Modes of Failure and Fos – Introduction to Mechanical Engineering Design – Machine Design 1


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from Ekeeda Hello friends in last video we have seen the basic principles that
are used in machine design in today’s session we are going to look at the
modes of failure and the factor of safety modes of failure of any machine
design part or any mechanical part or system this them many ways a machine can
fail you will say what why we are talking about the failure because we are
most concerned about the safety guys machine design is the only field where
you have to consider the failure first and after the success because we have to
avoid the failure you know in order to avoid the failure you must know what are
the ways of failure so in today’s session we are going to look at this
different important types of modes of failure of machines or machine parts so
let us begin as you can see the very first type is normal stresses normal
stresses are induced resistance given to the applied forces or these are the
intensities which are produced against the applied forces now there are two
ways your normal stresses will be induced either your stresses will be
induced in tensile manner or they will induce in compressive manner so friends
we know that tensile stresses are induced when the body is subjected to
full forces means body is subjected to elongation of that particular part right
and compressive stresses are induced when the body is being compressed means
the body is being contracted by the applied forces so it is the opposition
to the tension and it is a position to the compression which is nothing but
your tensile stresses and compressive stresses no body can fail in normal
manner so when the surface area on which the force is acting is pappan
to that particular force direction they will be called normal stresses like we
said first type is of stress our tensile stress second is of compressive stress
so in case of tensile stress if this material is not sufficient to carry
these the forces which are applied on it in whole manner then the body will fail
or it be for the material is not sufficient to carry the compressive
forces which are acting on them which are push forces in such cases I will
call or we will call that the materials fail in tension or the material is
failed in compression so this is the very first and basic mode of failure
which we consider almost everywhere almost in each and every part which is
subjected to loading as you can see the second type is shear stresses as we know
that shear happens when two successive planes try to slide against each other
in such a case when the separation happens between two successive planes
the failure is called shear failure and the stress which is responsible for this
or the stress which is generated while this shear happens is called shear
stress so second mode of failure is shear failure
the third type let us move to the third type that is called a rupture no rupture
happens when something is you know when a body subjected to loads in such a
manner that the body will be destroyed in uneven manner like if a hammer is a
hammer is objected on certain small object or the hammer is filled or a pile
on some small object the object will destroy in an uneven manner you can’t
decide whether it is failing in compression or whether it is tension
whether it is shear in such a case that the body or the bodies fail in the
rupture so that is a third mode of failure the fourth mode of failure you
can see is tearing airing we know we tear the page with
tear these thin sheets so in such a manner where the the particle or the
object or the thin sheet lamina which fails in two separate parts but
application of forces in parallel manner to the through the plane of failure
it’s called tearing the best example is steering of page we can demonstrate it
in simple manner a page which we have kept in this way there is a line along
which I want to tear it off I’ll apply the forces in parallel manner only to
this lines the only thing is the nature and the direction of forces will be
different can you see this object is failing along the line so this
particular phenomenon is called theory so this is just an another way of
failure of the object let’s move ahead a click so there is next type is buckling
and as we have already learned this topic in strength of materials buckling
of columns and struts so something or some part which is which have which has
its length larger than its lateral dimensions for example the length is
larger than its diameter there are chances that if axial forces are applied
on such object it will buckle buckling is nothing but bending of the
things in vertical manner generally we consider bending of beams but bending of
the things which have length in a y direction after application of axial
loads is called bugling so there is another way where things can buckle now
huge shafts which are vertical in manner can buckle right so that’s another way
of failure of next type is bending called obvious beams are the members
which which are subjected to bending after application of transverse loading
yes so this is the another way of failure where in bending not only
deflection will be larger but also the bending stresses will be induced
a last topic of the last type of failure is fatigue no fatigue is called the
deadliest failure nano because in case of sly cyclic loadings what are the
cyclic loadings when some object or the part is subjected to loadings which
continue for n number of cycles or which come back to the object after ‘s equal
in interval of time are called cyclic loading for example a wristwatch
there are many gears in the wristwatch when they work when they function they
perform some cyclic motion or the oscillating motion so they undergo
cyclic loading situations the parts the objects which undergoes such kind of
situation the cyclic loading they may fail in fatigue because during the
cyclic loading their plastic deformation keeps on happening the problem is due to
the machine is running we can’t identify we can’t trace this plastic deformations
so at certain point it fails in fatigue that means it fails without any prior
identification or without any prior information so all such kind of failures
are called fatigue failures guys in today’s session
we have seen so far are the modes of failure there are different modes of
failure so there are different ways in which the loadings that happen on the
product so guys remember forever whenever you go for the designing of a
machine element very first thing you have to look at is the mode of failure
of different objects in different manner once you have mode of failure is desired
decided for that particular topic or particular product you can decide the
design method for that particular product let us move ahead and let us
look at the last part of this topic that is factor of safety guys
factor of safety is something we can call as the margin let us say we have
purchased a bottle water bottle it has the real capacity of 5 kilo 5 litres
right but instruction if it is written that just pour in 1.4 liter or just pour
in 4 litres of water inside it so why that one litre margin is given guys Bo
the water is designed to carry 5 litres it is desired I mean according to the
manufacturer it is desired to pour in only 4 litres of water so that one liter
margin is nothing but your factor of safety yes guys the extra margin we must
give and why that margin is required this when we designed some product we
consider some boundary conditions we apply some constraint to that design we
assume that the product is going to sustain or the product is going to
experience these conditions but that is not always the case for example if you
design a product to carry the in cell stresses of course the loads are acting
in the normal manner in that case but somehow by any chance the loads at in
shear manner that means the load acting perpendicular manner in that case not
only the tensile stresses but also the shear stresses will be induced in that
product but a product is designed only to carry stencil services so that
inclusion of shear stresses may fail the product so in such cases where you have
even a slight idea whether apart from normal stresses shear stresses may also
come into picture you have to design it with certain margin so that the product
will not fail second example argue if you design a product to carry 10 Newton
of force but somehow by some means the load act on it is actually 15 Newton in
that case the product is going to fail so when you have even a slight idea
the force actually is going to act we’ll be having some slightly larger magnitude
than the design value in such case you have to design it in some / manner so
that even after 15 Newton of force is acted on it by mistake the product will
not fail yes so that was the basic idea of factor of safety let us look at it
properly that is the types of stresses of course we have we have already seen
the types of stresses one of the stress-strain curve I will draw like
this and the second one like this this is the stress so this is this trace and
this is this strain this crease and strain can you identify this one yes you
are right this is the stress-strain curve for the pile materials and this
one is for brittle materials of course we have learned this we know
these salient points also the important points also this curve of course is the
starting point this is the point of elastic limit this is a primal point
this is lower yield point this is ultimate point and this is breaking
point for any ductile material whereas in brittle material yes there is some
elastic point and beyond that you can’t have distinguishing points so what do
you have is ultimate point and the breaking point so I will call it
breaking BR point ultimate you point and this is elastic
point whereas here we have elastic point upper yield point lower yield point a
limit point and breaking point again BR point guys when the things are designed
this traces are taken into consideration to certain extent yes the yield points
and the ultimate points as you need to understand this thing very important see
whatever we design they actually undergo different forces when they undergo
forces stresses induced in them stress can be shear stress it can be a tensile
stress and can be a compressive stress similar stress-strain diagrams are used
for their analysis know when I say my object has reached its elastic point I
have the chances if I remove the lower which is acting right now in the product
it will trace the same line back to the origin and it will regain its original
shape because the section which lies up to this is called elastic section or
elastic region the region thereafter is called plastic region now what do we
mean by elastic region when your object is stretched or it is elongated or it is
compressed in the elastic region there are chances it will regain its original
shape and size but once it starts going ahead of it
it actually goes into its plastic deformation area what does it mean at
certain points after this elastic point if I start reducing the load on the body
it will not trace the exact go back to its original shape that means it will
come to certain shape like this so can I say this small amount of permanent
deformation is there yes it is called plastic deformation so we can conclude
that after elastic point there will be always some plastic deformation the
retain inside the body this plastic deformation goes on increasing as we
goes on increasing the force applied so one thing is very clear that after
elastic point or after yield point it is impossible for the material to regain
its original shape and size that is if we keep on adding this kind of forces
which introduce the stresses which are greater than a per yield point the body
is going to experience the permanent plastic deformation which in other way
going to help only failure failure of the product not other productive thing
we don’t want the failure of the product so that we should consider that the only
possible stresses are only up to this which is up to yield point guys when we
design a material we’ll consider that the material or the body or the
particular part will have only limitation that is Sigma of a per yield
point means we consider this as the extreme stress that the metal can handle
whereas it can handle up to the ultimate stress we’ll consider and this is the
extreme aim and we will design the product in this manner only that is
first thing but when we switch back to or brittle stresses it’s not that simple
brittle faces somewhat show the elastic point but thereafter they don’t give us
the significant points like a foil point lower yield point
directly give us the ultimate stresses and then breaking stress so in case of
brittle material I have to consider ultimate stress as the reference point
so I’ll consider that I can apply the forces on brittle material or a brittle
element to the extent of ultimate stress can you see the distinguishing thing yes
for ductile materials my maximum possible stress is upper yield stress
and for my brittle it is ultimate stress now when I consider these two stresses
what I’m supposed to do is guys we know that these two things these two things
are nothing but the extremities that are the maximum possible stresses a material
can handle so what is factor of safety factor of safety is a ratio ratio of ratio of ideal to the actual so that
ideal is the possible extreme and shall say extreme value divided by actual
value we know that extreme value is always greater than the actual value so
in such case factor of safety is always greater than one there is always
remember this thing this is very important thing your factor of safety is
always greater than one so factor of safety should be greater than one that
is one condition now we have seen that the extreme ends or extreme values for
ductile and the brittle metals are different that means the factor of
safety formula will be different for both of them so in case of the tail
material and in case of brittle material factor of safety is equal to extreme
value that we use in the pel material is its upper limit Sigma of per year that
means stress that is induced at the upper yield point divided by actual
value so I will call it working stress whereas
in case of brittle material FS will be defined by extreme value brittle which
is ultimate Sigma ultimate divided by actual value this value also I’ll called
working value that’s it factor of safety so these are the formula we use for
factor of safety but what do we find out in that case there are two ways you can
use this concept if you already know see guys we already know what is ultimate
tail point for a material and we already already know what is ultimate point or
ultimate stress point for a material we need to know this Sigma working that is
the maximum stress that can be induced in the part or in the object once we
know these two things we can find out factor of safety but guys the working
stress generally is experienced by the product it’s not a defined thing because
empirical formally sometimes fail in descending the actual stress that is
going to be induced so in such a case that you have to work with factor of
safety guys these are the things of experience and judgment through judgment
we can suggest the factor of safety required for that particular for the
particular product or for that particular element so as of now we have
the values with us of ultimate yield point or appeal point and factor of
safety now using this two ratios I can definitely find out Sigma working so
that I will define the extreme value the distress can be induced in the body so
that with the margin of factor of safety my body will be kept safe the same thing
we can apply here we already know Sigma ultimate for brittle we through
experience already know what is factor of safety for brittle material we can
find out the value of Sigma working using these two relations so what is my
purpose my purpose is to find out Sigma working first
and then design the product accordingly or if the product is already design
where I know Sigma working I can define factor of safety for example if my
machine is capable of carrying 100 Newton but when I specify the
specification of this particular product I’ll say maximum capacity is only 50
Newton in that case my F face becomes the maximum possible extreme which is
hundred divided by the maximum working which is specified which is 50 so in
this case it becomes 2 now guys you will say you are not talking about stresses
you are talking about forces as we know that stress is directly proportional
with the force and hence factor of safety can be defined in terms of forces
as well as of loads and stresses so FS can be specified as extreme force divided by working force or it can be
defined as extreme stress divided by working stress now the same thing is
applicable not only for Britain but also for ductile materials now there are some
specified factor of safety s which are predetermined by the agencies and the
air towed of course we are going to look at them at a time we are going to solve
the problems we are going to solve the numerical zuv machine design
so let me conclude this topic we looked at different types of failures they can
be failure in shear failure in normal and tensile stresses in compressive
stresses failure in buckling failure in bending the tearing failure the fatigue
failure etcetera we all or we also looked at the thing that the failure
modes are important while designing the products the next thing we looked at is
factor of safety we went to the definition of factor of safety we looked
at the expression of factor of safety and then we define the factor of safety
expressions not only for ductile but also for brittle materials now the same
concept we are going to use while solving numericals on machine design so
that was for this particular topic thank you so much for watching the video if
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