Stress Ratio - Fatique Failure
The most commonly used stress ratio is R, the ratio of the minimum stress to the maximum stress (Smin/Smax).
If the stresses are fully reversed, then R = -1.
- If the stresses are partially reversed, R = a negative number less than 1.
- If the stress is cycled between a maximum stress and no load, R = zero.
- If the stress is cycled between two tensile stresses, R = a positive number less than 1.
Variations in the stress ratios can significantly affect fatigue life. The presence of a mean stress component has a substantial effect on fatigue failure. When a tensile mean stress is added to the alternating stresses, a component will fail at lower alternating stress than it does under a fully reversed stress.
Preventing Fatigue Failure
The most effective method of improving fatigue performance is improvements in design:
- Eliminate or reduce stress raisers by streamlining the part
- Avoid sharp surface tears resulting from punching, stamping, shearing, or other processes
- Prevent the development of surface discontinuities during processing.
- Reduce or eliminate tensile residual stresses caused by manufacturing.
- Improve the details of fabrication and fastening procedure