A machine element has ultimate strength $\sigma_u=600$ N/mm² and endurance limit $\sigma_{en}=250$ N/mm². Its fatigue curve is shown on a log–log plot. For a life of 10000 cycles, find the maximum amplitude of completely reversed operating stress in N/mm².
The straight fatigue segment has $S=0.8\sigma_u=480$ N/mm² at $N=10^3$ and $S=250$ at $N=10^6$. At $10^4$ cycles, one third of the logarithmic life interval has elapsed. Thus $\log S=\log480+(\log250-\log480)/3$, giving $S=480(250/480)^{1/3}=386.196$ N/mm². Linear interpolation of stress rather than log stress would not follow the specified plot.
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