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Title:
Microstructure and Fracture Surface Analysis
Lecturer:
Written By:

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Date: Wednesday, 05 February 2014

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Table of Contents
Table of Contents .......................................................................................................................................... 3
Table of Figures ............................................................................................................................................. 4
List of Tables ................................................................................................................................................. 5
Introduction .................................................................................................................................................. 6
1.0
Part 1 ................................................................................................................................................. 7
1.1
Gear Design ................................................................................................................................... 7
1.2
Gear Materials .............................................................................................................................. 8
1.3
Gear Manufacture......................................................................................................................... 9
1.4
Causes of Gear Failure .................................................................................................................. 9
1.5
Observations and Analysis ............................................................................................................ 9
1.6
Failure Prevention ....................................................................................................................... 11
2.0
Part 2 ............................................................................................................................................... 12
2.1
Activity 1: Observation of Fracture surfaces ............................................................................... 12
2.2
Activity 2: Creep Testing of Lead Alloys ...................................................................................... 17
2.3
Activity 3: Library/Literature Search (Stress Corrosion Cracking)............................................... 22
2.4
Activity 4: Scanning Electron Microscope ................................................................................... 23
3.0
References ...................................................................................................................................... 28

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Table of Figures
Figure 1: Failed Land Rover Gear ................................................................................................................ 7
Figure 2: Failed Tensile Test Specimen ...................................................................................................... 12
Figure 3: Failed Tensile Test Specimen ...................................................................................................... 12
Figure 4: Ductile Fracture Mechanism (Callister and Rethwisch, 2011) .................................................... 13
Figure 5: Hydraulic Hammer Head............................................................................................................. 15
Figure 6: Failed Weld ................................................................................................................................. 16
Figure 7: Creep Test Results for a Lead Specimen of Width 7.79mm and 1.78 mm Thickness and a
Constant Temperature of 250 C ................................................................................................................... 20
Figure 8: Creep Test Results for a Lead Specimen of Width 5.27mm and 1.97 mm Thickness and a
Constant Temperature of 290 C ................................................................................................................... 20
Figure 9: Scanning Electron Microscope (ASM International, 2003) ........................................................ 24
Figure 10: fatigue striations (Ramachandran, 2005) ................................................................................... 26
Figure 11: inter-granular fracture caused by hydrogen embrittlement in high strength steel
(Ramachandran, 2005) ................................................................................................................................ 26
Figure 12: Brittle Fracture in TiCN (Brandon & Kaplan, 2008) ................................................................ 27

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List of Tables
Table 1: Creep Test Results for Test 1 ........................................................................................................ 18
Table 2: Creep Test Results for Test 2 ........................................................................................................ 19

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Introduction
Failure analysis is an essential function in the field of engineering. It enables engineers
and other relevant specialists to determine the cause of failure, to improve the performance of the
component and to prevent such failure in future. It can also improve future engineering and
management decisions. An effective failure analysis requires a thorough understanding of the
operating conditions of the failed component, its design, internal microstructure, and method of
manufacture among other material processing aspects. This report is about analysis of the
microstructure and fracture surfaces of failed components.

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1.0 Part 1
In this part, a failed part (Land Rover gear) is analysed in terms of design, materials
selection, manufacture, microstructure, mechanical properties and in-service performance and
durability. The study identifies the possible causes of failure and how the failure can be
prevented in future. The failed gear is shown in figure 1 below:
Figure 1: Failed Land Rover Gear
1.1 Gear Design
Gears are critical machine elements and are used for various applications such as
multiplication or reduction of speed and torque and changing the direction of motion. They are
also used to transmit force and motion over a distance. Gear failure is very critical hence gear
design considerations should go beyond meeting the normal working conditions such as the

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required torque, speed and design life. This is because there are several unpredictable factors that
can speedup gear failure. Some of the factors are related to manufacturing errors in gears and
shafts, elastic deformations of the support structures and misuse.
Gear design should consider the power to be transmitted, gear speed and the expected
velocity ratio. The gear teeth should be designed such that they have sufficient strength to
prevent failure from either static or dynamic loading under normal working conditions. The teeth
must also have sufficient hardness to prevent wear. The design of the overall gear train should
also be considered in order to avoid misalignments of the shafts, bearings and other support
structures.
1.2 Gear Materials
Material selection for gears depends on the nature of application. The selected material
must satisfy the service conditions such as wear and noise as well sufficient strength. Gear
materials include wood, polymers, composites and metals. Non-metallic gears are used for noise
reduction and in applications where the power to be transmitted is relatively low. They can also
be appropriate in corrosive environments.
Metallic gears are the most common gears and can be made from cast iron, steel and
bronze. Cast iron is used in gear manufacture because of its excellent machinability, ease of
manufacturing complex shapes through casting and good wearing properties. On the other hand,
phosphor bronze is preferred for manufacturing worm gears in order to minimize wear of the
worms. Gears made of bronze are also preferred in a corrosive working environment (Khurmi &
Gupta, 2008).
Steel is the most preferred gear material for high strength applications, that is,
applications that involve transmission of high torques and power. There are also different steels

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used in gear manufacture depending on the required strengths. Nickel-chromium-molybdenum
steels like the SAE 4320 and SAE 8620 alloys are widely used in gear manufacture. The strength
and toughness characteristics of these alloys can be enhanced by addition of silicon and
vanadium. SAE 8620 is the most preferred gear material for manufacturing automotive gears.
1.3 Gear Manufacture
Gears can be manufactured by either cutting processes or forming process. Gear cutting
processes include hobbing and milling while gear forming processes include extrusion, rolling,
powder metallurgy, stamping, casting and forging (Marinov, 2008). The gears are then heat
treated and surface hardened depending on the application.
1.4 Causes of Gear Failure
The main cause of gear failure is fatigue. This is as a result of cyclic loading of contact
and bending stresses. These stresses yield various types of fatigue failure. Other common causes
of failure include bending failure, scuffing, micro-pitting and pitting. Pitting and micro-pitting
are caused by surface contact stresses while scuffing occurs in form of welding of contact
surfaces. Scoring, abrasive wear and corrosive wear are other forms of gear failure.
1.5 Observations and Analysis
Visual examination of the Land Rover gear showed in figure 1 above revealed several
broken teeth. Since this an automotive helical gear, it is probably manufactured from hardened
steel alloy. Examination of the fractured surface revealed the presence of beachmarks which are
associated with fatigue failure. Therefore the mode of failure is probably root bending fatigue.
A through observation of the fracture surface also revealed that the region of final fracture was
between two areas of fatigue propagation. This is a clear indication of bending stresses. Root

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bending fatigue causes total fracture of gear teeth. This mode of failure is similar to the
conventional fatigue failure which begins with crack initiation, crack propagation then fracture.
It is worth to note that each gear tooth is like a cantilever beam; hence maximum tensile
stresses are located at the root which is the point of support (Fernandes, 1996). Since gears are
usually exposed to fatigue loading, fatigue cracks can develop at the root. Continuous loading of
the tooth eventually leads to failure. The bending fatigue in the root can also be facilitated by
misalignments. Gear mesh misalignment changes the load distribution of a gear pair. Causes of
gear mesh misalignment include lead slope error, shaft bending and torsion deflections, bearing
and housing deflections and centrifugal forces.
The fatigue failure observed in figure 1 above could also have been promoted by pitting,
scoring, abrasive wear or corrosive wear. Pitting, scoring, abrasive wear and corrosive wear are
all related to the lubricant used. Unfavourable lubrication conditions at the point of teeth contact
may lead to surface cracks and extreme wear which finally initiates fatigue failure. For instance,
presence of foreign particles and corrosive additive in the lubricant can cause abrasive wear and
corrosive wear respectively. On the other hand, scoring occurs when the lubrication system fails
leading to excessive heating. Increase in temperature increases the rate of fatigue failure.
This gear teeth failure can also be attributed to the microstructure of the gear. This
depends on the method of manufacture. Forging and rolling produces the best microstructure.
For instance, forged gears have a fibrous microstructure which provides excellent mechanical
properties in the plane of maximum strain. During forging the gear material also undergoes
recrystallization which produces finer grains than those of cast gears.

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1.6 Failure Prevention
In order to prevent gear teeth fatigue failure, extreme care should be taken during the
manufacture, assembly and use of these gears. The manufacturing process should involve heat
treatment and other practices such as shot peening in order to enhance the fatigue strength of the
gears. Shot peening improves the fatigue strength of a metal component by inducing surface
compressive residual stresses. The gear should also be surface hardened by carburising in order
to increase the fatigue strength.
Another important consideration is the lubrication of the gears during operation. The
lubricant used should be free from impurities and corrosive additives. Only manufacture’s
recommended lubricant should be used.
Gear teeth failure can also be avoided by using the most appropriate manufacturing
process and heat treatment method. Forging and rolling should be preferred to casting in order to
obtain a good microstructure. If casting must be used, then adequate care should be taken to
prevent solidification shrinkage which cause internal porosity.

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2.0 Part 2
2.1 Activity 1: Observation of Fracture surfaces
In this section several broken parts were examined and their possible causes of failure
identified. Possible ways of preventing such errors are also suggested. The first component to be
examined was a tensile testing specimen shown in figures 2 and 3 below.
Figure 2: Failed Tensile Test Specimen
Figure 3: Failed Tensile Test Specimen

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A keen observation of the fracture surface indicates that the specimen failed due to
ductile fracture resulting from a simple tensile overload. Ductile fracture can be caused by
overload or manufacturing errors. Ductile fracture surfaces have distinctive features as shown in
figure 4a and 4b below.
Figure 4: Ductile Fracture Mechanism (Callister and Rethwisch, 2011)
Figure 4a shows highly ductile fracture in which there is a considerable amount of
necking to a point. On the other hand, figure 4b shows moderately ductile fracture which occurs
after a small amount of necking. Highly ductile fracture only occurs in very soft materials like

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gold and lead. However, moderately ductile fracture shown in figure 4b is the most common
mode of ductile fracture in most ductile materials.
Ductile fracture starts with initial necking that is followed by formation of small cavities
inside the cross section. If loading is continued, these cavities increase in size and finally merge
to form an elliptical crack. This crack is oriented in such a way that its long axis is normal to the
stress direction. When loading is continued the, the cavities continue to merge hence enlarging
the crack in a direction parallel to its major axis. Finally, fracture occurs due to the rapid growth
of a crack around the outer perimeter of the neck (Callister and Rethwisch, 2011).
Ductile failure can be prevented by ensuring that the working load does not exceed the
manufacturer’s recommended load. It is also important to select the right material for the
application depending on the expected loads and the corresponding stresses. Proper
manufacturing process and heat treatment should also be used in order to avoid manufacturing
defects which can accelerate failure.
The second part to be examined was a hydraulic hammer head shown in figure 5 below.
A hydraulic hammer is a powerful percussion hammer that is usually attached to an excavator for
breaking rocks. The hammer exerts high impact forces on rocks and concrete in order to break
them. During operation, the hammer is exposed to high and repeated impact loads. This
repeated impact loading can lead to development of cracks and eventually fracture (Owolabi,
2013). This failure can be referred to as impact fatigue. The hammer head failed because it was
exposed to stresses beyond its fracture toughness. The fracture surface has signs of brittle
fracture, that is, the fracture surface does not show any appreciable deformation. This means that
there was rapid crack propagation which occurs in brittle fracture.

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Possible causes of this failure may include poor material selection, manufacturing errors
or misuse. For instance, choosing a material of lower fracture toughness will result to premature
failure. In addition, manufacturing errors can induce pores in the material which can serve as
crack initiation sites. Finally, the operator may have misused the hammer by exposing it loads
that are beyond the manufacturer’s specifications leading to premature failure.
To prevent such failure, proper material selection should be done during the design stage
to ensure that the material has the required fracture toughness value. Quality control should also
be properly applied when manufacturing the head to prevent development of pores. Finally, the
operating conditions should comply with the manufacturer’s specifications.
Figure 5: Hydraulic Hammer Head
The final part to me examined was the welded metal bar shown in figure 6 below.

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Figure 6: Failed Weld
An examination of the weld shows cracks which are indications of welding defects. These cracks
can either be cold cracks, hot cracks or fatigue cracks depending on the cause and time of
formation. Cracks develop in welded joints due to factors such as stress concentration defects,
tensile stress and low fracture toughness. Since all welds have some level of microscopic defects
that can cause cracks, engineers are only left with toughness and environmental conditions to
control failure of welds. Engineers should therefore control toughness and working environment
in order to prevent the development and growth of cracks (Jeffus, 2003). Toughness of the welds
can be enhanced by controlling alloy chemistry and post-heating. Welded joints should also be
properly designed to ensure that the weld is under low tensile stresses or is under compression.
To prevent cold cracks due to hydrogen embrittlement the welded joint should be preheated and
later post-heated. This will promote diffusion of hydrogen from the weld (Jeffus, 2003).
In order to prevent corrosion related weld cracks, the joint should be protected from
coming in contact with an ionic liquid that can induce galvanic cell corrosion attack.

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2.2 Activity 2: Creep Testing of Lead Alloys
When a loaded material is exposed to high temperatures for an elongated period, the
material can fail due to creep. If a tensile specimen is loaded with a constant load and is exposed
to high temperatures it will elongate continuously and finally fail even if the applied stress is less
than the yield strength of the material at that temperature. Creep is therefore an important
engineering consideration when designing engineering parts that are exposed to elevated
temperatures during operation.
Creep test is usually carried out by subjecting a specimen to a fixed load and a fixed high
temperature. The elongation and time are recorded and the corresponding graph of elongationversus-time is drawn. This experiment investigates the creep performance of lead. Lead is one of
the metals that creep at room and slightly elevated temperatures. The collected results are shown
in table 1 and 2 below.

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Table 1: Creep Test Results for Test 1

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Table 2: Creep Test Results for Test 2
A graph of elongation against time was plotted for each of the creep test 1 and creep test
2. The resulting plots are shown in figure 7 and 8 below.

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6
Creep Test Results for a Lead Specimen of Width:7.79mm and
1.78 mm Thickness and a Constant Temperature of 250 C
5
Elongation (mm)
Rupture
4
3
Third
Stage
2
Second Stage
First Stage
1
0
0.5
1
1.5
2
2.5
3
3.5
4
4.5
5
5.5
6
6.5
7
7.5
8
8.5
9
Rupture Time
Time (min)
Figure 7: Creep Test Results for a Lead Specimen of Width 7.79mm and 1.78 mm
Thickness and a Constant Temperature of 250 C
6
Creep Test Results for a Lead Specimen of Width 5.27mm and 1.97
mm Thickness and a Constant Temperature of 290 C
Elongation (mm)
5
Rupture
4
3
2
Third
Stage
Second Stage
First Stage
1
0
1
1.5
2
2.5
3
3.5
4
Time (min)
4.5
5
5.5
Figure 8: Creep Test Results for a Lead Specimen of Width 5.27mm and 1.97 mm
Thickness and a Constant Temperature of 290 C
6
6.3
Rupture Time

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As expected, plotting the elongation against time generated a graph with an instantaneous
elongation zone and three other distinct regions. The three distinct stages are usually known as
the primary (1st) stage, secondary (2nd) stage and finally the tertiary (3rd) stage. The
instantaneous elongation is attributed to the elastic response of the material to the applied load.
After instant elongation, primary creep starts and it is characterized by continuous reduction in
creep rate (Callister and Rethwisch, 2011). These characteristics are attributed to increase in
creep resistance of the specimen due to strain hardening.
After the primary stage, the secondary stage begins. This stage has a constant creep-rate
hence generating a near linear plot. The linear plot is caused by the balance between strain
hardening and recovery. In this stage the materials softens but still retains its ability to undergo
deformation Callister and Rethwisch, 2011). The secondary stage has the minimum creep rate
and it is this value that is considered when designing for creep.
The tertiary stage immediately follows the secondary stage and is characterised by high
creep rate followed by abrupt failure that is normally known as rupture. Final rupture is caused
by microstructural changes such as grain boundary separations as well as the development of
internal cracks and cavities. Some level of necking can also be experienced in the third stage.
This reduces the cross sectional area hence increasing the creep rate.
A comparison of the plots for the first test (25 0 C) and the second test (290 C) revealed
that the first test had an elongated secondary stage than the second test. This is attributed to
increase in temperature i.e. from 250 C to 290 C. This is also revealed by the difference in creep
rate. An analysis of the data within the secondary stage for both the first and the second creep
tests revealed a difference in the creep rate. The second test had a minimum creep rate of
0.28mm/hr within the secondary stage while the first test had a creep rate of 0.24mm/hr.

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2.3 Activity 3: Library/Literature Search (Stress Corrosion Cracking)
Stress corrosion cracking is a form of material deterioration process that occurs due to
simultaneous application of tensile stresses and a corrosive medium. This process is divided into
three distinct stages namely the incubation, crack growth and finally failure. The incubation
period involves initiation of a crack. The cracks can develop from scratches or dents at surface
locations where the stress levels are high. They can also be initiated by corrosion pits that occur
when a metal coating is destroyed (Ananya, 2008). The initiated cracks then spread in a direction
that is perpendicular to the stress. It is important to note that even materials that are inert in
certain environments may undergo stress corrosion cracking when they are stressed while in that
environment. Similarly, a material can fail when exposed to low stress levels while in a
corrosive environment.
Failure due to stress corrosion cracking is similar brittle fracture even if the material
involved is ductile. Most alloys undergo stress corrosion cracking in a specific environment. For
instance stainless steels undergo stress corrosion cracking in environments containing chloride
ions, while brasses can undergo stress corrosion cracking in presence of ammonia (Callister and
Rethwisch, 2011). On the other hand, high tensile steels stress corrodes when hydrogen is
induced in the crack.
The stress necessary to initiate stress corrosion is not always applied externally. Residual
stresses in the material due to rapid cooling can also induce cracks in the material. In addition,
gaseous or solid impurities within the material can be the source of internal stresses. Some of the
areas where stress corrosion cracking is common include pressure vessels, pipework and
structures exposed to marine environment.

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There are several ways to prevent or minimize stress corrosion cracking. One of these
techniques involves selecting a material that is not vulnerable to the environment where it is
applied (Callister and Rethwisch, 2011). Any change in the environment through activities such
as cleaning should be checked to ensure that it does not induce a corrosive environment. To
achieve this, corrosion inhibitors should be used during cleaning in order to regulate the
corrosiveness of the surrounding environment
Additionally, every component should be properly designed to ensure that it can endure
the service stresses. Care should also be taken to ensure that there are no stress concentration
sites due to impurities or manufacturing defects. On the other hand residual stresses should be
reduced by heat treating the component. Another common technique for reducing stress
corrosion cracking is coating the material to completely isolate it from the environment (Cheng,
2013).
2.4 Activity 4: Scanning Electron Microscope
The operation of a scanning electron microscope resembles that of an optical microscope
except that a scanning electron microscope uses a beam of electrons instead of light. A scanning
electron microscope is usually made up of an electron source (electron gun), electron lenses,
sample stage, detectors and a display device. The general layout of a scanning electron
microscope is shown in figure 9 below.

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Figure 9: Scanning Electron Microscope (ASM International, 2003)
In a scanning electron microscope, the electron gun generates a stream of monochromatic
electrons by heating the filament to a high temperature. The stream is then accelerated towards
the specimen. Before the stream reaches the specimen, it first passes through the first condenser
lens that condenses it to form a beam and reduce the current in it. At this stage, a condenser
aperture is used to remove high-angle electrons from the beam. The beam is then passed through
a second condenser lens that condenses the electron beam into a thin and coherent beam. This
condenser lens is controlled using a fine probe current knob. The beam then passes through an
objective aperture to remove high-angle electrons (ASM International, 2003).
A set of coils are then used to scan the beam across the specimen before passing the
beam through the last objective lens. The last objective lens focuses the electron beam onto the
desired part of the specimen. The sample is usually placed on a stage located in a vacuum

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enclosure. When the beam strikes the sample, signals are produced. These signals include
secondary emission electrons and backscattered electrons. The composition and nature of these
signals vary depending on the surface topology of the specimen. The secondary emission
electrons are found near the beam impact zone and allows for high resolution imaging. The
working magnification of a conventional SEM ranges from 10 to 100,000 diameters (ASM
International, 2003). Additionally, the microscope can achieve a resolution of up to 100
Angstroms.
The scanning electron microscope is preferred in failure analysis because it can image
non-flat samples from low to high magnification (Masters, 1992). Moreover, this microscope
can provide more information about the fracture surface. This includes topography (texture of the
surface), morphology (shape and size of the particles of the object), composition and
crystallographic information (how atoms are arranged in the object). All this information is
important in identification of causes of failure. The microscope can therefore be used to identify
fatigue failure, brittle fracture, and ductile fracture among other causes of failure. For example,
figure 10 shows fatigue striations, figure 11 shows inter-granular fracture caused by hydrogen
embrittlement in high strength steel and figure 12 shows brittle fracture in TiCN. These images
are obtained using a scanning electron microscope.

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Figure 10: fatigue striations (Ramachandran, 2005)
Figure 11: inter-granular fracture caused by hydrogen embrittlement in high
strength steel (Ramachandran, 2005)

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Figure 12: Brittle Fracture in TiCN (Brandon & Kaplan, 2008)

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3.0 References
Ananya, B. (2008). Stress Corrosion Cracking of Duplex Stainless Steels in Caustic Solutions.
Michigan: ProQuest.
ASM International. (2003). Characterization and Failure Analysis of Plastics. Materials Park,
OH: ASM International.
Brandon, D. G., & Kaplan, W. D. (2008). Microstructural Characterization Of Materials.
Chichester, England: John Wiley.
Callister, W. and Rethwisch, D. (2011). Materials science and engineering. New York, NY:
Wiley.
Cheng, Y. F. (2013). Stress Corrosion Cracking Of Pipelines. New Jersey: Wiley.
Fernandes, P.J.L. (1996). Tooth Bending Fatigue Failure in Gears. Engineering Failure Analysis,
3(1996), pp. 219-225
Jeffus, L. F. (2003). Welding: Principles and Applications. Clifton Park, N.Y: Thomson/Delmar
Learning.
Khurmi, R. S., & Gupta, J. K. (2008). A Textbook Of Machine Design (S.I. UnitsNew Delhi:
Eurasia Publishing House.
Marinov, V. (2008). Manufacturing Processes For Metal Products. Dubuque: Kendall Hunt Pub
Co.
Masters, J. E. (1992). Damage Detection in Composite Materials. Philadelphia, Pa: ASTM.
Owolabi, G., et al. (2013). Occurrence of Dynamic Shear Bands in AISI 4340 Steel under Impact
Loads. World Journal of Mechanics, 3(2), pp. 139-145. Available at:
http://www.scirp.org/journal/PaperDownload.aspx?paperID=30873 [Accessed: 16 Nov
2013].

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Ramachandran, V. (2005). Failure Analysis Of Engineering Structures Methodology And Case
Histories. Materials Park, OH: ASM
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