mme445: lecture 19 materials selection the...
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A. K. M. B. Rashid Professor, Department of MME
BUET, Dhaka
MME445: Lecture 19
Materials Selection – The Basis
Learning Objectives
Knowledge &
Understanding Understanding the design process and the role of material on it
Skills & Abilities Ability to translate resign requirements into constraints on material
properties
Values &
Attitudes
Appreciation of design-led decision-making and systematic
selection strategy
Resources • M F Ashby, Materials Selection in Mechanical Design, 4th Ed., Ch. 05
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Outline of this lecture
Introduction
The selection strategy
translation - screening - ranking – documentation
Introduction and synopsis
• A material has attributes: density, strength, cost, resistance to corrosion, …
• A design demands a certain profile of these: a low density, a high strength, a modest
cost, good resistance to sea water, …
In this lecture, we set out the basic procedure for
selection, establishing the link between material
and function.
The task of selection is that of
1. identifying the desired attribute profile
2. comparing this profile with those of real engineering materials to find the best match
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The first step in the selection process is translation
examination of the design requirements and express them as constraints
that they impose on material choice and objectives to meet
The immensely wide choice is narrowed first
by screening out the materials that cannot meet the constraints
Further narrowing is achieved by ranking the survivor candidates
using the objectives by their ability to maximize performance
The material property charts can be used with these criteria
constraints and objectives can be plotted on them, isolating the subset
of materials that are the best choice for the design
The whole procedure can be implemented using software
as a design tool, allowing computer-aided selection
the procedure is fast and makes for lateral (“what if...?”) thinking
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Product specification
Concept
Embodiment
Detail
Market need
Problem statement
Review of the Design Process
Material data needs
Data for material family (metals, ceramics, polymers..)
Data for material class (Steel, Al-alloy, Ni-alloy…..)
Data for single material (Al-2040, Al-6061, Al-7075…..)
design flow chart
Concepts Need
Embodiments
Direct pull Levered pull Spring-assisted pull Geared pull
Need – Concept – Embodiment
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Detail Embodiment
How are those choices made?
Embodiment in detail
Design requirements:
expressed as
Constraints and
Objectives
Data:
Material attributes
Process attributes
Documentation
Final selection
Comparison engine
Screening
Ranking
Documentation
Density
Price
Modulus
Strength
Durability
Process compatibility
More…….
Able to be molded
Water and UV resistant
Stiff enough
Strong enough
As cheap as possible
(As light as possible)
The Selection Strategy: Materials
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The selection strategy
1. Translation: use design equations, including materials related equation
to maximise or minimise objective function to develop an
expression which consists of materials properties, functional
properties and design variables for the component
2. Screening: set minimum or maximum values on properties which all
candidates must meet and eliminate materials that cannot
3. Rank: use the materials selection charts to narrow the choices
down to a few candidates that do the job best
4. Documentation: Detailed information of the top-ranked candidates
to select one material
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Translation – Defining boundary conditions
(a) Function: what does the component do?
(b) Constraints: dictated by design
• What specific requirements must be met (hard constraints) e.g., stiffness, strength, dimensions, thermal conductivity …
• Are there other constraints that are desirable, but not compulsory, to fulfill (soft constraints)
e.g., cost, finish, colour …..
(c) Objective: what is to be minimised or maximised e.g., mass, dimension, cost, or a combination of these (some of which may be in conflict)
(d) Free variable(s): what is the designer free to change e.g., material, dimension, colour
1. Define design requirements
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(a) List the constraints of the problem e.g., no buckling, high stiffness, no yielding, fracture toughness
(b) Develop constraint equation(s) this must be satisfied in the design
2. Developing constraint equation(s)
(a) Develop equation(s) for the design objective in terms of functional requirements, geometry and materials properties
(b) One such equations is a objective equation which indicates
the quantity that must be maximised or minimised e.g., mass: m = ρ A L
3. Developing objective function(s)
which are related to the design objectives e.g., area
4. Defining and isolating free variables
from the objective equation(s) into the constraint equation(s)
5. Substituting free variables
by grouping the variables into three groups:
(1) functional requirements (F)
(2) geometry (G)
(3) materials (M)
minimise: P ≤ f1(F), f2(G), f3(M)
maximise: P ≥ f1(F), f2(G), f3(M)
6. Developing performance metric, P
using materials index (M) with the help of materials selection charts
7. Maximising / minimising performance metric, P
At this point in time, all possible materials are candidates to fulfill the needs of the application
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Screening – Applying attribute limits
All designs have non-negotiable limits which all candidate materials
must meet in order to be considered
e.g. electrical resistivity, transparency, yield strength, etc.
Screening allows us to eliminate materials that do not meet
these requirements (a.k.a. attribute limits)
E ≥ 10 GPa (stiff)
r < 3000 kg/m3 (light)
KIC ≥ 15 MPa·m1⁄2 (tough)
ρelect ≤ 105 Ω·m (good conductor)
An unbiased selection requires that all materials be considered
candidates until shown to be otherwise
E > 10 GPa
Search Area
Modulus – Density Chart showing lower limit for modulus and upper limit for density
r < 3000 kg/m3
Attribute Limits
E ≥ 10 GPa (stiff)
r < 3000 kg/m3 (light)
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One should not be too hasty in applying attribute limits
• it may be possible to engineer a route around them
• a component that gets too hot can be cooled ;
one that corrodes can be coated with a protective film
Many designers apply attribute limits for fracture toughness, K1C, and ductility, εf,
insisting on materials with, as rules of thumb
K1C > 15 MPa.m1/2 and εf > 2%
in order to guarantee adequate tolerance to stress concentrations
By doing this they eliminate materials that the more innovative designer is
able to use to good purpose
• the limits just cited for K1C and εf eliminate all polymers and all ceramics,
a rash step too early in the design
At this beginning stage, it is wise to keep as many options open as possible
Ranking – Materials indices
Materials indices and the materials selection charts are used to select
a smaller number of candidates whose performance is optimised
with respect to the application
performance is sometimes limited by a single property,
sometimes by a combination of them
Materials indices are specific functions, the criteria of excellence,
derived from design equations that involve only material properties
this can be used with materials selection charts to form an objective function
Example:
• Thermal insulation – minimise l (l = thermal conductivity)
• strong, light tie rod in tension – maximise σYS / ρ
Attribute limits do not help with ordering the candidates that remain.
To do this we need optimization criteria.
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Mp = E1/3/r Mb = E1/2/r Mt = E/r
guidelines for minimum mass
design
Modulus – Density Chart showing three material indices for stiff, lightweight design
5 1
0.1
M = E1/3/r (GPa)1/3 / (kg/m3)
Increasing value of index
E1/3/r
Search Area
Modulus – Density Chart showing a grid of lines for the material index M = E1/3/r
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Selected Area
A selection based on the index M > 2 (GPa)1/3/(kg/m3) and the property limit E > 50 GPa
Index, M = E1/3/r
> 2 (GPa)1/3/(kg/m3)
Modulus, E > 50 GPa
Selected Area
A selection based on the index M > 2 (GPa)1/3/(kg/m3) and the property limit E > 50 GPa
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Index, M = E1/3/r
> 2 (GPa)1/3/(kg/m3)
Modulus, E > 50 GPa
Selected Area
A selection based on the index M > 2 (GPa)1/3/(kg/m3) and the property limit E > 50 GPa
The materials contained in the search area become the candidate for the next stage of the selection process.
To summarize:
• screening isolates candidates that are capable of doing the job
• ranking identifies those among them that can do the job best
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Documentation
From the ranked short list of candidate materials,
one material that is the most suitable is chosen
To proceed further, we seek a detailed profile of each candidate:
its documentation
• Such information is found in handbooks, suppliers’ data sheets,
case studies of use, and failure analyses
Why not just choose the top-ranked candidate?
But what do we know about this candidate?
• Does it be shaped, joined, or finished easily?
• What are its strengths and weaknesses?
• Does it have a good reputation?
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Local conditions
The final choice between competing candidates will often depend on
the local conditions:
• in-house expertise or equipment
• the availability of local suppliers, and so forth
A systematic procedure cannot help here;
the decision must instead be based on local knowledge.
This does not mean that the result of the systematic procedure
is irrelevant.
It is always important to know which material is best,
even if for local reasons you decide not to use it.
The selection procedure summary of 4 steps
1. Translation and deriving the index
• Identify the material attributes that are constrained by the design,
• decide what you will use as a criterion of excellence (to be minimized or maximized)
• substitute for any free variables using one of the constraints, and
• read off the combination of material properties that optimize the criterion of excellence
2. Screening: Applying attribute limits
• Any design imposes certain non-negotiable demands (“constraints”) on the material of which it is made
• Translate these constrains into attribute limits and plot them as horizontal or vertical lines on material
selection charts
3. Ranking: Indices on charts
• Seek those materials, from the subset of materials that meet the property limits, that maximize
performance and rank them
4. Documentation
• Explore the characters that cannot be expressed as simple attribute limits in depth of the shortlisted and
ranked candidate materials
• Many of these relate to the behavior of the material in a given environment or to aspects of the ways in which
the material can be shaped, joined, or finished
• Such information can be found in handbooks, manufacturers’ data sheets and computer-based sources.
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