Short Answer:
The stages of creep represent the different phases of deformation that a material undergoes when subjected to a constant load and high temperature over time. Creep occurs gradually and can be divided into three main stages — Primary creep, Secondary creep, and Tertiary creep.
In the primary stage, the rate of deformation decreases with time; in the secondary stage, the rate becomes steady; and in the tertiary stage, the deformation rate increases rapidly until failure occurs. These stages help engineers understand the long-term performance and safety of materials used in high-temperature applications such as turbines, boilers, and engines.
Detailed Explanation:
Stages of Creep
When a material is subjected to a constant load or stress and high temperature for a long period, it undergoes time-dependent deformation known as creep. The rate of this deformation is not constant but changes with time, giving rise to three distinct stages known as the stages of creep. These stages can be clearly represented on a creep curve, which is a graph between strain (deformation) on the vertical axis and time on the horizontal axis.
The three stages of creep are:
- Primary or Transient Creep
- Secondary or Steady-State Creep
- Tertiary or Accelerating Creep
Each stage has its own characteristics and behavior, which depend on the material, temperature, stress level, and time duration.
- Primary or Transient Creep
The primary stage of creep occurs immediately after the material is subjected to a constant load and temperature.
- In this stage, the creep rate is initially high but decreases gradually with time.
- This happens because the material undergoes strain hardening, which increases its resistance to further deformation.
- During this period, the internal structure of the material (mainly the dislocations and grains) adjusts itself to the applied load.
Key features of primary creep:
- Rapid initial deformation.
- Gradual decrease in creep rate.
- Dominated by elastic and plastic strain adjustment.
- Short duration compared to the total life of the component.
Mathematically, this stage is described by a decreasing creep rate, where the slope of the strain-time curve reduces continuously.
Primary creep is important because it defines how quickly the material adjusts to the working conditions before reaching a stable condition.
- Secondary or Steady-State Creep
After the primary stage, the material enters the secondary stage, which is also called the steady-state creep stage.
- In this stage, the rate of deformation becomes almost constant with time.
- This happens when the effects of strain hardening and recovery (softening) processes in the material balance each other.
- As a result, the material deforms steadily without any sudden changes in strain rate.
Key features of secondary creep:
- Constant creep rate (steady-state condition).
- Longest duration among all stages.
- Governs the useful life of a component in service.
- Controlled by diffusion and dislocation motion within the material.
The steady-state creep rate, often represented as , is used for design and material selection because it directly affects the lifetime of high-temperature components. The relationship is often expressed using Norton’s law:
Where,
= material constant,
= applied stress,
= stress exponent,
= activation energy for creep,
= gas constant,
= absolute temperature.
The secondary stage is the most stable and predictable part of creep, and engineers use it for calculating safe operating limits for components like turbine blades, boilers, and pressure vessels.
- Tertiary or Accelerating Creep
The tertiary stage is the final stage of creep, occurring just before the failure of the material.
- In this stage, the creep rate increases rapidly with time.
- The reason for this acceleration is the progressive internal damage in the material, such as the formation of voids, cracks, grain boundary separation, and necking.
- The effective cross-sectional area of the material decreases, which causes the stress to rise further, leading to rupture or fracture.
Key features of tertiary creep:
- Rapid increase in creep rate.
- Short duration compared to secondary stage.
- Visible damage such as cracks, voids, or bulging.
- Ends with failure (fracture or rupture).
In this stage, grain boundary sliding and intergranular cracking dominate, particularly in metals and alloys exposed to high temperatures for long periods. The end of tertiary creep marks the rupture point of the material, also known as creep rupture.
Creep Curve Representation
A typical creep curve (strain vs. time) includes the three stages:
- The initial concave curve represents the primary stage (decreasing rate).
- The linear portion with constant slope represents the secondary stage (steady rate).
- The final convex upward curve represents the tertiary stage (increasing rate leading to failure).
The total strain during creep is the sum of strains from all three stages.
Factors Influencing the Stages of Creep
- Temperature:
- Higher temperatures make creep start earlier and proceed faster, reducing the time spent in the secondary stage.
- Stress Level:
- High applied stress accelerates the transition from primary to tertiary creep.
- Material Structure:
- Fine-grained materials resist creep initially but may fail faster at high temperatures.
- Time Duration:
- Long service times allow more deformation accumulation.
- Environment:
- Oxidizing or corrosive environments weaken materials and shorten the secondary stage.
Significance of the Stages of Creep
- Understanding the stages of creep helps engineers predict the life span of a component.
- It allows selection of materials with better creep resistance for high-temperature service.
- Helps in setting maintenance schedules before failure occurs.
- Plays an important role in the design of steam turbines, jet engines, pressure vessels, boilers, and nuclear reactors.
Conclusion
The stages of creep describe how a material deforms over time under constant stress and temperature. They include:
- Primary Creep – Decreasing creep rate due to strain hardening.
- Secondary Creep – Constant creep rate; most significant and stable phase.
- Tertiary Creep – Increasing creep rate leading to failure.
Among these, the secondary stage determines the service life of components. Understanding each stage helps engineers design safer and more reliable systems that can withstand long-term exposure to heat and stress.