selection process overview - computer-aided...
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Selection Process Overview
• At concept (preliminary) level
– Analyze material requirements (from PDS)
• Conditions of service and operating environment
• Translate conditions to material properties
– Screen for candidate materials
• Compare req’d properties with a materials database to select candidate materials
• During embodiment phase
– Analysis of candidate materials in terms of tradeoffs in performance, cost, manufacturability
Material Selection and Design
• Selecting an appropriate material is a critical part of almost all engineering designs.
• A methodology for selecting materials and processes which is driven by design, uses as inputs the functional requirements of the design.
• There are many factors to consider
– Mechanical Properties:• Strength, stiffness, ductility, fracture toughness , fatigue, creep…
– Physical Properties:• Density, melting point, thermal/electrical conductivity…
– Other• Cost, corrosion, formability, etc
Material Selection
Basic Methods
Translation:
express design requirements as constraints and objectives.
Screening:
eliminate materials that cannot do the job.
Ranking:
find materials that best do the work using performance indices.
Research Supporting Info:
handbooks, expert systems, web, etc.
Translation (express design requirements as constraints and objectives)
Function: What does the component do?
Objective: What essentials conditions must be met?
Constraints: What is to be maximized or minimized?
Free Variables: Identify which design variables are free?
• Which can be modified?
• Which are desirable?
Screening(Eliminate materials that cannot do the job)
• Need an effective way to evaluate a large range of material properties and classes.
• Methods to evaluate materials
– Material Bar Charts
– Material Property Charts
(for example, density vs. Young’s Modulus)
Material Bar Charts
Metals Polymers Ceramics Hybrids
PEEK
PP
PTFE
WC
Alumina
Glass
CFRP
GFRP
Fibreboard
Steel
Copper
Lead
Zinc
Aluminum
Material Selection Charts
Screening Example:Heat Sink for Power Electronics
Retain materials with:
1. Max service temp > 200 C
2. “Good insulator”, or R > 1019 mohm.cm
3. “Good T-conductor” or T-conduct. > 100 W/m.K
• Function:
• Heat sink
•Constraints:
• operate at 200 C
• be electrical insulator
• conduct heat well
• Free variable:
• Choice of material
Therefore…
2000C
Screening using Bar Charts (max service temperature >200oC)
Metals Polymers Ceramics
Composites
PEEK
PP
PTFE
WC
Alumina
Glass
CFRP
GFRP
Fibreboard
Steel
Copper
Lead
Zinc
Aluminium
Screening using Property Charts
R=1019
λ=100
Screening using Material Property Charts (>100W/m.K, e>1019m.cm)
Ranking(find the materials that do the job best)
• What if multiple materials remain after screening?
• Which one is best?
• What if there are multiple material parameters for evaluation?
Use the Material Performance Index
Start with Translation
Rank on Objectives because objectives define performance metrics
Example: Tie Rod
• Function: Support a tensile load
• Objective: Minimize mass
• Constraints: Required length (L)
Load carrying capability w/o failure
• Free Variables: Cross-sectional area
Material
Write design requirements as equations.
• Objective
m = A*L*
• Constraint
F/A < sy
Eliminate free variable
m ≥ FL (/sy)
therefore, minimize weight by maximizing sy /
Using the Performance Index
• Method
1) Identify function, constraints, objective and free variables (as before)
2) Write equation for objective (the Performance Equation)
3) Define combination of material properties that maximize performance (the Material Index)
4) Use these for ranking
The Performance Equation
P ,properties
Material
G ,parameters
Geometric,
F ts,requiremen
FunctionalP
or P= f (F,G,M)
Use constraint to eliminate free variable
Performance equation for Tie Rod example
m ≥ FL (/sy)
Example: material for stiff, light beam
• Function:Support a bending load
• ObjectiveMinimize mass
• ConstraintsLength
Carry load F with minimal deflection
• Free VariablesCross-sectional area
Material
F
Deflection, dBeam has length = L and area = A
Objective:
m = AL
Constraint:
3L
CEIFS
d
Beam Example con’t
• Objective:
m = AL
• Constraint:
• Solve for “m” substitute and rearrange.
3L
CEIFS
d
2/12/1
2/52/14
d
C
LFm
Material Indices
Minimize weight by minimizing
or by maximizing
2/1E
2/1E
Material Index Calculation Process Flow
Tie Rod
Beam
Shaft
Column
Function
Stiffness specified
Fatigue limit
Geometry specified
Constraints
Minimize cost
Minimize weight
Maximum energy
Storage
Minimum
eco-impact
ObjectiveStrength specified
Could be mechanical,
thermal, electrical…
(Each combination has a characterizing material index)
2/1EM
Index
Material Index Examples
• An objective defines a performance metric
For example mass or resistance
• Equation for performance metric contains material properties
Sometimes a single property*
Sometimes a combination*
*either is a material index
Material Index Examples
• Material Indices for a Beam
Loading Stiffness Limited
Strength Limited
Tension E/ sf /
Bending E1/2/ sf 2/3/
Torsion G1/2/ sf 2/3/
Objective:
minimize mass
Performance Metric:
mass
Maximize indices
Optimized Selection using Material Indices and Property Charts
Example:
Tension load, strength limited
Maximize: M = s/
In log space:
log s = log + log M
This is a set of lines with slope =1
Materials above the line are candidates
Source
• Material Selection in Mechanical Design, Michael F. Ashby, Butterworth-Heinemann