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

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  1. Introduction
  2. Algebraic Models
  3. Haigh Diagram

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 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