TORSION MEMBERS
Strength, sf (MPa)
4
10

Ceramics

Cermets
3
10

PMCs
Steels


Metal

grain
2
10

alloys
Adapted from Fig. 22.2, Callister 7e. (Fig. 22.2
adapted from M.F. Ashby, Materials Selection in
Mechanical Design, Butterworth-Heinemann Ltd.,
1992.)
wood
Polymers
10


grain

1

0.1


0.1

1

10

30

Density, r (Mg/m3)
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DATA STRONG LIGHTBENDING MEMBERS
Maximize the Performance Index
Increasing P for strong bending
members


4

10

Ceramics

Cermets

3
10

PMCs

Steels

Strength, sf (MPa)

grain
Metal

2
10

Adapted from Fig. 6.22, Callister 6e. (Fig. 6.22
adapted from M.F. Ashby, Materials Selection in
Mechanical Design, Butterworth-Heinemann Ltd.,
1992.)
alloys

wood
Polymers

10



1

slope 2
0.1

0.1

1

10

3
0



Density, r (Mg/m3)
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DETAILED STUDY I STRONG, LIGHT TORSION MEMBERS
Maximize the Performance Index
Other factors --require sf gt 300 MPa.
--Rule out ceramics and glasses KIc too small.
Numerical Data
material CFRE (vf 0.65) GFRE (vf 0.65) Al
alloy (2024-T6) Ti alloy (Ti-6Al-4V) 4340 steel
(oil quench temper)
r (Mg/m3) 1.5 2.0 2.8 4.4 7.8
P (MPa)2/3m3/Mg 73 52 16 15 11
tf (MPa) 1140 1060 300 525 780
Data from Table 22.1, Callister 7e.
Lightest Carbon fiber reinforced epoxy
(CFRE) member.
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DETAILED STUDY II STRONG, LOW COST TORSION
MEMBERS
Minimize Cost Cost Index M /P
(since M 1/P)
where M mass of material
relative cost
Lowest cost 4340 steel (oil quench temper)
Need to consider machining, joining costs also.
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SUMMARY
Material costs fluctuate but rise over the
long term as -- rich deposits are
depleted, -- energy costs increase.
Recycled materials reduce energy use
significantly. Materials are selected based
on -- performance or cost indices.
Examples -- design of minimum mass, maximum
strength of shafts under torsion,
bars under tension, plates
under bending,
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ANNOUNCEMENTS
Ch22-Case Study Failure of an Automobile Rear
Axle, Callister, web page W101.