Fatigue in Engineering Design Notes

Engineering Failure: Fatigue

1. Introduction to Fatigue

  • Fatigue refers to failure due to fluctuating or dynamic loading, occurring at stress levels below the yield stress.

  • Common in engineering structures subject to cyclic stresses:

    • Aircraft structures

    • Bridges

    • Pressure vessels

    • Axles and shafts

  • Fatigue results in brittle fracture, even in generally ductile materials.

2. Importance of Fatigue

  • Example: Boeing 737-200 emergency landing due to engine failure, emphasizing the real-world relevance of understanding fatigue in engineering.

  • Fractures, such as that of a cone bolt in an engine mount, illustrate how low cycle fatigue can lead to catastrophic failures.

3. Characteristics of Fatigue

  • Cracks originate from:

    • Surface stress concentrations

    • Large internal defects

  • Crack growth forms concentric beach marks, indicating progressive failure.

  • When cracks reach critical size, rapid, catastrophic failure occurs, exceeding fracture toughness.

4. Fatigue Loading Classifications

  • Types of load cycles include:

    1. Fully Reversed Loading: like in rotating axles of rail cars.

    2. Repeated Loading: such as in batch pressure vessels.

    3. Full Spectrum Loading: encountered in ships or oil platforms.

5. Fatigue Testing

  • Testing is crucial to determine fatigue behavior:

    • Subject samples to cyclic loads

    • Examine number of cycles to failure (N)

  • Generate relationships between stress/strain and cycles to failure using an S-N curve.

6. S-N Curves

  • S-N Curve: plots stress against number of cycles to failure. Key concepts include:

    • Endurance Limit: Stress below which fatigue failure does not occur (found in ferrous alloys and some polymers).

    • Fatigue Strength: Maximum stress at which failure can occur at a specified number of cycles (for materials without an endurance limit, such as aluminum).

7. Fatigue Behavior Types

  • Low Cycle Fatigue (LCF): below about 10^4 cycles, characterized by plastic strain.

  • High Cycle Fatigue (HCF): above about 10^4 cycles, where elastic strain is dominant, crucial for most engineering applications.

8. Stress and Strain Analysis in Fatigue

  • Important stress and strain parameters include:

    • Stress Range (∆σ): Difference between maximum and minimum stress.

    • Stress Amplitude (σa): Half of the stress range.

    • Mean Stress (σm): Average of maximum and minimum stress.

    • Stress Ratio (R): Ratio of minimum to maximum stress.

  • Understanding these parameters is critical for predicting fatigue life.

9. Practical Applications of Fatigue Data

  • Fatigue data from tests is applicable under specific loading types and atmospheric conditions, typically for fully reversed loading.

  • Safe loading conditions fall below the S-N curve, while unsafe conditions exceed it.

10. Challenges in Fatigue Testing

  • Sample variability affects fatigue values; testing must account for this to establish reliable S-N curves.

  • Fatigue behavior can differ significantly under different loading conditions and mean stresses.

11. Future Considerations in Fatigue Research

  • Factors affecting fatigue performance include:

    • Notch sensitivity due to stress concentrations.

    • Impacts of welds on structural integrity.

    • How mean stress conditions affect overall fatigue life.

  • Strategies like Goodman’s rule help adjust loading conditions for accurate fatigue assessment.

12. Summary

  • Fatigue is a crucial failure mode in engineering, demanding thorough understanding for designing safe and reliable structures.

  • Continuous evaluation of materials under various loading conditions remains essential in fatigue research and real-world applications.