GATE 2023 ME – Question 18

Machine Design · Fatigue strength and S-N diagram · 1 mark · Multiple choice

The S-N curve from a fatigue test for steel is shown. Which one of the options gives the endurance limit?

an S-N curve of the fatigue strength $S$ against the number of cycles $N$, from $S_{ut}$ falling through the points 1, 2, 3 to the point 4, after which the curve is horizontal. The stress levels are marked $S_1, S_2, S_3$ and $S_4$ (the lowest, on the horizontal part).

$S_{ut}$
$S_2$
$S_3$
$S_4$

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Correct answer: (D) $S_4$

Explanation

For steel the S-N curve becomes horizontal beyond a certain number of cycles. The stress level of this flat part, $S_4$, is the endurance limit: below it the material can in principle withstand an unlimited number of cycles.