# Equivalent Stress Amplitude

Overview of Equivalent Stress Amplitude (EQSTSAMP).

In S-N fatigue analysis, equivalent stress amplitude ( ${S}_{eq}$ ) is a stress amplitude that corresponds to reported life ( ${N}_{f}$ ) in an adjusted stress-life curve:

${S}_{eq}=SRI1{\left({N}_{f}\right)}^{b}$

In E-N fatigue analysis, equivalent strain amplitude is first obtained from an adjusted strain-life curve:

${\epsilon }_{eq}=\frac{{S}_{f}^{\text{'}}}{E}{\left(2{N}_{f}\right)}^{b}+{\epsilon }_{f}^{\text{'}}{\left(2{N}_{f}\right)}^{c}$

The corresponding elasto-plastic stress amplitude can be obtained by solving the following equation for elasto-plastic stress amplitude ( ${\sigma }_{eq}$ ):

${\epsilon }_{eq}=\frac{{\sigma }_{eq}}{E}+{\left(\frac{{\sigma }_{eq}}{K\text{'}}\right)}^{1}{n\text{'}}}$

Finally, Neuber correction gives the elastic equivalent stress amplitude:

${S}_{eq}=\sqrt{E{\sigma }_{eq}{\epsilon }_{eq}}$

## Output Request

EQSTSAMP = SET ID gives the equivalent stress amplitude result in supported format. If multiaxial fatigue is carried out, critical plane angle is also available in the output files. The angle is measured from the local X-axis of a plane where plane stress was calculated.