Short Answer:

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

In simple words, creep is a time-dependent deformation that happens when a material is exposed to both steady stress and elevated temperature for an extended period. This phenomenon is important in components such as turbine blades, boilers, and steam pipes, which operate under high temperature and stress for long durations.

Detailed Explanation:

Creep

Creep is defined as the progressive and permanent deformation of a material under a constant load and constant temperature over time. It is a time-dependent plastic deformation that occurs mainly in metals and alloys when operating at temperatures typically above 0.4 times their melting point (in Kelvin).

At normal room temperatures, most metals exhibit elastic and plastic behavior — the deformation stops when the load is removed. However, at high temperatures, the atoms in the material gain sufficient energy to move slowly over time, even under constant stress. As a result, the material continues to deform gradually — this is known as creep deformation.

Creep is especially significant in mechanical components that operate for long periods under both high stress and temperature, such as turbine blades, jet engines, pressure vessels, steam boilers, and power plant pipelines.

Stages of Creep

The process of creep deformation can be divided into three distinct stages, as shown in a typical creep-time curve:

1. Primary (Transient) Creep:
   • This is the initial stage that begins when the material is first loaded.
   • The creep rate is high at the beginning but decreases with time as the material undergoes strain hardening.
   • During this stage, the internal structure adjusts itself to the applied load.
2. Secondary (Steady-State) Creep:
   • After the primary stage, the material reaches a stable condition where the creep rate becomes nearly constant.
   • This is the longest stage of the creep process and determines the useful life of the component.
   • The steady-state rate depends on stress, temperature, and material properties.
3. Tertiary Creep:
   • In this stage, the creep rate increases rapidly until failure occurs.
   • It is caused by internal damage, such as cracks, voids, or necking.
   • The cross-sectional area reduces, and the material eventually ruptures.

The overall creep curve (strain vs. time) shows an initial deceleration, a steady region, and a final acceleration leading to failure.

Factors Affecting Creep

Several factors influence the rate and extent of creep in a material:

1. Applied Stress:
   Higher stress increases the creep rate and reduces the time to failure.
2. Temperature:
   Creep becomes more severe at high temperatures (generally above 0.4–0.5 of the material’s melting point in Kelvin).
3. Material Properties:
   Materials with a higher melting point (like nickel alloys, tungsten, and molybdenum) have better creep resistance.
4. Grain Size:
   Larger grain size materials resist creep better because grain boundaries act as paths for atomic movement.
5. Time Duration:
   The longer the time under load, the greater the total creep deformation.
6. Environment:
   Corrosive or oxidizing environments can accelerate creep failure.

Creep Mechanisms

Creep occurs due to atomic diffusion and dislocation motion within the material. Depending on temperature and stress, different mechanisms dominate:

1. Dislocation Creep:
   • Occurs at moderate to high stresses and temperatures.
   • Dislocations move slowly through the lattice structure, causing deformation.
2. Diffusion Creep:
   • Dominant at lower stresses and higher temperatures.
   • Atoms diffuse through the lattice (Nabarro-Herring creep) or along grain boundaries (Coble creep), leading to elongation.
3. Grain Boundary Sliding:
   • At very high temperatures, grains slide past each other, contributing to deformation and eventual rupture.

Creep Equation

The steady-state creep rate is often expressed by an empirical relationship known as Norton’s Law:

Where:
= steady-state creep rate,
= material constant,
= applied stress,
= stress exponent,
= activation energy for creep,
= gas constant,
= absolute temperature.

This equation shows that the creep rate increases with stress and temperature.

Creep Testing

Creep tests are performed to study material behavior under constant load and temperature over time.

Procedure:

• A specimen is subjected to a constant tensile load.
• The temperature is kept constant (usually high).
• The elongation is measured with time.

Results:
A creep curve is plotted between strain and time to determine:

• Primary, secondary, and tertiary creep regions,
• Creep rate, and
• Rupture time.

These tests help engineers select materials that can withstand specific stress and temperature conditions without failure.

Applications of Creep Concept

1. Power Plants:
   Design of steam turbine blades, boilers, and pipes that operate continuously at high temperatures.
2. Aerospace Engineering:
   Jet engine and rocket components experience very high operating temperatures, making creep a critical factor.
3. Automobile Industry:
   Used in exhaust valves and manifolds that operate under heat and pressure.
4. Nuclear Reactors:
   Fuel rods and reactor pressure vessels must resist creep for safe operation.
5. Metal Forming Processes:
   Understanding creep helps in hot-working processes like extrusion and forging.

Creep-Resistant Materials

Materials with high melting points and strong atomic bonding are preferred where creep resistance is required. Examples include:

• Nickel-based superalloys
• Stainless steels
• Tungsten and molybdenum alloys
• Titanium alloys
• Ceramic materials

These materials maintain their strength and shape even under long-term exposure to heat and stress.

Conclusion

Creep is a time-dependent and temperature-dependent deformation of materials under constant stress. It becomes significant at high temperatures and long service durations. The process passes through three stages — primary, secondary, and tertiary — and eventually leads to rupture. Creep is an essential consideration in designing components that operate under high temperature and pressure, such as turbines, boilers, and engines. By selecting creep-resistant materials and using proper safety factors, engineers can ensure long-lasting and safe operation of mechanical systems.