During a high cycle fatigue test, a metallic specimen is subjected to cyclic loading with a mean stress of +140 MPa, and a minimum stress of –70 MPa. The R-ratio (minimum stress to maximum stress) for this cyclic loading is _______ (round off to one decimal place)

The correct answer is -0.2.

To find the stress ratio (R-ratio) for the cyclic loading, we need to determine the maximum stress (${\sigma }_{\text{max}}$) first.

We are given the following values:
Mean stress, ${\sigma }_{\text{mean}}=\mathrm{+140}\text{MPa}$
Minimum stress, ${\sigma }_{\text{min}}=-70\text{MPa}$

The mean stress is defined as the average of the maximum and minimum stresses:
${\sigma }_{\text{mean}}=\frac{{\sigma }_{\text{max}}+{\sigma }_{\text{min}}}{2}$

Substituting the given values into the equation:
$140=\frac{{\sigma }_{\text{max}}+(-70)}{2}$

Solve for the maximum stress (${\sigma }_{\text{max}}$):
$140\times 2={\sigma }_{\text{max}}-70$
$280={\sigma }_{\text{max}}-70$
${\sigma }_{\text{max}}=280+70=350\text{MPa}$

The stress ratio (R-ratio) is defined as the ratio of the minimum stress to the maximum stress:
$R=\frac{{\sigma }_{\text{min}}}{{\sigma }_{\text{max}}}$

Substituting the values of ${\sigma }_{\text{min}}$ and ${\sigma }_{\text{max}}$:
$R=\frac{-70}{350}=-0.2$

Thus, the R-ratio for this cyclic loading is -0.2.