A perfectly polished round steel specimen with ultimate strength S _ut at room temperature, as shown below, is subjected to a completely reversed stress cycling with at an operating temperature . Determine the endurance limit at the given conditions and the number of cycles to failure.

300 × (1 + (P ₁ + P ₂ )/20) [MPa]

400 × (1 + (P ₃ + P ₄ )/20) [MPa]

300 × (1 + (P ₂ + P ₄ )/20) [°C]

-3 in 0.30 in- 9 in R. Where P1=4 , P2= 5, P3= 6, P4=7.