Stress Ratio
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Stress Ratio - Fatique Failure

The most commonly used stress ratio is R, the ratio of the minimum stress to the maximum stress (Smin/Smax).

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  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