Structural Integrity 3
Series Editors: José A. F. O. Correia · Abílio M. P. De Jesus
Jorge Luis González-Velázquez
Fractography
and Failure
Analysis
Structural Integrity
Volume 3
Series editors
José A. F. O. Correia, Faculty of Engineering, University of Porto, Porto, Portugal
Abílio M. P. De Jesus, Faculty of Engineering, University of Porto, Porto, Portugal
Advisory editors
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Matthew Hebdon, Virginia Tech, Blacksburg, USA
Andrei Kotousov, University of Adelaide, Adelaide, Australia
Grzegorz Lesiuk, Wroclaw University of Science and Technology, Wroclaw, Poland
Yukitaka Murakami, Kyushu University, Fukuoka, Japan
Shun-Peng Zhu, University of Electronic Science and Technology of China,
Chengdu, Sichuan, China
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Jorge Luis González-Velázquez
Fractography and Failure
Analysis
Jorge Luis González-Vel ázquez
Department of Engineering in Metallurgy
and Materials
Instituto Politécnico Nacional
Mexico City
Mexico
ISSN 2522-560X
ISSN 2522-5618 (electronic)
Structural Integrity
ISBN 978-3-319-76650-8
ISBN 978-3-319-76651-5 (eBook)
https://doi.org/10.1007/978-3-319-76651-5
Library of Congress Control Number: 2018933022
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Printed on acid-free paper
Preface
The study of fractured surfaces has been a fundamental part of materials research,
practically since the formalization of the study of material mechanical behavior in
the second half of the nineteenth century. Nevertheless, it was considered a minor
specialty, usually subordinated to fracture mechanics until the post-Second World
War years, when high impact fractures involving mechanical and structural components, particularly in the aerospace industry, showed the usefulness of studying
fractured surfaces. Since the introduction of the term Fractography by Carl A.
Zapee at the 26th Annual Convention of ASM in 1944, it was recognized as a key
discipline for scienti fic research and new material development. But it was not until
the publication of the Metals Handbook, 8th Edition, Vol. 9 Fractography and
Atlas of Fractography in 1974, by the American Society for Metals International
(ASM), the leading organization in the publication of technical texts in the field of
metallic materials for engineering use, that Fractography became a consolidated
engineering specialty; likewise, the close relationship between fracture examination
and failure analysis encouraged ASM to publish, in 1987, the Metals Handbook,
Vol. 12, Fractography, the Metals Handbook, Vol. 11 Failure Analysis and
Prevention, and the Metals Handbook, Vol. 19 Fatigue and Fracture, completing
a series of handbooks related to fracture and material failure.
The fracture and failure analysis Metals Handbooks of ASM are comprehensive
compendiums of technical articles and numerous case studies, but their technical
level is often very high, and the amount of information contained in them is difficult
to process for those without previous experience in the area. This is the reason why,
nowadays, the industrial and academic areas need a textbook that may be used for
technical staff and students in their training for incursion into the fields of fracture
studies and failure analysis. It is precisely this need that motivated me to write the
book Fractography and Failure Analysis, with the intention of offering
easy-to-read material, even for people with a basic knowledge of engineering, and
to introduce the reader to the correct fracture examination methodology and to the
“
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”
“
”
“
”
“
“
”
”
1.4
Photographing Fractures
13
Fig. 1.6 Example of hollow resolution. a Photograph at the original magni fication, as taken in an
SEM. b Enlargement of the original image. Note that the enlargement makes the image bigger, but
does not reveal fine details
by the optical instrument. Even when it is possible to obtain higher magni fication,
the images will not show smaller details; such an effect is known as hollow
resolution.
Resolution depends mostly on the qualit y of the optical system, but it is often
confused with the image size in mega pixels. 2 The image size represents the number
of pixels that form the image, for example, a one mega pixel image is composed of
one million pixels. If the image is 10  10 cm in size, each pixel will be a square of
0.01 cm (10 l). At first, it may be thought that the greater the image size in mega
pixels, the greater the resolution, but this is not necessarily true. If the image is out
of focus, blurred or beyond the resolution of the instrument, it will be a very
heavy image, but the resolution will be poor.
An idea that is frequently used by novice photographers is to take a high mega
pixel image and enlarge it to reveal finer details, but if the resolution of the original
picture is not good enough, the result will not be good either, because of the hollow
resolution. The degree of enlargement is also controlled by the pixel size: if the
enlargement is large enough to make the pixels visible, the image will look like a
mosaic; this is called pixelization. The images in Fig. 1.6 shows an example of
this.
Light Sensitivity- Light sensitivity is the amount of light that has to hit the
recording system (film or image sensor) to make a visible image. If the light
sensitivity is low, longer exposure times are needed, and vice versa. Long exposure
times are not good for fractography, because the movements of the camera and
variations of light may affect the quality of the photograph. In digital photography,
light sensitivity depends on the lens diameter (the larger the diameter, the greater
“
”
“
”
“
”
14
1
The Fractographic Examination
the sensitivity, as it allows more light to enter) and the intrinsic sensitivity of the
image sensor that makes up the digital recording system. In photographic films, the
ASA/ISO number determines the light sensitivity, resolution (grain size) and
contrast. A low ASA/ISO means lower sensitivity, but higher resolution, and vice
versa. In digital photography, light sensitivity or ASA may be adjusted through the
camera settings, but the limits are set by the manufacturer (a minimum/maximum
ASA). In addition, the digital images can be improved in regard to brightness and
contrast by image processing systems.
Magnification- Magnification is the size at which an image is originally captured, but not the print size. Magni fication is determined by the optic system, and
the purpose in fracture photography is to get the greatest magni fication without
losing resolution (avoiding hollow magnification). Generally speaking, the greater
the magnification needed, the more complex and costly the system will be and the
more care will be necessary in taking the photo, especially considering vibrations,
stability and cleanliness of the optic system. Due to the hollow magnification effect,
the best option will be to get the picture at the desired magni fication from the very
beginning.
Depth of field- This is the distance along the optical axis at which objects remain
in focus. Depth of field depends on the optic system, and the rule is that the greater
the magnification, the smaller the depth of fi eld. In any optic system, the depth of
field is controlled by the diaphragmatic opening of the objective lens; the smaller
the opening, the greater the depth of field. In fractography, it is advisable to get
greater depths of field, but this always leads to a compromise with magni fication, so
the fractographer will have to decide what magni fications are best in order to get the
desired depth of field.
Illumination- Illumination determines the brightness and the contrast, which,
combined with focus and resolution, make up the picture s quality. For fractography, first of all, an adequate source of light must be chosen. Of course, the best
source is natural light (sunlight), as it contains the whole color spectrum visible to
the human eye, but it has the limitation that it cannot be controlled, therefore it is
rarely used for professional photography, which prefers artificial lighting. The most
common artificial sources of light are: incandescent tungsten lights, halogen lamps,
fluorescent lights, LED and xenon lights. All of these have the advantage that the
intensity and direction can be controlled. Nonetheless they also have their disadvantages: first, as they come from a relatively small source, the illumination is not
even, and second, they provide a limited range of wavelengths, which affects the
contrast, especially in color photography. Both tungsten and xenon lights produce
yellowish tonalities, so the details in this color will fade, whereas fluorescent, LED
and xenon lights give off an excess of blue light or ultraviolet light, thus producing
a whitish tonality over color, all of which gives them a washed out aspect. In
practice, such problems may be corrected or minimized by filters, diffusers or other
accessories. For the fractographic job, the best results are obtained by trial and
“
”
’
“
”
1.4
Photographing Fractures
15
Illumination angles- Another important variable in illumination is the tilt angle
of the light source with respect to the fractured surface plane. The angle can be
perpendicular (also known as vertical), which is when the source is placed directly
above the surface, or oblique, when light beams come tilted onto the surface. In
fractography, oblique illumination is the best, because it reveals the topographic
details of the fractured surface, while vertical illumination produces less contrast,
but reveals better colored details of the fractured surfaces. The tilt angle and distance of the light source should be adjusted according to trial and error, to find the
combination that best shows the fracture details and roughness features. It is
advisable not to rely on one single light source location, because some details may
be visible under one condition and not clearly visible under another. Figure 1.7
16
1
The Fractographic Examination
shows an example of the effect of variations in the illumination angle on the
photographs of a fracture surface.
Based on the aforementioned photography principles, the steps to success in
photography of fracture surfaces can be summarized as follows:
1. Select the zone to be photographed. After having completed the macroscopic
examination, determine the magnification and perspective of the surface details
that you wish to record. That is, make a photography plan.
2. Stabilize the camera. The camera must be well fixed and free of vibrations.
Make the adjustments on the camera (diaphragmatic opening, speed shutter,
resolution and light sensitivity) according to the desired magni fication, illumination and resolution.
3. Make illumination adjustments- Make sure that there is adequate and sufficient
illumination to obtain the desired brightness and contrast. Make sure that the
angle and position of the light source is such that the details to be recorded will
be highlighted.
4. Focusing- Focus on the center of the desired image to be captured and shoot.
To achieve steps 2 and 3 in a successful way, photography stands are of great
help. They have a neutral background (which can be a piece of canvas or a sheet of
paper) along with a series of pedestals and bars to fix the camera and lights. Among
the main characteristics that photography stands should have are: (1) correct camera
base with adjustable height, (2) several lamp supports with adjustable angle and
height, and (3) diffusing screen frames. Figure 1.8 shows a homemade photography
stand suitable for photography of small size fractured specimens.
1.5
1.5
Replicas
17
Replicas
A replica is a reproduction of a fractured surface obtained by making an imprint of a
malleable plastic material, which later hardens and can be removed integrally,
without leaving any residue. A replica is called one step, or direct, when the imprint
of a surface is obtained. In this case, the replica will be a negative of the surface,
that is, the crests will be valleys and the valleys will be crests, as illustrated in
Fig. 1.9.
The two-step replica, or indirect, is obtained by using the first replica as a mold
to make a second replica, in this way obtaining a positive imprint of the original
surface, as shown in Fig. 1.10.
Although ideally, it is best to make two-step replicas, since the reproduction of
the fracture is identical to the original, two-step replicas may not turn out as well as
expected, because the infiltration of replicating material is not perfect and there are
distortions and defects introduced in each step.
Replicas are required when it is not possible to take the fractured piece for
examination at the lab, whether because it is too big, cannot be cut out or simply
because the original piece is not provided. When such is the case, the macroscopic
examination may be carried out with almost the same results as with the original
piece, provided it is a high-quality replica. A replica is also made when the
examined piece must be returned and it is desirable to have a reproduction of it.
Replicant material
Finished replica
Piece
Fig. 1.9 Schematic representation of the process for obtaining a one-step replica
Replicant material
Step 1
Direct replica
PIECE
Step 2 Second replica
REPLICA 1
Finished replica
20
•
•
•
1
The Fractographic Examination
allowed to harden for a few minutes. This material is very stable, so it can be
cut, painted or coated. For its observation in the SEM, it will be necessary to
metalize it in order to make it into a conductor, and it should be observed at low
voltage
voltage (5–10 kV). Its resolution is good for mid-magni fications (up to 1000).
Algi
Algina
natete- Also
Also a mate
materi
rial
al for
for dent
dental
al use,
use, this
this is chea
cheap
p and
and easy
easy to use.
use. Its
Its
resolution is not as good as silicon, but it is good enough to carry out macroscopic examinations. It comes in the form of a powder and is mixed with water
until
until a viscou
viscouss fluid is obtained. The fluid is poured onto the surface to be
reproduced without applying any pressure or heat. Once hardened, it must be
carefu
carefully
lly detach
detached,
ed, because
because it is not mechani
mechanical
cally
ly resist
resistant
ant.. To improv
improvee its
appearance, it might be metalized or painted. It is good for two-step replicas.
Plaste
Plasterr- For two-ste
two-step
p replic
replicas
as and big pieces,
pieces, plaste
plasterr may be a good option,
option,
though its resolution is very limited. It is better to use high quality plasters for
dental moldings instead of construction plaster.
WaxWax- This
This is not an engineer
engineering
ing materia
material,
l, but it can be used when there is no
other option available. The wax is heated until it becomes fluid and then is
poured onto the surface to be reproduced. It hardens after cooling off, so it can
be removed carefully. The replica must be handled with extreme care, because
wax
wax is a weak
weak mate
materi
rial
al and its
its boil
boilin
ing
g poin
pointt is clos
closee to room
room temp
temper
eratu
ature
re,,
meaning that it may melt, even while being held with bare hands.
The appearance of all replicas is improved
improved substantia
substantially
lly when they are metalized
metalized
or painte
painted.
d. Metali
Metalizin
zing
g throug
through
h evapor
evaporati
ation
on of a noble
noble metal
metal (gold,
(gold, platin
platinum
um or
rhodium) or graphite is the best option. The evaporation technique most recommended is by incandescence in vacuum bells, because the heat generated with other
evaporation methods, such as plasma, may damage the replica. The following table
shows the most common defects in replicas, their causes and corrective actions
(Table 1.1
1.1).
).
Table 1.1 The most common defects on replicas,
replicas, causes and corrections
corrections
Defect
Cause
Correction
Filaments
and
tongues
Adherence of replicating material due
to removal before full hardening
Wait until the replica fully hardens
Porosity
Trapped air
Avoid excessive stirring or replica
material that is too soft
Poor
resolution
Application of the replica material
after it has begun to harden
Follow the manufacturer s
recommendations for replica material
preparation
Void
Voidss
Impr
Improp
oper
er appl
applic
icat
atio
ion.
n. Appl
Applic
icat
atio
ion
n of
the replica material after it has begun
to harden
Apply the replica material in a
and uniform fashion
Tear
Tearin
ing
g
Repl
Replic
icaa mate
materi
rial
al trap
trappe
ped
d in cavi
caviti
ties
es
without an exit angle
Make the replica from areas without
cavities. If the tearing is excessive,
’
fluid
Chapter 2
Elements of Fractography
Abstract This chapter begins with a description of the different classi fications of
fracture, according to mechanism and extent of plastic deformation. The mechanical
aspects
aspects of fracture,
fracture, from the continuum
continuum mechanics and fracture
fracture mechanics points of
view, are brie fly described. Based on the mechanical aspects of fracture, a General
Fracture Model is introduced in order to facilitate the systematic study of fractures.
The
The main
main feat
feature
uress obser
observe
ved
d in the
the macr
macros
osco
copic
pic exam
examin
inat
atio
ion
n of frac
fractu
ture
ress are
are
desc
descri
ribed
bed,, alon
along
g with
with the
the form
format
ation
ion mech
mechani
anism
smss used
used to iden
identi
tify
fy and
and anal
analyze
yze
the fracture sequence, initiation sites and relations to mechanical properties. The
chapter finish
nishes
es with
with a prop
propos
osed
ed proc
proced
edure
ure for
for the
the exam
examin
inat
atio
ion
n of frac
fractu
ture
ress at
the microscopic level and a description of the main micromechanisms of fracture.
2.1
Classi
ssification of Fractures
The first classi fication of fracture is according to the amount of plastic deformation
present in a solid body after being fractured. The types of fracture based on this
criterion are:
(a) Brittl
Brittlee Fractu
Fracturere- This
This is charac
characteri
terized
zed by showin
showing
g little
little or no plasti
plasticc defordeformation at all.
(b) Ductile
Ductile FractureFracture- This is the one that shows an appreciable
appreciable plastic deformatio
deformation
n
associated with fracture.
The aforementioned classi fication is valid only from the engineering point of
view, because
because a fracture
fracture may be brittle at the microscopic
microscopic level,
level, but show an intense
intense
plastic deformation at a macroscopic scale. The opposite may also occur, that is,
when, after extensive plastic deformation, the fracture occurs in a brittle fashion,
although this is rather rare. Figure 2.1 illustrates this classi fication.
The next fracture classi fication is according to the fracture mechanism , which
means the process that led to fracture. The most common fracture mechanisms are:
22
2
Elements of Fractography
% AREA REDUCTION
BRITTLE
FRACTURE
DUCTILE FRACTURE
0
5
100
CUP AND CONE
Little or no plastic
deformation
Neck formation
The
fragments
adjust
perfectly
σ
σF
σ
= σo
100%
R. A.
Generalized
plastic
deformation
σ
σF
σu
< σo
σO
σO
σF
ε
ε
= σu
ε
Fig. 2.1 Schema
Schematic
tic repres
represent
entati
ation
on of fractu
fracture
re classi
classificati
cation
on acco
accord
rdin
ing
g to amou
amount
nt of plas
plasti
ticc
deformation
1. Overload- This refers to a fracture caused by a single application of load that
made the stresses in the material exceed the mechanical resistance in components with no cracks or with signi ficant fracture toughness. It is also known as
static fracture.
2. Fatigue- This is the formation and propagation of a crack due to the action of
cyclic or fluctuant stresses, with a sufficient number of repetitions, resulting in
the component s fracture.
3. Stress corrosion cracking- Also called environmental fracture and commonly
know
known
n as SCC.
SCC. It is the
the form
format
atio
ion
n and
and prop
propag
agat
atio
ion
n of a crac
crack
k due
due to the
the
combined action of stress, a corrosive environment and a susceptible material.
4. Creep- This is a fracture caused by thermally-activated deformation processes
and internal damage, when the material is subject to constant stress for a prolonged time and at high temperature. It is considered high temperature above
approximately 0.4 of the material s melting point in absolute degrees.
’
’
2.1
Classification of Fractures
23
material, thus causing microstructural alterations and a reduction of the
mechanical properties, especially fracture toughness, leading to cracking.
6. Radiation cracking- This comes from exposing the material to different forms
of radiation, which cause alterations at the molecular level, resulting in superficial multiple cracking. This may happen with or without applied stress. This is
commonly observed in polymers exposed to solar ultraviolet radiation. It has
also been reported to happen in metallic components exposed to intense nuclear
radiation.
Finally, fractures can be classi fied according to the micro-mechanism that leads
to the formation and growth of the main crack and that takes place at the tip of the
crack, within the plastic zone just in front of it. Such mechanisms are further
described at the end of this chapter and are:
1.
2.
3.
4.
Cleavage
Plastic Flow
Decohesion
Void Coalescence
In all materials, the fracture mechanism is determined by the crystalline or
molecular structure first, and by the temperature second, which is calculated by the
relation Ta /Tf, where Ta is the service temperature and Tf is the fusion temperature,
both in absolute degrees (Kelvin or Rankine). These are in fluenced by the applied
stress level and the strain rate. The fracture mechanisms occurring as a function of
the temperature and the strain rate can be shown through the Ashby deformation
mechanism maps (introduced by Ashby in the 1990s), like the ones shown in
Fig. 2.2. These charts show that face cubic centered materials ( fcc) do not show
10-1
10-1
Ductile
Ductile
σ/E
Rupture
ε
-1
s
High
temperature
fracture
Creep intergranular
fracture
Cleavage
10-6
10-6
0
1.0
0
10
24
2
Elements of Fractography
cleavage, whereas brittle forms of fracture are favored by low temperatures and high
strain rates. High temperature fracture (cavitation and viscous fluid) is favored by
low strain rates.
2.2
Mechanical Aspects of Fracture
Continuum mechanics. A fracture is the result of an increase of the stress within a
localized zone in the material, which activates a process of rupture of the atomic
bonds, so promoting the formation of new surfaces. Since fracture will eventually
take place in a relatively narrow zone of the material, it always refers to local stress
concentration. Such stress concentration is the result of a geometric discontinuity
that previously existed in the piece (for example, a sharp corner, a corrosion pit or a
preexisting crack), or that was formed by the very same fracture mechanism (neck
formation in the tensile rupture). Likewise, due to the directional nature of stress,
the fracture s trajectory will always be associated with the stress direction within the
zone of fracture.
The directional nature of the stresses implies that, with any state of stress, except
the hydrostatic, there will always be normal and shear components of stress within
the material. The response of materials to the tension or shear components of stress
is different, therefore the materials will show a different resistance to tension, as
compared to the resistance to shear, which, somehow, are independent. The basic
rule is that fracture will occur in the first plane, where resistance is overcome, so
then, materials can have greater resistance in tension than in shear, like most
engineering metals, while other materials may resist greater shear stresses, but show
little resistance in tension, as is the case with ceramics.
Depending on which resistance is stronger in a given material, fractures will be
controlled by either of these two stresses, and will fall into either of the following
two categories:
’
1. Fracture controlled by tension stress- The material has low resistance to
tension, but high resistance to shear, therefore the fracture will occur on the
plane where maximum principal stress is located. Usually, this type of behavior
is shown by brittle materials.
2. Fracture controlled by shear stress- Here, the material offers high resistance to
tension, but low resistance to shear, and so fracture will take place on the plane
where maximum shear stress is located. Usually, this type of behavior is shown
by ductile materials.
To illustrate this behavior, the following experiments can be easily done:
Experiment 1: Break a chalk bar in pure shear, for example, by twisting it until it
breaks. Chalk is a brittle material, and therefore it will fail in tension. By twisting
2.2
Mechanical Aspects of Fracture
τ
τ max
Mohr‘s
circle in
pure shear
25
Pure
torsion
Direction of
σ max
σ
Fracture
plane
σ max
Fig. 2.3 Schematic representation of a fracture in pure shear of a brittle material. Note that the
fracture plane is oriented 45° from the twisting direction, which is the direction of the maximum
principal stress
τ
τmax
Pure shear
Mohr’s
circle
Pure
torsion
Direction of
τ
max
σ
σ
max
Fracture
plane
Fig. 2.4 Schematic representation of a fracture in pure shear of a ductile material. Note that
fracture plane is parallel to the twisting direction, which is in the direction of the maximum shear
stress
is in the transversal plane. Thus, the fracture plane will be at 45° from the bar ´s
longitudinal axis, as shown in Fig. 2.3. The opposite behavior is observed if a play
dough clay bar is twisted up to its rupture. Play dough is a ductile material, thus it
will fail by shear. In a pure shear stress state, the maximum shear stress is parallel to
the direction of twisting, therefore the fracture will be in a plane perpendicular to
the bar´s longitudinal axis, as shown in Fig. 2.4.
Experiment 2: Break a play dough bar in pure tension, with the load parallel to
the longitudinal axis of the bar. In this case, the material will start flowing plastically in the direction of the maximum shear stress, which will be at 45° from the
tension axis. Assuming that the material will behave as an ideal plastic (no strain
hardening) and that it is homogeneous, the transversal section will be continuously
38
2
Elements of Fractography
Fig. 2.18 Macroscopic cleavage in a brittle fracture of a metallic material. The arrows indicate the
crack propagation direction
the direction in which the ranges align, having a greater roughness as the crack
advances to the final separation point.
This type of fracture is formed by exceeding the materials of cohesive strength,
so the fracture orientation is always perpendicular to the maximum tension stress
plane, however, the principal stress direction on the crack ´s front may vary as the
crack moves forward, so the cleavage plane may get curved or wavy in that
instance, as in the example given in Fig. 2.19.
Macroscopic cleavage in amorphous materials may result in rough fractures,
with many steps, ridges and ranges and even with fragment detachments in the form
of fish scales, as shown in Fig. 2.20. The direction of propagation corresponds to
the direction in which small ranges align themselves, having greater roughness as
the crack advances forward.
Fig. 2.19 Wavy cleavage plane due to the variation in the orientation of the maximum tension
2.6
Multiple Cracking
39
Fig. 2.20 Macroscopic cleavage on brittle amorphous material
2.6
Multiple Cracking
Multiple cracking occurs frequently in brittle material fractures when there is a large
enough energy supply. Fracture mechanics energy criterion predicts that under
constant load conditions, the energy release rate (G) increases as the crack grows.
This great availability of energy can favor multiple cracking in Stage III of the
fracture, as shown in Fig. 2.21. The crack starts propagation when G = 2R and its
size is critical.
The formation of crack branching will demand twice as much energy (R) for its
propagation, which is attained when the crack has reached a size of a c þ Da2; G ¼
2R, When the crack s length extends up to ac þ Da2; G ¼ 3R, there will be enough
energy to get three cracks propagating. This process will not go on inde finitely,
because, as was said before, this occurs only at the fracture s final stages and before
many branches are formed, thus the piece will be fully separated or broken.
Under impact conditions, on the other hand, the initial energy input is great and
it causes multiple propagations in Stage I of the fracture. However, due to the
system´s demand for energy, multiple cracks will eventually stop and just one of
them will continue propagation. During crack branching, it is common for a crack
to take a Y or T con figuration, as shown in Fig. 2.22.
The Y rule says that the crack s propagation direction is always the direction of
the Y s branches, as shown in Fig. 2.23.
When a crack intersects with another one at an angle close to 90°, they form a T
type intersection. The rule is that the hat over the T comes first, and then the pole of
the T intersects with the former one, as illustrated in Fig. 2.24. It is also a rule that
the intersecting crack (pole) stops at the intersection.
’
’
’
’
40
2
Elements of Fractography
G, R
G
3R
2R
R
∆a
∆a1
a
∆a2
ac
Fig. 2.21 R curve (energy criterion) for crack branching in a brittle material
2.7
Microscopic Fractography
41
Fig. 2.23 Type Y branch
Fig. 2.24 Type T intersection
2.7
Microscopic Fractography
As mentioned in Chap. 1, a fracture mechanism is the sequence of deformation
processes and rupture of atomic bonds that cause the formation and propagation of a
crack. The fracture mechanism takes place in the process zone, which is located
right in front of the crack tip, so it happens at the microscopic scale and is closely
related to the material ´s microstructure. In order to study the fracture mechanisms, it
is necessary to go on a microscopic examination, generally with a scanning electronic microscope (SEM).
The microscopic examination of a fractured surface is carried out according to
the following specific objectives:
•
•
•
•
Identify the fracture mechanism in each zone of the fracture.
Identify those fracture characteristics resulting from interaction with the environment, the microstructure and defects present in the material.
Identify and analyze the composition of and types of deposits, debris and second
phase particles on the fractured surface.
Determine the microscopic propagation directions of the crack.
In order to achieve these objectives, the microscopic examination must be
42
2
Elements of Fractography
1. Sectioning. Cut out samples of the fractured piece to a size that fi ts into the SEM
specimen chamber.
2. Cleaning. Eliminate cutting debris, oil and dirt from the microscopic examination specimen according to the procedures and recommendations given in
Chap. 1.
3. Specimen preparation. If required, prepare the specimen for observation in the
SEM, by evaporation coating with a noble metal or graphite.
4. Microscopic examination. Once the specimen is in the SEM, first have a look at
low magnification in order to recognize the zones of interest. At this point, it is
advisable to have at hand a macro-photograph of the piece or a drawing,
because, once in the microscope, the change of perspective and contrast (in
SEM, the image is always black and white) may confuse the observer. If the
sample is large enough, or there is more than one sample, a few fine marks can
be scribed in the zones previously identi fied through the macroscopic examination, to use them as guidance for quick spotting and as a measuring reference.
The marks can be made using a fine point marker or a needle, and care must be
taken not to make marks on zones of interest. Marks should never be made on
fractured surfaces that are being examined for the first time, those that are
unique or those that are part of a judicial investigation.
5. Photography. Once the area to be observed has been selected, begin at low
magnification and increase amplification up to a level that allows for clear
identification of the desired features. Here, it is recommended to pick out a
single area and increase amplification in sequence, from 100, 500 and
1000. Greater ampli fications will be decided according to the level of resolution needed to reveal fine fractographic features. It is also recommended that at
least one shot be taken with low, middle and high ampli fications of the zone of
interest.
6. Representativeness. An important recommendation is that attention should be
focused on those characteristics that appear more often on the fractured surface,
with little to no attention being paid to rather small or rare details that might be
attractive, but are not representative of the main fracture mechanism. The
researcher must bear in mind that he or she is just observing a little fraction of a
whole, and there is always the risk of making incorrect generalizations.
Likewise, it is not advisable to take photographs of every detail on the fractured
surface, because, in addition to making the examination longer and more costly,
the investigator may end up tired and confused when preparing a written report.
7. Micro-Analysis. Once the fractographic observations and their photographing
have concluded, the investigator can proceed to carry out both the chemical and
physical analyses required. It is advisable to do this at the end of the examination, because occasionally, the analysis techniques may damage the surface or
alter the material itself.
8. Preservation. The final step in the microscopic examination of fractured surfaces
3.2
Ductile Fractures
61
Fig. 3.18 Macroscopi
Macroscopicc appearance
appearance of a ductile
ductile fracture in uniaxial
uniaxial tension.
tension. Left, cup fracture.
Right, cone fracture
(a) Plane
Plane stressstress- In this
this case,
case, the compone
components
nts of the maximum
maximum shear strain
strain are on
planes transversal to the fractured surface. This produces two necks at each
crack side and the fracture will show a shear lip parallel to the crack growth
direction, as shown in Fig. 3.19
3.19a.
a.
(b)
(b) Plan
Planee stra
strain
in-- Here,
Here, the
the maxim
maximum
um shea
shearr stra
strain
in comp
compone
onent
ntss are
are on incl
inclin
ined
ed
planes in front of the crack tip. This produces a neck parallel to the front of the
crac
crack
k and
and the
the frac
fractu
ture
re will
will have
have a neck
neck para
parall
llel
el perp
perpen
endi
dicu
cula
larr to the
the crac
crack
k
growth direction, as shown in Fig. 3.19
3.19b.
b.
In the case of very thin plates, the fracture may be fully controlled by the shear
strain and the orientation of the fracture plane will correspond to the orientation of
the maximum shear plane, thus creating a fracture with one or two sides inclined at
about 45° from the tension direction, as shown in Fig. 3.20
3.20..
Plastic
zone
Crack
Plastic
zone
Neck
Neck
Crack
62
3
+
3
=0
Brittle and Ductile Fractures
1
+
2
Fig. 3.20 Fracture
Fracture plane orientation
orientation in thin plates
plates fractured
fractured in tension
tension
Fig. 3.21 Macroscopic
appearance of a ductile
fracture in pure shear
(torsional fracture)
When the ductile fracture is pure shear, the fractured surface will be on a plane
parallel to the maximum shear stress direction and the shear lips will not form. The
most common pure shear fractures are torsional fractures of cylindrical bars, in
which case the fractured surface is as shown in Fig. 3.21
3.21..
3.2
Ductile Fractures
3.2.2
63
Ductile Fracture Mechanism
The basic
basic mechani
mechanism
sm of ductil
ductilee fractu
fracture
re is void
void nuclea
nucleatio
tion
n and growth
growth,, which
which
consists of the following: once a neck is formed, a triaxial stress state is created at
the mid-thickness section of the solid body. This triaxial stress induces the nucleation of voids, usually at inclusions or second phase particles. The voids grow by
plasti
plasticc deforma
deformatio
tion
n until
until the ligame
ligament
nt between
between the neighbo
neighborin
ring
g voids
voids become
becomess
small
small enough
enough to produc
producee their
their coales
coalescen
cence,
ce, formin
forming
g an interna
internall cavity
cavity.. In that
that
moment, the shear stress in the remaining transversal section (ligament) increases
until a failure by sliding shear occurs. This mechanism is shown in Fig. 3.22
3.22..
The voids causing the ductile fracture mechanism are of microscopic size and
look like little craters or dimples on the fractured surface. The shape and size of the
dimples are directly related to the size, shape and distribution of the inclusions or
second
second phase particles
particles that are their nuclei, as well as to the mode of loading, as will
be described in the next paragraphs.
Effect of particle size: The rule is that fine and closely spaced particles produce
smal
smalll dimp
dimple
les,
s, beca
becaus
usee the
the void
voidss grow
grow a litt
little
le befo
before
re coal
coales
esce
cenc
nce;
e; howe
howeve
ver,
r,
whereas coarse particles produce large dimples, the final size of the dimple will
depend on the particle spacing and the material s ductility. Normally, metals have a
comb
combina
inati
tion
on of larg
largee and
and smal
smalll part
partic
icle
les,
s, ther
therefo
efore
re it is comm
common
on to obser
observe
ve a
mixture of dimple sizes, as seen in Fig. 3.23
3.23..
Effec
Effectt of partic
particle
le shape:
shape: Spheri
Spherical
cal partic
particles
les produce
produce more
more or less
less equiaxi
equiaxial
al
dimples, whereas long particles produce long and narrow dimples, like the ones
shown in Fig. 3.24
3.24..
’
Triaxial
stress
max
Void
Neck
Nucle ation at
inclusion
particles
Void growth
and
coalescence
Final
separation
by shear
Fig. 3.22 Schematic representation of the ductile fracture mechanism. i Void nucleation, ii void
growth and iii final separation by plastic shear
64
3
Brittle and Ductile Fractures
Fig. 3.23 Microscopic view of dimples in a ductile fracture under uniform tensile loads. Notice
that larger dimples are produced by larger particles, and vice versa
Fig. 3.24 Long and narrow dimples produced by long nucleating particles
Effect of ductility: High ductility materials produce deep and narrow dimples,
whereas less ductile materials produce shallow and wide dimples. Figures 3.25 and
3.26 shows an example of each.
3.2
Ductile Fractures
65
Fig. 3.25 Deep dimples in a very ductile material
Fig. 3.26 Shallow dimples in a low-ductility material
Effect of load application mode: Tensile loads uniformly distributed across the
66
3
Brittle and Ductile Fractures
Equiaxial dimples
Uniform stress
Lateral load
p a ra b o li c d imp le s
Fig. 3.27 Ductile fracture dimple shape according to the loading mode
3.2.3
Void Nucleation and Growth Mechanisms
The best known mechanisms of void nucleation in ductile fractures are:
(a) Fracture of brittle particles: According to this mechanism, relatively large and
brittle particles with strong matrix cohesion are deformed along with the matrix,
but due to their brittleness, they quickly break, and as the matrix continues
deforming, a cavity is formed, as shown in Fig. 3.27a. Due to the high cohesive
strength, the voids nucleated by particle fracture grow by plastic deformation of
their walls, thus forming large and spaced dimples.
(b) Inter-phase decohesion: If the cohesive strength of the matrix-particle interface
is not high, the particle may simply separate from the matrix, forming a void, as
shown in Fig. 3.27b. This is the most common mechanism of void nucleation
and occurs favorably in spherical oxide particles of small size (1 10 l).
3.2
Ductile Fractures
67
(c) Dislocation pile-up: In materials with non-cutting particles dispersed in the
matrix, dipolar dislocation rings are formed around the particles during plastic
deformation by a mechanism known as the Orowan s. Under the action of the
resolved shear stress, the dislocations form heavy pile-ups, and when a critical
pile-up size is reached, the dislocations next to the particle suffer an annihilation
process that creates small voids, as illustrated in Fig. 3.27c. At the same time,
the pile-up creates high tension stresses around the particle that may cause
decohesion of the interface.
’
The void growth mechanism in a ductile fracture is basically by plastic deformation along the inclined dislocation bands where the slip takes place. Once a void
has been nucleated, the maximum shear stress planes will be at an inclined angle
with respect to the maximum principal stress direction. Plastic slip bands usually
start at the void wall surface, so the emission or annihilation of dislocations takes
material away from the surface, thus causing the void to grow, as schematically
shown in Fig. 3.28.
Dislocation slip bands in the dimple walls of very ductile materials will form
surface steps that are clearly visible at the microscopic level in the SEM, as shown
in Fig. 3.29.
3.2.4
Ductile-Brittle Transition
Although it is clear that ductile fracture is preceded by plastic deformation, the
ductility level needed to consider a fracture as ductile is not speci fically defined and,
in most cases, is determined by judgment. Generally speaking, when the elongation
(a)
(b)
Void
nucleus
Particle
cleavage
(c)
Dislocation pile-up
Void
nucleus
Particle
decohesion
Distorted
crystal
Void nucleus
74
4
Fatigue Fracture
The first case (elastic strain) is the most interesting in research and practice,
because all structural and mechanical components prone to fatigue are designed to
operate under an elastic strain regime. When cracks caused by fatigue appear in
these components, the conditions of linear elastic fracture mechanics (LEFM )
are satis fied, so the fatigue crack growth can be characterized by the stress intensity
factor amplitude ( DK).
The shape of the load cycle is the first major mechanical
mechanical variable of fatigue,
fatigue, as it
determines the magnitude and strain rate in the crack tip. As mentioned earlier, the
load can be either fluctuant, cyclic or random, as shown in Fig. 4.2
4.2..
In the
the thre
threee stag
stages
es of fatig
fatigue
ue,, the
the crac
crack
k growt
growth
h rate
rate depe
depends
nds on the
the stre
stress
ss
amplitude at the crack tip, DK, which, under elastic linear conditions, is determined
as follows:
The general equation for K, according to LEFM, is
K
p
¼ Pb
ffi ffi ffi;
pa
where P is the load, a is the crack size and b is a geometric factor.
The stress intensity amplitude is
DK
p
ffiffiffi
¼ Kmax  Kmin ¼ Pmax b
pa
Since K depends linearly on the load,
DP
p
ffi ffi ffi:
Pmin b pa
¼ Pmax  Pmin:
Thus,
DK
p
¼ DPb pa:
ffiffiffi
The Fig. 4.3 graphically shows the value of
L
o
a
d
DK.
Time
Cyclic
Repetitive
Fluctuant
4.1
General Aspects of Fatigue
75
½ frecuency
Kmax
ΔK
K
= K máx - K mín
Kmin
Fig. 4.3 Fatigue
Fatigue load cycle variables
variables
The load cycle is completely de fined by the load ratio R, which is de fined as
R
¼ Pmin =Pmax:
Based on the R value, the following load cycle types are de fined. Note that the
sign of R is de fined by the sign of the applied loads and not by the K values, since
there are no negative values of K (Table 4.2
4.2).
).
During Stage I, the crack usually initiates in a stress concentrator, which, in a
generic way, is a notch of radius q. It has been found that the ratio DK/q controls
the number of crack initiation cycles Ni. The limit below which fatigue cracks do
not grow (DKth), and therefore the fatigue life of the component is in finite, is related
to both the yield strength r 0 and the notch root radius of the stress concentrator by
the following empiric relation:
DKth
¼ 10pqro:
ffiffiffiffi
In Stag
Stagee II,
II, at cons
consta
tant
nt load
load ampl
amplit
itude
ude,, as the
the crac
crack
k prop
propag
agat
ates
es,, the
the stre
stress
ss
intensity factor increases, as does the crack growth rate. This process goes on until
the maximum stress intensity factor value equals the material fracture toughness KIC
and the final fracture occurs, meaning the crack growth rate (d a/dN) is a function of
DK. The Fig. 4.4 schematically shows this behavior.
Paul C. Paris, in 1962, proved that in a logarithmic plot of d a/dN versus DK data,
there are three well-de fined regions that correspond to the three stages of fatigue
Table 4.2 Load cycle types
according to the value of R
Cycle type
R value
Tension–tension
0<R<1
Tension compression
−∞ < R < 0
76
4
ΔK
ΔP
Fatigue Fracture
a
= Cte.
da / dN
N
a
Fig. 4.4 Variation of DK as a function of crack size in fatigue crack growth
crack growth, as shown in Fig. 4.5
4.5;; he also found that in Stage II, the following
relation is valid:
da
¼
C DK m ;
dN
where C and m are empiri
empiricc consta
constants
nts.. This
This equati
equation
on has been
been fundame
fundamenta
ntall for
fatigue research and has allowed us to analyze the effect of multiple factors, both
internal and external, in fatigue crack growth. These studies have shown that the
most
most import
important
ant influence
uence comes
comes from
from the micros
microstruc
tructure
ture and the enviro
environme
nment.
nt.
Figure 4.5 summarizes such in fluences.
Stage I
Near threshold
. Crystalline
fracture.
. Effect of
microstructure
Stage II
KMAX = KIC
Paris region
. Transgranular
fracture
.
Influence of
environment
Stage III
da/dN
da/dN = C ΔKm
Fatigue threshold
Δ Kth
ΔK
Unstable
. Combination
with static
modes of
fracture
4.1
General Aspects of Fatigue
77
Stress
Monotonic
strain
Cyclic strain
loop
Cyclic
plastic zone
Strain
Crack
Monotonic
plastic zone
Δε plastic
Δε elastic
Fig. 4.6 Cyclic plastic and monotonic zones at the tip of a fatigue crack and the strain-stress cycle
in both zones
From the mechanical behavior point of view, the most important zone of a
fatiguing component is the plastic zone, initially generated by the stress intensi fication at the crack tip. Due to the fluctuant nature of stresses, the plastic zone is
divided into two regions, as shown schematically in Fig. 4.6. The smaller region,
located right in front of the crack tip, is a cyclic strain zone, where strain goes from
tension to compression, due to the change of direction of the load during each cycle.
The cyclic strain zone is surrounded by a larger strain zone, where the deformation
is monotonic, and its size depends on the maximum stress value.
The crack propagation basically depends on the behavior of the cyclic zone, but
it is strongly in fluenced by the size of the monotonic zone. Paris demonstrated that
the cyclic plastic zone size ( rc) is approximately
rC
¼
 
DK
8 p r0
1
2
;
where ΔK is the stress intensity factor amplitude and r0 is the yield strength. The
size of the monotonic plastic zone can be calculated according to the Irwin correction formula
r0
1
¼p
 
Kmax
r0
2
:
Based on the aforementioned formula, the cyclic zone is about eight times
smaller than the monotonic zone. The actual size of the cyclic zone, of course, is
affected by several factors, like strain hardening, anisotropy and strain rate, among
others.
78
4.2
4.2.1
4
Fatigue Fracture
Fractography of Stage I Fatigue
Macroscopic Characteristics of Stage I Fatigue
Stage I fatigue fracture surfaces do not exhibit signi ficant macroscopic features:
they are smooth, flat and shiny, with very fine lines. The most outstanding characteristic is the ratchet marks in the initiation zone, due to the simultaneous
nucleation of several small cracks. Most frequently, the crack initiation zone will be
found in a free surface and is typically connected to a stress concentrator, which
might be a sharp corner, a hole or a notch. Usually, the limit of the initiation zone
and the slow crack growth zone is well-de fined by a propagation front line (a beach
mark). This is due to the change of fracture mechanism. These characteristics are
schematically shown in Fig. 4.7.
As has been mentioned, fatigue cracks usually start in the free surface and are
connected to stress concentrators. However, in pieces with severe internal defects,
such as cast iron shrinks or needle-like particles, fatigue cracks can nucleate within
the material bulk. In this case, the same macroscopic characteristics of the fractured
surface, such as smooth and shiny surfaces and ratchet marks, will be observed
around the initiating defect.
Fig. 4.7 Macroscopic aspect of a Stage I fatigue fracture
4.2
Fractography of Stage I Fatigue
4.2.2
79
Microscopic Characteristics of Stage I Fatigue
At the macroscopic level, the crystalline nature of Stage I mechanisms leads to the
formation of faceted fractures with dense and well-de fined river patterns, as shown
in Fig. 4.8. This type of fracture is called pseudo-cleavage, due to its similarity to
cleavage. Pseudo-cleavage can be very hard to differentiate from cleavage, so a
novice fractographer may formulate an incorrect interpretation. In order to prevent
confusion, it is recommended that a detailed macroscopic inspection of the fractured component be carried out before proceeding to the microscopic observation.
If the latter is not possible, the safest way to identify pseudo-cleavage is by
carrying out a crystallographic trace analysis to identify the planes and directions of
the river pattern edges, which should correspond to the slip system of the particular
fatigued alloy, and not to cleavage systems. The same analysis can be done on the
slip lines that appear on the free surface of the piece, as long as it has a mirror-like
polish, as shown in Fig. 4.9.
In Stage I fatigue of polycrystalline material, a common characteristic of the
fractured surface is the formation of a high number of ridges parallel to the crack
propagation direction. These marks are formed due to the presence of a component
of Mode II displacement in the crack displacement, especially in planar slip
materials. Figure 4.10 shows an example of this. These fractures are difficult to
interpret, and the best recommendation, again, is to carry out a good macroscopic
examination, to make sure that it is a Stage I fatigue fracture.
4.2.3
Fatigue Crack Nucleation Mechanisms
In pieces with neither pre-existing cracks nor stress concentrators, the basic
nucleation and slow propagation fatigue crack mechanism is by dislocation slip;
80
4
Fatigue Fracture
Fig. 4.9 Slip lines around a pseudo-cleavage fracture on a nickel alloy single crystal fatigued in
Stage I
Fig. 4.10 Stage I fatigue fracture in a polycrystalline material. Stainless steel fatigued in air at
room temperature
nonetheless, when there are either pre-existing cracks or sharp stress concentrators,
Stage I is suppressed.
The most widely accepted model of fatigue crack nucleation is known as the
4.2
Fractography of Stage I Fatigue
81
Free
surface
Intrusion
Nucleated
crack
Slip plane
Extrusion
Back and
forth slip
Fig. 4.11 Intrusion-Extrusion mechanism of fatigue crack nucleation
metal extrusions on planes where dislocations reach a free surface. In order to
maintain continuity, in a nearby strip of material, the opposite process takes place,
that is, the dislocation slip sends material inwards, making an intrusion. When the
intrusion is sharp and deep enough, it turns into a crack. Figure 4.11 shows a
scheme of this process.
This mechanism is supported by widespread experimental evidence, and it is
favored by planar slip conditions, whether by a favorable slip plane orientation or
by a limited number of slip systems, so the deformation preferably occurs along
dense slip bands.
Another highly accepted model is the one by Forsyth, proposed from the
observation of polished surfaces in pure and ductile materials subject to cyclic
loads. In these cases, the formation of extrusions can be highly localized. Forsyth
observed, in pure aluminum samples, that a slip band close to an extrusion could go
on to decohesion and form a micro-crack. This mechanism can be considered a
variation of Wood´s mechanism and is shown schematically in Fig. 4.12.
To explain the nucleation of cracks in pure shear (Modes II and III), Mott
proposed a model based on a double cross-slip of screw dislocations, to explain the
observation of extruded tongues and debris in pure Mode II fatigue fractures.
According to Mott, a screw dislocation in alternating pure shear loading can go
through a double cross-slip when the dislocation is located in a free corner; after
completing the double cross-slip process, a metal fragment is ejected (Fig. 4.13).
Figure 4.14 shows evidence of the metal fragments ejected from a step in a
crystalline fracture of a Nickel Base Superalloy single crystal, fatigued under
Modes I and II, in vacuum at room temperature.
It is an accepted fact that corrosive environments, moderate or severe, shorten
the stage of crack initiation as compared to fatigue in inert or vacuum environments.
An experimental observation shows that an aggressive environment notably
diminishes the slip activity, and this makes the nucleation of fatigue cracks easier.
Nowadays, this effect has not been totally clari fied, and some of the following
4.3
Fractography of Stage II Fatigue
85
Fig. 4.18 Typical characteristics of a Stage II fatigue fracture: smooth surface covered by beach
marks
Fig. 4.19 Macroscopic appearance of a fatigue fracture in a polymer material
polymers, as shown in Fig. 4.19, where the typical features of fatigue can be clearly
observed.
The fractured surface extension in Stage II depends on the maximum stress and
86
4
Kt
High
Fatigue Fracture
Simple Bending
Rotating Bending
Stress Amplitude
Stress Amplitude
Low
High
Low
Low
High
Fig. 4.20 Effect of stress amplitude and stress concentration level on the appearance of bending
fatigue fractures. Kt is the stress concentration factor
during the first half of the cycle and then the load is released. Since the outer radius
of the bent bar is in tension, the cyclic load is tension-tension and the crack will
start at the surface of the outer radius, because that is where the highest tensile stress
is located. As the crack propagates towards the center, the neutral axis moves
forward, until the critical crack size is reached and the piece fails. One special case
is rotation bending: in such a case, there are two initiation sites located at opposite
sides of the piece, so the final fracture zone will be at the middle section of the
piece. Figure 4.20 schematically shows the appearance and extension of the fatigue
fractured surface of a round bar in simple or rotation bending as a function of the
stress concentration level and the load amplitude.
In alternating torsion, the maximum tension stresses are at ±45° from the longitudinal shaft axis, therefore two groups of cracks will be formed in planes at 45°
from the shaft axis and will be perpendicular to each other, forming a star-type
fracture, as shown in Fig. 4.21.
4.3.2
Microscopic Characteristics of Stage II Fatigue
The most noticeable characteristic of Stage II fatigue fractured surfaces is the
4.3
Fractography of Stage II Fatigue
87
Torsion
Direction of
σmax
+45
Long. axis
-45
Cracks
Fig. 4.21 Fracture by fatigue in pure alternating torsion
Fig. 4.22 Microscopic striations on Stage II fatigue fractured surfaces
spacing is close to the macroscopic crack growth rate, thus it is believed that each
striation corresponds to one load cycle. The presence of striations indicates, without
doubt, that the observed fracture was caused by fatigue, but their absence does not
mean the opposite, because striations may fail to form under a variety of conditions.
Depending on their appearance, the striations are classi fied into two groups:
1. Ductile striations: The striation profile is wavy and smooth, as in Fig. 4.22.
2. Brittle striations: The striation pro file is irregular or saw tooth-like.
88
4
Fatigue Fracture
Fig. 4.23 Brittle type striations
Fig. 4.24 Correspondence
between crest and valley of
striations in opposing fatigue
fractured surfaces
Crack growth direction
Crack tip
Valley
Crest
In general, the matching of the striations between opposite fracture surfaces in
the same crack is crest –to-crest and valley-to-valley, as shown in Fig. 4.24.
Normally, striations do not cover the entire Stage II fractured surface; it is most
common to find areas with striations separated by shear lips or areas where the
fracture mechanism is not well-de fined. It also common for the striation spacing not
to be uniform in the same area, as seen in Figs. 4.22 and 4.23. Likewise, the
striations local direction is not always the same, and there could be different
striation directions, as seen in Fig. 4.25. If the striations directions in the same area
are overly different, it is most likely due to the intersection of secondary cracks, and
not because the propagation front has sudden changes.
The cause that makes striations spacing variable, even within the same fatigue
fracture area, is that the crack growth mechanism is quite sensitive to microstructural changes. One of the most common cases is when the crack passes through a
’
’
’
4.3
Fractography of Stage II Fatigue
89
Fig. 4.25 Local variation of striation direction
crack propagation front and cause the crack to locally accelerate or retard, while the
rest of the crack front grows at a uniform rate in a single phase zone.
A generally accepted observation is that striation spacing matches the macroscopic crack growth rate (da/dN). This has not been fully confirmed, mainly
because, at slow growth rates (less than 10 −6 mm/cycle), the resolution limit of the
scanning electron microscopes does not allow for measuring the striation spacing.
Table 4.3 allows for a better appreciation of the relation between striation spacing
and the magnification required for their observation.
Figure 4.26 shows the relation between striation spacing measured in an MEB
and the macroscopic crack growth rate d a/dN. It can be seen that striation spacing is
constant below da/dN values of around 10−5 mm/cycle, but this has to do with the
electron microscope resolution used for studying these fractures. In order to observe
Table 4.3 Fatigue crack growth rate and ampli fication needed to observe striations (in case they
do exist)
Macroscopic crack
growth rate
Typical
scale
Magnification necessary for
observation
Observation technique
10−3 mm/cycle
1l
100
Optical microscope (poor
resolution)
10−4–10−5 mm/cycle
0.1 l–
10 nm
1000
10−6–10−7 mm/cycle
5A
Approx. 150,000

 a 10,000

Scanning electron
microscope (SEM)
High resolution SEM or
102
5
 
iox
gox ¼ box ln
i0
Environmentally-Assisted Fracture
;
where box and i0 are experimental constants. The Tafel equation for the reduction
reaction is
 
gRed ¼ bRed ln
iRed
;
i0
where bRed and i0 are experimental constants.
By plotting the half-cell potential as a function of the current density logarithm
for each Tafel equation, two straight lines with opposite slopes are obtained, representing the oxidation and reduction kinetics, respectively. According to the
principle of charge balance, the corrosion reaction kinetics, i.e., the amount of
transferred charges, in absence of external potentials, is given by the intersection of
the two straight lines, and is called the exchange current density ( icorr). This graph is
called the activation control Evans ´ diagram, and is shown in Fig. 5.4.
When the cathodic reaction kinetics is controlled by the reducing agent diffusion,
the Tafel equation for the reduction reaction has an additional term that takes into
account the diffusion of the oxidizing agent:
gRed
 
iRed
¼ bRed ln
i0


1  iRed
RT
ln
þ 2:303
;
zF
iLim
where R is the ideal gas constant, T is the temperature, z is the transferred load, F is
the Faraday constant, and i Lim is the boundary current density in the diffusion layer.
When this equation is plotted in the Evans diagram, the curve for the cathodic
reaction bends downwards, as shown in Fig. 5.5.
A very important phenomenon in corrosion occurs when a thin and very resistant
corrosion product film is formed on the metal s surface. This film isolates the
’
E X/X -
M
M N+ + e-
(Oxidation)
E
E M/M +
X n+ + n e-
(Reduction)
X
5.2
Fundamentals of Metal Corrosion
103
Oxidation
-
E
Reduction
+
log (i)
iLim
Fig. 5.5 Evans diagram for a diffusion-controlled cathodic reaction
E
Transpassive zone
Passive zone
Transition zone
E X -/X
Epp
Active zone
E M/M +
ip
icor
ipp
log i
Fig. 5.6 Evans diagram with passivation
surface from the electrolyte, strongly reducing the corrosion rate. Such a phenomenon is known as passivation, and it modifies the Evans diagram as shown in
Fig. 5.6.
The Evans diagram for passive behavior is divided into four regions:
1. Active zone: Corrosion kinetics is controlled by the electric charge exchange.
104
5
Environmentally-Assisted Fracture
3. Passive zone: The corrosion kinetics is reduced to a minimum ip, and it is
independent of the potential over an ample interval.
4. Transpassive zone: The active behavior is established.
Because the crack tip is subject to continuous deformation, it is expected that it is
always active, because the passive film is continuously broken, while on the
fractured surfaces, the passive film is stable. This produces a very complex equilibrium in the crack cavity that severely affects the crack growth mechanism.
5.3
5.3.1
Stress Corrosion Cracking
Mechanical Aspects of Stress Corrosion Cracking
The usual SCC laboratory test consists in determination of the time of rupture as a
function of the applied stress in a speci fic environment. The test piece is similar to a
smooth tensile test specimen. The results are plotted on a semi-log scale, so as to
obtain a graph like the one shown in Fig. 5.7. At first, it is observed that at low
stresses, the rupture time is very long, and there is a stress under which the rupture
time is practically in finite. This stress is called SCC threshold stress, and its value
depends on the combination of material and environment.
Just like in fatigue, the SCC general condition is that the applied stress is below
the yield strength, so the crack propagates within an elastic environment. This
means that the crack growth behavior can be analyzed according to linear elastic
fracture mechanics. In such a case, the stress level at the crack tip depends on the
stress intensity factor K . When the crack growth rate (d a/dt) versus K data is plotted
on a log-log scale, a graph similar to the Paris graph is obtained, as shown in
Fig. 5.8. In this plot, three clearly de fined stages can be observed.
Fig. 5.7 Typical SCC test
results plot
σu
Stress
σ th
5.3
Stress Corrosion Cracking
STAGE I
Kth
105
STAGE II
STAGE III
log (KI)
KIC
Fig. 5.8 Crack growth rate by ACE, according to the stress intensity factor
Stage I: This shows a K threshold value below which the crack does not
propagate. It strongly depends on the microstructure, and brittle fracture dominates.
The crack growth rate can be estimated by an equation similar to the Paris Law.
Stage II: The crack growth rate shows little or no dependence on K. It is said
that the crack growth rate is then controlled by corrosion reaction kinetics, since the
crack grows by anodic dissolution of the crack tip, and therefore the crack growth
rate depends on the corrosion rate ( i ).
Stage III: The crack growth rate is very sensitive to the stress level. Static modes
of fracture are observed. The process ends when K reaches the fracture toughness
value in the testing environment.
The cracking mechanism, and consequently the characteristics of the fractured
surface, varies from one stage to the next, as shown in Table 5.1.
The effect of the microstructure in SCC is the most complex of all fracture
mechanisms. The highly localized cracking process makes it very sensitive to local
corr
Table 5.1 SCC stages and mechanisms
Stage
Mechanism
Fractographic features
I
Formation and rupture of
passive lavers
Process zone embrittlement
Cleavage
High roughness, facets, secondary cracking,
intergranular fracture
106
5
Environmentally-Assisted Fracture
changes in the microstructure. Microstructural variations affect the stability of
passive films, modify the corrosion potential, promote the formation of micro
galvanic cells and affect the local stress distribution. Among the most important
microstructural factors that affect SCC, are:
1.
2.
3.
4.
5.
6.
7.
8.
9.
Chemical composition of the matrix and second phases.
Content and spatial distribution of second phases.
Segregation.
Grain size.
Dislocation density and cold strain.
Slip modes.
Micro- and macro-segregation.
Inclusions content.
Grain boundary precipitation.
To analyze each one of these factors and identify their effect on the fractured
surface s features is a complex task, which demands deep knowledge of fractography, metallurgy, materials science and electrochemistry. In addition, the experimental set-ups require expensive equipment and highly trained personnel, in order
to accurately reproduce the environmental in-service conditions. The description of
these interactions and experiments goes beyond the scope of this book.
’
5.3.2
Characteristics of SCC Fractures
SCC cracks generally initiate at corrosion pits (unless there is a preexisting crack or
a sharp stress concentrator), as shown in Fig. 5.9. Usually, the nucleated cracks are
very short, but a stable crack growth mechanism is quickly established, making
Stage I very brief.
SCC fractured pieces have a brittle appearance, and there is clear evidence of
corrosive attack on the fractured surface. The crack path is tortuous, which makes
108
5
Environmentally-Assisted Fracture
Fig. 5.12 Example of a tortuous and branched SCC crack making a very rough fractured surface
Fig. 5.13 Transgranular SCC fractured surface
As to their trajectory through polycrystalline materials, SCC cracks can be
transgranular (TG) or intergranular (IG). Although intergranular fractures dominate,
because the grain boundaries are energetically favorable paths for cracking, both
TG and IG cracking can appear on the same fractured surface, with a sharp transition from one to the other. Examples are given in Figs. 5.13, 5.14 and 5.15.
The transition from TG to IG fracture has been explained by the passivation
phenomenon. In the active regions of the Evans diagram, as shown in Fig. 5.16, the
grains have an active electrochemical dissolution behavior, so the crack takes a path
5.3
Stress Corrosion Cracking
109
Fig. 5.14 Intergranular SCC fractured surface
Fig. 5.15 Mixed transgranular and intergranular SCC fracture
through the anodic type grains, which are dissolved, making the crack TG. While in
the passive region, the grain boundaries have more energy, so the passive film is not
formed or it is easily broken, turning the grain boundaries themselves into anodes,
thus favoring an IG fracture.
110
5
Fig. 5.16 Evans diagram
showing the predominance of
IG and TG fracture
Environmentally-Assisted Fracture
Transgranular (TG)
E
Intergranular (IG)
Transgranular (TG)
Log i
5.3.3
SCC Mechanisms
Stress corrosion cracking is a stable or delayed fracture mechanism that occurs
through the synergistic interaction of a corrosive environment, a susceptible
material and a sustained stress that causes a crack to initiate and grow. As in fatigue,
SCC makes a crack grow up to a critical size, thus causing failure.
SCC is a more general phenomenon than is thought, indeed, it can be said with
even more certainty that any structural component that cracks or fractures after
being stressed over some time has been a victim of SCC. From the fractography
point of view, in SCC, there is no clear difference between the initiation and the
crack growth stages, so the differences observed on the fractured surface as the
crack advances are minimal, and perhaps only an increment of roughness may be
observed due to the increment of the size of the plastic zone.
There are two types of SCC:
Controlled by environment: The crack growth mechanisms are predominantly
by anodic dissolution. The fractured surface has thick layers of corrosion products
and plenty of pitting and selective attack.
Controlled by stress: The predominant mechanism is brittle fracture. The
fractured surface has facets, river patterns and thin films of corrosion products.
Considering the great variety of environment-material combinations that may
exist, there is no single mechanism of SCC, but the main processes that occur are
shown schematically in Fig. 5.17.
It is important to note that the external environment and the internal crack
environment may be quite different. For example, the pH in the crack cavity might
be very acidic, whereas the external environment may be near neutral. Another
important fact is that the already formed fractured surface can chemically react with
the environment, affecting the corrosion and fracture processes that take place at the
crack tip.
5.3
Stress Corrosion Cracking
111
Free surface
External
environment
(Temp., pH,
pressure,
chemicals)
Adsorption
Deposit layer
formation
Film
formation
and rupture
Anodic
dissolution
Process
zone
M
-
A
M
Internal
environment
Transport of oxidant
A-
A
A-
AFractured surface
M
Embrittlement
and plastic
strain
M
M
Diffusion of
chemical spices
Crack
Extension by
dissolution
Extension by
brittle fracture
Fig. 5.17 Main processes that occur during SCC
The main processes of SCC, shown in Fig. 5.17 are:
(a) Anodic dissolution. The material at the crack tip is dissolved by the corrosive
action of the internal environment, causing the crack to advance.
(b) Process zone embrittlement. Certain chemical species, mainly hydrogen,
formed in the internal environment diffuse into the process zone, causing its
embrittlement, which is fractured by the acting stresses, making the crack grow.
(c) Film formation and rupture. The corrosion product films formed in the crack tip
break and the crack advances by a little more than the thickness of the broken
layer, exposing new unprotected metal, which is rapidly attacked, forming a
new film. Then, the process is repeated.
5.4
5.4.1
Creep Fracture
General Aspects of Creep Fracture
112
5
Environmentally-Assisted Fracture
temperature deformation mechanism, creep fractures get less attention, nonetheless
they are an important failure mechanism. In theory, creep may occur at any temperature, but it is at high temperatures that it becomes signi ficant. The definition of
high temperature is relative: typically, it is assumed that high temperatures are those
over 0.4TM, where TM is the melting point temperature in absolute degrees.
The conditions of temperature, time and stress under which creep occurs depend
on the material s mechanical properties and its microstructure. The exposure of a
material to high temperatures has several effects, which, all together, are the cause
of creep. The main effects of high temperature exposure in engineering materials
are:
’
•
•
•
•
•
•
•
Reduction of yield strength and tensile strength.
Increase of dislocation mobility.
Recovery and recrystallization and grain growth.
Increase of diffusion rate.
Dissolution and precipitation of second phases.
Incipient fusion.
Oxidation.
If the deformation of a tensile test specimen under constant stress and high
temperature is recorded continuously as a function of time, a curve like the one
shown in Fig. 5.18 is obtained. Such plots are known as creep curves.
In a typical creep curve for metallic materials, like the one shown in Fig. 5.18,
the strain does not start at zero, because there is an instantaneous initial strain
caused by the applied stress. It is also observed that the time of rupture is reduced as
the temperature or the applied stress increases.
STAGES
Strain
I
II
III
ε
(dε/dt)
Decreasing
(d ε/dt)
Constant
εO
Time
Fig. 5.18 Idealized creep curve for a metallic material
(d ε/dt)
Increasing
5.4
Creep Fracture
113
The creep curve shows three stages:
I. Primary creep . In this stage, the initial strain rate is high, but gradually
diminishes until it reaches a constant value. In this stage, there is high
dislocation mobility and interaction that leads to strain hardening, therefore
the strain rate is gradually reduced. Some microstructural transformations
may also occur.
II. Secondary creep. In this stage, there is a dynamic equilibrium between the
strain hardening and dislocation rearrangement and annihilation, which leads
to a constant strain rate.
III. Tertiary creep. The microstructural transformations, dislocation pile-ups
and cavitation of grain boundaries increase the strain rate and weaken the
material at the same time, causing specimen rupture. This process is generally localized, causing the formation and growth of cracks. The material
within the process zone suffers an accelerated formation and growth of grain
boundary voids, up to the point at which they interconnect, causing the
extension of the crack.
From the engineering point of view, secondary creep is the most important stage,
because it represents the longest portion of the rupture time, but also because it
allows for estimation of the rupture time. The strain rate ( de/dt) is directly related to
the applied stress r, according to the creep power law, which has the form
d e=dt ¼ Crn ;
where C is a constant and n is the creep exponent. According to this law, the greater
the stress, the greater the strain rate. In secondary creep, the constant strain rate
mechanism is a thermally activated process, and therefore the strain rate can be
represented by an Arrhenius type equation:
de
dt
 H
¼ Aeð RT Þ ;
D
where DH is the activation energy, T is the absolute temperature, R is the ideal gas
constant and A is a constant that depends on the material. This equation predicts that
higher temperatures exponentially increase the strain rate, thus it can be stated that
temperature has a much higher effect than the stress in the creep rupture time.
5.4.2
Creep Fracture Mechanism
Creep fracture is usually intergranular, so its fractured surfaces show a granular
aspect, where the grain facets are covered by little voids or craters that are the
114
5
Environmentally-Assisted Fracture
Fig. 5.19 Test bar showing a creep failure
At the macroscopic scale, creep fractures present mild plastic deformation, little
neck strain, multiple cracking and surface roughness. The intense intergranular
cracking in creep frequently causes the fracture to occur with little plastic deformation, as shown in Fig. 5.19. This may be mistaken for creep fractures with brittle
or SCC fractures, however, the observation of intergranular cavitation in a metallographic specimen, plus the typical high temperature oxidation in the fractured
piece, should help in clearly identifying a creep failure.
As has been mentioned, creep fracture is typically intergranular. The basic
mechanism is nucleation, growth and interconnection of grain boundary voids. The
formation of grain boundary voids may be due to three mechanisms:
1. Grain boundary sliding.
2. Vacancy condensation at grain boundaries.
3. Creep deformation.
The grain boundary sliding is a consequence of the loss of mechanical strength
experienced by grain boundaries at high temperatures. The grain boundaries oriented favorably to the local maximum shear stress will slide, causing decohesion at
the triple joints, forming wedge-like voids known as W type. Conditions that
favor W type voids are: high temperatures, above 0.6T M, and high stresses. The
Fig. 5.20 schematically shows the formation of W voids.
Vacancy condensation at grain boundaries, on the other hand, leads to the formation of spherical voids along the grain boundaries, called r type. Their growth
is controlled by vacancy diffusion and stress, according to the following equation:
“
”
“
”
dr=dt ¼ C DV r m rn ;
where dr/dt is the r void growth rate, Dv is the vacancy diffusivity, r is the void
size, is the power law creep exponent and
is an experimental constant. The
5.4
Creep Fracture
115
σ
Grain
boundary
slip
Tripe
joint
Wedgelike void
Fig. 5.20 Type W voids formed by grain boundary sliding
Fig. 5.21 Type r cavities
Direction of máximum
principal stress
Type r voids
the direction of the maximum tensile stress, as shown in Fig. 5.21. The presence of
precipitates on the grain boundaries restrains the grain slip, and thus type W cavity
formation, but has no effect on the formation of type r voids. Figure 5.22 shows
grain boundary cavitation in a piece of steel with creep failure.
Creep fractured surfaces are typically intergranular and show cavities on the
grain boundary surfaces, as shown in the example in Fig. 5.23.
5.4.3
Creep Crack Growth
Creep failure may occur in a localized way in pre-cracked components or where
6.5
Examples of Failure Analysis
157
Fractured surface
surface of a tooth, taken from a zone identi fied as Stage II fatigue showing
Fig. 6.15 Fractured
severe mechanical damage; even so, ill-de fined striations can be seen
¼ 2800 KW ¼ 3753 hp;
rpm ¼ 666;
R ¼ 50 = 11 ¼ 4:55;
F ¼ 10 plg;
d ¼ 12 plg:
HP
w
Substituting into the corresponding equation, a value of K = 601 is obtained,
which is considerably greater than the limit value of 440; therefore, it is concluded
that pinion teeth are prone to pitting according to AGMA.
As no corrosion, gouges, distortion or pre-existing flaws were observed on the
teeth,
teeth, and their
their micros
microstruc
tructur
turee and hardne
hardness
ss are normal
normal for the applic
applicati
ation,
on, the
hypot
hypothes
heses
es of inco
incorr
rrec
ectt fabr
fabric
icat
atio
ion
n or insu
insuffficient
cient mainte
maintenan
nance
ce are discard
discarded,
ed,
leading to the conclusion that the pinion failure was caused by service loads in
excess of the original design of the reducer.
To verify whether the reducer box of the cement mill, powered by a 2800 kw
motor with input rate of 546 –666 rpm and output of 15.14–18.45 rpm, is adequate
for service, a mill cement reducer manufacturer catalog was consulted. It was found
that a reducer with a 2800 kw motor is recommended for an output rate of 27 rpm,
so, considering that the failed reducer ’s maximum output rate is 18.45, the reducing
box would be limited to a maximum power of approximately 2200 kw, because the
lower the output rate, the less power required.
158
6
Failure Analysis of Fractured Components
the
the moto
motorr supp
suppli
lies
es the
the need
needed
ed powe
powerr to keep
keep the
the mill
mill table
table at a stea
steady
dy spee
speed,
d,
independent of the milling load (tons per hour) and the hardness of the material
being ground. The maximum power is limited by motor power, so if the material ’s
hardness demands the full 2800 kw power from the motor, it will supply it, but if
the reducer is designed for 2000 kw, it will be overloaded, thus causing the pitting,
fatigue cracking and spalling.
Failure sequence.- According to the specialized literature, pitting, cracks, fatigue and spalling of gears occur during service, therefore the failure sequence of the
pinion gear was as follows:
1. Since the beginning
beginning of the service,
service, the high cyclic
cyclic loads induced
induced a mechanism of
surface fatigue on the teeth ’s surface below the pitch line, forming pits at least
2 mm in diameter, and both the number and size of the pits increased until a
larger portion of material became detached.
2. The
The larg
largee pits
pits acte
acted
d as stre
stress
ss conc
concen
entr
trat
ators
ors that
that promo
promote
ted
d the
the form
format
atio
ion
n of
macroscopic fatigue cracks (more than 10 mm long) as the number of cycles
continued to accumulate.
3. The fatigue
fatigue cracks
cracks grew until they reache
reached
d their
their critic
critical
al size
size and caused
caused the
detachment of larger sections of the teeth, or the fatigue cracks interconnected
with other cracks to cause the fall of a tooth.
4. The teeth and chips
chips fell,
fell, obstru
obstructi
cting
ng gear
gear moveme
movement,
nt, thus causin
causing
g the overall
overall
failure of the mill reducer.
Conclusion.- The failure of the pinion gear was caused by the selection of a
ceme
cement
nt mill
mill redu
reduce
cerr mode
modell with
with insu
insuffficient
cient power
power capaci
capacity
ty (2800
(2800 kw),
kw), which
which
induced
induced excess
excessive
ive loads
loads on the gears,
gears, thus
thus causin
causing
g pittin
pitting,
g, spalli
spalling
ng and fatigu
fatiguee
failures.
6.5.
6.5.3
3
In-S
In-Ser
ervi
vice
ce Rupt
Rupture
ure of a Gasoli
Gasoline
ne Pipe
Pipeli
line
ne
Background.- A 10-i
10-in.
n. nomi
nomina
nall diam
diamet
eter
er gaso
gasoli
line
ne tran
transpo
sport
rt pipe
pipeli
line
ne had
had an
in-service rupture at a highway crossing. Before failure, it was reported that the
pipeline operation had been interrupted many times because of maintenance work
to correct illegal tappings, with up to 39 interruptions in the month prior to the
reported
reported failure. In addition,
addition, the pipeline
pipeline had been packed with nitrogen for a period
of about six months in the previous year.
The operating conditions of the pipeline are: pump station discharge pressure
68 kg/cm2, pressure at the point of failure 63.5 kg/cm 2, normal operation temperature 25 C. The fabrication material is API 5L X52 pipe steel. The majority of the
pipes are SAW seam, with isolated sections of seamless or ERW seam pipe. As the
failed
failed section
section was located
located at a crossroads,
crossroads, it had a sleeve of 14-in. nominal
nominal diameter
6.5
Examples of Failure Analysis
159
failure, the pipeline had a little over 32 years of service and it had been internally
inspected at least two times with MFL tools.
Figure 6.16 show
showss the
the fail
failed
ed pipe
pipe sect
sectio
ion
n as it was
was
Visual examination
examination.- Figure
rece
receive
ived
d in the
the labo
laborat
rator
ory.
y. The
The pipe
pipe segme
segment
nt dime
dimens
nsio
ions
ns are:
are: 25.4
25.4 cm (10
(10 in.)
in.)
diameter, 431 cm (169 in.) length and the measured average wall thickness was
6.54 cm (0.257 in.).
The main feature of the failed pipe is a fish mouth-type fracture, 133 cm long
and with a 5 cm opening, localized along the ERW seam. The fracture consisted of
a single crack, with no branching or secondary cracks. The fracture plane is parallel
to the longitudinal direction, without de flections at the ends. The fractured surface
was covered with atmospheric corrosion products.
No local metal loss was observed on the external and internal pipe surfaces, and
the uniform corrosion metal loss was negligible. The pipe segment was free of
mechanical damage, such as punctures, gouges, dents or scratches.
The expansion of the pipe perimeter in the maximum opening zone was 3.1,
while the pipe perimeter away from the fracture was 86.1 cm. Thus, the pipe plastic
expansion
expansion was 3.6%. The thickness
thickness in the middle of the fish mouth crack, measured
by ultrasound on a square grid of 5
5 cm, 150 cm long by 30 cm wide, showed a
minimum wall thickness of 6.1 mm (0.240 in.), located at the edge of the crack,
which represents a 2.7% loss of the average thickness of 6.54 mm (0.257 in.).
Figure 6.17 shows a metallographic preparation of the transversal section of the
crack plane, showing
showing that the fracture grew along the ERW seam. It is observed
observed that
the crack path is right along the fusion line of the ERW seam, and the heat-affected
zone
zone adjace
adjacent
nt to the fractur
fracturee plane
plane shows
shows long
long non-met
non-metall
allic
ic inclus
inclusion
ions,
s, aligned
aligned
parallel to the ERW fusion line; these non-metallic inclusions go across the full
thickness, as shown in Fig. 6.17
6.17..
Fractographic examination.- Samples of the fracture’s center were cut, cleaned
and prepared for observation in the stereoscope and the SEM. Figure 6.18 shows
that
that the fractu
fracture
re has two zones with
with differ
different
ent appear
appearance
ance,, one,
one, locali
localized
zed in the
middle thickness, is a stripe of approximately 2.0 mm wide, with a granular and
shiny aspect,
aspect, and other, located
located at each side of the pipe wall in the form of stripes, is
covered with corrosion products and a series of parallel lines that are preliminarily
identified as beach marks.

160
6
Failure Analysis of Fractured Components
Fig. 6.17 Metallographic preparation of the transversal section of the fracture end of the failed
pipe, showing that the crack path was along the fusion line of the ERW seam
O.D
6.54 mm
I.D.
Fig. 6.18 Fractured surface of the center of the “fish mouth” fracture, as viewed in the
stereoscope, showing a shiny and granular fracture in the center and fatigue-like fractures on both
sides of the pipe wall
Figure 6.19 shows the microscopic appearance of the fractured surface of the
crack growing from the ID to the mid-thickness zone at the center of the fish mouth
crack. Despite the corrosion products covering most of the surface, poorly de fined
striations can be observed, which allow us to identify the fracture mechanism in that
zone as corrosion-fatigue. The appearance of the fractured surface in the
mid-thickness zone (the shiny stripe), as seen in the SEM, is shown in Fig. 6.20; it
6.5
Examples of Failure Analysis
161
20
m
Fig. 6.19 Fractured surface close to the external edge of the central zone of the
identified as corrosion-fatigue fracture
fish
mouth,
An elemental chemical analysis of the non-metallic inclusions found on the
fracture zone was carried out by EDS in the SEM. An example of the results is
shown in Fig. 6.21, indicating that the inclusions are magnesium sulphur ( MnS).
Analysis of results.- The chemical composition, hardness, tensile strength and
microstructure of the failed pipe meet the requirements of speci fication API 5L
X52, which is the design speci fication of the pipe, and therefore the failure cause is
not related to a defective or incorrect material speci fication.
Regarding the failure type, as mentioned earlier, it is a fish mouth-type rupture,
which is the typical ductile tearing failure of a pressurized component. This is
confirmed by the plastic expansion detected at the fracture mid-section of the fish
mouth.
The following calculation is done to estimate the burst rupture pressure that
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