We discuss about a real case of failure and tried to find out the phenomenon and reason of the failure.

1. Heaven’s Light is Our Guide
Rajshahi University of Engineering & Technology
Course No.: MSE 2212
Course Title: Crystal Defect, Deformation and fracture
Sessional
Experiment No.: 07
Experiment Name: Case Study of Material’s Failure Due to
Fatigue Fracture
Date of Performance: 04 April, 2021
Date of Submission: 20 June, 2021
Submitted By:
Name: Abdul Al Mamun
Student ID: 1813060
Group No.: 4B
Group Member’s ID: 1813045
1813052

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Abstract:
Fatigue has traditionally been associated with the failure of metal components which led to the term metal
fatigue. It is defined as a degradation of mechanical properties leading to the failure of a material or a
component under cyclic loading. fatigue is a problem that affects any structural component or part that
moves. Automobiles on roads, aircraft (principally the wings) in the air, ships on the high sea constantly
battered by waves, nuclear reactors and turbines under cyclic temperature conditions (i.e., cyclic thermal
stresses), and many other components in motion are examples in which the fatigue behavior of material
assumes singular importance. It is estimated that 90% of service failures of metallic components that
undergo a movement of one form or another can be attributed to fatigue. In this case study, the most salient
discussion is to know how fatigue fracture is introduced in materials and what should we need to do to
mitigate fatigue fracture.
Objective:
The main purpose of this report is to find and study a material failure case that is caused by fatigue. We
will try to find out the actual reasons for this failure and comment on how can be addressed this kind of
structural disaster or accident from failure.
Introduction
A fatigue fracture is a material failure that occurs as a result of excessive cyclic loading. Before final fatigue
fracture, many different microfractures are created, and eventually, the repeated dynamic loading
propagates the cracks. When a fatigue fracture occurs depends largely on the type and shape of the material.
A fatigue fracture is caused when stress is applied, then removed, then reapplied. This process can repeat
millions of times before a fatigue fracture is significant enough to cause material failure. Every time the
stress is reapplied, micro-cracks on the surface of the material are allowed to grow. Once the growth of
these micro-cracks and the stress applied is sufficient, one or more of the cracks will propagate throughout
the thickness of the material; this ultimately results in material or component failure. Fatigue fracture is
determined by many variables. Material selection is very important. For example, aluminum has less fatigue
resistance than a material such as steel. Also, materials with sharp corners fracture easier than materials that
have rounded edges because the sharp corners act as stress concentrators that are nucleation points for the
initial micro-cracking. Another cause is the stress being cyclically applied. If the stress can be reduced
either in magnitude or in multitude, then the risk of a fatigue fracture diminishes.
Theory
In materials science, fatigue is the weakening of a material caused by cyclic loading that results in
progressive, brittle, and localized structural damage. The majority of engineering failures are caused by
fatigue. Fatigue failure of materials refers to their failure under the action of cyclic elastic stress. The
majority of engineering failures are caused by fatigue.

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Fatigue generally involves the formation and gradual growth of cracks and ultimately to fracture as a result
of reduced load-carrying capacity. Once a crack has initiated, each loading cycle will grow the crack a
small amount, even when repeated alternating or cyclic stresses are of an intensity considerably below the
normal strength. The stresses could be due to vibration or thermal cycling. Fatigue damage is caused by:
simultaneous action of cyclic stress, tensile stress (whether directly applied or residual), plastic strain. If
any one of these three is not present, a fatigue crack will not initiate and propagate. Although the fracture
is of a brittle type, it may take some time to propagate, depending on both the intensity and frequency of
the stress cycles. Nevertheless, there is very little, if any, warning before failure if the crack is not noticed.
The number of cycles required to cause fatigue failure at particular peak stress is generally quite large, but
it decreases as the stress is increased. For some mild steels, cyclical stresses can be continued indefinitely
provided the peak stress (sometimes called fatigue strength) is below the endurance limit value. A good
example of fatigue failure is breaking a thin steel rod or wire with our hands after bending it back and forth
several times in the same place. Another example is an unbalanced pump impeller resulting in vibrations
that can cause fatigue failure. The type of fatigue of most concern in nuclear power plants is thermal
fatigue. Thermal fatigue can arise from thermal stresses produced by cyclic changes in temperature. Large
components like the pressurizer, reactor vessel, and reactor system piping are subject to cyclic stresses
caused by temperature variations during reactor startup, change in power level, and shutdown.
Figure 1: A Classic Example of Fatigue Failure
Although the primary cause of the phenomenon of fatigue failure is not well known, it arises from the
initial formation of a small crack resulting from a defect or microscopic slip in the metal grains. The crack
propagates slowly at first and then more rapidly when the local stress is increased due to a decrease in the
load-bearing cross-section. The metal then fractures. Fatigue failure can be initiated by microscopic cracks
and notches, and even by grinding and machining marks on the surface; therefore, such defects must be
avoided in materials subjected to cyclic stresses (or strains). Plant operations are performed in a controlled
manner to mitigate the effects of cyclic stress. Heat up and cooldown limitations, pressure limitations, and
pump operating curves are all used to minimize cyclic stress.
Applied Force:
It is an important factor in fatigue failure that how the force is applied in a particular cross-sectional area
such as cyclic tensile-compression force, load-unload force, stress change with temperature, etc. which
can be shown by a sinusoidal wave. A schematic figure is shown in figure 2. This type of applied force is
continued for a long time but can be shortened by increasing the applied force. The applied stress is

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gathered in the defect of material and causes the initiation of crack and crack propagation and after a time
being the material is no longer capable to resist the force and at last failed by catastrophic failure.
Figure 2: Wave of Applied Load in the Load Bearing Cross-Section
Fatigue Life – S-N Curve
The American Society for Testing and Materials defines fatigue life, Nf, as the number of stress cycles of
a specified character that a specimen sustains before failure of a specified nature occurs. Fatigue life is
affected by cyclic stresses, residual stresses, material properties, internal defects, grain size, temperature,
design geometry, surface quality, oxidation, corrosion, etc. For some materials, notably steel and titanium,
there is a theoretical value for stress amplitude below which the material will not fail for any number of
cycles, called a fatigue limit, endurance limit, or fatigue strength.
Figure 3:S-N Curve
Engineers use several methods to determine the fatigue life of a material. One of the most useful is the
stress-life method is commonly characterized by an S-N curve, also known as a Wöhler curve. This method
is illustrated in the figure It plots applied stress (S) against component life or number of cycles to failure
(N). As the stress decreases from some high value, component life increases slowly at first and then quite
rapidly. Because fatigue like brittle fracture has such a variable nature, the data used to plot the curve will
be treated statistically. The scatter in results is a consequence of the fatigue sensitivity to several test and
material parameters that are impossible to control precisely.
The following terms are defined for the S-N curve:
Fatigue Limit: Fatigue limit (also sometimes called the endurance limit) is the stress level, below which
fatigue failure does not occur. This limit exists only for some ferrous (iron-based) and titanium alloys, for

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which the S–N curve becomes horizontal at higher N values. Other structural metals, such as aluminum
and copper, do not have a distinct limit and will eventually fail even from small stress amplitudes.
Fatigue Strength: The ASTM (American Society for Testing and Materials) defines fatigue strength, as
the value of stress at which failure occurs after some specified number of cycles.
Fatigue Life: Fatigue life characterizes a material’s fatigue behavior. It is the number of cycles to cause
failure at a specified stress level, as taken from the S–N plot.
Effect of Mean Stress on Fatigue Life
The mean stress σm can have an important effect on the fatigue strength of a material. A simple and crude
way to demonstrate the effect of σm would be to present S-N curves of a given material for different values
of σm on the same graph.
Figure 4: Effect of Mean Stress
Figure 3, shows such curves schematically. Note that, for a given stress amplitude σm, as the mean stress
increases, the fatigue life decreases.
Forms of Fatigue
Fatigue failures occur in several forms; however, they can be generally classified in one of two categories:
mechanical fatigue or thermal fatigue.
Mechanical Fatigue:
Fatigue damage develops as a result exposure to of cyclic stresses for an extended period. It’s worthy to
note that numerous classes of components (e.g. rotating equipment and pressure reduction valves)
designed for alternating mechanical stresses are not usually subject to fatigue failure unless damaged by
some other mechanism (e.g. foreign object impact damage or corrosion). The following types of fatigue
can generally be classified as mechanical.
• Vibration Fatigue
Vibration fatigue is a type of mechanical fatigue caused by the vibration of equipment or piping during
operation. As an example, vibration fatigue could occur as a result of operating equipment beyond
designated integrity operating windows. The amplitude and frequency of vibration are critical factors for
vibration fatigue damage that leads to crack initiation and crack propagation.

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• Corrosion Fatigue
Corrosion fatigue occurs from the simultaneous actions of chemical attack and mechanical fatigue.
Corrosive environments are known for deteriorating metal. As corrosion develops, the area of damage
serves as a point of stress concentration and results in the initiation of a crack. Thin films and coatings are
applied to protect equipment from corrosion; however, mechanical fatigue will frequently damage these
films and expose the equipment to the surrounding conditions.
Thermal Fatigue
Thermal fatigue is simply a failure that is induced by cyclic temperature changes. This mechanism is most
often encountered in the tube assemblies of fired heaters. Mechanical fatigue may or may not be present.
In most services, thermal fatigue is caused by start-ups and shut-downs. Sudden temperature changes are
referred to as thermal shock and result in immediate failure. Start-ups and shut-downs increase the
susceptibility to thermal fatigue. Rapid heating and cooling rates also increase susceptibility.
Fatigue Testing
A fatigue test helps determine a material’s ability to withstand cyclic fatigue loading conditions. By design,
a material is selected to meet or exceed service loads that are anticipated in fatigue testing applications.
Cyclic fatigue tests produce repeated loading and unloading in tension, compression, bending, torsion, or
combinations of these stresses. Fatigue tests are commonly loaded in tension–tension, compression–
compression, and tension into compression and reverse.
The purpose of the investigation is the most important item for the research and of course, this is known
when starting the investigation. Hence, it makes sense to base the classification on the purpose of the test,
type of stress, means of producing the load, operation characteristics, type of load, etc. Classification based
on the type of stressing method:
1. Rotating bending testing machine
The type of S-N curve created by this machine is identified as a rotating-bending, stress-controlled fatigue
data curve. The rotating bending test machine is used to create an S-N curve by turning the motor at a
constant revolution per minute, or frequency. To create a failure on the specimen, a constant-stationary
force is applied to the specimen, which creates a constant bending moment. A stationary moment applied
to a rotating specimen causes the stress at any point on the outer surface of the specimen to go from zero
to maximum tension stress, back to zero, and finally to compressive stress. Thus, the stress state is
completely reversed in nature.
Figure 5: Rotating Bending Test

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2. Reciprocating bending test machine
The type of S-N curve produced is identified as a tension-compression, strain-controlled fatigue data curve.
This machine type is capable of zero mean cyclic stresses by positioning the specimen clamping vice
concerning the mean displacement position of the crank drive.
Evaluation of Fatigue Fracture
The process of fatigue failure is characterized by three distinct steps:
Stage 1: Crack initiation — Fatigue cracks almost always initiate at a free surface and near a stress riser.
The initial development of a fatigue crack occurs at localized discontinuities in the metal’s crystal
structure. The generation and movement of discontinuities strengthen the metal through plastic
deformation. This phenomenon is called work hardening. As the metal becomes work hardened, it loses
its ability to plastically deform in the localized area where cyclic stresses are evident. Once the metal
reaches its plastic deformation limit, the discontinuity becomes a small crack.
Stage 2: Crack propagation — Once a crack is initiated, continued cyclical stresses repeat the process,
slowly growing the micro-crack, which becomes a threat to structural integrity.
Stage 3: Failure — The final failure event (i.e. catastrophic fracture) can be ductile or brittle, depending
on the material, thickness, temperature, and the applied stress. Fatigue failures typically occur suddenly.
The fatigue-induced fracture surfaces of a broken component are typically smooth and show no evidence
of plastic deformation.

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Case Analysis
Background
Our case is the failure of the tubewell pump connecting rod bolt. A few days ago all of a sudden I heard a
sound while fetching water from a tubewell pump and that was the breaking sound of that nut of the
connecting rod and my sister was injured by this catastrophic failure. We had analyzed the case to find out
the reason for this sudden failure and what type of failure it was.
Figure 6: Parts of Tubewell Figure 7: Fractured Nut
Analysis of the Failure:
When a material fails to load, several factors can cause the incident. These causes and the reasons should
be analyzed to know why the accident occurred. Analysis of failure is a process belonging to some
important sequential stages. In this section, we try to find out, what are reasons working behind the
accident:
Inspection and macroscopic view:
The early stages of failure analysis include the collection of background information and try to understand
are factors for the accident after collecting the sample. When the accident occurred, I collected the bolts.
From the inspection, we found that the surface is characterized by the presence of multiple and superficial
nucleation sites, radial ratchet marks in the crack propagation region, and a rough feature in the final
fracture zone due to overloading.
Figure 8: Fractured Bolt

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Type of applied force:
The type of applied force on the bolt is tensile and compressive. The bolt was joined with the plunger and
the lever of the tubewell. When the pump handle is pressed down a tensile force is applied in the plunger
but compressive in the bolt. But when released the pump handle a compressive force is introduced in the
plunger but tensile in the bolt. And this process is ongoing in a cyclic way.
Microscopic examinations:
A macroscopic visual examination of the fracture surface and external surfaces of the part begins the
investigation and will be followed by microscopic examinations. An optical stereo microscope
examination at magnifications of 50X or less will help to reveal fracture surface details, confirm fracture
initiation locations and mode of failure, and reveal possible evidence of surface damage at the locations of
fatigue crack initiation. For this case study when going through the microscopic view of the sample, we
noticed that the surface is divided into two regions; one is smooth but there are some racks and the other
region is rough with lots of cavities.
Figure 9: Microscopic Observation
Metallographic examination:
The metallographic examination is the preparation of specimens for microscopic examination and the
study of microstructures about the physical and mechanical properties of a particular material. An etching
process would reveal the microstructure of the metal. Due to the limitation and the pandemic situation we
cannot test the sample in the lab. However, we take the microscopic image from the net which is quite
similar to our case topic.
Figure 10: Metallographic examinations a) Without etching (100x) b)Etched with 2% nital (100x) c) Etched with 2% nital (1000x)

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Scanning Electron Micrograph:
A scanning electron microscope is a type of electron microscope that produces images of a sample by
scanning the surface with a focused beam of electrons. The electrons interact with atoms in the sample,
producing various signals that contain information about the surface topography and composition of the
sample. In our sample, we found the following micrograph. Also in case, Due to the limitation and the
pandemic situation we cannot test the sample in the lab. So, we take the Scanning Electron Micrographic
image from the net which is quite similar to our case topic.
Figure 11: Scanning Electron Micrographic View
Alloying elements:
Fasteners are made from a variety of aluminum alloys, with elements such as manganese, silicon, iron,
magnesium, zinc, copper, and silicon being added to increase strength and melting point. In our country,
most of the nuts which are used in tubewell are processed from cast iron where the percentage of carbon
elements is greater than 2% of weight composition.
Defects:
As the nut was used in open environments and contact with water, we observed that the nuts are
encountered by corrosion. And it is a type of planar defect. Corrosion reduces the cross-sectional area and
by this crack is initiated. The stress was localized on the free surface and when the applied load is not
bearable the material fails to resist the load and fractures.
Figure 12: Reduced Cross-Sectional Area

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The decision from the analysis:
We know that fatigue failure occurs based on crack formation and crack propagation. This crack initiates
from vibration, corrosion, thermal or mechanical processes. As a crack initiates, it may be hard to notice
much change until the crack size reaches a critical limit. By then it may be too late. A quick analysis of
the fracture surface of a fatigue failure will often show features casually referred to as “Fatigue Striation”.
These indicate the propagation of the failure from the initial cracks. Once the crack size has reached a
critical level, it will propagate very rapidly until the fracture is complete. In the analysis of the failure
section, our findings are matched with the discussed points in this section. So, we can say that the case
which we selected indicates the fatigue failure of materials.
Mitigations of Fatigue Failure
In our daily life, fatigue fractures in the oil and gas, chemical industry, nuclear and fossil-fuel power
generation, and other industries are common. When they occur, they may result in a sudden, unexpected,
and catastrophic failure. A few of the most important factors are material selection, stress concentration,
surface finish, and material discontinuities. Recommendations to reduce the potential risk of fracture may
include the following:
Changing materials:
When it is observed that the existing materials of construction are mostly to corrosive attack, it is normally
decided to change the materials of construction and select alternate materials to suit the specific need.
Material selection:
Material selection is paramount to all design considerations. Material selection may be limited by any of
several factors including economic, environmental, and service restrictions. Selecting a material with a
high endurance limit is good practice. We can use steel bolts or stainless steel bolts despite cast iron steel.
Alloying elements:
Alloying elements also have a great impact on materials selection. Silicon with a percentage of more than
0.1% helps to resist corrosion while sulfur is the reason for hot shortness. In cast iron often it is seen that
the percentage of manganese and phosphorus is in the range of 0.4% to 1% and 0.1% to 0.9% successively
which act as a carbon stabilizer. And carbon is one of the major reasons for corrosion and thus fatigue
failure. So, the percentages of alloying elements should strictly be maintained by the manufacturer.
Heat-treatment:
In the heat treatment process, the manufacturer should be concerned about the cooling temperature. In
rapid cooling, often some air bubbles are stuck in the matrix in which stress is localized and initiates to
crack. Minimizing differential thermal expansion in dissimilar metal joints is also an important factor in
fatigue mitigation.
Stress concentration:
Stress concentration is another key factor. Essentially, all sharp corners should be made into a radius if at
all possible. Sharp corners provide stress concentration and are often responsible for the initial crack and
thus fatigue failure occurs.

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Surface finishing:
Surface finish is another critical component. Strength of Materials classes teach a very important lesson:
in many loading configurations, like bending and torsion, the critical stress is located on the surface.
Therefore, a blemish-free surface will generally lend itself to good fatigue life.
Lastly, material discontinuities are inevitable on the microscopic level, but a good forming process will
help to reduce them.
Conclusions:
Fatigue was not always well understood in the engineering world. It was long thought that if the material
was deformed within its elastic range it would not carry any of the residual stresses or deformation when
the stress was removed. Sometimes this type of the fracture causes an enormous danger for our lives and
properties and also has an impact on our nature. So, it is needed to be careful to avoid this type of trouble.
In this context, the selection of materials and the process of how the surface properties of materials can be
enriched; are very crucial. Furthermore, the investigation of used materials is a must after a while which
can act as a safety guard from facing an unwanted accident.
References
1. Materials-science-and-engineering-8th-edition- a book by Calister
2. Mechanical Behavior of Materials Book by Krishan Chawla and Marc A. Meyers
3. https://www.sciencedirect.com/topics/engineering/fatigue-failure
4. https://inspectioneering.com/tag/fatigue
5. https://www.lsptechnologies.com/what-is-fatigue-failure/
6. https://www.materials.unsw.edu.au/study-us/high-school-students-and-teachers/online-
tutorials/crack-theory/fatigue-failure/example-fatigue-failure
7. https://www.engineersedge.com/material_science/fatigue_failure.htm
8. https://www.nde-ed.org/Physics/Materials/Mechanical/S-NFatigue.xhtml
9. https://www.boltdepot.com/fastener-information/materials-and-
grades/materials.aspx#:~:text=Fasteners%20are%20made%20from%20a,as%20the%20primary%
20alloying%20element.
10. https://academic.uprm.edu/pcaceres/Courses/MatEng3045/EME8-4.pdf