When an S-N curve for a specific stress ratio is not available, the program linearly interpolates between S-N curves to extract data for a specific stress ratio.
Suppose you have defined only two S-N curves in the material database for a given material for stress ratios 0.5 and 1.0. Based on your analysis, you want to calculate the number of cycles N for stress ratios 0.25 and 0.7.
The program interpolates linearly between the S-N curves defined for specific stress ratios, if the calculated stress ratio falls in the range of the defined stress ratios. For example, for the calculated stress ratio of 0.7, the program calculates N1 from the first curve and N2 from the second curve. It interpolates linearly between N1 and N2 to find the number of cycles N for stress ratio 0.7.
It the stress ratio falls outside the range of the given S-N curve stress ratios, then the S-N curve with the closest stress ratio is considered.
In the above example, the stress ratio 0.25 falls outside the range of the defined ratio 0.5 to 1.0. The S-N curve with the closest stress range is 0.5, and therefore, the first curve is considered in the computation of the number of cycles.
In the absence of multiple S-N curves for a specific material, you can use a single S-N curve with a stress ratio of -1 (zero-mean stress ratio). To adjust the value from the zero-mean stress ratio curve to the actual intended stress ratio (calculated from the actual stress conditions, apply one of the mean stress correction methods (Goodman, Gerber, or Soderberg).