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


 * 1) Introduction
 * 2) /Algebraic Models/
 * 3) /Haigh Diagram/

=Introduction=

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 $$R = \frac{\sigma_{min}}{\sigma_{max}}$$. This measure offers a convenient way to classify different stress constellations, as follows:


 * $$R = 1 \Rightarrow$$ constant stress
 * $$R = \pm \infty \Rightarrow$$ no tension, only cyclic compression
 * $$R = 0 \Rightarrow$$ no compression, only cyclic tension