So far we have talked about various types
of materials, but in this last series we will
enter into the application of this knowledge
domain.
That means now we are going to talk about
how all this knowledge that you have acquired
so far, on the basis of that knowledge how
you can basically select a particular material
for some engineering design application.
So there will be various case studies that
we will carry out and this will be really
the test of how much knowledge to have acquired
towards the material properties.
So what we are going to talk about one example
is the general, today we will discuss all
these things in principle and from the next
class we will talk about that how to apply
it.
So today we are going to talk about the general
aspects of the mechanical design, design flowchart
and the doubling time issues, resource availability,
eco-efficiency and finally the Ashby chart.
The knowledge of Ashby chart which is the
culmination of today’s lecture is very very
important for us to apply from the next lecture
onwards.
So let us 1st talk about that what do we mean
by mechanical design, because I said that
we are going to talk about material selection
for mechanical design.
Now, mechanical design refers to the mechanical
component design which has definitely some
mass and which will carry some load that is
functional and also it may have other functionalities
such as thermal or electromagnetic requirements
and then it must be manufactured.
So that is the way this kind of design would
qualify.
And the design for say would refer to the
selection of engineering material based on
a set of defined properties, so it will be
naturally an iterative process.
What we will try to do is that, corresponding
to some functional requirements let us say
you have functional requirement A, so you
have a particular personal requirement let
us say then corresponding to this requirement,
we will you say F1 is the functional requirement
we will try to find out that what is the performance
index that we should look for and this performance
index we should look for from the material
property point of view.
So this will be from the material property
point of view.
And then we will go through all our knowledge
of the material and we will see that which
group of materials can give you the best performance
index that means that satisfies this function
in the very best manner, so that is what is
the sense of the whole direction.
Now in any particular design when we do what
exactly we actually carry out in this design
process.
So we 1st start with the market need, what
are the design requirements in a market?
Let us say tomorrow I want to develop I find
out that there is an energy crisis, so there
is a market need of developing a micro scale
energy solution.
Then we have to find out what is the design
requirement, do I need to actually design
a system which is which can extract energy
from say wind energy or from water or maybe
from both or from vibration, so you have to
find out what are the design requirements,
based on that we have to go for a conceptual
solution.
So at this stage, we have to determine what
is the function structure that means how many
functions this particular concept has to certify
and then we have to see what are the work
principles corresponding to this function
structure and we have to evaluate and select
the concept, because you can have multiple
concepts right, so one can give many design
concepts C1, C2, C3.
So each concept you have to find out that
in a something called PU chart, you have to
find out that which concept is good for a
particular work and which concept is not so
good.
So once we rank these concepts then we choose
one concept from that and then we go ahead
with the next part which is known as the embodiment
design.
In the embodiment design part, we actually
develop the layout, scale, forms, etc. and
then we also work on the system model, we
analyze the system and then we evaluate and
select the layout.
So embodiment design is something like a detailed
analysis that you have to carry out in order
to know that what sort of materials system
you will need in terms of materializing the
concept.
Then you have to come to the detail, and in
the detail part we are going to talk about
each of some assemblies, look into it in details,
then optimize the performance, minimize the
cost and then choose the final set of material
and the process.
So, when we are at the conceptual stage generally
we do not select the material, but at the
embodiment stage we have many candidate materials,
so at this stage we have many candidate materials.
And when we are doing the detailed design,
here we finalize the material selection and
then we see that how the products specifications
are satisfied and we can iterate that process.
Suppose we are not happy with it, we may go
back to the concepts; we may look into the
other concepts like we can then work with
concept 2 or concept 3 and then carry out
the same process again and again until and
unless we are satisfied with the solution
of the system.
Now, when we will be selecting a particular
material for a particular function, what are
the points that you have to keep in your mind?
The 1st important point you have to keep in
your mind is the money that is the price and
availability of the material, so that is the
1st important part we have to keep in our
mind.
Then the 2nd is we have to look into the group
of mechanical properties and in this group
of mechanical properties we have to think
of things like density of the material because
many times your system is to be of lightweight,
then you have to think of the modulus because
modulus is related to stiffness of the system,
we will see it through some examples in the
next class.
We may have to think of the damping because
for dynamic works damping is very important,
then the yield strength, the tensile strength,
the hardness, fracture toughness, fatigue
strength, the thermal fatigue resistance particularly
if it is for a high temperature application
as well as the creep strength, so these are
like certain very important mechanical properties,
other than that there could be some other
some more properties very specific to the
application, but these are definitely some
of the very important mechanical properties
that we need to look into before we select
a particular material.
Next, we are going to talk about the other
properties like thermal, like optical, magnetic
and electrical properties.
This comes because a machine for example today
is usually multifunctional and it has to interact
with a very complex environment, so you need
to know about properties which are other than
the so called mechanical properties of the
system.
Thermal property is one of the examples for
example, these gas turbine blades, it has
to work in a high temperature environment,
so you have to look into the conductivity
of the material, so thermal property comes
in to a very important property.
There are some applications where the for
example a kind of a transparent oven say if
I have to design, then I have to think of
that the optical property should not be sacrificed
with respect to temperature, so then that
also comes into picture.
Or let us say, you have to design a microscope,
and then the optical property comes into the
picture.
Magnetic and electric property comes into
picture for example, just a simple example
that you have to design a motor housing, now
the magnetic and the electrical properties
of the motor becomes important for you.
So thus even for mechanical design, all these
things actually get covered and this becomes
important for us.
Next is there is chemical environment change,
the oxidation, corrosion becomes important.
There could be things like there could be
friction in the system, so the frictional
properties are important, there could be abrasion
or wear and tear, so the properties becomes
important for us.
Now, that is the property number set number
4 that we have to look into it.
Then in the property set number 5, we have
to look for the ease of the manufacturing
and the joining so this is related to the
manufacturability of the system, so design
for manufacturing.
And then finally, we have to think of the
appearance, we have to think of the texture,
feel, etc., so the aesthetics become important,
right.
So thus this 6 property set becomes important
for us.
Now, once we know that these are the property
sets, then we have to start to look into that
what are the specific property level for each
of the property set.
For example, price and availability are the
1st important property set I told you.
So if you look into the relative prices of
a system, you would see that diamond, platinum,
gold, silver, these are all very expensive,
right so till this point we are at a very
expensive level of the property set.
So naturally you cannot use them unless you
have a very-very compelling situation of using
such materials.
So the next interesting material which can
be used is actually carbon fiber reinforced
plastic, is highly functional I have told
you when we have discussed about carbon fibers
that is one of the very high performance system
are designed using carbon fibers, so this
we can use.
And then further for example, tungsten is
one of the materials for many space grade
applications you will see that the tungsten
is used, then there will be titanium alloys
once again for space grade applications and
then polyamides, polyamides are like kevlar
if you remember we have talked about Kevlar’s,
right or bullet-proof jackets this is like
polyamides.
And then once again magnesium alloys, then
nylons, polycarbonate, so where we are in
terms of for example, steel?
Steel is one of the cheapest materials, so
if you look into this list for example, you
will see that presence of steel or such material
let us say here we have the aluminium alloys
in this list.
Even below the aluminium alloys you can get
epoxies, polyester and glasses that mean you
can make GFRPs Glass Fiber Reinforced Plastics.
And then down this line in fact, somewhere
below this because it is not here covered
I can see that the steel would come into picture,
so this is one of the cheapest material basically
it is somewhere here as you can see that this
is where is the steel.
So you can use also that as a baseline of
your material selection.
Let us say, if steel is 100 units of in any
units if this particular set would be in terms
of basically pound or dollar so however, if
it is 100 units in comparison to that if you
look at the aluminium alloys, then this is
somewhere around 650 units.
So that means it is that many times expensive
aluminium as a material.
So you have to be cautious of that what is
the going to be the cost of the system as
you are trying to improve the performances
using a better material.
And similarly, from aluminium let us say if
you want to jump to something like magnesium
alloys this is about thousand units, so this
is 10 times more expensive than steel.
If you want to go for something like tungsten
or super alloys, 5000, so it is like 50 times
more expensive.
If you want to go for something like CFRPs,
then it is 20,000 so that means it is 200
times more expensive than steel.
This is very-very important for us because
we will indeed save lot of weight by using
carbon more expensive than steel, by using
carbon fiber reinforced plastic, but have
to be ready to spend a 200 times more money
for that system.
Now, let us look into the availability of
the materials.
There are 2 groups of it that is availability
in the earth’s crust and availability in
ocean and in atmosphere.
Of course, atmosphere you get mostly gaseous
substances, but in the oceans there are certain
things which are very peculiar for example,
magnesium good supply from the ocean comes,
so that is also used as one of the structural
materials.
However, most of the materials that we use
they actually come from the earth’s crust
itself.
And in terms of abundance you can see that
aluminium is about 8%, so aluminium is highly
abundant.
Next to that is in fact steel, but aluminum’s
processing cost is more that is why aluminium
is actually more expensive than steel, so
this actually gives us a broad picture of
what is the abundance of different materials.
Now, once the abundance there is the particular
knowledge which is very important at this
stage that is known as the doubling time.
So what is the doubling time?
The definition of doubling time is that it
is the period of time required for a quantity
to double in size or value.
That means, if you think of it that this is
like X axis is time and Y axis is the demand
of a particular material and let us say that
the demand is increasing exponentially.
So if it is increasing exponentially, than
at a particular time t1 if the demand is D1,
how much of time would it take say t2 to go
to D2 where D2 is actually twice of D1.
So that time that is needed between this t1
to t2 that is what is the doubling time that
is needed.
Now if you assume an exponential growth, in
many cases the growth rate will be given to
you and hence by knowing the growth rate and
logarithmic base of with respect to 2, you
can actually approximately find out the doubling
time to be about 70 over R, so that means
if I know the growth rate, then I can find
out that what is the doubling time and if
I know the doubling time, then I know that
a particular material is of very high demand,
which means as I will be designing, I have
to be get ready of a situation where its cost
may increase because it’s doubling time
is small.
So material whose doubling time is more, essentially
is a either a cheaper material or that material
is completely rare, so either of the cases
but the doubling time helps you to take a
decision on that.
Now this also in terms of the availability,
this also brings us to the very famous McElvey
diagram.
So what this diagram tells us is that this
is like engineering materials this Ashby and
Jones book, you can use it for reference for
this particular beautiful diagram that this
actually tells us that there are 2 things;
one is called reserve and the other one is
actually the total resource base.
So, you can have actually a very big resource
base like this.
But a good part of the resource base is maybe
actually undiscovered or maybe actually not
economic.
So it is the economy level and the level of
discovered known part actually makes the reserve
and the rest of the things are your resource
base which includes the reserve of course.
So for example, if you consider aluminium
another point here it is important that why
some of the things are not economic?
Like if you consider aluminium, it takes about
280 Giga Joules per ton, plastics 85 to 180
Giga Joules per ton, so these are all high
energy consuming, copper 140.
So even if you find it may not be economically
feasible to use such a material, so hence
your even though you have the resource, you
may not be able to consider it as your reserve
because it is not profitable at that particular
point of time.
Now a broad picture I want to tell you.
For example, copper, silver, tungsten, tin
and Mercury they are rarely available, so
you should avoid them by all means from your
general design.
Iron and aluminium are most widely available
materials, so you should try to use them more.
Steel consumption is doubling every 20 years,
so it is taking time and it is cheaper.
Aluminium consumption on the other hand is
doubling in every 9 years, so it is doing
going at an even faster rate.
And polymers in every 4 years, so you have
to be cautious of this particular fact that
at this point of time polymer is the most
popular material is of highest demand, next
is aluminium and next is steel, so accordingly
you should select the material.
How to encounter shortages of material?
First of all, you have to go for a material
efficient design that means use less amount
of material for example, for a good surface
property use cheap substrate and good surface
finish, so there are certain tricks of using
more material efficient designs.
Substitution, substitute rarer materials by
the more available ones for example, substitute
copper by aluminium in the electrical wires
we have seen that this has already taken place.
Recycling, use such materials which are recyclable
materials like aluminium, this is also being
done today, so these are certain strategies
that you want to take in order to encounter
the shortages of materials.
Next is a very important point which is known
as eco-efficiency.
Today, we are very much conscious about the
nature around us, so it is the eco-efficiency
which means a margin of ecological and economic
goals together we have to think before selecting
a particular material.
So we have to think of improving the productivity
of energy and material inputs to reduce the
resource consumption, also cut the pollution
per unit of output.
So we have to not only think of reducing my
own cost, but also I have to think of it that
it should not affect the nature.
So a win-win approach in this direction would
benefit both the bottom line design and the
environment itself.
Just one example in 1989, Procter and Gamble,
they introduced a concentrated detergent powder,
which is also called Ultra detergents.
They took up half the volume of traditional
detergents, this product cleans the same amount
of clothes but were more convenient for consumers
to handle, used 30% fewer materials, required
30% less packaging and substantially cut the
energy needed to transport them, so as a result
this material was actually much more eco-efficient
material.
There are 7 dimensions of eco-efficiency,
one is to reduce the material intensity of
goods and services, then reduce the energy
intensity of goods and services, then reduce
the toxic dispersion, enhance the material
recyclability, and all of them are very simple
so I will not explain them.
Maximize sustainable use of renewable resources,
extend the product durability, increase the
service intensity of goods and services that
means increase the kind of life of the material.
So greater the improvement in each or any
of these dimensions, it is considered that
your system is more eco-efficient in terms
of a product design.
So that means you see you are not only thinking
of the material to serve the particular function,
but also you are thinking what is the outcome
of the use of that material through the environment
around us, so that is what forces us to look
into the 7 dimensions.
Now, we have talked about the 7 dimensions.
Finally, you have to select the material property,
to do that a fantastic chart I will introduce
you tomorrow is called Ashby chart.
So in this chart, what you will get is that
you will get this is like you can develop
a N dimensional picture out of it, but in
each one of the dimension you will get a comparison
between supposed 2 properties; Young’s modulus
and strength or suppose density and something
like Young’s modulus, Young’s modulus
versus density.
Or say density versus some other property
of the system like strength of the system,
strength-density.
So buy looking at this comparison chart, let
us say if I consider one of the slice of this
chart here like this is strength versus density,
so this is density and this is the strength.
So if I look at it, then I will see that the
very low-density materials are like foams,
then from there we are going to natural materials,
then from there we are going to polymers,
composites, so gradually as I am increasing
the density, the strength is also increasing
and then the ceramics.
However, the problem is that if I have to
go for too much of high-density, then the
structure may be very heavy in weight.
So if I want to go for low-density relatively,
but strength then we choose something like
CFRPs because the metals are so this is where
it is, but the metals at the same strength
level if you look at it that they are actually
an order of magnitude higher at the same strength
level.
So that is why this 2D charts actually help
a lot in terms of selecting the material from
a particular function point of view.
This function also you can plot on the Ashby
chart like somewhere supposed if it is Sigma
F by Rho that is what your point is.
Then any material if it is here, it is discarded
because that is what your functional chart
level is; you are searching for properties
on that line beyond this level, not below
this level.
So thus it helps us in terms of the choice
of the material, so this we are going to discuss
in the next class.
I am going to talk about how to use Ashby
chart and I am going to talk about the numerical
problems on material selection, thank you.
Keywords- doubling time, McElvey diagram,
eco-efficiency and Ashby charts
