What is the difference between high-cycle and low-cycle fatigue?

Short Answer:

High-cycle fatigue and low-cycle fatigue are two types of fatigue failure based on how many times a material is loaded before it breaks. High-cycle fatigue (HCF) happens when the material faces low stress for a large number of cycles—usually more than 10,000 to millions of times. Low-cycle fatigue (LCF) occurs under higher stress levels but with fewer cycles—usually less than 10,000 cycles.

In high-cycle fatigue, the material behaves mostly elastically (no permanent deformation), while in low-cycle fatigue, plastic deformation happens in every cycle. Understanding the difference helps engineers choose correct materials and designs for different loading conditions.

Detailed Explanation:

Difference between high-cycle and low-cycle fatigue

In mechanical engineering, fatigue failure is a common problem where a material fails after being subjected to repeated loading and unloading. The number of cycles a component can withstand before failure depends on how much stress it experiences. Based on this, fatigue is divided into two main types: high-cycle fatigue (HCF) and low-cycle fatigue (LCF).

Knowing the difference between these two is important when designing machines, tools, structures, and vehicles, as it directly affects how long the components will last and how they should be maintained.

High-cycle fatigue (HCF)

High-cycle fatigue occurs when a material is subjected to relatively low stress levels but for a large number of cycles—usually more than 10,000 cycles, often going up to millions. In this type of fatigue:

  • The stress is low and below the yield strength.
  • The material behaves elastically, meaning it returns to its original shape after each load.
  • It takes many cycles before cracks start forming.
  • The failure occurs due to the growth of tiny cracks over time.

Common examples of high-cycle fatigue:

  • Aircraft wings experiencing vibrations during flight.
  • Rotating shafts in electric motors.
  • Turbine blades exposed to continuous light loading.

In HCF, the fatigue life is mainly determined using S-N curves (stress vs. number of cycles).

Low-cycle fatigue (LCF)

Low-cycle fatigue occurs when a material is subjected to high stress levels, usually above the yield strength, but for a smaller number of cycles—usually less than 10,000 cycles. In this case:

  • The material undergoes plastic deformation, meaning it changes shape permanently with each cycle.
  • Cracks can start quickly due to the high strain.
  • Failure happens faster, and the component must be designed to handle large deformations.

Common examples of low-cycle fatigue:

  • Steam turbine components during start-up and shutdown.
  • Engine parts facing thermal and mechanical stress together.
  • Pressure vessels during pressure fluctuations.

In LCF, the fatigue life is studied using strain-based methods, because both elastic and plastic strains are important.

Key differences

  1. Number of cycles:
    • HCF: More than 10,000 cycles (often up to 10⁶ or more)
    • LCF: Less than 10,000 cycles
  2. Stress level:
    • HCF: Low stress, below yield strength
    • LCF: High stress, above yield strength
  3. Deformation:
    • HCF: Elastic deformation only
    • LCF: Elastic + plastic deformation
  4. Design focus:
    • HCF: Based on stress amplitude
    • LCF: Based on strain amplitude
  5. Failure behavior:
    • HCF: Crack initiates slowly, grows over time
    • LCF: Crack initiates faster due to plastic damage

Importance in engineering design

Both HCF and LCF are important depending on the application:

  • High-cycle fatigue is important in parts that operate continuously under small repeated loads (like fans, rotating shafts, etc.).
  • Low-cycle fatigue matters in components that go through start-stop cycles, thermal expansion, or overloads.

Understanding the type of fatigue helps engineers select suitable materials, set safe stress limits, and plan maintenance or replacement intervals.

Conclusion

High-cycle fatigue and low-cycle fatigue are two different types of fatigue failure based on the number of load cycles and the level of stress applied. High-cycle fatigue occurs at low stress over many cycles and involves elastic behavior, while low-cycle fatigue involves higher stress and plastic deformation but over fewer cycles. Knowing the difference is essential for designing components that are both safe and long-lasting under different working conditions.