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.