3. INTRODUCTION
What is creep?
Creep in ceramics refers to the time-dependent
deformation they experience under constant stress at
elevated temperatures. ceramics are particularly
susceptible to creep at high temperatures, typically
exceeding 0.4 to 0.5 times their melting point. At these
elevated temperatures, the internal atomic structure of
the ceramic becomes more susceptible to
rearrangement under stress.
What is high temperature creep?
High-temperature creep is a specific type of creep that
occurs when a solid material is subjected to both
elevated temperatures and constant stress. Although
high temperature is crucial, the stress needs to be
below the yield strength for creep to occur. Although
high temperature is crucial, the stress needs to be
below the yield strength for creep to occur.
• CREEP
• HIGH TEMPERATURE CREEP
• DIFFERENCE BETWEEN
CREEP AND HIGH
TEMPERATURE CREEP
5. CREEP TEST
A creep test is a fundamental method used to assess a
material's behavior under sustained stress at elevated
temperatures. It provides valuable insights into a
material's creep resistance and helps engineers predict
its long-term performance in real-world applications.
Test Setup:
• Specimen: A carefully prepared sample of the material
to be tested is used. The specimen geometry is
standardized based on the material and testing
standards.
• Temperature Control: The test chamber is precisely
controlled to maintain a constant, desired high
temperature throughout the test duration.
• Constant Stress: A constant load is applied to the
specimen throughout the test. This load is typically
chosen to be a specific percentage of the material's
yield strength at the test temperature.
• Strain Measurement: The deformation (strain) of the
specimen is meticulously measured over time using
extensometers or other displacement transducers
6. Test Stages:
The creep test data typically exhibits three distinct
stages:
1.Primary Creep: In this initial stage, the creep rate
is relatively high but gradually decreases over time.
The material undergoes some initial adjustments
and microstructural changes to accommodate the
applied stress.
2.Secondary Creep: This is the longest and often the
most important stage. The creep rate reaches a
relatively constant value. The mechanisms like
vacancy diffusion and dislocation movement
become dominant, causing the material to elongate
at a steady rate.
3.Tertiary Creep: In this final stage, the creep rate
accelerates rapidly. Microstructural damage
accumulates within the material, and micro voids or
cavities can form. This can ultimately lead to failure
of the specimen.
CREEP TEST
8. MECHANISM
Thermal Activation:
At high temperatures, atoms vibrate with much greater
intensity, providing them with more thermal energy. This
allows them to overcome these energy barriers and move
around more readily within the crystal lattice. This increased
atomic mobility is crucial for creep to occur at a significant
rate.
Vacancy Diffusion:
• As temperature rises, the intense atomic vibrations create
more vacancies (empty spaces) within the crystal lattice.
These vacancies act like steppingstones for surrounding
atoms to move. Under the influence of stress, these
vacancies preferentially move in the direction opposite the
applied stress.
9. Dislocation Movement:
• Dislocations are imperfections in the atomic
arrangement that act like weak spots within the
material. High temperatures further enhance the
mobility of these dislocations. Under stress,
dislocations can glide or climb, allowing entire
planes of atoms to slide past each other. This
contributes to the overall deformation of the
material.
Grain Boundary Sliding:
• These grain boundaries are regions of higher
energy compared to the crystal interiors.At high
temperatures and stress, the grain boundaries
can slide past each other, accommodating the
deformation. This movement contributes to the
overall creep of the material.
MECHANISM
10. FACTORS
Temperature:
• As the temperature rises Thermal vibrations intensify, allowing
atoms to overcome energy barriers and move more readily
within the crystal lattice. More vacancies (empty spaces) are
created, providing steppingstones for atoms to shuffle around
under stress.
Stress:
• Constant stress is necessary for creep to occur, but the level of
stress also plays a role .Higher stress levels generally lead to
faster creep rates. At high temperatures, even modest stress
levels can cause significant creep compared to lower
temperatures.
Material Properties:
• Grain size: Finer grain sizes generally offer better creep
resistance by hindering dislocation movement.
• Microstructure: Certain microstructural features can
impede vacancy diffusion and dislocation movement.
Time:
• Creep is a time-dependent phenomenon. The longer a material
• TEMPERATURE
• STRESS
• MATERIAL PROPERTIES
• TIME
12. TYPES
High-temperature creep can be categorized into different types based
on the dominant mechanism causing the deformation. Here are some
common classifications:
Diffusion Creep:
• Diffusion creep is typically dominant at very high temperatures
(often exceeding 0.6 times the melting point of the material).
• There are two sub-categories of diffusion creep:
• Nabarro–Herring Creep: This occurs when vacancies diffuse
through the crystal lattice itself.
• Coble Creep: This occurs when vacancies preferentially diffuse
along grain boundaries, which are regions of higher energy
compared to the crystal interiors.
Dislocation Creep:
• Dislocation creep is generally dominant at intermediate
temperatures (between 0.4 and 0.6 times the melting point).
• There are different mechanisms by which dislocations can move
under stress, leading to sub-categories like:
• Power-law creep: This is the most common type of dislocation
creep where the creep rate is proportional to a power of the
applied stress.
DIFFUSION CREEP
• Nabarro-Herring Creep
• Coble Creep
DISLOCATION CREEP
• Power-law Creep
GRAIN BOUNDARY SLIDING
CREEP
13. TYPES
Diffusion creep is a specific type of high-temperature
creep where the diffusion of vacancies plays the dominant
role in causing the material to deform. Here's a breakdown
of the key aspects:
The Role of Vacancies:
• Imagine a material like a lattice of marbles, with some
empty spaces (vacancies) scattered throughout. At high
temperatures, atoms vibrate intensely, creating more
vacancies.
Mechanisms of Diffusion Creep:
There are two main mechanisms of diffusion creep, both
relying on vacancy movement:
• Nabarro–Herring Creep: In this mechanism, vacancies
diffuse through the crystal lattice itself. Imagine the gaps
(vacancies) between the marbles moving throughout the
lattice.
• Coble Creep: Here, vacancies preferentially diffuse along
grain boundaries. Grain boundaries are the regions
DIFFUSION CREEP
17. Examples of Applications:
• Turbine blades in jet engines and gas turbines: These
blades operate at very high temperatures and experience
significant centrifugal forces. Understanding and mitigating
creep is crucial for ensuring their structural integrity and
preventing catastrophic failures.
• Boiler tubes in power plants: These tubes carry hot
pressurized fluids and are susceptible to creep over time.
Selecting creep-resistant materials and managing operating
conditions are essential for safe and efficient power
generation.
• Heat exchangers: These components transfer heat
between fluids, often at elevated temperatures.
Understanding creep helps ensure the exchanger's
structural integrity and maintains efficient heat transfer over
its lifespan.
• Welding processes: Welding involves localized heating
and introduces residual stresses in the material.
Understanding creep behavior helps predict potential issues
like weld cracking or distortion during high-temperature
service.
APPLICATIONS
20. SUMMARY
The Phenomenon:
• High-temperature creep is the gradual deformation of a
solid material under constant stress at elevated
temperatures. It's a time-dependent process, meaning the
longer the material is under stress and heat, the more it
creeps.
The Mechanism:
• At high temperatures, atoms vibrate more intensely,
creating vacancies (empty spaces) within the crystal lattice.
These vacancies and imperfections like dislocations (weak
spots) can move under stress, causing the material to
slowly deform in the direction of the stress.
Types of High-Temperature Creep:
• Diffusion Creep: Vacancies act as steppingstones for
atoms to move, causing deformation. Dominant at very high
temperatures.
• Dislocation Creep: Dislocations move more easily under
stress and high temperature, leading to material
rearrangement. Dominant at intermediate temperatures.
21. Factors Affecting Creep Rate:
• Temperature: The single most significant factor. Higher
temperatures exponentially increase creep rate.
• Stress: Higher stress generally leads to faster creep rates.
• Material Properties: Grain size, microstructure, and
composition influence creep resistance. Finer grains and
specific alloys can offer better resistance.
• Time: The longer the material is under stress at high
temperatures, the more it creeps.
Applications:
• Understanding material performance and predicting
lifespan for components in power plants, jet engines, and
other high-temperature applications.
• Selecting materials and designing components to minimize
creep and ensure safe operation.
SUMMARY
