LECTURER 4
Fundamental Mechanical Properties
Fatigue
Creep
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Fatigue
Fatigue is caused by repeated application of stress to
the metal. It is the failure of a material by fracture when
subjected to a cyclic stress.
Fatigue is distinguished by three main features.
i)
Loss of strength
ii)
Loss of ductility
iii)
Increased uncertainty in strength and
service life
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Fatigue
Fatigue is an important form of behaviour in all materials including
metals, plastics, rubber and concrete.
All rotating machine parts are subjected to alternating stresses.
Example: aircraft wings are subjected to repeated loads, oil and gas
pipes are often subjected to static loads but the dynamic effect of
temperature variation will cause fatigue.
There are many other situations where fatigue failure will be very
harmful.
Because of the difficulty of recognizing fatigue conditions, fatigue
failure comprises a large percentage of the failures occurring in
engineering.
To avoid stress concentrations, rough surfaces and tensile residual
stresses, fatigue specimens must be carefully prepared.
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Fatigue
The S-N Curve
A very useful way to visual the failure for a specific material is with
the S-N curve.
The “S-N” means stress verse cycles to failure, which when plotted
using the stress amplitude on the vertical axis and the number of
cycle to failure on the horizontal axis.
An important characteristic to this plot as seen is the “fatigue limit”.
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Fatigue
The point at which the curve flatters out is termed as fatigue limit
and is well below the normal yield stress.
The significance of the fatigue limit is that if the material is loaded
below this stress, then it will not fail, regardless of the number of
times it is loaded.
Materials such as aluminium, copper and magnesium do not show a
fatigue limit; therefore they will fail at any stress and number of
cycles.
Other important terms are fatigue strength and fatigue life.
The fatigue strength can be defined as the stress that produces
failure in a given number of cycles usually 107.
The fatigue life can be defined as the number of cycles required for
a material to fail at a certain stress.
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Factors affecting fatigue properties
Surface finish:
Scratches dents identification marks can act as stress raisers and so
reduce the fatigue properties.
Electro-plating produces tensile residual stresses and have a
deterimental effect on the fatigue properties.
Temperature:
As a consequence of oxidation or corrosion of the metal surface
increasing, increase in temperature can lead to a reduction in fatigue
properties.
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Factors affecting fatigue properties
Residual stresses:
Residual stresses are produced by fabrication and finishing
processes.
Residual stresses on the surface of the material will improve
the fatigue properties.
Heat treatment:
Hardening and heat treatments reduce the surface
compressive stresses; as a result the fatigue properties of the
materials are getting affected.
Stress concentrations:
These are caused by sudden changes in cross section holes or
sharp corners can more easily lead to fatigue failure. Even a
small hole lowers fatigue-limit by 30%.
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Stress Cycles
There are different arrangements of fatigue loading.
The simplest type of load is the alternating stress where the stress
amplitude is equal to the maximum stress and the mean or average stress
is zero. The bending stress in a shaft varies in this way.
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Fatigue Failure
Fatigue fracture results from the presence of fatigue cracks, usually
initiated by cyclic stresses, at surface imperfections such as machine
marking and slip steps.
The initial stress concentration associated with these cracks are too
low to cause brittle fracture they may be sufficient to cause slow
growth of the cracks into the interior.
Eventually the cracks may become sufficiently deep so that the
stress concentration exceeds the fracture strength and sudden
failure occurs.
The extent of the crack propagation process depends upon the
brittleness of the material under test.
In brittle materials the crack grows to a critical size from which it
propagates right through the structures in a fast manner, whereas
with ductile materials the crack keeps growing until the remaining
area cannot support the load and an almost ductile fracture suddenly
occurs.
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Fatigue Failure
Failure can be recognized by the appearance of fracture.
For a typical fracture ,Two distinct zones can be distinguished – a
smooth zone near the fatigue crack itself which, has been
smoothened by the continual rubbing together of the cracked
surfaces, and a rough crystalline-looking zone which is the final
fracture.
Occasionally fatigue cracks show rough concentric rings which
correspond to successive positions of the crack.
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Design for Fatigue
To secure satisfactory fatigue life
 Modification of the design to avoid stress concentration eliminating
sharp recesses and severe stress raisers.
 Precise control of the surface finish by avoiding damage to surface
by rough machining, punching, stamping, shearing etc.
 Control of corrosion and erosion or chemical attack in service and to
prevent of surface decarburization during processing of heat
treatment.
 Surface treatment of the metal.
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Creep
• The creep is defined as the property of a material by virtue of which
it deforms continuously under a steady load.
•
Creep is the slow plastic deformation of materials under the
application of a constant load even for stressed below the yield
strength of the material.
• Usually creep occurs at high temperatures.
• Creep is an important property for designing I.C. engines, jet
engines, boilers and turbines. Iron, nickel, copper and their alloys
exhibited this property at elevated temperature.
• But zin, tin, lead and their alloys shows creep at room temperature.
• In metals creep is a plastic deformation caused by slip occurring
along crystallographic directions in the individual crystals together
with some deformation of the grain boundary materials.
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Creep
The creep curve usually consists of three \ stages of creep.
Primary Stage:
 In this stage the creep rate decreases with time, the effect of work
hardening is more than that of recovery processes. The primary stage is of
great interest to the designer since it forms an early part of the total
extension reached in a given time and may affect clearness provided
between components of a machine.
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Creep
Secondary Stage:
In this stage, the creep rate is a minimum and is constant with
time. The work hardening and recovery processes are exactly
balanced. It is the important property of the curve which is used
to estimate the service life of the alloy.
Tertiary Stage:

In this stage, the creep rate increases with time until fracture
occurs. Tertiary creep can occur due to necking of the specimen
and other processes that ultimately result in failure.

The “Creep Limit” is the stress at which a material can be formed
by a definite magnitude during a given time at a given
temperature. The calculation of creep limit includes the
temperature, the deformation and the time in which this
deformation appears.
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Types of Creep
Creep are classified based on temperature

Logarithmic Creep

Recovery Creep

Diffusion Creep
At low temperature the creep rate decreases with time and the
logarithmic creep curve is obtained.
At high temperature, the influence of work hardening is
weakened and there is a possibility of mechanical recovery. As a
result, the creep rate does not decrease and the recovery creep
curve is obtained.
At very high temperature, the creep is primarily influenced by
diffusion and load applied has little effect. This creep is termed
as diffusion creep or plastic creep.
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Factors affecting Creep
Heat Treatment
• Creep resistance of steel is affected by heat treatment.
• At temperatures of 300°C or higher maximum creep resistance is usually
produced. But the quacking and drawing decreases the creep resistance.
Grain size
• The major factor in creep is grain size.
• Normally large grained materials exhibit better creep resistance than fine
grained one based on the temperature.
• At temperatures below the lowest temperature of recrystallisation, a fine
grained structure possesses the greater resistance whereas at temperature
above this point a large grained structure possesses the greater resistance
and we must select it for high temperature applications.
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Factors affecting Creep
Strain Hardening
 Strain hardening of steel increases its creep resistance.
 Particularly below the equicohesive temperature at which the fracture
changes from intra crystalline to inner-crystalline strain hardening increases
the creep resistance and hence there is no measurable creep. So the
second stage of creep curve is almost horizontal.
 At temperature above the equicohesive temperature yield rate exceeds the
strain hardening rate and creep will proceed even under low stresses.
Alloying additions
 At temperatures, below the lowest temperatures of recrystallation the creep
resistance of steel may be improved by the finite forming elements like
nickel, cobalt and manganese or by the carbide forming elements like
chromium molybdenum, tungsten and vanadium.
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Mechanism of Creep
Some mechanisms that play vital roles during the creep process are:
Dislocation climb
Vacancy Diffusion
Grain boundary sliding
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Mechanism of Creep
At high temperature, the appreciate atomic movement causes the
dislocation to climb up or down.
By a simple climb of edge dislocation the diffusion rate of vacancies
may produce a motion in response to the applied stress.
Thus edge dislocations are piled up by the obstacles in the glide
plane and the rate of creep is governed by the rate of escape of
dislocation.
Another mechanism of creep is called diffusion of vacancies.
In this mechanism, the diffusion of vacancies controls the creep rate
but does not involve the climb of edge dislocations.
It depends on the migration of vacancies from one side of a grain to
another. In response to the applied stress, the vacancies move from
surfaces of the specimen transverse to the stress axis
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Mechanism of Creep
The third mechanism of creep is sliding of grain boundaries.
It means sliding of neighboring grains with respect to the boundary
that separates them.
Grain boundaries become soft at low temperature as compared to
individual grains.
Grain boundaries play a major role in the creep of polycrystals at
high temperatures as they side past each other or create vacancies.
At high temperature, ductile metals begin to lose their ability to strain
– harden and become viscous to facilitate the sliding of grain
boundaries.
As the temperature increases the grain boundaries facilitate the
deformation process by sliding, whereas at low temperature, they
increase the yield strength by stopping the dislocations.
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