Macro examination in fatigue failure, practical

1. Macro Structure Examination And
Identification Of Fracture Surface
EXPERIMENT NO.:
INSTRUCTED BY:
GROUP MEMBERS:
NAME :
COURSE : BSc. Engineering
INDEX NO. : 150131A
GROUP :
DATE OF PER. :
DATE OF SUB. :

2. Abstract
In engineering world, mostly deals with the materials and their products especially in the machinal
engineering field. So that these products are going under compressions, tension, torsion situations so
that these parts will deform or break.
The failure of a product can be predicted with the use of expected life time. But some of them will
fail before it comes to the end of that expected life time. There are more cases as well as reasons that
will cause for such system.
In this practical, we are going to identify different types of cracks and failures also, the safety factors
and methods we have to follow in order to avoid catastrophic failure. At the end of the practical we
come to know that the mode of failure and method of overloading, as well as safety precautions we
can take.
Introduction
Human life cycle is heavily related with the products that are used to ease the life. So that there are
many types of materials are going to use in order to make different products as well as components of
them. As an example, in a car, there are about millions of components, use to get the proper
functioning of the individual component as well as overall component.
Materials have several properties like strength, hardness, ductility or brittleness, fracture type etc.
These properties basically based on the structure of the molecule arrangement in the material. We can
improve the strength of the material and material properties through proper controlling of the surface
treatments, strain hardening like methods.
The properties of the material can be observed through naked eye, or with the help or instrument. So
that there are two types of examination, reverently Macro structure examination as well as
microstructure examination. In this practical we are going to observe the macro structure of the failed
materials.
Fractures will occur from the scratches on the material, inclusions, voids, flashes etc. in Macro
structure examination we can observe that the outer surface cracks and defects only. We cannot say
that the part or component has no errors. Because we cannot see the inner surface defects like voids,
internal cracks, blowholes. But we can get some idea about the quality of the product.
We can see that in solids, basically there are two types of materials, ductile and brittle. With the
application we are going to use the type of material and material properties also changed. So that can
have desired properties as well as functionality of the part.
When we consider about the failure modes, that also interrelated with the material type. Material like
iron, will fail because of ductile failure. And material like high carbon steel will fail because of brittle
failure. But there are some other failure modes that will forced on both types of materials. Fatigue,
creep are the examples for them.
We have sound knowledge of failures as well as the protective actions, when we have to deals with
the materials. So that we have to get the pre-understanding about the failures via the examples and
experiences.

3. Theory
There are mainly three types of failures, namely as ductile failure, brittle failure, fatigue failure. We
have to know these types of failures in details.
1. Ductile failure
When ductile material has a gradually increasing tensile stress, it behaves elastically up to a
limiting stress and then plastic deformation occurs. As stress is increased, the cross-sectional
area of the material is reduced and a necked region is produced. With a ductile material, there
is a considerable amount of plastic deformation before failure occurs in the material, there is
a considerable increase of shear stress and fracture will propagate through the necking
surface. There is a considerable deformation occurs before the fracture occurs.
2. Brittle failure
Brittle failure takes place without any appreciable deformation, and by rapid crack
propagation to the direction of the applies tensile stress and yields a relatively flat fracture
surface. When gradual tensile load is applied on material in tensile test, at the end of elastic
limit, without any prior indication material breaks.
3. Fatigue failure
This failure is defined as the tendency of a material to fracture by means of progressive brittle
cracking under repeated alternating or cyclic stresses of an intensity considerably below the
normal strength. There will be very little warning before cracking rather than the brittle
failure. The number of cycles before want to crack the material is related with the magnitude
of the load and the strength of the material. For an example, when we want to break a steel
wire rod, we can apply very little bending moment and cyclic loads, then the rod will break
easily.
Equipment
There is no need of special equipment because we are going to observe the material outer surface by
our naked eye.
Procedure
• First get a sample of material and observe the fracture surface.
• With the knowledge of the failure and the help of guidance of the instructor can identify the
mode of failure.

4. Results
In ductile failure, there will be a cup and cone structure that was made during the necking procedure.
And some of the steel plates will get distortion and get ductile failure, without shaping but with the
necking behavior.
In brittle failure, there will no such necking behavior in the surface. The surface will be polished
behavior.
In fatigue failure, part of the fracturing surface will have the polished behavior because of the contact
of two metal plates through some times. Then another part is getting brittle behavior, sudden cracking
in the surface.
Ductile Failure
Figure 1Brittle Failure

5. Discussion
Observation reveals the material properties
Basically, we can identify most of the material failures using naked eye because of the patterns in the
fracture surface. In the ductile material there is a necking behavior. In the fracture surface cross
section is smaller than the original cross-sectional area because ductile failure occurs after that plastic
deformation occurs. So that material will pass the yield stress, ultimate tensile stress also. So that in
stress strain curve we can see that applying force continuously in the material the cross section should
get rescued in order to increase the stress on the material. If the material part gets cylindrical shape,
the material will have the cup and cone structure in the fracture surface. That is a unique observation
for the ductile material. There is no shine or polished surface in the material, because the material
surfaces won’t get rubbed each other.
In brittle fracture surfaces, there won’t any shiny or polished behavior but there will be flat surface.
The cracks are propagating through the grain boundary so that the surface will not get polished
surface, there is an uneven surface. Because of the brittle failures are occurred in suddenly, there is
no time for necking, so that there will not any noticeable change of cross sectional area in the fracture
surface.
In fatigue surfaces, part of the surface gets polished behavior because the time between the crack
initiation and the final crack is sufficient to rub and get polished. And also, part of the surface will get
brittle behavior because total load applied cannot withstand by the shaft or part, after crack is
propagated some times. Then that component will get suddenly failed.
But some of the fatigue failures occur in the shafts, that are in round shapes and transmit torsions.
The fracture surface will get cup and cone shape. But this is not ductile failure behavior. Because of
the torsion, the material mass gets distorted and so that material surface ell rubbed together until
failure. In between the propagation and the final failure, part of the material in one side will bounded
with the other part of the shaft, then gets cup and cone behavior.
Modified the structure in order to achieve desired properties.
We can change the type of material with the type of application as well as with the loading
conditions. First, we have to select the material type relative to the loading conditions. in high loading
conditions normally used ductile materials. because of there is failure occurred there are visible
identification prior to the final failure, so that can avoid dangerous thing.
By selecting the good heat treatment method, the surface structure can be changed. There for the
outer surface can be protected by the scratches and severe corrosions. Also, when the part has skin,
that can bear higher loads, when it is stressed skin.
There are several types of tempering methods. Annealing, quenching, normalizing etc. each type has
their own characteristics and some of them are used to increase the ductility of the material and some
of them are used the brittle properties of the material with the type of application. As an example,
quenching, fast cooling rate gives high hardness, but increase the brittleness. In annealing is
happened under slow cooling so that give the balanced properties.
Also in some materials can undergo with work hardening. So that ductility of the material will get
increased. And the surface hardening, case hardening are the other types of hardening of the material.
In ceramic materials use low thermal and thermomechanical treatment. The heating and cooling rates
are used in order to reduce the crack formation.

6. Action need to be taken by the engineer at the design stage to avoid each type of failure mode
In ductile failure, first we have to get an idea of the stresses developed in the part and mode of
loading. So that material must be selected in order to bear such stress, that should below the yield
stress. Most of the applications are operates below plastic area, so that the part should be within the
elastic region. Also, can select service factor as safety precautions.
In brittle failures, the same as ductile material should not pass over the yield stress, so that material
will not going in plastic region. That will help for not face into sudden failures. Also, there should be
used a safety facture definitely because the crack formation and final failures are very rapid. In
manufacturing process, notches, voids, inclusions should be minimized in the design of the product.
That will minimize the crack propagation in a part.
In fatigue failures there should be less skinned outer surface. The shafts should have corrected
diameters and reduce length due to minimize the bending moments. Also, as described in the brittle
failure, there also should avoid notches, voids, for that the design should have proper heat rejection
methods and uniformness. Also, for reduce the stress concentration, should have the uniform
thicknesses or gradually reducing surfaces. The analyzed stress lines should not be braked through
the proper analysis, so that punchers, breaks, stampings, should be minimized. Also with the help of
mass reduction methods, should use in order to reduce the weight of the part, then self-weight of that
can be minimized.
Conclusion
• Failures are occurred in various ways, some of them are catastrophic.
• By visual inspection we can identify the type of failure mode
• Modifying the structure can avoid failures and increase the life of the part
References
Kalpakjian, S. (n.d.). Manufacturing Engineering and technoligy. Prentice Hall.
R.S. Khurmi, J. G. (2005). Machine Design. New Delhi: Eurasia publishing house .
Structure change process. (1995). In Unit manufacturing processes: issues and oppertunities in
research. The national acadeies of science and medicine.