Mechanical Engineering Motivational Multimedia Video. Video Lecture

hi everybody its Rob Smith here and I'd
like to welcome you to fundamentals of
mechanical engineering topic 1.1 first
of all a couple of checks before we get
underway we do have a busy session today
so without further ado can you all hear
me clearly please indicate that you can
hear me
you
you
positive response a good start and now
can you see the arrow pointing to the
letter M in mechanical
you
cool so we've got sound and we've got
vision well in this webinar we're going
to look at some basic concept concerning
work power energy force torque and we're
also going to look at some drawings and
friction amongst other things let's
start off with the force well force is a
push or a pull on on an object that
results from its interaction with
another object so here we've got a force
100 Newtons because that's the units
that we measure force in and we're
applying this to this column at an angle
of 45 degrees five meters from the
bottom here so engineers like to do cool
things so engineers could work out how
much this column moves because of this
force applied here and we could even
work out things like bending stresses
but not don't worry you just know more
about that later now the force is a
product of mass times acceleration
that must being was it measured in
kilograms and our friend acceleration of
course being measured in meters per
square second and this equation here F
equals mass times acceleration is known
as Newton's second law of motion
if we're going to do work it means that
we have to do and/or produce a move of
force through a distance so the
definition here of work is when a body
moves under the influence of a force
work is done so we calculate work by
multiplying force times the distance
through which we move the object energy
well we know that energy can't be
created or destroyed and the energy as
unit of energy is the Joule and there
are basically two formats that energy
can take a forming the first one is
potential energy that's the energy
stored in Springs for example the energy
that a body has a due to its height if
you think about a tank a water tank
suspended above a tap the higher the
tank is suspended above the tap the more
water will come out as the the tap is is
opened more pressure will have kinetic
energy well as comes from when the day
the the Greek cannae meaning motion
kinetic energy is the energy possessed
by a body by virtue of its motion and
these are basic things that we've gone
through now guys so we've got to become
familiar with them because we will be
using them later and potential energy we
can calculate that by taking the mass of
the object multiplying the distance
between the object and some datum that
we set at x G the gravitational constant
kinetic energy well we can calculate
that by taking 1/2 times the mass of the
object times its velocity squared and
the velocity will be in meters per
second
okay now we're getting to some useful
stuff and out power well power is the
rate of doing work and the way we can
calculate power is by measuring the
force multiplied by its average velocity
and the unit of power is the watt well
in industry there's not many things that
we measure in what unless you're in
electronics and non machinery that we're
utilizing for driving pumps and
compressors for example or or conveyors
is measured in kilowatts net torque is
basically is force times radius of
application so you can see here we've
got a 10 kilogram force on a spanner and
we're applying that force at a distance
of 1 meter from the center of this this
bolt here so we get a talk of 10
kilogram force meters so the further
away this force is applied the more
torque we get the higher this force
becomes here over a given distance the
more torque we get so torque is a
product of Force Times distance of
application from in this case the center
of the bolt that's useful if you want to
tighten spanners tighten the bolt over
now
friction is something that engineers
love and hate
friction is it is the enemy when we're
looking at rotating equipment friction
is the friend when we want to stop that
rotating equipment from moving and the
resisting force is called friction force
now there are some laws of friction
there's basically four laws that we need
to be familiar with the first law is
frictional forces directly proportional
to the normal load between the surfaces
for any pair of materials second law
says that friction force depends on the
material from which the contact surfaces
are made so we have really rough
surfaces then we will have a high
friction force between them when we try
to move them and the second law says the
friction force for pure sliding is
independent of the area of contact of
the surfaces and then the fourth law is
that friction force is independent of
the velocity of the sliding of one body
relative to the other and here's a
couple of examples if we looked under
the microscope at surfaces that might
look to the human eye to be a smooth we
see we've got this kind of surface here
that looks like the moon and the
friction is caused by these tiny little
bumps called asperities and that the
friction is caused by these bumps
rubbing each other now what did you
think the purpose of the lubricant is
guys
what do you think one of the purposes of
a lubricant is if we take a look at this
little sketch here what do you think one
of the purposes of the lubricant would
be or obviously to reduce friction but
what what would it be
aha someone's on the ball that's right
well done guys we would want to fill
these little gaps and separate separate
the two surfaces so they do not come
into contact and that is one of the
purposes of a lubricant and once these
surface are separated then obviously we
reduce the coefficient of friction
between the two surfaces so the say
engineers are in constant turmoil as to
whether they want friction or they don't
one fraction depends which side of the
fence you're on and here we've got a
block with a normal reaction here of N
and we've got a frictional force F that
we're applying parallel to this this
this base if you like and this
frictional force is the amount of force
that we need to apply to this block in
order to get it moving
now the friction and between the two
blocks and depends on the surface
roughness and we've got something called
the coefficient of friction and it's
denoted by the Greek letter nu and we
can calculate the coefficient of
friction between any two surfaces if we
know the force that's required to drag
the in this case the block along and we
know what the normal reaction force is
and here's a table I don't know if you
can see this very clearly on your
screens but let's take a couple of
examples here
let's take cast iron cast iron and when
we're sliding has a coefficient of
friction of point of 0.15 if we take
graphite rook sliding well it's it's
point one it's point one it's a lot less
than cast iron and cast iron is actually
very good because it's filled with
graphite let's take let's take iron iron
as a coefficient of friction there of
about 1 so you can see we can get these
values for mu for different materials I
just want to explain all of this is not
on your slides you see that we've got
two columns here once the static one
such sliding well in order to get the
block if we go back a couple of slides
in order to get this block moving we
have to overcome friction static
friction so if I drew a little graph
here and here's the force what will
happen is is as I increase the force I
increase the force to try and get it
moving and then once I've got it moving
that's the force to keep it moving so
this here if I can just show you here is
the static friction and once it's moving
that is the friction here in this table
yet that's the sliding friction there's
my little pointer
so that static friction
this value here and then where the curve
flattens out
that is the sliding friction so that I
hope that explains this to their
different values then so as I said
engineers always have this dilemma
someone friction others don't want
friction and friction is pretty useful
if you're walking or you're running also
if you're driving it happens to be very
useful if you want to stop your car in a
hurry if we didn't have friction you'd
be in big trouble as this sketch shows
here we have a belt transmitting drive
from the engine to various components
the alternator the air conditioner the
fuel pump etc etc and not realize on
friction so in this case friction is
good okay as I said at the beginning
we've got a lot of stuff to get through
and it's very diverse here guys so let's
move on to the next subject which is
drawings drawings now drawings are the
main line of communication between the
craftsman the originator of the drawing
itself and the end user and the example
given here is between the designer and
the machine operator and it's important
that we're able to communicate
communicate what the the draftsman wants
the operator the machine operator
whatever to produce so the things that
are really important in drawings are the
type of projection that we use any
sections cross section is shown on the
drawing as we'll see they're very very
useful in supplying information and then
we're going to look at assembly drawings
and now we're going to discuss
tolerances and fits so let's look at
projections well we've got two lane
projection types like isometric and
oblique and the difference is that an
isometric and projection you can see
that the cube here that shown here is at
an angle of 30 degrees to the horizontal
so it's the side here is 30 degrees and
on oblique projection we actually see a
true representation on the side of the
cube and one side is two is at 45
degrees at 45 degrees there now just to
further complicate things we have two
systems that we use first system is
called first-angle projection which is
mainly used in Europe and you can see
here we've got front view side view and
plan view
similarly in third angle projection
which our American friends favor we've
got three views as well but look at the
placement
we've got plan view here on the top
front of you below it and sign for you
now how I remember is I've got a little
trick that I use and if something it
goes like this
if I can find a color pen I don't know
what this is going to look like I divide
the drawing or up into three quadrant
four quadrants sorry this is the first
this is the second this is the third and
this is the fourth now where is the
front view
in third angle projection
where is the front view in third-angle
projection
in which quadrant is it
okay everybody passed that tough
question now let's look at first angle
projection divide us into four again
here's my first quadrant second quadrant
third quadrant and fourth quadrant
where is my front view now which
quadrant is it in first so that's a
little trick I use guys you know
everybody has their own own methodology
but for these sort of things that's how
I remember when I'm looking at a drawing
whether you know I don't have to look at
the name or where it was printed in
order to ascertain what type of
projection is now I hope you'll find
that useful to you
moving on sectional views are very very
useful there is so powerful guys they
really help you understand what's going
on what you're looking at and when you
look at a drawing and a sectional view
is an imaginary slice through an object
so that you can see what it looks like
now here and the top view here what
we're looking at is we're looking at
what looks like a cylinder with a hole
drilled in the middle and some kind of
hidden object here but look if we didn't
have a sectional drawing we think that
that hole went right through wouldn't we
the natural fact that doesn't this full
section view and these these lines here
are called cross hatching this full
section view shows us that battle is
only in the top half of this object it
doesn't actually go all the way through
and here is a half section which we
sometimes use now let's take a look at
this one here on the right this is
mind-blowing
look at this view here so what we have
is in this part the first part of the
object here we've got two solid lines
now that could be a groove
or it could be a raised portion like
it's shown here then we've got a center
line here what do you think this could
be what what section do you think this
could be just looking up is drawing here
what type of section
do you think that could be because two
lines there what what could they
represent
you
well we see one of these things is that
yeah it's round but what other section
could it be
Stefan it wouldn't be a scallop I know
what you're saying is that it's like a
you section you wouldn't see a
centerline man it could be an oval yes
it could be good one
the other thing what about it it could
be a rectangular or it could be a square
couldn't it what we're saying is two
lines here and this are a four lines
here so by taking a cross-section and
this this case here Bibi Bibi shows us
the section is actually round how about
this guy here it shows you a square hole
and there what else could this be here
represented by two dotted lines here
what else could that be shows here in
the section such a square but what else
could it be here
you
well it could be around halkan it could
be drilled and I think yes yeah it could
be round here at shone square a natural
fact so can you see how this is all
hidden detail here it's all hidden
detail but how using sectional views how
a lot more information is revealed and
that's why these are very powerful
details and on a drawing very useful to
someone that's got to manufacture this
how about these sectional views let's
take a look at this plate here Wow well
this this is counter sunk but is it is
it a square counter sunk or is it a back
counter sunk because look at this hole
here on the right and look at that hole
there on the right they look exactly the
same in this view don't Bay but now take
a look in the cross-section we can see
this is actually counter sunk square
counter sunk pay but how about this one
this obviously it's shown in in detail
in in dotted lines that would go
actually 3/4 of the way around there to
represent a capped hole look at this
that looks the same as that then the
same as that doesn't it but in this case
it's actually a raised face here
it's a raised section that's been spot
faced in machined and it's round it
happens to be round so Wow
do I do need to explain anymore about
how powerful sectional views are they do
reveal a lot of information don't they
let's talk about dimensions well why do
we need guy dimensions guys why do we
need time mentions now don't give me any
of these answers here
because it's all that's all in front of
me as well as in front of you but what
other reason do you think
apart from what's here on the slide do
we need dimensions on a drawing
you
you
you
well we yes we've got some pretty good
answers there but folks the answer that
I was looking for is if you have
drawings on a drawing with dimensions on
we said that the purpose of drawings are
to communicate between the draftsman and
the operator the machinist now the
operator the machinist makes the part
according to the drawing how do we know
it's right
how do you how do we know how do we know
the machinist has made the part
correctly
you
you
yes God on the money bull's-eyes yes
Bruce yes ROI we compare the dimensions
that we measure on the actual
manufactured part against those stated
in the drawings now just a word about
drawings here before we start talking
about limits and tolerances because I'm
just bringing us into the next subject a
little bit prematurely take a look at
this drawing can anybody tell me why we
would say this particular imagine this
we've got two drawings mirror image of
each other this first part here there's
a mirror image of that part take a look
at the way that they're dimensioned what
strikes you as being the reason we've
labeled this side the left-hand side
trying to be correct and this one
incorrect
we've got all the dimensions that we
need here on this this part here on the
right all the dimensions we need on the
left but why would we say that this
drawing de mentioned here is correct and
this one is incorrect
you
well I'll give you a little bit well
yeah the first thing that that strikes
you is the dimensioning here on the
right is nowhere near as clear as the
dimensioning on the left it's a lot
neater also we're working from a datum
if you're a machinist I need to know how
far away from this side here to put this
hole in and that hole in there and in
this hole in here so it's very easy I
don't have to add up the dimensions in
this drawing but you guys got it right
you said we are aware that the reason
that this is correct is because the
drawing is a lot tidier it's a lot
easier to understand and we don't have
any dimensions that are actually on the
drawing they're away from the drawing so
we can actually see the part much
clearer I mean this is really confusing
what do you think would happen if we
sent this down to the machine shop for
the machinist to manufacture what do you
think that that might happen there
you
you
you
absolutely people 100% correct well yeah
if I was a machinist I wouldn't be
impressed by the drawing either yeah you
should and good answer that's but
there's always a there's a much greater
risk of producing the wrong article the
machinist would have to sit there and
try and work through this maze of
dimensions as to how to produce this
article and yes Justin yeah it takes
time and time as we know is money so
yeah bigger chance here of producing
scrap when compared to this dimensioned
part of the drawing good one very good
okay moving on I'm another useful
drawing for us especially if we're going
to put parts together as an assembly
drawing and an assembly drawing shows
how things are put together and
generally and that they want one can
assemble it a triangle Lego set and if
we if we look at this now the assembly
drawing shows how things go together and
we have a parts list along with the
drawing what do you think the parts list
helps us to achieve
you
but what purpose does the parts list and
serve should I say yes god yes it
identifies the components because bear
in mind people this is an assembly
drawing but each individual part has to
be machined so in this column here it
says description there's a part part
number that will actually that's the pod
description really to be to be strictly
as strict that this is the the
description it should be there should be
a number in here and a machining drawing
number in here so that we can identify
the component also when you look at the
parts list it tells you how many of each
part we need and that's also useful
isn't it the x-axis no and Daniel the X
is a section it's just to show that this
drawing as a section view viewing on a a
so in other words I take a hacksaw I cut
this imaginary hacksaw I cut this in
half and what I'm seeing here on the
right of the drawing is the view that I
would get if I looked at the component
once it's cut in half from left to right
any idea what this might be guys what
this component might be
you
you
yeah it's a hydraulic piston it's a
hydraulic piston I actually believe it's
sort of a brake master so well done
Bruce yeah I believe itself a brake
master cylinder Mullen doesn't tell you
that they're well done you guys are on
the ball today Wow look at this exploded
assembly I did say it shows us how
things go together and I think this is
pretty good in determining where where
we are how how things fit together where
they should go and you'll notice we've
also got item numbers on here what is
the purpose of those item numbers why do
we have nine item numbers and unexploded
assembly yeah okay yep you guys are
definitely awake it's tied n defy the
pot and to go back and look at them so
we can go back refer to the actual
manufacturing join
good now before we talked about you know
dimensioning things and manufacturing
things and hopefully we'd get them get
them right now we're going to talk a
little bit now about limits and fits and
the limits of size are the extreme
dimensions the maximum size the minimum
size which is permissible for a
component to be to be made now designers
often calculate and design a shaft to
fit an air hole now the cost of the
manufacturing of this depends on the
limits that we apply to the shaft and to
the hole if we make the the the
dimensions of the shaft extremely tight
in other words we allow very very very
little for manufacturing errors then
that shaft will be expensive to
manufacture and often the designers
decide what the final dimensions should
be for for mating parts and now limits
are normally used for cylindrical holes
and shafts but they can use be used on
any part or parts rather that fit
together
regardless of what they look like now
typically a limit is upper and lower
tolerance for a shaft or hole now we've
got different types of fit between
components we can have a clearance fit
where a pump shaft fits into a plane
bearing and that shaft rotates in the
bearing hopefully with some kind of
lubrication then and and that is a
clearance fit an interference fit is
where as a negative allowance and you
may want to put two halves of a tool
together with a dowel and the dowel size
is slightly larger
than the the whole size in one half of
the tool and then we put something
that's in between a parent's foot and in
the difference that it's called a
transitional fit let's take a look at an
example here so we're looking at a
drawing of a housing a beragon bush and
a shaft here and a shaft what type of
fit do you think that there is between
the bearing bush and the housing
so this shaft will rotate in this
bearing bush and this is fitted inside a
housing will yes it's an interference
that it's pressed the bush would be
pressed into the housing so that it
doesn't rotate now let's look at the
shaft we've got the shaft rotating and
this bearing here now ignore the drawing
what type of fit would you think that
the shaft is in the bearing bush or some
of your yet you got it already
you got it well done guys it's a
clearance fit it's a clearance fit so
what we would do in this case of this
assembly is loop put nominal sizes for
the shaft the bearing bush and the
housing and put clearances absolutely
plot
good tolerances on to ensure that we get
the correct fit and our designers refer
to ISO limits in fits and there is a
standard which is British landed 4500
which is an abbreviated table if you'd
like and you'll see the table in a
moment and you'll understand immediately
why we use letters it's a shorthand
basically on on a drawing now what we
have to know for the sake of this
webinar is that holes are designated by
a capital letter and shafts are
designated by a lowercase so we've got
HL Evan h9 h 8 h 7 which refers to holes
and then we've got C 11 D 10 e 9 etc etc
which refers to shafts and here's the
table here guys now can you imagine
producing a drawing and then having to
write Freerunning fit not for use where
accuracy is essential a good for
temperature varies high reading speeds
are heavy for I'm getting tired to
saying it imagine having to put all of
this text on a drawing when you can just
do this h99 which would you prefer to do
write all of this out or put this in
your on your drawing I think I know what
the answer is going to be before I even
see and you reply to that one so under
clearance fits we have all of these
symbols that one would refer to and and
these go from moose running to location
clearance and all of these are defined
by
the large letter H and a lowercase
letter here in this case D transition
then we've got different types of
transition that's the locational
transition fit for accurate location and
it's a compromise between clearance
that's a rattling fit and an
interference that where it really has to
be pressed in and then we've got
location transit transition fit for more
accurate location where we've got very
very little clearance at all and we're
having to make sure that the parts that
are fitting together don't move and you
see the difference here we've got h7
what is a lot large letter designate
you
just checking that you're awake okay so
I knit in both cases we've got an H
seven and in the first case we've got
the case six and the second case we've
got an N six so this is all this is guys
is an abbreviated way of saying this to
the machinist and everybody that's
reading the drawing and here we've got
an example here there's a little
calculator that shows us that we've got
an H 8 h 7 here on nominal shaft science
is 10 millimeters and we can see here
the tolerance we've got here and H 8
which is your hole size can be 10
millimeters plus 22 microns and not any
smaller so it can be 10 millimeters plus
22 microns plus nothing so it's either
10 millimeters or ten point zero to two
millimeters that's your hole and
similarly for the shaft the shaft is not
going to be any bigger than ten
millimeters but it can be ten
millimeters
-
zero minus 15 and that's why or how with
tolerance things up sorry when in order
to get the desired fit who moving on
let's now discuss engineering materials
and this is important guys engineer you
need to know how materials behave under
different conditions now let's get
something out of the way the difference
between tension and compression is if
you compress something you press it
together push them push these loads are
pushing together in tension with pulling
apart with pulling apart so if a spring
is being compressed is it going to get
larger or smaller six dimensions its
length if a spring is being compressed
will it get smaller or larger well I'm
so glad you guys look smaller okay so
we've got the the terminology right now
force is applied to go over this in this
case is load P is applied to a rod of
cross-sectional area a zero and in this
case we've got a load that's pulling
these apart
that's in tension so both of those rods
would be under stress one would be being
compressed the other one would be being
stretched and the stress in both cases
would be the load that we apply divided
by the cross-sectional area strain is
the definition of deformation of the
component and divided by its original
length
now stresses can be shear stresses where
we're trying to chop something in half
or they can be torsional stresses where
we're trying to twist something or it
can be a tensile stress or a compressive
stresses we saw before now we can't
measure stress itself but we can measure
deformation we can measure the change in
cross-section so if we talk about normal
stress normal stress is the force per
unit area that's P divided by the
cross-sectional area a naught and the
Sigma is the stress in Newton's per
square meter so P must be a Newtons an
area must be in square meters Wow tough
muffs just laughs now this cartoon yep
and it's just going to be tensile stress
here or compressive stress do you think
this zebra is is undergoing we're
pulling something often pulling your
stripes off as a tensile or compressive
stress so I think it's which tensile
stress okay and here we've got a picture
of a lab and these are our pieces of
equipment called 10 kilometers now what
they do is they allow us to put samples
in between two jaws there's a lower jaw
another jaw there and then we apply a
load and we stretch the tensile test
object and we measure the load and the
reduction in cross-sectional area and
that helps us calculate the tensile
strength of the material
now I'm going to show you a sketch in a
moment that explains this this
particular slide but when we talk about
elastic elastic deformation in Steel's
or in materials what elastic deformation
means is is if we apply a load to a
piece of material and it stretches if we
reduce or take away the load that piece
of steel will go back to its original
shape
that's elastic different defamation and
we're able to measure this in the
laboratory and doing tensile tests
so if we want to find out the tensile
stress of a piece of material then we
apply this formula but we know that
Sigma is the stress E is Young's modulus
of elasticity that's measured in
gigapascals and then e this epsilon is
the strain and the strain is
dimensionless it's a change in length
divided into the original length that's
change so it's dimensionless and for
different materials the the value of
Young's ELA modulus of elasticity
capital e here can vary anywhere between
45 kilo Pascal's right up to four
hundred and seven bigger Pascal's it
really depends on the material now you
know we've got a stress-strain diagram
and there's strain this is this is
supposed to be little epsilon you can
see that it's the change in length
divided by the original length that's
why it's the dimensionless and here's
our stress that's load divided by the
cross-sectional area so if we look at
this solid line of this curve this is a
tensile test on a piece of material
where we're measuring stress that we
apply against strain that's the stretch
of the material and here you can see a
sample shape of a sample now in this
region here where we put a linear
relationship between stress and strain
this is called the limit of
proportionality and the word stress is
proportional to strain it's linear
it's linear then we get to a point here
that we call the elastic limit now
in other words elastic limit is that
stress apply to the material and that
will when it's removed allow the
material to go back to its original
diamond
once we get above that limit that
tensile stress limit we get what we call
plastic defamation so in other words if
I released the the stress if I release
the load my tensile test object here
would not go back to this original shape
and what happens is we get something
called necking and you can see we get a
reduction in diameter of the tensile
test sample and what's interesting is
that here we get something called the
UTS the ultimate tensile strength once
we get past that UTS point look what
happens here I don't have to apply any
more load the material is saying Oh
goodbye I'm failing and then we get
fracture of the the tensile test and
sample and I just want to show you in a
moment what does look like so the
proportional limit is stress at which
the linear relationship between stress
and pain and strain fails to exist that
is the limit of proportionality or
proportional limit as they have on this
slide here once you're past that point
it's the point of no return
now engineers when they design whether
it's a bridge whether it's a rope for a
drag and in a quarry they're always
designing to make sure that we never
ever stress a material past this point
here because that can lead to failure
now yield stress I've mentioned before
that's where we get yielding of the
material and then we get plastic
defamation appearing strain Harding is
if you look at the sample you could see
the grain structure you know under a
microscope you could see the grain
structure of the materials actually
starting to change and that's what's
doing the damage and let's talk about
engineering material properties for a
moment stiffness that's how much a
component will deflect or not deflect
under a given load we're given a slick
material here diamond diamond does not
change and very very heavy loads rubber
on the other hand it doesn't take much
to change the shape of a piece of rubber
so diamond would have a very very high
and Young's modulus of elasticity and
rubber of course would have a very very
low modulus of elasticity ductility
that's defined really as the ability of
a material to be drawn into wires
strength the strength of a material is
the ability of that material to resist
defamation
now when we're talking about failure
mechanisms engineers when they're
designing equipment have to take all of
these five failure mechanisms into
account when they're designing equipment
you
well fractures defined quite simply as a
separation of a body into two or more
pieces and let's hope if we design
something that we never ever see
fracture occurring
now fracture can occur under conditions
of compressive stress shear stress and
torsional stress and there's definitely
two steps in the process that occurs
before we have ultimately fracture on
our hands and that is the formation of
cracks and then the propagation of
cracks
and we've got two possible fracture
modes this fracture occurring under
ductile conditions and fracture
occurring under brittle conditions now
let me show you the samples of these
tensile test objects that I was talking
about before imagine those tensile test
machines at 1000 meters and producing a
load a on on samples until they're
actually fracture and this would show
you the different samples and of
different materials and the fractures
that we get depending on the ductility
of the the of the sample here we can see
that we've got necking that's really
really visible so in other words this
material is stretched and stretched and
stretched and then bang it's broken this
is indicative of a very highly ductile
and material something like copper
something like aluminium here we see
there's been a definite reduction in the
size of the sample up to the point of
fracture then at the point of fracture
now you can see this sawtooth pattern
here where the samples have torn apart
from each other so this is what we call
in water gray ductile fracture and you'd
see that in some stainless steels for
example and then on here on this
particular one here we've got brittle
fracture you can see no reduction in
cross-section here and what happens is
we load the sample it doesn't stretch
we love this example more it doesn't
stretch we load the sample more it
doesn't stretch and then what happens is
we reach the ultimate tensile strength
of the material without it having
stretched and then bang it just gives
way and this is indicative of a and
a highly brittle material any ideas what
that might be well can you think of a
highly brittle material early brittle
metal for example highly brittle metal
yep
well cast-iron is a very very common
material that is very brittle tungsten
you said is brittle as well titanium mm
I think in titanium you're going to see
this one is moderately ductile tiny
timing as is but high speed steel yep I
think it's going to be it's going to be
there on the on the on the brittle
fracture brass no you will see some
necking on the sample before it gives
way so Russ is is moderately ductile
plus whips definitely brittle you have
ever strip try to stretch a pane of
glass I haven't but would be interesting
to see if you could oh yeah glass is
definitely brittle now with ductile
fracture oh that the processes we've got
a very slow
crack propagation if we reduce the load
then the crack becomes stable yay and
it's not a great situation to be in but
we want we won't get any any any
fracture occurring unless we reapply the
load and carry on doing that until we
get to a certain stage where we've got
plastic deformation where the material
is visibly stretched and we know around
the corner fractures just waiting to
happen so that's that's ductile fracture
with brittle fracture it's the other
extreme what happens is you get very
very little if any deformation at the
fracture site
and you know as I said before we apply a
load keep on a plan and keep on applying
and then bang it goes and it goes very
very suddenly you get rapid crack
propagation and a brittle fracture is
extremely quick and it can be very
catastrophic other failures that we can
see in in construction or dynamic
stresses and and and fluctuating
stresses this is where the stresses are
going up and down and the dynamic
stresses are going back and forth and
what's happened here in this pretty
little picture is this boat has broken
in off it's broken in half due to
fluctuating stresses glad I didn't
design that here's some examples of
fatigue we've got a truck with a ladder
on here and oh dear
the ladder has broken and archaic bent
as well how about this is an example of
fatigue over a jet aircraft engine where
the blades of the jet engine flew apart
and they just burst the casing of the of
the jet engine these are things people
that engineers have to design out and so
we've got to really be aware of these
things
another form of stress is corrosion the
corrosion
basically people is what can can
somebody give me a definition of
corrosion but a very very simple one
you
you
okay we've got some cotton pillows to me
corrosion is a loss of material due to a
chemical and electrical reaction it's a
loss of material can be gradual or it
can be rapid but it's a loss of material
and if we use material then we lose
strength now there are two types of
corrosion that we're going to look at in
this webinar
that's uniform and localized corrosion
but uniform corrosion as the name
implies it happens
all of this all over the surface and by
they basically um the the corrosion rate
is the same over the surface of the
material and things that can can affect
corrosion uniform corrosion are low pH
things that are very very acidic
dissolved oxygen dissolved carbon
dioxide another one here is high
temperatures high temperatures can
accelerate corrosion and here's some
examples of uniform corrosion see this
for the steel floor has been eaten no
way and you can see the corrosion air is
uniform and I don't know who is standing
there before that gave way but anyway
you can see that that's that's that's
happened here in this construction site
and another example is where we've got a
pillar in sea water and you can see
where there where the high tide is here
can't you okay there we've got a good
example of uniform corrosion localized
corrosion is where we got non-uniform
loss of material over the surface of the
component and things that affect that or
carbon dioxide calcium chloride
especially in stainless steels and that
are are that contain and calcium
chloride and
that can can affect local corrosion and
here we can see an example of localized
corrosion which is so localized here
this is actually gone right through the
material and there's some examples here
on the right hand side here of localized
corrosion we've got something called
here so hick Soh I see does anybody know
what
so hick is
well the letters s co h IC Stanford
stress oriented hydrogen induced
cracking it's a fancy name for what
happens when we get hydrogen sulfide
corrosion
you know hydrogen sulfide that's the
stuff that that smells like bad eggs if
you've ever been in a refinery you can
smell it often and that's sort of sort
of said in English
hydrogen sulfide attacking weak points
in the material here
and galvanic corrosion can anybody give
me an example of where galvanic
corrosion occurs
no hint but I think the name might give
it away where do you think galvanic
corrosion occurs still in our minium yes
that's for sure
what I got aluminium and stainless steel
bolt yes
underwater on ships holes the propellers
yes good example whose brilliant example
you got what I was hinting out there
galvanized steel people that's where it
happens there so the in galvanized steel
for example the sink which is the
galvanizing
sacrifices itself to protect the steel
and we've got two dissimilar metals in
contact with each other steel and copper
and if we looked at the galvanic series
the table and in the galvanic series we
see that in actual fact zinc and steel
are fairly close together if you look at
that same table and looked at copper now
copper could protect steel but steel and
copper of further apart in the galvanic
series and what will happen is if you
copper plate steel and compared the life
of its ability to protect the steel
you'll find it's shorter than the sink
and that the same conditions
here we've got crevice corrosion and
this is where we we get a concentration
and where we convene to similar
materials and it can happen under the
heads of washes and I think someone gave
me an example there of aluminium and
steel steel bolts definitely could get
get that where you've got dissimilar
metals and you've got some kind of
electrolyte could be water it could be
liquid product whatever you can also get
it in gaskets you get the steel gaskets
that can accelerate corrosion if they're
not selected if the material is not
selected properly
and stress corrosion cracking SCC well
we see that in sulfuric acid tumped
tanks where the chlorides okay so in
hundred chloric acid tanks that are made
from stainless steel where the chlorides
and the acid actually attack the steel
where it's been stressed and yeah you
know when a tank is being filled up it
expands as so the steel is being
stressed it's being stretched and then
when the tank empties then the steel is
being compressed it's trying to get back
to its original shape and springs for
example mechanical seal manufactures to
reduce the cost of producing their
mechanical seals often make the springs
out of stainless steel now that's fine
if you are using them on water that has
very very low chloride content but if
you've got anything that's got any
significant amount of chlorides then
what will happen is that spring will
crack and it will fail and the
mechanical seal will ultimately fail how
about this example of fatigue corrosion
what you've got here is you've got the
presence of stress along with corrosion
so the parts are stressed psychically
and you've got the added devil of of
corrosion so that accelerates that they
they feed on each other and accelerate
at each other
oh and that brings us to the end now
you'll see there's a slide here of a
roller assembly your assignment question
asks you to show an assembled sectioned
view of this roller assembly so do
to explain what we're looking for in
that question because last time we run
this workshop a lot of people said we
don't understand what you're looking for
what we're looking for is you to do as a
drawing of this assembly sectioned so
imagine I took a hacksaw and I cut down
the center of this roller along this
axis and I cut right through there and
then all the way down through there so I
cut this whole assembly in half from top
to bottom what we're looking for is what
would the view look like with this thing
sectioned with this thing cut in half
are you all with me on that that one
that is is that because clear people
what we're looking for
great well then good luck with that
question and the idea a legend yep
yeah Justin it doesn't have to be damn
stuff doesn't have to be done on
computer and a hand-drawn sketch as long
as it's clear and it shows the
components and you can draw it by hand
what we're looking for people as you're
you can you can dodge by care if you
want and when you say Bruce what format
do you want to sin
please explain that
well it'll be well it remember it will
be well you can do it in PDF you can do
it on whatever remember it's going to go
on your exam paper so if you're going to
do it and you might want to do it by
hand and then then scan it and bring it
put it into your into your exam paper
whatever whatever suits you at the end
of the day what we're looking for is
your understanding of what assembly
would look like if we cut it in half and
we looked at it from the direction of
letter A so you've got one section it's
cut all the way through there
but don't forget it's one drawing it's
one drawing but it's going to show
different components sectionalized isn't
it and that's the only help and I'm
going to give you on this the rest
you're going to have to think through so
it'll be one drawing showing a
sectionalized
view of this roller assembly
and people I'm just a lecturer and the
assignments will be given to you by EIT
and and you'll find them normally on
Moodle to find them on Moodle okay
people are there any questions on
anything we've done in this module that
you would like to ask now I usually put
my email address on the very very last
slide but for you guys that well
actually you'll get all of these slides
on Moodle but if you want to make a note
of my email address there's anything
that you want to ask me and apart from
the answers to the this assignment
you're more than welcome
I'll try and get back to you I am away
from Saturday and I'm being sent to
Mauritius on business people on business
so I won't be able to answer anything
and go the week after next there must be
an easier way to there we go there's my
email address and yeah it did you're
right guys it's a tough job but
somebody's got to do it so as well do me
so people
once more and you'll know I don't want
to delegate once more people any
questions on anything we've done in this
webinar oh you're very welcome Maria
very welcome
okay Scott go well okay Daniel you're
welcome you're welcome sir k :
we are very welcome just YouTube Tom hi
glad you enjoyed it Justin okay guys
Cheers all the best and when the weekend
comes have a good one
okay folks I just say anything that you
might have an afterthought on there's
the email address this should be on the
slides in Moodle for you so from even
kareo guys all the best and we see you I
believe it's on the 4th of March 4th of
March
so until and stay well and it's goodbye
for me