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Fatigue Reduction

The effort of fatigue prevention/reduction can be summarized as

  • Use stronger, more capable materials
    • Select materials with high fracture toughness and slow crack growth.
    • Consider materials that are suitable for the major charllenge of the working environments, such as corrosion resistance, better/worse heat conductivity.
  • Reduce the margin of errors in assembly and manufacture
    • Specify tighter requirements when possible.
    • Works done in controlled an environment is usually better than in the field.
  • Avoid, soften when inevitable, stress concentrations
    • Change smoothly and gradually in geometry.
    • Avoid changes of geometry or openings at high stress area
    • Symmetry and simplicity of design is encouraged.
    • Pay extra attention on joins, double shear joints when possible
    • Rivets require less maintenance but not as durable as bolts.
    • Go a step further from nominal average stresses to considering stress concentration factors.
  • Keep residual stress at surface, if any, in compression
    • Choose proper surface finishes. Shot peening and cold rolling are good for the fatigue life in general.
    • Avoid metallic plating with widely different properties than underlying material
    • Consider prestressing when feasible.
  • Take service environment into account
    • Provide protection against corrosion.
    • Monitor extreme or frequent changes in temperature. Choose materials with less mismatch of thermal coefficients of expansion for mating parts.
    • Provide protection against UV light or other harmful sources.
    • The natural frequency of the structure must stay away from the frequency of its working environment or loading
  • Schedule routine maintenance, firm and thorough
    • Perform maintenance including inspections and protection against corrosion, wear, abuse, overheating, and repeated overloading
Fatigue Design Pholosophies

Fatigue is such a vast subject that is impossible to cover in a few pages of discussion. However, there are several philosophies cocerning design of fatigue that are notable along the way to deeper considerations.

  • Infinite-life design keeps all stresses below fatigue limit to achieve "infinite" service life. This approach is more conservative and used in the place where long-life and/or safety out weigh space and weight restrictions.
  • Safe-life design allows and expects that cracks occur in service but never grow to critical length during the life of service. High stress structures, such as pressure vessels, bearings, and aircraft, employ the safe-life design. The designed "safe-life" is often one forth or less of the predicted fatigue life.
  • Fail-safe design allows and expects that cracks occur in service but never lead to failure before the scheduled maintenance which will detect, repair or replace the damaged components. The design of newer aircraft employ this philosophy since the weight penalty is crucial and the regulations for maintenance is tighter than other industries.
  • Damage-tolerant design involves fracture mechanics and takes initial imperfection into account. It is expected to slim down the margin of safety further and better integrate the fatigue engineering and fracture mechanics.

Although equipped with all these great deal of design philosophies, fatigue situation may still occur at the short coming of the design. Simulated service testing is introduced to fulfill the uncharted area by analysis and design. Finally, the feedbacks and lessons from previous failures are extremely important for future improvements.

Glossary