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