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