Lecture 32- General procedure of FA: Determination of type of fracture III and chemical analysis


Hello, I welcome you all in this presentation
related with the subject failure analysis and prevention. And nowadays we are talking
about the general procedure of the failure analysis and under that we have talked about
so, many aspects like collection of the background information, preliminary examination of the
failed component, non destructive testing of the field component and destructive testing
microscopy, microscopy of the fracture surfaces and ah then ah metallographic aspects of the
field component and thereafter we also talked about the determination of the type of fracture.
Under that we have talked about the dimple fracture cleavage how to how to determine
the ah that the the way by which fracture has taken place, considering the microstructure
microscopy and micrography of the fracture surface.
So, whether it is ductile fracture having the dimple, dimples on the fracture surface
or it is brittle with the inter granular fracture or a cleavage facets or ah ah or any other
form of the fracture unlike hydrogen induced cracking or ah the creep ah .
Similarly, ah now ah the there is one more fracture; ah which about which we need to
talk is the fatigue fracture. ah This is related with the basically ah the determination of
the fracture only . So, here ah we know that ah even the fatigue can occur not just on
the the components with the stress regions like this , but also the components which
are very smooth . So, then a component with the stress regions
or unique features, which will be localizing the residual or localizing the applied stresses.
So, they will be basically increasing the stress concentration in those areas and thereby
facilitating the nucleation of the crack easily nucleation of the crack easily. Then once
the crack is nucleated it will be growing ah sequentially ah until the sudden fracture
takes place. So, the nucleation of the crack is a stage 1, the growth of the crack is this
stage 2 and the sudden fracture is the stage 3 . So, ah and each one will have the different
kind of the surface morphology. . So, in order to understand and appreciate
ah. So, the the all these stages you see if there is a notch then the nucleation stage
is completed earlier because of the localization of the stresses , but ah thereafter the material
properties will be determining the rate at which it will be growing. ah
On the other hand when the component is new even then also under the cyclic load conditions,
under the cyclic load conditions due to the very ah localized ah the slip which is taking
place at the micro level at the specially at the surface. So, it will be leading to
the ah creation of the ah intrusions and extrusion at the surface and these intrusions and extrusions
of after a sufficient number of the cyclic loading, ah it leads to the generation of
the irregularities at the surface like this. So, ah one which is ah below the surface will
which is termed as intrusions and other one ah is termed as extrusion . So, in any case
these will be creating the irregularities at the surface, and thereby they will be causing
the increase in the stress concentration . So, in this particular case since it takes
some time to create to nucleate a a crack like discontinuity, well in the case when
there is already notch ah which is acting as a stress raiser. So, the life fatigue life
in this case will be somewhat lower as compared to the case, when there is no a stress raiser
and this smooth streamline ah smooth finished component ah exists.
So, basically . So, this is one aspect that ah the surf crack nucleation is one stage
and whenever such kind of things happen, we will find that multiplicity of the crack is
present multiple multiple cracks are present a near the ah fracture ah ah initiation site;
ah while in this case ah there may be a sharp crack ah ah ah sharp zone where from it has
been ah initiated and ah subsequently it has a grown .
So, ah if we see these cross sections, ah which have failed in fatigue manner. So, the
location where such kind of nucleation has taken place that will be showing the different
kind of the ah morphology, and that is termed as the initiation zone. It takes life of about
10 to 15 percent depending upon the surface roughness or the kind of a stress raisers,
which are present on the fracture on the component. Thereafter ah such kind of the beach marks
and at micro level these are termed as striations are formed on the surface.
. So, the gap between the individual striations ah corresponds to the growth of crack, ah
roughly in each ah loading cycle . So, the spacing is also used to characterize the rate
at which the crack is growing . So, if the gap between these stations is large will be
indicating that crack growth rate was so high as compared to the case when the gap between
these striations is ah low . Variation in crack grow . So, there can be
a continuous ah change in spacing ah between the striations, ah in general ah the space
this ah gap ah keeps on increasing with the increase of the crack ah size . So, initially
the growth rate is less and then this spacing will keep on increasing, ah because of the
increasing the stress amplitude actual stress amplitude or ah the stress intensity factor
um. So, ah here the spacing will keep on increasing,
this this happens even in case when the load is the load fluctuation or the ah stress amplitude
is constant. But ah this can also happen when the load
is fluctuating when the maximum magnitude of the tensile load especially is ah wearing
significantly, in that case we will find ah lot of variation in the spacing between the
striation . So, the ah ah in over a narrow zone if the variation in the spacing between
the striations is large, that ah can indicate the a possibility of the a fluctuating load
conditions instead of ah ah instead of the ah the constant, ah constant stress ah amplitude.
ah Now ah the efforts have been made ah to to study ah um that how the crack grows ah
especially during the second stage so, that ah the life of the component under the fatigue
conditions can be estimated, can be determined. So, the da by dn is the typical formula, which
is used to indicate C is the constant and ah delta K is the stress intensity factor
range m m is the constant. So, ah depending upon the material system
value can vary from like 2 to 3 or ah ah even more it it can vary ah ah go up to the 5 also
and the stress intensity factor range is basically the delta sigma means the kind of stress variation,
which is taking place ah the the stress range ah under root pi C or . So, here we can write
ah like they ah a a is the crack length, crack length and ah the sigma is the stress range
basically the sigma delta sigma is the stress range .
So, as the um ah crack size grows in ah the delta k will be increasing even for the given
stress range and ah um the the rest of the things are other constants. So, ah this will
indicate the rate ah this will indicate that ah ah with the increase of the crack size
for a given stress range also ah there will be increase in the growth rate of the crack,
which we can show with the help of this ah very commonly used figure noise] where in
x axis we plot delta k and da by dN ah in the y axis .
So, this indicates the rate of the crack growth especially in the second stage .
So, minimum variation in the stress range is required for minimum the stress intensity
factor range is needed for growth of the crack they are after, ah the the before that ah
their crack will be non-propagating type . So, this is known as kth ah or you can say
the delta kth the minimum threshold stress intensity factor range, which is required
for ah crack to propagate and thereafter we have a diagram like this means the variation
like this where the crack growth rate increases with the increase of the stress intensity
factor range at particular rate at a fixed rate ah at a ah.
So, the rate will keep on increasing . So, this ah relationship is linear in its log
log scale and thereafter it is starts increasing. So, this is the commencement of the third
stage, this is the second stage and this is the zone one where in the crack behaves as
a non-propagating ah type. So, this is the second stage corresponding to which we write
this equation delta k raise to the power m .
. So, this is known as power ah law equation for the fatigue ah life ah estimation and
we know the if we know that initially the crack size is of a given ah value, and we
know the crack growth rate, then we can estimate the life of the component ah which has failed
. So, now, these things will be ah shown with the help of a certain micrographs, ah now
this is the diagram which is showing that ah when the even when the component is very
smooth under the localized slip conditions, there will be creation of the ups and downs
at the surface. So, that will be leading to the creation of
the intrusive and extrusion and these ah intrusions and extractions will be acting as a stress
raiser and ah thereafter once the sub crack a stress raiser or ah the crack like defect
has been formed of sufficient size, then ah the growth of the crack will be occurring
in a particular way . So, the zone where such kind of the crack
initiation takes place that ah will have all two other different morphology as compared
to the other cases, where other zones where ah um the growth of ah where crack ah grows
at a steady rate or ah the the sudden fracture occurs.
So, ah this is what I have just explained that the crack growth rate, can be expressed
using such kind of the equation which is known as a power law, and crack growth rate of course,
increases with the increase of this stress range, which is clear from this diagram ah
because delta ah stress range in as the stress range increases the stress intensity factor
range also increases, which in turn increases the crack growth rate .
Similarly, as the crack size increases that ah then also the delta a by delta N will be
increasing because here delta K is the function of the square root of a; a is the ah crack
size for the open cracks and the half crack length in case of the internal cracks and
ah increase of the loading frequency also increases the crack growth rate .
So, as as I was talking about that ah the striations are formed ah and which ah spacing
between, which indicates that roughly the value by which the crack will be growing in
each load cycle and ah um the it can be constant or variable the constant stress amplitude
ah ah results in the um constant spacing otherwise it can be fluctuating ah for given ah uniform
for the given material properties. And if there is a heterogeneity in metal properties
of course, the spacing can also vary according to the kind of material through which crack
is passing; ah the course ah these striations can be coarse or fine fine ah striations will
indicate that ah the crack is growing at a lower rate, while the courses is um striations
means the coarser or wider spacing between the striations, indicate that ah ah the the
ah the crack is growing at a faster rate . So, ah it is also possible, that it may
we may not be able to identify where striations are present on the fracture surface it requires
lot of ah ah efforts using the scanning electron microscope to establish the presence of the
ah such a kind of the striations sometimes it also becomes difficult to locate where
such a a kind of the striations are present. So, these are ah the examples of the poorly
ah defined or no striation case even under the fatigue condition. So, the such kind of
the lines will indicate the possibility of the striations, but these are not very properly
defined ah on the fracture surfaces. . So, um sometimes ah if the material is very
complex in in terms of the phases compositions, then and ah then it may not be possible to
really establish the presence of this striations so clearly .
Now, ah another aspect ah of the failure analysis is about. So, ah is the ah chemical analysis
. So, in the failure analysis procedure we have also to see ah the chemical ah composition
of the component which has failed . So, there are certain things which we want to ah um
ah investigate with regard to the chemical analysis of the failed, component chemical
analysis . Say the background colle information collected
about the component if it says that the component is made of this particular metal. So, if we
have the specification or the standard corresponding to which the component should be made so,
that ah specification is to be conformed through the ah chemical analysis.
This is one thing that ah ah we would like to do like if this is the failed component,
and ah if it was made of particular materials . So, we need to verify if really it was made
of actually of that material or there was some variability . So, confirmation of the
chemical composition is one thing that we want to do through chemical analysis, and
ah minor variation although is not attributed to the ah um deficiency in chemical aspects,
but ah if the if the variation is really significant, then this can be one of the aspects because
it affects lot of ah things related with the material ah structure, properties, cracking
sensitivity etcetera . So, the confirmation of the chemical composition
is one thing that is what we want to see. The second thing if the component has been
ah subjected to ah the effect of service effect of a service especially with regard to the
compositional modification effect of service with regard to the compositional modification.
This can be there in form of the loss of alloying elements this is one thing, which we commonly
see in terms of the like decarburize zation where loss of carbon takes place carbon loss
takes place or carburization where ah the carbon content is increased .
Ah Likewise ah high temperature exposure means exposure at a high temperature for prolonged
ah period can also lead to the loss of the other alloying elements also from the component.
And we know that the since the composition affects the micro structural aspects of the
component which in turn affects the mechanical properties, ah also affects the harden ability
and ah response to the heat treatment, ah cracking tendency all these are affected by
the chemical composition. So, it becomes important to really see if
the composition was as per the requirement or it it has been different from ah what has
been what was specified or what was expected to be there in the component .
Or sometimes loss of alloying element ah or enrichment of the alloying elements in the
component, during the service can also lead to the modification in properties which we
can see with regard to the two aspects, one is like if the it is the loss of alloying
elements then ah for example, loss of the carbon or ah chromium or ah molybdenum kind
of elements from the surface, it will be leading to the reduction in the hardness reduction
in the sigma y . So, a reduction in the sigma y and hardness
ah this can ah lead to the loss of strength and a loss of strength may lead to the deformation
or even the fracture . So, we need to really establish if there has
been any losses or enrichment has taken place enrichment of the alloying elements and if
addition of the carbon is taking place, then like say during the carburization then it
will be leading to the increase in hardness, ah increase in the brittleness reduction in
toughness ah reduction in ductility . So, it is possible that that that under the impact
load condition such kind of the components can fail in very brittle manner . So, this
will be increasing the ah cracking sensitivity or sensitivity ah for the fracture in presence
of the crack . So, ah it is important to consider this ah
aspect especially with regard to the macroscopic study of the fracture surface whether it has
been ah ductile or brittle accordingly it can suggest the possibility for the loss of
ah alloying elements. Not just this ah ah it is also important to it is not just important
to ah find out the numerical value of the chemical elements present in a given component
, but also we need to see what kind of the gas is present.
Because the gases can lead to the ah the change in the ah properties in form of embrittlement.
Embrittlement of the like presence of the hydrogen, ah nitrogen in iron leads to the
ah especially high strength steels it leads to the embrittlement .
Ah Similarly the presence of the oxygen ah also leads to the oxides or the pores in the
material. ah Apart from determining the not just amount, not just amount is amount of
the alloying elements or the elements present in the metal are important ah within limits
of the variation in amount, the distribution of the alloying element is more important
distribution of the elements is more important. ah Because ah like say if this is the metal
and if most of the things ah if a particular element has been segregated like this in particular
location, then it will not be able to offer its effect in the entire ah ah mass of the
component . While if the same one has been well distributed
in the matrix then it will be able to offer, the desired change in the microstructure desired
properties in the components. So, homogeneity is more important ah um then then the exact
amount , but of course, the amount variation is ah has to be within the limits .
. So, ah it is important therefore, to see the kind of ah distribution of the alloying
element which is there in the field component, and for that what will be the zone of interest
of course, the where from the fracture has been Initiated or the cross section where
failure has taken place. So, of course, we need to take the samples
from the fracture initiation site or the location near the fracture surface . So, that their
chemical composition can be established to see, if there has been a segregation of certain
kind of the elements gases ah or not. So, this is one thing apart from this ah the
segregation distribution related aspects of the alloying element, we need also to see
ah um if ah if the failed component has been exposed ah means has been in the service for
a longer period, then the service conditions will also have effect on the ah surface characteristics
of the component, which may be they are in form of the ah formation of the ah oxide.
So, the ah like failed component after prolonged use can show the presence of surface deposits
in form of the oxides, nitrides, carbides some shoot formation rusts etcetera.
So, all these things ah if these aspects have deteriorated the um the ah ah properties or
they have created some stress raises on the surface of the component, they have if they
have eaten out the material uniformly from the surface or they have caused the localized
damage in the component this may be in form of pits or crevices or ah um presence of ah
some ah causing the development of the irregularities on the surface .
So, ah now these are ah things which are ah presented the surface ah as a due to the conditions
which have been exposed or experienced by the component during the service, these need
to be these surface deposits need to be investigated deposits need to be investigated.
So, whatever is present at the surface and if it has a ah affected the surface properties
irregularities proper mechanical properties of the component at the surface, all these
need to be ah investigated. So, the impurity is presented the surface also should be investigated.
Why it is important? Because the chemical analysis has the direct effect on the microstructure,
which in turn affects the mechanical properties also affects the heat response to the heat
treatment . And ah not just that ah the corrosion resistance
of the material is also affected because if the homo composition is homogeneous, then
corrosion resistance will be good if there is segregation of some of the alloying elements
either the grain boundary or within the grain, then it will be it will be forming the galvanic
cells easily and which in turn will be reducing the ah corrosion resistance.
Similarly, the higher concentration of the alloying elements will be leading to the increased
harden ability in case of steels, and which in turn will be increasing the crack sensitivity.
Now there are various methods, which are used for the chemical analysis purpose depending
upon the purpose what is to be analyzed in what size what quantity which material is
there to be analyzed, ah we have the different things like ah optical emission spectrome
metry is used for determining the elements dissolved in the metal.
Then ah atomic ah um absorption spectrometer is used for determining the elements dissolved
in the metal as well as and most commonly it is used for ah determining the carbon sulphur
content apart from the other alloying elements also it is also used for ah determining the
concentration of the oxygen nitrogen, hydrogen. Then wet chemical analysis is used for determining
the concentration of the alloying elements which are higher in concentration. Elements
which are high in concentration can be established accurately with the bread chemical analysis,
the same time it can also be used for determining the presence of anions like ah chlori ah chlorine
ah chlorides nitrites and sulfides ah . Then other methods of the spectroscopy like
the x ray diffraction based spectroscopy is primarily used for the crystalline materials,
and x ray fluorescence spectroscopy is is used for both crystalline as well as the in
amorphous materials, which can be applied for both solid liquid and the gases. Then
infrared ultraviolet ah infrared or ultraviolet spectroscopy can be used for ah determining
the composition of the organic compounds such as oil grease rubber and plastics.
ah Then there are two ah good approaches which are used for determining the distribution
of the elements in the metal like ah EPMA which is called electron probe micro analyzer
or ion micro analyzer ah this is ah. So, ah EPMA is used for ah ah elemental analysis
over of the surface of the diameter of ah one micron ah for analyzing the components
analyzing the elements down up to the three atomic number ah and the content is up to
0.1 percent. So, the me the this is the threshold or ah
resolution of such ah device where the elements greater than 0.1 percent of the atomic number
greater than 3 can be analyzed using this approach and it analyzes the area of ah about
1 micrometer . Then ion microprobe analyzer is same as that
of the EPMA , but ah it ah has much lower resolution whether regard to much better resolution
as compared to that of the EPMA because it can analyze ah the elements, ah which are
present ah as low as 100 ppm . Then ah for analyzing the surface deposits
there are two techniques. So, Augar electron microscope ah is used for ah ah determining
the ah for measuring the elements up to the atomic number 3 over the area of 1 to 50 micrometer,
while the mossbauer spectrometer is used for ah analysis of the compounds are like sulfides
carbides and oxides present in the ferrous systems and which can analyze the depth up
to three ah from depth of varying from 3000 angstrom to the ah 0.00005 inch .
And ah then there are two techniques which are used for ah there are a few techniques
which are used for the analysis purpose and for most of these techniques, work in ah combination
with the. Scanning electron microscope ah for like EPMA
or ah um FESEM or ah ah ion electron mic ah ion ah microprobe analyzer these work in combination
with the sam . So, we need to magnify the sample to be analyzed sufficiently so, that
we are able to identify and recognize the phases which are to be analyzed.
. So, like say A B C D these are the four phases present in the sample, and if we have
to analyze them we can follow ah certain ah techniques and these are called a point analysis.
In case of the point we we just select a particular point of way for the analysis and it will
give us the and the percentage of different elements present in that ah at that particular
location or, but that particular point . So, here what ah the this phase is made of that
will be indicated by analyzing ah the very small area like a point .
But if we want that know we we need to have the better representative ah um ah zone of
the area being analyzed. So, we can select a particular area like this and then it will
be analyzed for ah area analysis and know if you want that know ah we have to have the
variation in the composition varying from one zone to another . So, then we can draw
one line ah between the two points; the points of our choice and then we can draw one line
like this and if there is a falling if there is something present at the grain boundary
that it also can be analyzed, what is done in this case? ah
We select the distance to be analyzed between the two points and then we also select the
the interval of the interval at which we ah the different points need to be analyzed.
So, this will be giving us the idea about the different ah the variation in the concentration
of the different elements, ah on moving from location one to the location another this
one is called line analysis ah. We may also be interested in that where what
is present and how it is distributed ah over ah given area. So, for that purpose we also
use the elemental maps. Elemental maps will be able to highlight where what is present
which element where it is present and what is the kind of the distribution within the
grain or away ah from the or ah near the grain boundary. So, this is what has been shown
in this ah um in these ah ah two three diagrams here this is the .
Ah This is the edax analysis ah which is carried out with the help of fesem and here this area
ah is analyzed to see what is present in what quantity . So, this is the kind of analysis
ah of the different elements of this particular area .
Now, we will see this is the line analysis where that the the between two points one
line is drawn and ah the analysis is carried out between these two points and the distance
ah between the points ah which are to be analyzed can be selected suitably . So, what it shows
that this white particle having the higher concentration of the tungsten this is what
can be seen here while ah at this location it has a lower concentration of the other
elements like ah ah the nickel. So, nickel is high in the matrix and the particle is
made of the tungsten . So, such kind of the um ah things can be done
for the line analysis and then it is the elemental mapping .
So, we need to choose the ah the location which is to be analyzed. So, this is the matrix
these are the particles which are being analyzed and these particles are being analyzed for
represents of the different elements like this one shows the chromium con and distribution
of the chromium . So, what it is showing that, wherever at these particles wherever the the
bright particles are present indicating the presence of chromium and wherever dark dark
zone exist indicating the presence means ah absence of the chromium or a presence of less
chromium . Similarly, this one shows the distribution
of the iron what it is showing that matrix is a dominated with the iron, and in these
particles the iron is in very limited quantity. So, wherever iron is missing or it is less
in concentration that zone will be shown by the dark area. Similarly the for the manganese
content ah ah almost to the distribution of the manganese is somewhat more means the manganese
content is somewhat more in these particles as compared to that in matrix.
So, wherever if the particular particle is present in greater quantity that location
will look brighter I will say it will look the darker one . So, this is how the elemental
list a maps can be used to see where what is present ah um.
So, now I will summarize this presentation, in this presentation basically I have talked
about the ah how to determine the fatigue fracture in ah using the suitable ah techniques
like microscopy and microscopy and ah also I have talked about the importance of the
chemical analysis in failure analysis and what are the different techniques which can
be used ah for carrying out various types of the chemical analysis
Thank you for your attention .

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