Failure Analysis : Laboratory studies

MM: 503 Deformation Behavior
and
Failure Analysis of Materials
Engr. Muhammad Ali Siddiqui
Assistant Professor
Course Teachers
NED University of Engineering and Technology
Department of Materials Engineering
Lecture 3 Series
“Laboratory Studies in Failure Analysis”
1

Laboratory Studies
a) cleaning of fracture surface
b) Preliminary Examination (Macro- fartrography)
c) Microscopic Examination (Micro -fractrography)
d) Metallography Examination
e) Chemical Analysis.
f) Mechanical Properties.
g) Nondestructive Evaluation
h) Any other special technique
2

 Perform fractographic examination.
 Perform chemical analyses and compare
results with specification or standards.
Analyze any important surface corrosion
products, deposits, or coatings.
 Determine mechanical properties and
compare with specifications or
standards.
 Perform macroscopic examination to
evaluate homogeneity, integrity, and
quality.
 Perform metallographic examination to
evaluate microstructural features.
Determine the deformation direction of
wrought products and its relationship to
the applied and residual stresses.
 Perform microhardness testing to
measure case depths, evaluate cold
working, determine quality of
weldments, and aid in identifying
phases.
 Perform high-magnification
metallographic examination using the
electron microscope to study phases
unresolvable with the light microscope.
 Microprobe any critical abnormalities,
such as inclusions and segregations, that
are too small for bulk analysis.
 Use x-ray techniques to determine: (a)
level of residual stress; and (b) the
relative amounts of phases, for example,
austenite or retained austenite, ferrite,
delta ferrite or martensite, sigma, and
carbides in steels.
 Perform simulation tests to evaluate
critical characteristics of the material
(such as the stress-corrosion cracking
tendency in a particular environment), to
determine the degree of embrittlement,
or to confirm the method of heat
treatment or hardenability.
3
Basic Approach to Failure Analysis (Important steps)

Fracture-Cleaning Techniques
4

Fracture-Cleaning Techniques
• Fracture surfaces exposed to various environments generally contain
unwanted surface debris, corrosion or oxidation products, and
accumulated artifacts that must be removed before meaningful
fractography.
• Before starting cleaning procedures, the fracture surface should be
surveyed with a low-power stereo binocular microscope, and the results
should be documented with appropriate sketches or photographs.
• The most common techniques for cleaning fracture surfaces are:
1) Dry air blast or soft organic-fiber brush cleaning
2) Replica stripping
3) Organic-solvent cleaning
4) Water-base detergent cleaning
5) Cathodic cleaning
6) Chemical-etch cleaning 5

1) Air Blast or Brush Cleaning:
• Loosely adhering particles and debris can be
removed from the fracture surface with either
a dry air blast or a soft organic-fiber brush,
such as an artist's brush, should be used on
the fracture surface because a hard-fiber
brush or a metal wire brush will mechanically
damage the fine details.
6

2) The replica-stripping cleaning technique
• It is very similar to that "Preservation Techniques". However,
instead of leaving the replica on the fracture surface to
protect it from the environment, it is stripped off of the
fracture surface, removing debris and deposits. Successive
replicas are stripped until all the surface contaminants are
removed.
• The given figure shows successive replicas stripped from a
rusted steel fracture surface.
7

• Note: The replicas can be retained, and the embedded
contaminants can be chemically analyzed, if the nature of
these deposits is deemed important.
• The one disadvantage of using plastic replicas to clean a
fracture surface is that on rough surfaces; it is very difficult
to remove the replicating material completely.
• However, if the fracture surface is ultrasonically cleaned in
acetone or methyl acetate. after each successive replica is
stripped from the fracture surface, removal of the residual
replicating material is possible.
• Ultrasonic cleaning in acetone or the appropriate solvent
should be mandatory when using the replica-stripping
cleaning technique.
8

3) Organic solvents
• Organic solvents such as xylene, naphtha, toluene, ketones,
and alcohols, are primarily used to remove grease, oil,
protective surface coatings, and crack-detecting fluids from
the fracture surface.
• Sample can be cleaned by 3 methods:
1. soaked in the appropriate organic solvent for an extended
period of time,
2. immersed in a solvent bath where jets from a pump
introduce fresh solvent to the fracture surface,
3. placed in a beaker containing the solvent and
ultrasonically cleaned for a few minutes
10

• The ultrasonic cleaning method is probably the
most popular of the three methods mentioned
above, and the ultrasonic agitation will also
remove any particles that adhere lightly to the
fracture surface.
• However, if some of these particles are inclusions
that are significant for fracture interpretation, the
location of these inclusions relative to the
fracture surface and the chemical composition of
these inclusions should be investigated before
their removal by ultrasonic cleaning.
11

• It is important to avoid use of the chlorinated
organic solvents, such as tri-chloro-ethylene
and carbon-tetra-chloride, because most of
them have carcinogenic (hazardous)
properties.
12

4) Water-base detergent cleaning:
• Water-base detergent cleaning assisted by ultrasonic agitation is
effective in removing debris and deposits from the fracture surface
and, if proper solution concentrations and times are used, does not
damage the surface.
• A detergent Alconox, has proved effective in cleaning ferrous and
aluminum materials.
Example:
a) The cleaning solution is prepared by dissolving 15 g of Alconox
powder in a beaker containing 350 mL of water.
b) The beaker is placed in an ultrasonic cleaner preheated to about
95 °C (205 °F).
c) The fracture sample is then immersed in the solution for about 30
min, cleaned in water then alcohol, and air dried.
13

• Fig: (a) shows the condition of a laboratory-tested fracture
toughness sample (AISI 1085 heat-treated steel) after it was
intentionally corroded in a 5% salt steam spray chamber for 6 h.
• Figure (b) shows the condition of this sample after cleaning in a
heated Alconox solution for 30 min.
14

5) Cathodic cleaning
General Requirement:
• Cathode = Fracture Sample
• Anode = carbon/Platinum
• Electrolyte = any current conducting solvent
• vibrate the electrolyte ultrasonically or to rotate the specimen (cathode)
with a small motor
15
 It is an electrolytic process in which the sample to be cleaned is made
the cathode,
 and hydrogen bubbles generated at the sample cause primarily
mechanical removal of surface debris and deposits.
 An inert anode, such as carbon or platinum, is normally used to avoid
contamination by plating upon the cathode.
 During cathodic cleaning, it is common practice to vibrate the
electrolyte ultrasonically or to rotate the specimen (cathode) with a
small motor.

• The electrolytes commonly used to clean ferrous fractures are
sodium cyanide, sodium carbonate, sodium hydroxide solutions,
and inhibited sulfuric acid.
Example-1:
• A study in which AISI 1085 heat-treated steel and EX16 carburized
steel fractures were exposed to a 100% humidity environment at 65
°C (150 °F) for 3 days.
• A commercially available sodium cyanide electrolyte, ultrasonically
agitated, was used in conjunction with a platinum anode for
cleaning.
• A 1-min cathodic cleaning cycle was applied to the rusted fractures,
and the effectiveness of the cleaning technique without altering the
fracture morphology was demonstrated.
16

• The below given figure shows a comparison of an as fractured surface
with a corroded and cathodically stable ductile cracking region in a
quenched-and-tempered 1085 carbon steel.
• The relatively low magnification (1000 ×) shows that the dimpled
topography characteristic of ductile tearing was unchanged as a result of
the corrosion and cathodic cleaning.
• High magnification (5000 ×) shows that the perimeters of the small
interconnecting dimples were corroded away
17
• The fractographs on the
left show the as-fractured
surface;
• those on the right show
the fracture surface after
corrosion exposure and
cathodic cleaning

6) Chemical Etching
• If the above techniques are attempted and prove ineffective, the
chemical-etch cleaning technique, which involves treating the
surface with mild acids or alkaline solutions, should be
implemented.
• This technique should be used only as a last resort because it
involves possible chemical attack of the fracture surface. In
chemical-etch cleaning, the specimen is placed in a beaker
containing the cleaning solution and is vibrated ultrasonically.
• It is sometimes necessary to heat the cleaning solution. Acetic acid,
phosphoric acid, sodium hydroxide, ammonium citrate, ammonium
oxalate solutions, and commercial solutions have been used to
clean ferrous alloys.
18

• Titanium alloys are best cleaned with nitric acid .
• Oxide coatings can be removed from aluminum alloys by using
a warmed solution containing 70 mL of ortho-phosphoric acid
(85%), 32 g of chromic acid, and 130 mL of water.
• However, it has also been recommended that fracture
surfaces of aluminum alloys be cleaned only with organic
solvents .
• Especially effective for chemical-etch cleaning are acids
combined with organic corrosion inhibitors. These inhibited
acid solutions limit the chemical attack to the surface
contaminants while protecting the base metal.
19

• Ferrous and nonferrous service fractures have been
successfully cleaned by using the following inhibited acid
solution:
a) 3 mL of hydrochloric acid (1.19 specific gravity),
b) 4 mL of 2-butyne-1,4-diol (35% aqueous solution), and
c) 50 mL of deionized water .
• This study demonstrated the effectiveness of the cleaning
solution in removing contaminants from the fracture surfaces
of a low-carbon steel pipe and a Monel Alloy 400 expansion
joint without damaging the underlying metal.
• Various fracture morphologies were not affected by the
inhibited acid treatment when the cleaning time was
appropriate to remove contaminants from these service
fractures. 20

Fractrography
• The word fractography origin from the Latin word fractus, meaning
fracture, and graphy derives from the Greek term grapho, meaning
descriptive treatment.
• “The science of studying the fracture surface is termed as fractography”.
• Thus, depending on the level of examination, one can have
macrofractography and microfractography.
1. Macroscopic Examination /Macro-fractrography
• Macroscopic examination is carried out with unaided eye or a simple
handheld magnifier, or a stereomicroscope with low magnification (up to
50X).
• In the stereo microscope, reasonably large specimens can be handled, and
the microscope can easily be adopted for examination of components in
the field or accident site.
22

• The usual sequence for the examination of fractured components is as
follows :
1. Visually survey the entire component to obtain an overall understanding
of the component and the significance of the fractured area
2. Classify the fracture from a macroscopic viewpoint as ductile, brittle,
fatigue, torsion, and so forth
3. Determine the origin of failure by tracing the fracture back to its starting
point or points
4. Based on the observed fracture features, estimate the manner of loading
(tension, compression, shear, bending, and so on), the relative stress
level (high, medium, or low), and the stress orientation.
5. Examine areas selected by macroscopic examination at higher
magnifications by light microscopy, SEM, or replica transmission electron
microscopy (TEM) to determine the fracture mode, to confirm the
fracture mechanism (observation of cleavage facets, ductile dimples,
fatigue striations, and so on), and to detect features at the fracture
origin
6. Examine metallographic cross sections containing the origin to detect
any microstructural features that promoted or caused fracture initiation,
and determine if crack propagation favors any microstructural
constituent
23

• The features revealed during macroscopic examination
are:
1. Type of fracture
2. Origin of fracture
3. Presence of secondary cracks
4. Presence of external debris or corrosion products
5. Discoloration
6. Presence of wear marks in the vicinity of fracture
7. Plastic deformation preceding fracture
8. Dimensional changes in the component
9. Evidence of any overheating
10. Post-fracture damage such as rub marks
24

Examples of Macrofractrography
25

26
A ductile tensile fracture in a component of circular cross section consists of three
distinct zones as shown in the figure.
1. The inner flat/fibrous zone with a fibrous appearance is where the fracture starts and
grows slowly.
2. The fracture propagates fast along the intermediate radial zone. The radial lines
extended backward point to the fracture origin. Sometimes the radial lines start from
the origin itself.
3. The fracture finally terminates at the shear lip zone that is the annular region near
the periphery of the fracture surface. The shear lip zone is at an angle of 45° to the
tensile stress direction.

27
Macroscopic appearance of ductile (a) and brittle (b) tensile fractures

28
Bolt that failed in fatigue. The smooth dark areas are from initial fatigue cracking,
now coated with dark adherent oxide. This initial cracking took place some years prior
to the later fracture, the surface of which is covered by a less adherent rust coating

29
Brittle Sample: series of chevron marks on the fracture surface
Compressor Blade in an Aircraft Engine: Beach Marks
4 mm

30
5 mm
The broken tie-rod of the
developmental aircraft towing
tractor:

2. Microscopic Examination / Microfractography
• The information thus gathered at the macroscopic level has to be
integrated with the observations on detailed examination at the
microscopic level so that meaningful conclusions can be drawn
regarding the cause of failure.
• Microscopic examination is carried out using optical microscopes
and electron microscopes, the choice dictated by the magnification
and resolution desired.
• Microscopic examination is carried out on the fracture surface to
study the fracture features and also on a section transverse to the
fracture surface to study the internal structure of the material.
• The latter is a destructive test and should be carried out only at the
end after recording all the microfractographic features.
31

• The additional information one can obtain through
microscopy includes:
1) Microstructure of the material through metallography
2) Path of fracture
3) Mode of fracture
4) Length of the crack that pre-existed and propagated by
fatigue
5) Length of the fatigue crack before it became critical
6) Presence of inclusions, pits, or other flaws at the origin
7) Striation spacing of the fatigue crack
8) Presence of corrosion products
32

• The fracture surfaces are generally rough and cannot
be easily studied entirely by an optical microscope
because of its limited depth of focus and resolution.
• For microfractography, instruments with better depth
of focus and resolution are necessary. These
requirements are met by the electron microscope of
which there are two types:
• The scanning electron microscope (SEM)
• The transmission electron microscope (TEM)
33

Examples of Microfractrography
34

35
SEM Fractograph Elongated and equiexed dimples:

36
TEM Fractograph Elongated and equiexed dimples:

39
Sequence of SEM fractographs:
80 X 950 X 1000 X
Fracture in an iron alloy containing 0.14% S and 0.04% O. The fracture was obtained by
bending at room temperature. Several spheroidal oxide inclusions are visible, most of
them having diameters in the range of 1 to 3 μm.
The 6-μm-diam oxysulfide particle in Fig. 8 shows a shrinkage cavity plus a white spot
from an electron beam impingement in fluorescent x-ray analysis.

Metallography of Fracture Specimen:
40

• The metallurgical microscope is yet another instrument very useful
to the failure analyst.
• After collecting all the information through fractography of the
failed component, a section of the component can be cut
transverse to the fracture surface.
• This section is then polished and examined in the metallurgical
microscope, both before and after etching. Inclusions present in the
material are observed on the as-polished surface.
• The inclusion rating can be determined by standard quantitative
microscopy techniques.
• By differences in color, reflectivity, and refractive index, they can
also be identified with some prior experience.
• The polished specimen is then etched with suitable etchants to
reveal the microstructure of the material.
41

• Abnormalities in the microstructure that may have been
responsible for the failure can be identified at this stage.
• The path of a crack, whether it is intergranular or transgranular, and
branched or not branched, will be clear in the microstructure.
• Cracks due to stress corrosion, hydrogen embrittlement, and liquid
metal embrittlement are generally intergranular with some
exceptional situations.
• Fatigue cracks are transgranular. If a stress-corrosion crack
propagates by fatigue, the transition from intergranular to
transgranular mode can be seen in the microstructure.
• Stress-corrosion cracks in certain stainless steels are transgranular
with extensive branching.
42

• Plastic deformation of the component prior to fracture can be
recognized in the microstructure by the elongated grains.
• Abnormal grain growth, segregation of brittle or weak phases
at the grain boundaries, and recrystallization are some of the
other features that can be identified by metallography.
43
Transgranular crack propagation
intergranular crack propagation

Chemical Analysis of Fracture
Specimen
44

• Chemical analysis of fracture component provides
information regarding any deviation from the standard
specifications, compositional inhomogeneities, impurities,
inclusions, segregations, also helpful in identifying the
nature of corrosion products, coatings, external debris, and
so on.
• Several cases of service failures are known to have been
caused by the presence of deleterious inclusions from
which cracks start in the component and propagate, leading
to fracture.
• Certain impurities are known to cause embrittlement in
metals. Segregation of constituent elements sometimes
provides an easy path for crack propagation.
45

• Hence, identification of these harmful
constituents is very important in failure
analysis. A variety of instruments are available
for bulk chemical analysis and microchemical
analysis as well…..
• A few features are briefly discussed here.
46

47
A- Techniques for average bulk chemical analysis
(accuracy, 2 to 5%)
● Spectrophotometry: Applicable to nearly all
elements; accessible range, 0.001 to 50%.
● Atomic absorption spectrometry: Applicable to
practically all elements; accessible range, 0.001 to 10%;
● Emission spectroscopy: Applicable to all elements;
accessible range, 0.005 to 10%;
● X-ray fluorescence analysis: Normally applicable to
elements heavier than sodium; accessible range, 0.005
to 10%;

48
B- Techniques for local composition variations
● Laser probe microanalysis: Applicable to nearly all
elements; accessible range, 0.01 to 100%; accuracy,
semiquantitative; resolution, 20 to 200 µm
● Electron probe microanalysis: Applicable to
elements heavier than boron; accessible range, 0.001
to 10%; accuracy, 5 to 10%; resolution, 0.2 to 1 µm

49
C- Techniques for surface chemical analysis
● Auger electron spectroscopy: Applicable to all
elements except hydrogen and helium; accessible
range, >0.1%; accuracy, 5 to 10%; analysis depth, 10
to 20 A˚
● X-ray photoelectron spectroscopy: Applicable to
all elements except hydrogen and helium; accessible
range, >0.01%; accuracy, qualitative and
semiquantitative; analysis depth, 5 to 25 Ao

Analysis of Mechanical Properties of
fracture component
50

• One of the important step in any failure analysis.
• This process enables the investigator to judge whether the
material with which the component is made meets the
strength specifications and whether the component was
capable of withstanding the service stresses.
• If the size of the failed component permits, samples can be
taken from the component, and the conventional
mechanical testing can be done by standard test
procedures.
• Tensile test is generally the most useful one in many cases.
Other properties such as impact strength, toughness, and
creep rupture provide clues for the mechanism of failure.
51

• Sometimes, even tests on miniature specimens would
provide vital information.
• If the condition of the component does not permit
tensile or other mechanical tests, even a hardness
measurement would help in estimating the tensile
strength.
• This method has been adopted in quite a few failure
cases.
• Defects due to improper processing or inadequate heat
treatment would result in poor mechanical properties.
52

Next Class
• NDT Techniques.
• Modes of Failure.
53

Reference
1. ASM Hand book Fractrography Volume 12
2. ASM Hand book Failure Analysis and Prevention,
Volume 11
3. Failure Analysis of Engineering Structures
Methodology and Case Histories V.
Ramachandran, A.C. Raghuram, R.V.
Krishnan, and S.K. Bhaumik
54

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