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CHAPTER 22: MATERIALS SELECTION AND DESIGN CONSIDERATIONS

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ISSUES TO ADDRESS... Price and availability of materials. How do we select materials based on optimal performance? Applications:-- shafts under torsion – PowerPoint PPT presentation

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Title: CHAPTER 22: MATERIALS SELECTION AND DESIGN CONSIDERATIONS


1
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.
2
MATERIALS ENGINEERING TETRAHEDRON
3
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.
4
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.
5
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 )

6
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)
7
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)
8
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)
9
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)
10
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)
11
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.
12
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.
13
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,
14
ANNOUNCEMENTS
Ch22-Case Study Failure of an Automobile Rear
Axle, Callister, web page W101.
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