One procedure for identifying good candidate materials for any specific application may be summarized as follows:
1. Using Table 3.1 as a guide, together with known requirements imposed by operational or functional constraints, postulated failure modes, market-driven factors, and/or man- agement directives, establish a concise specification statementas discussed in 3.2. If information about the application is so sketchy that a specification statement cannot be written, and the remaining steps cannot be executed, it is suggested that 1020 steel
3See ref. 3.
4Many more Ashby charts are presented in ref. 3, as well as a set of charts that are helpful in selecting a suitable manufacturing process.
Youngs modulus, E (GPa)
Density, (Mg/m3)
0.1 0.3 1.0 3 10 30
0.01 0.1 1.0 10 100 1000
1. MODULUS-DENSITY YOUNGS MODULUS E (G = 3E/8; K = E)
CORK
GLASSES Ge Be
SiC
B ALUMINAS
Si3N4
(m/s)
1/2 ENGINEERING
COMPOSITES
MEL PC
NYLON
TIN ALLOYS ZrO2
STEELS
W-ALLOYS WC-Co
PU POLYMERS
FOAMS
Mo ALLOYS
Ti ALLOYS BeO
SIALONS
Cu ALLOYS Zn ALLOYS DIAMOND
Ni ALLOYS
POROUS CERAMICS Mg
ALLOYS
Al ALLOYS KFRP
GFRP CFRP
HDPE PTFE LDPE
POLYESTERS PP
PS EPOXIES WOOD
PRODUCTS
ENGINEERING POLYMERS 3 103
3 102 103
104
LowerE limit for true solids E
=C E1/2
=C E
=C E Si
CFRP UNIPLY
FIR PINE
OAK ASH FIR
BALSA
PINE OAKASH
ROCK, STONE
BALSA SPRUCE
ENGINEERING ALLOYS LAMINATES
GFRP KFRP
PVC PMMA
HARD BUTYL
SOFT BUTYL
SILICONE PLASTICISED
PVC
CEMENT, CONCRETE LEAD ALLOYS POTTERY
PERPENDICULAR TO GRAIN WOODS
ENGINEERING CERAMICS
1/3
PARALLEL TO GRAIN
Guide lines for minimum weight
design
ELASTOMERS
Figure 3.1
Two-parameter Ashby chart for Young’s modulus of elasticity, E, plotted versus density, . The content of the chart roughly corresponds to the data included in Tables 3.4 and 3.9. (The chart is taken from ref. 3, courtesy of M. F. Ashby.)
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be tentatively selected as the “best” material because of its excellent combination of strength, stiffness, ductility, toughness, availability, cost, and machinability.
2. Based on the information from step 1, and the specification statement, identify all spe- cial needsfor the application, as discussed in 3.2, by writing a response of yes, no, or perhapsin the blank following each item in Table 3.1.
Matching Responsive Materials to Application Requirements: Rank-Ordered-Data Table Method 107
Strength,S(MPa)
Density, (Mg/m3)
0.1 0.3 1.0 3 10 30
0.1 1 10 100 1000 10,000
2. STRENGTH-DENSITY Metal and polymers: yield strength Ceramics and glasses: compressive strength Elastomers: tensile tear strength
Composites: tensile failure
Guide lines for minimum weight
design
=C S
=C
2/3 1/2
S
=C S
ENGINEERING ALLOYS CERMETS
GLASSES Si SiC
ZrO2
Ge B
SIALONS
MgO Al2O3
STEELS
Mo ALLOYS Ni ALLOYS Cu ALLOYS Ti
ALLOYS CAST IRONS Al ALLOYS
STONE, ROCK
ENGINEERING ALLOYS
PTFE OAKASH
PINE FIR
ASH PINEOAK FIR BALSA
HDPE
ELASTOMERS LDPE
POLYMERS FOAMS CORK
PU
SOFT BUTYL
PS PP
EPOXIES MEL PMMA ENGINEERING
COMPOSITES
LEAD ALLOYS Zn
ALLOYS
CFRP GFRP
KFRP Be
DIAMOND
SILICONE
UNIPLY
KFRP CFRP
POROUS CERAMICS ENGINEERING
POLYMERS NYLONS
Mg ALLOYS
WOODS
PVC
BALSA PERPENDICULAR
TO GRAIN
PARALLEL
TO GRAIN WOOD PRODUCTS
POTTERY ENGINEERING
CERAMICS
W ALLOYS
GFRP LAMINATES
CEMENT, CONCRETE POLYESTERS
Si3N4
Figure 3.2
Two-parameter Ashby chart for failure strength, S, plotted versus density, . For metals,Sis yield strength, Syp; for ceramics and glass,Sis compressive crushing strength; for composites,Sis tensile strength; for elastomers,Sis tearing strength. The content of the chart roughly corresponds to the data included in Tables 3.3 and 3.4. (The chart is taken from ref. 3, courtesy of M. F. Ashby.)
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3. For each item receiving a yesorperhapsresponse, go to Table 3.2 to identify the cor- responding performance evaluation index, and consult rank-ordered Tables 3.3 through 3.20 for potential material candidates (or similar information from other sources for specific materials data). Using these data sources, write a short list of highly qualified candidate materials corresponding to each identified special need.
Fracture toughness, KIc (MPa m1/2)
Density, (Mg/m3)
0.1 0.3 1.0 3 10 30
0.01 0.1 1.0 10 100 1000
3. FRACTURE TOUGHNESS-DENSITY Data for KIc valid below 10 MPa m1/2 Above 10 MPa m1/2 for ranking only
ENGINEERING ALLOYS Cu
ALLOYS
Ni ALLOYS
CAST IRONS
SiC MgO
DIAMOND
GLASSES POROUS CERAMICS ENGINEERING
POLYMERS
CORK POLYMER
FOAMS
LDPE
PS
PVC
PP NYLONS
HDPE
PC
COMMON ROCKS WC-Co STEELS
Al2O3
Si3N4 ZrO2
OAKASH PINE FIR
GFRP KFRP UNIPLY
Mg ALLOYS
Al ALLOYS GFRP
KFRP CFRP
GFRP
=C KIc
=C KIc
=C KIc
=C KIc
=C KIc
SIALONS
EPOXY ASH MEL
PINEOAK FIR PERPENDICULAR
TO GRAIN
BALSA
PARALLEL TO GRAIN
LAMINATES
ENGINEERING CERAMICS Guide lines
for minimum weight design
Ti ALLOYS
PMMA ENGINEERING
COMPOSITES
BALSA
PLASTER
POTTERY
CEMENT, CONCRETE
SiO2
W- ALLOYS
POLYESTER
ICE WOODS
4/3
4/5
2/3
1/2
Figure 3.3
Two-parameter Ashby chart for plane strain fracture toughness, KIc, plotted versus density, . The content of the chart roughly corresponds to the data included in Tables 2.1 and 3.4. (The chart is taken from ref. 3, courtesy of M. F. Ashby.)
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4. Comparing all rank-ordered lists written in step 3, establish the two or three better can- didate materials by finding those near the tops of all the lists. If a single candidate were to appear at the top of all lists, it would be the clear choice. As a practical matter, com- promises are nearly always necessary to identify the two or three better candidates.
5. From the two or three better candidate materials, make a tentative selection for the mate- rial to be used. This may require additional data, materials selection software packages,
Matching Responsive Materials to Application Requirements: Rank-Ordered-Data Table Method 109
Youngs modulus, E (GPa)
Strength,S (MPa)
0.1 1 10 100 1000 10,000
0.01 0.1 1.0 10 100 1000
4. MODULUS-STRENGTH Metals and polymers: yield strength Ceramics and glasses: compressive strength Elastomers: tear strength
Composites: tensile strength
=C S E
=C S2
E
=C S E
= 0.1 S
= 10– 4 E S E
= 10– 3 S
E S = 10– 2
E
ZrO2 Al2OSi33N4 SiC
MgO
WC BORON
Mo ALLOYS
CAST IRONS
OAKASH PINE
IPS PMMA
PP HDPE WOODS
TO GRAIN
LDPE
PU CORK
POLYMERS FOAMS
PINE
EPOXIES
⎪⎪ TO GRAIN
GFRP LEAD
Sn
BaO Ge
W
DIAMOND
Al ALLOYS COMMON
ROCKS
CFRP
WOOD PRODUCTS
PVC MEL
ENGINEERING POLYMERS ASH
SOFT BUTYL
Max energy storage per unit volume
Buckling before yield ENGINEERING
ALLOYS
Min. energy storage per unit volume Yield before buckling
ENGINEERING COMPOSITES
BRICK, ETC.
BALSA
CERMETS
SILICONE
HARD BUTYL
ELASTOMERS OAK
POLYESTER
NYLONS BALSA
POROUS CERAMICS
ENGINEERING CERAMICS GLASSES
SILICON
BERYLLIUM
Ni ALLOYS Cu ALLOYS Zn ALLOYS
CONCRETE
Mg ALLOYS
LAMINATES
3/2
CEMENT
STEELS
Ti ALLOYS CFRP UNIPLY GFRP
PTFE
Design guide
lines
Figure 3.4
Two-parameter Ashby chart for Young’s modulus of elasticity, E, versus density, . The content of the chart roughly corresponds to the data included in Tables 3.9, 3.3, and 3.11. (The chart is taken from ref. 3, courtesy of M. F. Ashby.)
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optimization techniques that are more quantitative, discussions with materials specialists, or additional design calculations to confirm the suitability of the selection.5
In some cases, mathematical optimization procedures may be available to help estab- lish which of the better candidates should be selected. Such procedures involve writing a
5See, for example refs. 16, 17, 18.
Fracture toughness, KIC (MPa m1/2)
Strength,S (MPa)
0.1 1 10 100 1000 10,000
0.01 0.1 1.0 10 100 1000
7. FRACTURE TOUGHNESS-STRENGTH Metals and polymers: yield strength Ceramics and glasses: compressive strength Composites: tensile strength
Process zone diameter KIc/S
Si4M4 KIc
S2
PINE OAK
ASH
TO GRAIN LDPE
BALSA
EPOXIES HDPE ENGINEERING
POLYMERS
WOODS
PS PC PMMA OAKASH PINE
POLYMERS ICE FOAMS
ENGINEERING CERAMICS GLASSES
POTTERY, BRICK, ETC.
DIAMOND ZrO2 SIC STEELS
Ti ALLOYS Cu ALLOYS
Al ALLOYS
CFRP W-ALLOYS
ENGINEERING ALLOYS
10
1
10–1
10–2
10–3
10
1 10–1 10–2 10– 4 mm
Fracture before yield GFRP
GFRP
LAMINATES CFRP UNIPY
Mg ALLOYS ENGINEERING
COMPOSITES
BALSA
AL2O3 MgO SIALONS
KIc S2
2
(mm)
=C KIc S
=C KIc
S
Yield before fracture
100
PP
100
⎪⎪ TO
GRAIN CAST IRONS
POROUS CERAMICS PLASTERS
CEMENT &
CONCRETE
POLYESTERS COMMON
ROCKS
WOOD PRODUCTS
NYLONS Ni ALLOYS
PVC Guide lines
for safe design
10–3
MEL
2
2
2
Figure 3.5
Two-parameter Ashby chart for plane strain fracture toughness, KIc, plotted versus failure strength, S(see legend of Figure 3.2 for definitions of Sfor various materials classes). The content of the chart roughly corresponds to the data included in Tables 2.1 and 3.4. (The chart is taken from ref. 3, courtesy of M. F. Ashby.)
merit function, defining performance parameters, documenting application constraints, and partially differentiating the merit function (within the constraints) to calculate a figure of meritfor each candidate material. The best figure of merit then establishes the best ma- terial choice. These optimization procedures are discussed in the literature6but are beyond the scope of this text.
6See, for example refs. 15 and 3.
Matching Responsive Materials to Application Requirements: Rank-Ordered-Data Table Method 111
Strength at temperature, S()(MPa)
Temperature, (K) 200
0 100 200 300 400
Temperature (C)
600 800 1000 1400
300 400 600 800 1000 1400 2000
0.1 1 10 100 1000 10,000
13. STRENGTH-TEMPERATURE
Metals and polymers: yield strength at temperature Ceramics: compression strength
Composites: Tensile strength at temperature
PC
PVC PF PP Zn ALLOYS
Mg ALLOYS POLYMIDES
PTFE
Time–
Independent yield strength
Upper limit on strength at
temperature LDPE
SILICONES WOODS
POLYMER FOAMS
HDPE TO
GRAIN TO GRAIN
Al ALLOYS
MULLITES SiC
MgO ENGINEERING
CERAMICS
POROUS CERAMICS
Al2O3
ZrO2
Si3N4 Ni ALLOYS
BRICK, ETC.
STEELS UNIPLY
KFRP
LAMINATES GFRP GFRP
CFRP
PMMA
EPOXIES ENGINEERING
COMPOSITES
ELASTOMERS ENGINEERING
POLYMERS
ICE
ENGINEERING ALLOYS
NYLONS
POLYESTERS
BUTYLS ⎪⎪ TO
GRAIN
COMPRESSION
Ti- ALLOYS
CFRP GLASSES
Range typical of alloy series
Figure 3.6
Two-parameter Ashby chart for failure strength, S(see legend of Figure 3.2 for definitions of Sfor various materials classes), plotted versus ambient temperature, . The content of this chart roughly corresponds to the data included in Table 3.5. (The chart is taken from ref. 3, courtesy of M. F. Ashby.)
®
Computer-aided materials selection systems (CAMSS) are also emerging rapidly as potentially powerful tools for materials selection. Such expert systems, capable of interfac- ing with design teams, consist of three integrated parts connected by search and logic de- duction algorithms: databases, knowledge bases, and modeling or analysis capabilities.
Proprietary in-house databasesexist in many companies, and some are commercially avail- able.7 Computerized knowledge basesare less well developed, requiring a wide range of formulas, design rules, “if-then” rules, manufacturability information, or company-specific
“lessons-learned” files.8
Example 3.1 Materials Selection: Rank-Ordered-Data Table Method
It is desired to select a material for a proposed design for the crankshaft to be used in a new, compact, one-cylinder air compressor. The crankshaft is to be supported on two main bearings that straddle the connecting rod bearing. A preliminary analysis has indicated that the most probable failure modes of concern are fatigue, wear, and yielding. Projected pro- duction rates are high enough so that cost is an important consideration. Select a tentative material for this application.
Solution
Following the five-step process of 3.4, a specification statement is first formulated as follows:
The crankshaft for this application should be short, compact, relatively rigid, fatigue resistant, wear resistant at the bearing sites, and capable of low cost production.
Using this specification statement as a basis, the “special needs” column of Table 3.1 may be filled in as shown in Table E3.1A.
Surveying these results, special needs have been identified for items 1, 6, 10, and 14.
For these special needs, Table 3.2 provides the corresponding performance evaluation in- dices shown in Table E3.1B.
Materials data for these particular performance indices are given in Table 3.3, 3.9, 3.13, 3.18, and 3.19. Making a short list of candidate materials from each of these tables results in the following array:
TABLE E3.1A Table 3.1 Adapted to Crankshaft Application
Crankshaft Application Requirement Special Need?
1. Strength/volume ratio Yes
2. Strength/weight ratio No
3. Strength at elevated temperature No
4. Long-term dimensional stability at elevated temperature No 5. Dimensional stability under temperature fluctuation No
6. Stiffness Yes
7. Ductility No
8. Ability to store energy elastically No
9. Ability to dissipate energy plastically No
10. Wear resistance Yes
11. Resistance to chemically reactive environment No
12. Resistance to nuclear radiation environment No
13. Desire to use specific manufacturing process No
14. Cost constraints Yes
15. Procurement time constraints No
7For example, CMS(see ref. 3) and PERITUS(see ref. 4). CMS implements the Ashby chart selection procedure discussed in 3.5, allowing successive application of up to six selection stages. PERITUSsupports the rank-ordered-data table method discussed in 3.4, with selection based on requesting “high,” “medium,” or
“low” values for pertinent properties.
8See 1.10.
Matching Responsive Materials to Application Requirements: Rank-Ordered-Data Table Method 113
TABLE E3.1B Performance Evaluation Indices for Special Needs
Special Need Performance Evaluation Index 1. Strength/volume ratio Ultimate or yield strength 6. Stiffness Modulus of elasticity 10. Wear resistance Hardness
14. Cost constraints Cost/unit weight; machinability
For high-strength/volume (from Table 3.3):
Ultra-high-strength steel Medium-carbon steel Stainless steel (age hardenable) Stainless steel (austenitic)
High-carbon steel Yellow brass
Graphite-epoxy composite Commercial bronze
Titanium Low-carbon steel
Ceramic Phosphor bronze
Nickel-based alloy Gray cast iron
For high stiffness (from Table 3.9):
Tungsten carbide Steel
Titanium carbide Stainless steel
Molybelenum Cast iron
For high hardness (from Table 3.13):
Diamond Case-hardened low-carbon steel
Sapphire Ultra-high-strength steel
Tungsten carbide Titanium
Titanium carbide Gray cast iron
For low material cost (from Table 3.18):
Gray cast iron Acrylic
Low-carbon steel Commercial bronze
Ultra-high-strength steel Stainless steel Zinc alloy
For good machinability (from Table 3.19):
Magnesium alloy Medium-carbon steel
Aluminum alloy Ultra-high-strength steel
Free-machining steel Stainless-steel alloy
Low-carbon steel Gray cast iron
Surveying these five lists, the materials common to all the lists are:
Ultra-high-strength steel
Low-carbon steel (case hardened) Gray cast iron
For these three candidate materials the specific data from Tables 3.3, 3.9, 3.13, 3,18 and 3.19 are summarized in Table E3.1C.
Example 3.1 Continues
Because the specification statement emphasizes short and compact design, the signif- icantly stronger ultra-high-strength steel is probably the best candidate material; however, the case-hardened low-carbon steel is probably worth a more detailed investigation since it has a higher surface hardness (better wear resistance), is cheaper, and is more easily ma- chined prior to heat treatment. If compact design were not an issue, cast iron would prob- ably be the best choice.
9The complete procedure, as presented by Ashby, is beyond the scope of this text. It involves writing per- formance indices as a function of performance parameters, geometric parameters, and materials properties;
writing a merit function (objective function); and optimizing the merit function to determine a figure of merit. Guidance is given in ref. 3 for determining appropriate mathematical expressions for many useful performance parameters and merit functions, but since a design nearly always involves optimization with respect to manydesign goals (often contradictory), judgment is frequently required to rank the goalsbefore selecting the better materials candidates.
TABLE E3.1C Evaluation Data for Candidate Materials
Candidate Material
Ultra-High- Low-Carbon Steel Gray Evaluation Index Strength Steel (case hardened) Cast Iron
Ultimate strength, , psi 287,000 61,000 50,000
Yield strength, , psi 270,000 51,000 —
Modulus of elasticity, E, psi
Hardness, BHN 560 650 262
Cost, dollars/lb 0.65 0.50 0.30
Machinability index 50 65 40
13-24 * 106 30 * 106
30 * 106 Syp
Su