Official Answer
Answer
Fatigue life is calculated using the Goodman relation or the Gerber criterion. For 17-7PH CH900, the operating stress $\sigma$ is calculated as $\sigma = \frac{3 \pi P D_m}{4 b t^2 N^2}$. The alternating stress $\sigma_a = \frac{\sigma_{max} - \sigma_{min}}{2}$ and mean stress $\sigma_m = \frac{\sigma_{max} + \sigma_{min}}{2}$ are compared against the material's endurance limit $S_e$ and ultimate tensile strength $S_u$. The safety factor $n_f$ is given by $\frac{1}{n_f} = \frac{\sigma_a}{S_e} + \frac{\sigma_m}{S_u}$. For high-cycle applications ($>10^6$ cycles), the maximum stress should generally not exceed 50% of the minimum tensile strength to account for surface finish effects and potential residual stresses from the coiling process.
