Failure of Materials in Service

MME195: Engineering Materials - I

Ahsiur Rahman Nirjhar

Lecturer
Department of
MME, BUET
Failure
Introduction
❑Failure of materials is almost always an undesirable event -
• human lives are in jeopardy, economic losses
• interference with the availability of products and services
❑ Usual causes of failure -
• improper materials selection and processing
• inadequate design of the component, misuse
❑ It is the responsibility of the engineer -
• to anticipate and plan for possible future failure
• if failure does occur, to assess its cause and then take appropriate preventive
measures against future incidents
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Failure Modes
❑Failure due to breaking or destruction of material
• By plastic deformation or yielding
• By impact fracture
• By fatigue (delayed fracture)
• By creep (temperature-assisted delayed fracture)

❑Failure due to gradual loss of material

• By corrosion (dissolution in liquid media)
• By oxidation (formation of non-metallic scale or film)
• By degradation (in electromagnetic radiation)
• By wear (surface damage)
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Ductile Vs Brittle Fracture
Ductile and Brittle fracture Modes
❑Ductile mode –
• Example: most metals (not too cold). Extensive plastic
deformation prior to and during the propagation of the crack
• Crack is “stable”: resists further extension unless applied
stress is increased
❑Brittle mode –
• Example: ceramics, cold metals. Relatively little plastic
deformation (no gross deformation)
• Crack is “unstable”: propagates rapidly without increase in
applied stress (rapid rate of crack propagation)
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Ductile and Brittle fracture Modes

A. Very ductile, soft metals (e.g. Pb, Au) at room

temperature, other metals, polymers, glasses at high
temperature.

B. Moderately ductile fracture, typical for ductile

metals

C. Brittle fracture, cold metals, ceramics.

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Ductile and Brittle fracture Modes
Ductile Failure Brittle Failure
Extensive plastic deformation Very little plastic deformation
ahead of advancing crack at the crack front

High energy absorption before Little energy absorption before

failure (high toughness) failure (low toughness)

Process proceeds relatively Crack advances extremely

slowly as the crack length rapidly
extended

Such crack is stable (i.e., it Such crack is unstable and

resists any further deformation crack propagation, once
unless an increased stress is started, continues
applied) spontaneously
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Ductile fracture
Steps in Ductile Fracture:
(a)Necking
(b) Formation of micro-voids (cavities)
(c) Coalescence of micro-voids to form
elliptical crack
(d) Crack propagation by shear
deformation
(e) Fracture

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Ductile fracture
Steps in Ductile Fracture:
(a)Necking
(b) Formation of micro-voids (cavities)
(c) Coalescence of micro-voids to form
elliptical crack
(d) Crack propagation by shear Typical cup-and-cone
deformation fracture in ductile aluminum

(e) Fracture

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Ductile fracture
Steps in Ductile Fracture:
(a)Necking
(b) Formation of micro-voids (cavities)
(c) Coalescence of micro-voids to form
elliptical crack
(d) Crack propagation by shear
deformation
(e) Fracture

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Brittle fracture
❑No appreciable plastic deformation.
❑Crack propagation is very fast.
❑Crack propagates nearly
perpendicular to the direction of the
applied stress and yields relatively
flat fracture surfaces.
❑Very limited dislocation mobility.

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Brittle fracture
• The fracture surface may
show features called river
lines (or chevron marks),
which point back to the
initiation point of the fracture.
• These river lines can help to
show which feature or defect
on the fracture face initiated
the brittle fracture.

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Types of Brittle fracture

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Transgranular fracture
• Crack passes through grains.
• For most brittle crystalline materials,
crack propagation corresponds to the
successive and repeated breaking of
atomic bonds along specific
crystallographic planes; such a process
is termed cleavage.
• Fracture surface have grainy or faceted
texture because of different orientation of
cleavage planes in grains.
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Intergranular fracture
• Fracture crack propagates along
grain boundaries.

• Grain boundaries are usually

stronger

• But sometimes, it is weakened or

embrittled by impurities
segregation or other means.

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Ductile to Brittle Transition
Impact test
Two standard tests, the Charpy and Izod, measure the impact
energy (the energy required to fracture a test piece under an impact
load).

18
Impact test
❑The load is applied as an impact
blow from a weighted pendulum
hammer that is released from a fixed
height h.
❑The specimen is positioned at the
base as shown.
❑Upon release, the pendulum strikes
and fractures the specimen at the
notch, which acts as a point of
stress concentration.
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Impact test
❑The pendulum continues its
swing, rising to a maximum height
h’, which is lower than h.
❑The energy absorption, computed
from the difference between h and
h’, is a measure of the impact
energy.
❑The primary difference between the
Charpy and Izod techniques lies in
the manner of specimen support.
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Ductile to Brittle Transition
• The phenomenon of ductile to
brittle transition refers to a change
from the ductile to the brittle type
of behavior.
• It is important to note that ductility
or brittleness is not an absolute
property of a metal.
• A sudden brittle type of fracture can
also occur in ordinarily ductile
metals in lower temperature.
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Ductile to Brittle Transition
❑In metals, plastic deformation at room temperature occurs
by dislocation motion.
❑If the stress required to move the dislocation is too high,
the metal will fail instead by the propagation of cracks and
the failure will be brittle.
❑Thus, either plastic flow to ductile failure or crack
propagation to brittle failure will occur, depending on which
process requires the smaller applied stress.
❑As temperature drops, dislocation motion becomes
increasingly difficult.
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DBTT
❑The temperature at which a ductile material changes its
behavior to a brittle material is known as Ductile to Brittle
Transition Temperature (DBTT).
❑The impact test can be used to determine whether a material
experiences a ductile-to-brittle transition as the temperature is
decreased.
❑In such a transition, at higher temperatures the impact energy is
relatively large since the fracture is ductile.
❑As the temperature is lowered, the impact energy drops over a
narrow temperature range as the fracture becomes more brittle.
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DBTT
❖The impact energy needed for fracture
drops suddenly over a relatively narrow
temperature range – temperature of the
ductile-to-brittle transition.
❖For pure materials, the transition may
occur very suddenly at a particular
temperature.
❖For many materials the transition
occurs over a range of temperatures.

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Effect on Crystal structure
❑The ductile-brittle transition is
exhibited in bcc metals, such as low
carbon steel, which become brittle at
low temperature.
❑ This is due to limited active slip
systems operating at low
temperature and low ductility.
❑ Increasing temperature allows more
slip systems to operate, resulting in
more plastic deformation.
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Effect of Crystal structure
❑The yield stress in bcc crystal is markedly temperature
dependent.
❑However, the crack propagation stress is relatively
independent of temperature.
❑Thus, the mode of failure changes from plastic flow at high
temperature to brittle fracture at low temperature.
❑ FCC and HCP metals do not experience ductile to brittle
transition; therefore, they give the same energy absorption at
any temperatures.

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Effect of Crystal structure
❑The yield stress in bcc crystal is markedly temperature
dependent.
❑However, the crack propagation stress is relatively
independent of temperature.
❑Thus, the mode of failure changes from plastic flow at high
temperature to brittle fracture at low temperature.
❑ FCC and HCP metals do not experience ductile to brittle
transition; therefore, they give the same energy absorption at
any temperatures.

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Effect of composition
❑Increasing the carbon content,
while increasing the strength of
steels, also raises the DBTT.
❑Higher DBTT means the steel
becomes ductile only at higher
temperatures.
❑Increasing carbon content also
lowers the ductility.

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Effect of grain size
❑Refining the grain size not only
strengthens (Hall-Petch
equation) but also toughens steel.
❑Reducing grain size shifts the
DBTT curve to the left .
❑Therefore, grain size refinement
enables the material to perform
reliably over a wider range of
service temperatures.

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Fatigue Failure
Fatigue failure: Introduction
Definition:
Fatigue failure occurs due to the prolonged
application of dynamic or fluctuating stresses,
which are significantly lower than the material's
tensile or yield strength under static loading.
Common Applications Affected:
Bridges, aircraft structures, rotating
machinery, and mechanical components.
Prevalence:
Accounts for approximately 90% of all
material failures in engineering applications.
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Fatigue failure: Introduction
Mechanism:
Initiated by repeated cycles of stress or strain, leading to progressive
damage over time.
Nature of Failure:
Fatigue failure is sudden and catastrophic,
often occurring without visible warning signs. It mimics brittle fracture
behavior, even in materials that are typically ductile.
Fracture Characteristics:
Initiates from surface cracks and propagates inward. The fracture
surface is usually perpendicular to the direction of the applied stress.

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Fatigue failure: Introduction
Mechanism:
Initiated by repeated cycles of stress or strain, leading to progressive
damage over time.
Nature of Failure:
Fatigue failure is sudden and catastrophic,
often occurring without visible warning signs. It mimics brittle fracture
behavior, even in materials that are typically ductile.
Fracture Characteristics:
Initiates from surface cracks and propagates inward. The fracture
surface is usually perpendicular to the direction of the applied stress.

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Alternating stress
𝝈𝒎𝒂𝒙 +𝝈𝒎𝒊𝒏
Mean stress, 𝝈𝒎 =
𝟐

Reversed stress

The range of stress, 𝝈𝒓 = 𝝈𝒎𝒂𝒙 − 𝝈𝒎𝒊𝒏

𝝈𝒓 𝝈𝒎𝒂𝒙 −𝝈𝒎𝒊𝒏
Repeated stress Stress amplitude, 𝝈𝒂 = =
𝟐 𝟐

𝝈𝒎𝒊𝒏
The stress ratio 𝑹, =
𝝈𝒎𝒂𝒙

Random stress 34
Rotating bending test

During rotation, the lower surface of the specimen is subjected to a

tensile stress, whereas the upper surface experiences compression.
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Rotating bending test

https://www.youtube.com/watch?v=LBKQISx4mmI

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The S-N curve

The higher the magnitude of the stress, the smaller the number
of cycles the material is capable of sustaining before failure 37
The fatigue limit
❑For some ferrous (iron base) and
titanium alloys, the S–N curve
becomes horizontal at higher N
values.
❑There is a limiting stress level,
called the fatigue limit (also
sometimes the endurance limit),
below which fatigue failure will
not occur
❑For steels, Sfat  35-60% of TS
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The fatigue limit
❑Most nonferrous alloys (e.g.,
aluminum, copper,
magnesium) do not show any
fatigue limit.
❑The S–N curve continues its
downward trend at
increasingly greater N values.
❑Thus, fatigue will ultimately
occur regardless of the
magnitude of the stress.
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Fracture surface

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Stages of fatigue
1. Crack initiation:
A tiny crack initiates or nucleates often at a
time well after loading begins at some point of
high stress concentrations.
Normally, nucleation sites are located at or near
the surface, where the stress is at a
maximum, and include surface defects such
as scratches or pits, sharp corners due to
poor design or manufacture, inclusions, grain
boundaries, or dislocation concentrations.

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Stages of fatigue
2.Crack propagation:
Crack advances with each stress cycle.
Stage I – initially slow, involving few grains
Stage II – faster propagation perpendicular to
the applied stress by repetitive blunting and
sharpening of process of crack tip.
3.Final failure:
Occurs very rapidly once the advancing crack
has reached a critical value when the remaining
cross-section of the material is too small to
support the applied load.
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Improving fatigue life
Mean Stress
❑The dependence of
fatigue life on stress
amplitude is represented
on the S–N plot.
❑Mean stress, however, also
affects fatigue life.
❑Increasing the mean
stress level leads to a
decrease in fatigue life.

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Surface effects
❑Maximum stress often occurs at the surface under common
loading conditions.
❑Fatigue cracks typically initiate at surface stress
concentrators.
❑Fatigue life is highly sensitive to surface condition and
geometry.
❑Improving surface quality enhances fatigue resistance.
❑Key influencing factors: design parameters and surface
treatments.

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Design factor
❑Design features like notches and holes
act as stress concentrators.
❑Any notch or geometrical discontinuity
can act as a stress raiser and fatigue Poor design
crack initiation site
❑Sharper geometries increase fatigue
crack initiation risk.
❑Fatigue resistance improves with
smooth transitions and rounded fillets.
Good design
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Surface treatment: Polishing
❑During machining operations, small
scratches and grooves are
invariably introduced into the
workpiece surface by cutting-tool
action.
❑These surface markings can limit
the fatigue life.
❑It has been observed that improving
the surface finish by polishing
enhances fatigue life significantly.
Machining Marks in bolt

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Surface treatment: Compressive stress
❑Fatigue resistance improves with
residual compressive stresses.
❑These stresses reduce the effect of
external tensile loads.
❑Residual stresses are introduced in
a thin surface layer.
❑Shot peening is a common method
using high-velocity metal particles
to introduce residual compressive
stress.
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Surface treatment: Compressive stress

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Surface treatment: Case hardening
❑A technique by which both surface hardness
and fatigue life are enhanced for steel alloy.
❑This is accomplished by a carburizing or
nitriding process: A carbon- or nitrogen-rich
outer surface layer (or case) is introduced.
❑The case is normally on the order of 1 mm deep
and is harder than the inner core of material.
❑The improvement of fatigue properties results
from increased hardness within the case, as
well as from the desired residual compressive
stresses.
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Creep
Creep failure: Introduction
• Creep is a time-dependent and permanent deformation of
materials when subjected to prolonged constant
load/constant stress at a high temperature (T > 0.4 Tm).
• It is a slow process, where deformation changes with time.
Hence, time dependent.
• Creep is important in applications such as: turbine blades (jet
engines), gas turbines, turbines, steam generators, power
plants (boilers and steam lines) which must operate at high
stresses and high temperatures without any changes in
dimensions.

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Creep failure: Introduction
❑Creep fracture is defined as the fracture which takes place
due to creeping of materials under steady loading.
❑It occurs in metals like iron, copper & nickel at high
temperatures.
❑The tendency of creep fracture increases with the increase
in temperature and higher rate of straining.
❑The creep fracture takes place due to shearing of grain
boundary at moderate stresses and temperatures and
movement of dislocation from one slip to another at higher
stresses and temperatures.
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GB sliding
❑The movement of whole grains
relation of each other causes cracks
along the grain boundaries, which
act as point of high stress
concentration.
❑When one crack becomes larger it
spreads slowly across the member
until fracture takes place.
❑This type of fracture usually occurs
when small stresses are applied
for a longer period.
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Creep curve
Instantaneous deformation
Mainly elastic.
Primary creep
Decreasing creep strain rate with time due to work-
hardening.
Secondary (steady-state) creep
Rate of straining is constant: balance of strain
hardening and recovery.
Tertiary creep
rapidly accelerating strain rate up to failure due to
micro-structural changes (formation of internal
cracks, voids, cavities, grain boundary separation,
necking, etc.)
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Effect of stress and temperature
With either increasing stress
or temperature, the result is:
I. The instantaneous strain
at the time of stress
application increases
II. The steady-state creep
rate increases
III. The rupture lifetime
decreases.

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Enhancing creep resistance
❑Selecting high melting point
materials. Since the general
requirement for creep is T > 0.4 Tm.
❑Selecting materials with high
Young’s modulus.
❑Selecting coarse-grained structure,
as it reduces grain boundary sliding.
Smaller grains permit more grain
boundary sliding resulting in higher
creep rates.
❑Designing super-alloys. Typical microstructure of single-
crystal nickel-based superalloy
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