Bab 3 S
Bab 3 S
Bab 3 S
MATERIALS
SELECTION
BDA20402
1
▪Course coordinator
TEST 2 :
Most of the material selection charts are plotted using logarithmic scales. We’re not going to go
into the details of the maths of logarithms here, but will just give an idea of how and why we use
them.
Logarithms are related to ‘times’ scales - for example in the Richter scale for earthquakes, an increase in 1
point on the scale corresponds to an increase of 10 times more energy released. The decibel scale used for
sound is another common example of a log scale.
So we need a scale which looks something like this:
Copper alloys
Lead alloys
Nickel alloys
Molybdenum alloys
Melamines
Polycarbonate
Polyesters
Polyformaldehyde
Polymethylmethacrylate
Polytetrafluoroethylene
Teflon
Polyvinylchloride
E=Stress/Strain
E Ceramic
E Metal or
Materials Selection 22
Polymer
BDA 20402
Strength
F F
The mass : m = . V = A l
The strength : Ff = A f, A= Ff/f
m = l . F . ( / f)
F/m = l . (f / )
All materials with same f/ ratio
behave equally well.
Materials Selection BDA 20402 48
The material Index
In the objective function the function
can be split into a geometry index
and a material index, M
Slope 1
Equal performance
poorer
1. < 1 Mg/m3
2. f > 10 MPa
3. weight
design,
M <= f2/3/
= 27
Mpa/Mgm-3
1. < 1 Mg/m3
2. f > 10 MPa
3. weight
design,
M <= f2/3/
= 27
Mpa/Mgm-3
1. < 1 Mg/m3
2. f > 10 MPa
3. weight
design,
M <= f2/3/
= 27
Mpa/Mgm-3
1. < 1 Mg/m3
2. f > 10 MPa
3. weight
design,
M <= f2/3/
= 27
Mpa/Mgm-3
1. < 1 Mg/m3
2. f > 10 MPa
3. weight
design,
M <= f2/3/
= 27
Mpa/Mgm-3
1. < 1 Mg/m3
2. f > 10 MPa
3. weight
design,
M <= f2/3/
= 27
Mpa/Mgm-3
Selected areas
for potential
materials
T1/2/ =2
= T1/2/2
At
T = 400 K, = 10 MPa
T = 1600 K, =20 MPa
Slope :
log M = ½ log T –
log
slope, m = 1/2
Stiffness = F/ = C . E. I / l3 =
= C E A2 / 12 l3
beam
rod
1. < 3 (Mg/m3)
2. M = f2/
M≤20(MPa/Mgm-3)
Done by:
- examining the design requirement to identify the
constraints that they impose on material choice.
–Supporting Information
–Property Limits
–Material Indices
Screening
– elimination of candidates materials which cannot do
the job at all because one or more of their attributes
lies outside the limits imposed by the design.
Ranking
– listing of materials according to their capability to
give the best performance for certain application.
Subset of Materials
Supporting Information:
Handbooks, specialized software, expert systems, CD-ROMS,
WWW
(Search “family history” of candidates)
Prime Candidates
Local conditions
(Does the choice match local needs, expertise?)
Figure 3.5: The strategy for materials selection. The main steps are enclosed in
bold boxes (M.F. Ashby, 1999)