3. INTRODUCTION
When a metal or alloy is under a constant load
or stress, it may undergo progressive plastic
deformation over a period of time, even though
applied stress is less than the yield strength at that
temperature . This time dependent strain is called
CREEP.
It can occur as a result of long-term exposure to high levels of
stress that are still below the yield strength of the material.
Creep is more severe in materials that are subjected to heat
for long periods, and generally increases as they near their
melting point.
Depending on the magnitude of the applied stress and its
duration, the deformation may become so large that a
component can no longer perform its function
4.
For example creep of a turbine blade will cause the blade
to contact the casing, resulting in the failure of the blade.
Creep is usually of concern to engineers and
metallurgists when evaluating components that operate
under high stresses or high temperatures.
Creep is a deformation mechanism that may or may not
constitute a failure mode.
For example, moderate creep in concrete is sometimes
welcomed because it relieves tensile stresses that might
otherwise lead to cracking.
Unlike brittle fracture, creep deformation does not occur
suddenly upon the application of stress.
Instead, strain accumulates as a result of long-term
stress.
Therefore, creep is a "time-dependent" deformation
5. EFFECT OF TEMPERATURE
The temperature range in which creep deformation
may occur differs in various materials.
For example, tungsten requires a temperature in
the thousands of degrees before creep
deformation can occur, while ice will creep at
temperatures near 0 °C (32 °F).
As a general guideline, the effects of creep
deformation generally become noticeable at
approximately 35% of the melting point (as
measured on a thermodynamic temperature scale
such as Kelvin or Rankine) for metals, and at 45%
of melting point for ceramics.
Virtually any material will creep upon approaching
its melting temperature.
Since the creep minimum temperature is related to
the melting point, creep can be seen at relatively
low temperatures for some materials.
Plastics and low-melting-temperature metals,
including many solders, can begin to creep at
6. CREEP TESTING AND ANALYSIS.
A creep-testing machine measures the
creep (the tendency of a material after
being subjected to high levels of stress,
e.g. High temperatures, to change its form
in relation to time) of an object.
It is a device that measures the alteration
of a material after it has been put through
different forms of stress.
Creep machines are important to see how
much strain (load) an object can handle
under pressure, so engineers and
researchers are able to determine what
materials to use.
The device generates a creep time-
dependent curve by calculating the steady
rate of creep in reference to the time it
takes for the material to change.
7.
The specimen to be tested is placed in the electric
furnace where it is heated to a given temperature.
The usual method of creep testing consists of
subjecting at constant tensile stress at constant
temperature and measuring the extent of
deformation or strain with the time.
Even though ,it appear to be simple, it requires
considerable laboratory equipment, great care and
precision in performance.
The data is presented by plotting creep curve as
deformation(or strain) verses time at constant
temperature and stress.
Period of testing does not exceed 10,000 hours.
If creep is continued until fracture occurs, the test
is called as CREEP RUPTURE test .
8.
9.
10. Creep occurs in three stages:
•
•
•
Primary, or Stage I.
Secondary, or Stage II and
Tertiary, or Stage III.
11.
Stage I, or Primary creep occurs at the beginning of the
tests, and creep is mostly transiently, not at a steady rate.
Resistance to creep increases until Stage II is reached.
In Stage II, or Secondary creep, The rate of creep becomes
roughly steady. This stage is often referred to as steady
state creep.
In Stage III, or tertiary creep, the creep rate begins to
accelerate as the cross sectional area of the specimen
decreases due to necking or internal voiding decreases the
effective area of the specimen.
If stage IIIis allowed to proceed, fracture will occur.
The creep test is usually employed to determine the
minimum creep rate in Stage II.
Engineers need to account for this expected deformation
when designing systems.
12. MECHANISM OF
CREEP
Some mechanisms that play
vital roles during the creep
process are:
a) Dislocation climb
b) Vacancy Diffusion
c) Grain boundary sliding
13.
14. DISLOCATION CLIMB:-
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.
15. VACANCY DIFFUSION:-
Another mechanism of creep is
called diffusion of vacancies.
The diffusion of vacancies
control creep rate.
In the mechanism grain
boundary acts as a source and
sinks for vacancies.
The mechanism depends on the
migration of vacancies from one
side of a grain to another.
16. GRAIN BOUNDARY SLIDING:-
he grain
The third mechanism of creep
is sliding of grain boundaries.
The sliding of neighboring grains
with respect to the boundary that
separates them.
Figure shows that ,grain
boundaries lose their strength at
lower temperature than the grain
themselves.
This affect arises from non-
crystalline structure of t
boundaries.
17. TYPES OF CREEP
The creep may be classified depending on the
temperature as:
(iT)he logarithmic creep
(iTi)he recovery creep
(iTii)he diffusion creep
At low temperature the creep rate usually decreases with time
and logarithmic creep curve is obtained.
At high temperature in the range of 0.5 to 0.7 of tmk(melting
point temperature on the absolute scale), 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.
Beyond 0.7 tmk, i.e. , At very high temperature, the creep is
primarily influenced by diffusion and load(stress) applied has
little effect. This creep is termed as diffusion creep or
plastic creep.
18. CREEP CURVE
ENGINEERING CURVE IS DETERMINED BY
APPLYING A CONSTANT LOAD AT A CONSTANT
TEMPERATURE , AND STRAIN OF THE
SPECIMEN IS DETERMINED AS A FUNCTION OF
TIME.
SLOPE OF CREEP CURVE IS REFERRED TO AS
CREEP RATE.
FOLLOWING RAPID ELONGATION OF
SPECIMEN, CREEP RATE DECREASES WITH
TIME,THEN REACHES STEADY STAE AND
FINALLY THE CREEP RATE INCREASES RAPIDLY
WITH TIME UNTIL FRACTURE OCCURS
19. For plotting creep curve of a
metal , a constant load is
applied to a tensile specimen
maintained at a constant
temperature and the
extension of the specimen is
determined as a function of
time.
21.
Creep Curve exhibits three distinct
ranges.
Stage 1 (Primary range ) of the creep
curve, the strain rate ( the slope of
the creep curve) decreases until it
reaches some minimum rate.
Stage II ( Secondary range) , this
minimum rate is maintained , more or
less , until a time at which the strain
rate begins to increase .
Stage III ( Tertiary range) , the strain
rate continues to increase under the
sustained stress and temperature
until at time t = tR , the specimen is
22. CREEP FRACTURE:
At low temperature grain
boundaries are stronger and at
high it becomes weak as compared
to grains.
The temperature at which the
strength of boundaries are equal to
the strength of grain is called Equi-
cohesive Strength .
Material always fails at weak region
by initiating crack.
23. When crack occurred below
equi-cohesive temperature ,
fracture is transgranular or
transcrystalline i.e. the crack
moves through the grains .
When crack occurred above
equi-cohesive temperature,
fracture is intergranular or
intercrystalline and it moves
along the grain boundaries .
24.
25.
Creep failures are characterized by:
Bulging or blisters in the tube
Thick-edged fractures often with very
little obvious ductility
Longitudinal "stress cracks" in either
or both ID and OD oxide scales
External or internal oxide-scale
thicknesses that suggest higher-than-
expected temperatures
Intergranular voids and cracks in the
microstructure
26.
CREEP RESISTANCE
This can be increased by restricting the
motion of dislocations, in a metal and by
inhibiting the formation of new
dislocations.
The presence of alloying elements which
do not diffuse rapidly will ‘pin down’
dislocations effectively.
Presence of small dispersed particles of
strong hard, strong constituent will have
a ‘particle hardening ‘ effect by acting as
barriers to the movement of dislocation
front .
27.
CREEP RESISTANT MATERIALS.
Creep resistant materials are required for
structural and machine components used at
elevated temperatures.
They should be capable of withstanding these
temperature without undergoing creep beyond
the specified limit. Following are the
requirements
It should have high melting point because the
creep becomes significant above 0.4 Tm .
It should have course grained structure.
It should be precipitation hardenable.
Dispersion hardening improves creep resistance.
It should have high oxidation resistance.
28. APPLICATIONS :
Though mostly due to the reduced yield strength
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weather and loading conditions and
founded that by 1990, a physical
menon called creep had caused the
bridge to sag 1.2 meters.
29.
The design of tungsten light bulb filaments
attempts to reduce creep deformation.
Sagging of the filament coil between its
supports increases with time due to the
weight of the filament itself.
If too much deformation occurs, the
adjacent turns of the coil touch one another,
causing an electrical short and local
overheating, which quickly leads to failure of
the filament.
The coil geometry and supports are
therefore designed to limit the stresses
caused by the weight of the filament, and a
special tungsten alloy with small amounts of
oxygen trapped in the crystallite
30. In steam turbine power plants, pipes
carry steam at high temperatures
(566 °C (1,051 °F)) and pressures
(above 24.1 MPa or 3500 psi).
In jet engines, temperatures can
reach up to 1,400 °C (2,550 °F) and
initiate creep deformation in even
advanced-design coated turbine
blades.
Hence, it is crucial for correct
functionality to understand the creep
deformation behavior of materials.
31.
32.
Creep deformation is important not only in
systems where high temperatures are
endured such as nuclear power plants, jet
engines and heat exchangers, but also in
the design of many everyday objects.
For example, metal paper clips are stronger
than plastic ones because plastics creep at
room temperatures.
Aging glass windows are often erroneously
used as an example of this phenomenon:
measurable creep would only occur at
temperatures above the glass transition
temperature around 500 °C (932 °F).
33. Preventing creep
There are four general ways to prevent
creep in metal.
One way is to use higher
melting temperature metals.
The second way is to use materials with
greater grain size.
The third way is to use alloying.
Finally, the last way is to use intelligent
design to reduce the possible factors of
creep
