Report on Microstructure and fracture surface analysis

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Microstructure and Fracture Surface Analysis

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Date: Friday, 14 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 250C ................................................................................................................... 20

Figure 8: Creep Test Results for a Lead Specimen of Width 5.27mm and 1.97 mm Thickness and a

Constant Temperature of 290C ................................................................................................................... 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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1.0Part 1In 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

1.1Gear DesignGears 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.

1.2Gear Materi alsMaterial 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.

1.3Gear M anufactureGears 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.

1.4Causes of Gear FailureThe 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.

1.5Observations and Anal ysisVisual 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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1.6Failu re PreventionIn 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 manufactures

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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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

manufacturers 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 manufacturers 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 manufacturers 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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2.2Activity 2: Creep Testing of Lead Al loysWhen 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 elongation-

versus-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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Figure 7: Creep Test Results for a Lead Specimen of Width 7.79mm and 1.78 mm

Thickness and a Constant Temperature of 250C

Figure 8: Creep Test Results for a Lead Specimen of Width 5.27mm and 1.97 mm

Thickness and a Constant Temperature of 290C

0

1

2

3

4

5

6

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

Elongation(mm)

Time (min)

Creep Test Results for a Lead Specimen of Width:7.79mm and

1.78 mm Thickness and a Constant Temperature of 250C

Rupture Time

Second Stage

Third

Stage

Rupture

First Stage

0

1

2

3

4

5

6

1 1.5 2 2.5 3 3.5 4 4.5 5 5.5 6 6.3

Elongation(mm)

Time (min)

Creep Test Results for a Lead Specimen of Width 5.27mm and 1.97

mm Thickness and a Constant Temperature of 290C

Rupture Time

Second StageThird

Stage

Rupture

First Stage


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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 (2

nd) stage and finally the tertiary (3

rd) 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 (250C) and the second test (29

0C) revealed

that the first test had an elongated secondary stage than the second test. This is attributed to

increase in temperature i.e. from 250C to 290C. 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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2.3Activity 3: L ibrary/L iterature 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).

2.4Activity 4: Scanning Electron M icroscopeThe 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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3.0ReferencesAnanya, 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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