What is creep?

Short Answer:

Creep is the slow and continuous deformation of a material when it is subjected to a constant load or stress over a long period of time, especially at high temperature. It occurs even when the applied stress is below the material’s yield strength.

In simple words, creep means the gradual elongation or deformation of a material when it is kept under a steady load for a long duration. This property is very important for materials used in boilers, turbines, and engines where they are exposed to high temperature and stress for extended periods.

Detailed Explanation:

Creep

Definition and Meaning:
Creep is a time-dependent deformation that occurs when a material is exposed to constant stress and elevated temperature for a long time. Unlike instantaneous deformation that happens immediately after loading, creep develops slowly with time.

It is a significant property in materials used in high-temperature environments, such as gas turbines, power plants, jet engines, and boilers. Creep can lead to dimensional changes, distortion, or failure in components if not properly controlled or designed for.

The temperature at which creep becomes noticeable varies for different materials. Generally, for metals, creep becomes significant at temperatures above 0.4 times their melting temperature (in Kelvin).

Stages of Creep

Creep deformation occurs in three main stages over time, which can be shown by a creep curve (strain vs. time):

  1. Primary (Transient) Creep:
  • This is the initial stage of creep when the rate of deformation is high but gradually decreases with time.
  • During this stage, strain hardening occurs because the material’s internal structure resists further deformation.
  • It represents the period of adjustment where the material begins to respond to the applied stress.
  1. Secondary (Steady-State) Creep:
  • This is the most important stage and is characterized by a constant creep rate.
  • The rate of deformation becomes steady due to a balance between strain hardening and recovery (softening) processes.
  • This stage determines the useful life of the material under high-temperature service conditions.
  • The steady-state creep rate is often used to evaluate the material’s resistance to creep.
  1. Tertiary (Accelerating) Creep:
  • In this final stage, the creep rate increases rapidly, leading to necking and fracture.
  • The material’s internal structure gets damaged due to grain boundary sliding, cavity formation, and void growth.
  • Eventually, the material fails under the constant load.

The overall creep curve shows a slow, time-dependent strain growth that becomes faster as the material approaches failure.

Mechanism of Creep

Creep deformation occurs through several mechanisms depending on the temperature and applied stress. These include:

  1. Dislocation Movement:
    At moderate temperatures, creep occurs due to the movement of dislocations through the crystal lattice. The dislocations overcome barriers by diffusion or thermal activation.
  2. Diffusion Creep:
    At high temperatures, atoms move from regions of high stress to regions of low stress through diffusion. This leads to a change in shape without significant dislocation movement.
  3. Grain Boundary Sliding:
    The grains of a polycrystalline material can slide past one another, causing creep deformation, especially at high temperatures.
  4. Void Formation and Coalescence:
    In the final stage, voids form at grain boundaries, which join together and lead to fracture.

Thus, creep is a complex process influenced by temperature, stress, time, and microstructure of the material.

Factors Affecting Creep

  1. Temperature:
    • Creep rate increases significantly with temperature.
    • At high temperatures, atoms move more easily, causing faster deformation.
  2. Applied Stress:
    • Higher applied stress increases the creep rate and reduces the life of the material.
    • Creep occurs even below the yield stress if the stress is maintained for a long period.
  3. Material Structure:
    • Fine-grained materials resist creep better at low temperatures.
    • Coarse-grained or single-crystal materials perform better at high temperatures.
  4. Time Duration:
    • Longer exposure to stress increases total creep strain.
  5. Environment:
    • Oxidation or corrosion at high temperatures can accelerate creep damage.
  6. Alloying and Heat Treatment:
    • Adding alloying elements like chromium, molybdenum, or vanadium improves creep resistance by strengthening the material at high temperatures.

Measurement of Creep

Creep is measured using a creep test, in which a specimen is subjected to a constant load and constant temperature over a long time. The strain (deformation) is measured at regular intervals.

The creep curve obtained from the test helps determine:

  • The primary, secondary, and tertiary stages.
  • The steady-state creep rate, which indicates the material’s creep resistance.
  • The rupture life, which is the time taken for the specimen to fail under the given conditions.

Such data are used to design materials and predict their long-term behavior in service.

Creep Resistant Materials

Materials that resist creep even at high temperatures are called creep-resistant materials. Examples include:

  • Stainless steels (e.g., 304, 316 grades)
  • Nickel-based alloys (e.g., Inconel, Nimonic)
  • Cobalt-based alloys
  • Chromium-molybdenum steels
  • Tungsten and molybdenum for very high-temperature applications

These materials are used in components like turbine blades, boilers, engine valves, and pressure vessels.

Effects of Creep

  1. Dimensional Changes:
    Components gradually deform, affecting clearances and tolerances.
  2. Loss of Strength:
    Continuous strain can weaken the material and reduce its load-carrying capacity.
  3. Crack Formation:
    Voids and cracks may develop at grain boundaries, leading to premature failure.
  4. Reduced Life of Components:
    Long-term exposure at high temperature decreases service life.
  5. Failure of Critical Parts:
    In engines, turbines, and boilers, creep failure can cause dangerous breakdowns or accidents.

Prevention and Control of Creep

  1. Material Selection:
    Use creep-resistant alloys or heat-treated materials.
  2. Temperature Control:
    Keep operating temperatures below the creep limit of the material.
  3. Stress Reduction:
    Reduce working stress levels by optimizing design or increasing cross-sectional area.
  4. Protective Coatings:
    Apply coatings to reduce oxidation and corrosion at high temperatures.
  5. Regular Inspection:
    Periodic monitoring and replacement of components before creep failure occurs.

Example of Creep in Real Life

  • Turbine Blades: Constant exposure to high temperature and stress leads to gradual elongation.
  • Steam Pipes: Over years of service, pipes may sag or distort under their own weight.
  • Lead Wires: Show creep even at room temperature due to the softness of lead.
  • Concrete Structures: Exhibit creep under sustained load, especially in bridges and dams.
Conclusion:

Creep is the slow, time-dependent deformation of a material under constant stress, especially at elevated temperatures. It progresses through primary, secondary, and tertiary stages, eventually leading to fracture. Creep is influenced by temperature, stress, material properties, and duration of loading. To prevent failure, engineers must choose creep-resistant materials, control operating conditions, and monitor high-temperature components regularly. Understanding creep behavior is crucial for designing safe and reliable structures in power plants, turbines, engines, and other high-temperature systems.