# Advanced Structural Analysis/Part I - Theory/Failure Modes/Fatigue/Crack Initiation/Loading/Influence of Mean Stress

< Advanced Structural Analysis | Part I - Theory | Failure Modes | Fatigue | Crack Initiation | Loading

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# Introduction[edit]

The mean stress during a load cycle can be compressive, zero or tensile. A compressive mean stress reduces the rate of micro crack growth, and consequently increases the fatigue life. A tensile mean stress on the other hand has the reversed effect on fatigue life. The impact of mean stress can be very significant.

There are several potential causes of non-zero mean stress, including: biased external loading, residual stresses and structural nonlinearities.

One frequently used term is the stress ratio . This measure offers a convenient way to classify different stress constellations, as follows:

- constant stress
- no tension, only cyclic compression
- no compression, only cyclic tension