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

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

• ${\displaystyle R=1\Rightarrow }$ constant stress
• ${\displaystyle R=\pm \infty \Rightarrow }$ no tension, only cyclic compression
• ${\displaystyle R=0\Rightarrow }$ no compression, only cyclic tension