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Lec 31, Page 1/18
MME131: Lecture 31
Materials Selection
Prof. A.K.M.B. Rashid Department of MME BUET, Dhaka
Topics to discuss … The design process
General problems in materials selection
How do we select materials? Materials selection charts
Steps in materials selection Setting up the problem Boundary conditions Performance and materials indices
Case studies
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Introduction
The selection of materials is an extremely important
part of engineering design.
In many technological fields, the design of engineering
components and structures is limited by the available
materials.
The 20th Century has been a period of
unprecedented evolution of materials. This will
continue in the 21st Century.
New materials enable new designs.
Design is....
“...the process of translating a new idea or a
market need into detailed information from which
a product can be manufactured.”
invention Design innovation
What is a design?
Lec 31, Page 3/18
Market pull vs. Technology push
e.g., cellular phone, high capacity hard drives
e.g., Teflon, amorphous metals, quasicrystals
Mechanical Design
Industrial Design
Original Design new idea or working principle
e.g. CD vs Tape
Types of design
Adaptive or Development Design takes existing product and seeks an incremental advance
in performance through a refinement in working principle
e.g. beverage cans, automobiles,…
Variant Design change in scale/dimension without change of function
e.g. desktop to laptop computer
Question: Microsoft Windows 2007 – What type of design is this?
Lec 31, Page 4/18
Design problems are open ended - no single correct answer
Design is an iterative process
Products are technical systems composed of
assemblies and components
Vocabulary of design
Must formulate a “need statement”:
“a device is requested to perform task x”
solution is neutral
Design and production of Boeing 767
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Market Assessment
Specification
Concept Design
Detail Design
Manufacture
Sell
THE
DESIGN
CORE
The design flow chart
Concept
Embodiment
Detail
Product Specification
Market Need
Define specification Determine function structure Seek working principles Evaluate and select concepts
Develop layout, scale, form Model and analyse assemblies Optimise the functions Evaluate and select layout
Analyse components in detail Select processing route Optimise performance and cost Prepare detailed drawing
Iterate
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Concept
Embodiment
Detail
Product Specification
Market Need
Design Tools
Function modeling
Viability studies
Geometric analysis
Simulation
Optimisation methods
Cost modeling
Component modeling
Finite element analysis
Materials Selection
All materials (low precision)
Subset of materials (high precision)
One material (best available data)
Design Tools and Materials Data
• Engineering design uses the methods and tools of engineers and materials scientists
• Different tools are required at different stages of the design process
Metals, ceramics, glasses
MATERIALS polymers
composites... Casting , moulding
PROCESSES powder methods,
machining...
Flat and dished sheet
SHAPES prismatic
3-D
At each materials selection stage, decisions need to be made about what (material) is to be used to make the product, and how to make it (process and shape)
General problems in materials selection
Lec 31, Page 7/18
The Materials kingdom
MATERIALS DATA IN THE DESIGN PROCESS
ALL MATERIALS SHORT LIST SPECIFIC MATERIALS
Rough Data Accurate dataExample:
Metals/Ceramics/Polymers/Composites
Example:Steel/Titanium/
Aluminium
Example:Aluminium Alloy 2024
Heat Treatment T6 or T4
Method #1: Data browsing Find material data by reference books, software and/or internet
How do we select materials?
Lec 31, Page 8/18
Method #2: Property bar chart
Plot property data as bar charts showing range of properties for a given material
Method #3: Materials selection chart
For designs requiring the optimization of two (or more) attributes, plot one property versus the another
Ashby Charts
Lec 31, Page 9/18
Can be qualitative (or, yes/no) material must be transparent
Often quantitative material must be able to operate above 1000oC
material must be less dense than water
material must have a Young’s modulus > 100 GPa
Can use Property Charts to find subset of
materials that satisfy the screening criteria
Steps in materials selection First Step: Screening
By applying primary constraints
Screen materials to reduce the number of candidates – a “subset” is formed
Second Step: Comparison of materials
How can we compare two material properties ? Are we going to compare apples with oranges ?!?!?
Problem: How do we quantitatively compare materials within the active subset ?
which maximise the performance
Need More Information!
A more formal approach, employing performance indices and constraints (with a dose of common sense!), is required
Lec 31, Page 10/18
Setting up the problem
Consider the “need statement” e.g., “device is required to allow access to juice in a corked bottle”
Concept stage
Solution neutral: doesn’t specify how to do it
Sets the playing field: what are the basic constraints (only considering juice bottles, only considering corked bottles, only considering bottles,….)
Develop concepts based on need statement (e.g. screw, shear, pressure,.…)
Once we have chosen a single concept, to develop further, we need to re-examine in terms of…
What does the component do?
What do we want to maximize/minimize?
What are our constraints?
Function
Objectives
Constraints
Lec 31, Page 11/18
Boundary conditions of materials selection
Function: support a load, contain a pressure, transmit heat,…
Objective: to make it as cheap as possible, to make it as light as possible, to make it as strong as possible, …
Constraints: length is fixed, component must carry a certain load, component must operate above a certain temperature,…
Free Variables: materials choice (always for us!), cross sectional area is free, cross sectional shape is free,…
Hard Constraints (non negotiable): component MUST meet the specification
– often mechanical in nature; e.g. beam must support the weight of a 300 lb person.
If a design can’t meet the soft constraints, no one dies !!
Soft Constraints (negotiable): Component MAY meet the specification
– often aesthetic or cost related; e.g. beam should cost less than $50.
Lec 31, Page 12/18
Performance and Materials Indices
“The Material Index (M) is a combination of material properties which characterizes the performance of a material in a given application”
Structural members perform a function We want to know what materials optimize the performance of the member
When M maximises, performance is also maximised
Design of structural element specified by:
Functional Requirements, F
Geometric Parameters, G
Material Properties, M
Thus, the performance index, P, of a structural element can be written mathematically as
P = f (F, G, M) eq.(1)
Aspects of the performance of a component can be described by its functional requirements, geometry and material properties.
Lec 31, Page 13/18
Optimum choice of material is often independent of geometry and functional requirements.
So, we can select optimum materials independent of the details of F and G.
If F, G and M are independent to one another then we can re-write eq.(1) as:
P = f1(F) • f2(G) • f3(M) eq.(2)
Thus, the equation for performance index P becomes,
P = f (M) eq.(3)
Step 0: Initial screening
Step 1: Identify function, constraints, objective and free variable(s)
Step 2: Write down equation for objective – “performance equation”
Step 3: If performance equation contains a free variable identify the constraint that limits it
Step 4: Use this constraint to eliminate the free variable in the performance equation
Step 5: Read off the combination of properties that maximize performance
The 5-Step Program to Material Selection
Lec 31, Page 14/18
Case Study 1: Deriving Materials Index for a Light, Strong Tie Rod
Strong structural member of minimum mass loaded in tension
1. Function: Tie rod
2. Objective: Minimise mass
m = (AL)r eq.(1)
4. Free variable: Material choice Cross-sectional area (A)
3. Constraints: Length (L) is specified Must not fail under load F Must be tough
Constraint on area:
F/A < σf eq.(2)
Combine eq.(1)
& eq.(2) to give
m = FL r sf
M = sf r
Best material minimum m, maximum M
Use of Ashby charts to select material:
Charts plotted on log-log scale.
Materials falling on this line give equal performance
Rearranging the materials index as:
M = (sf /r)
log M = log sf – log r
log sf = log r + log M
Lec 31, Page 15/18
Case Study 2: Deriving Materials Index for a Stiff, Light Panel
2
2
L
EInFbuckling
4/4/ 24 ArI
rALm
24
2
2
22
44 r
L
Emn
L
EAnFbuckling
En
LFm
r
4
2
f1(F) · f2(G) · f3(M)
Search Region
Case Study 3: Deriving Materials Index for a light, stiff column (circular)
So, to minimize mass m,
maximise M = E½
r
Lec 31, Page 16/18
How do we make materials selection in a rational manner?
Design requirements are translated into a prescription for selecting a material by analysing (i) the function of the component, (ii) the constraints must meet, and (iii) the objective of the design.
Simple constraints are applied as limits on material attributes, screening out materials that can’t do the job.
Constraints that limit objectives must be combined with the objective to identify a material index.
The objective is best displayed on a material-property chart, allowing optimised selection
Summary Design is iterative.
Starts with the market pull (or, technological push).
First describe market need with need statement.
Concept Stage: ideas stage, test ideas.
Embodiment Stage: working principles selected, size and layout decided on estimates of cost and dimensions.
Detailed Design Stage: full analysis of all components, production methods.
Materials Selection enters at all stages. At the end an optimum selection must be made.
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Lec 31, Page 18/18
MME131: Lecture 32
Non-Destructive Testing
Prof. A.K.M.B. Rashid Department of MME BUET, Dhaka
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