Fatigue Life Calculator

If your stress amplitude is at or below the modified endurance limit, the classic S-N model for steel assumes the material can withstand unlimited stress cycles without fatigue failure — that's the physical basis of the endurance limit itself. This assumption is valid for steels but doesn't hold for aluminum and most non-ferrous alloys, which lack a true endurance limit and will eventually fail at any stress level given enough cycles, so infinite life shouldn't be assumed for those materials even where the calculator reports it.

Your entered endurance limit typically comes from lab specimens with a fine ground finish, but real parts have rougher surfaces (which can nucleate fatigue cracks more easily) and often larger cross-sections (which statistically have more chance of a flaw somewhere in the stressed volume). The surface factor (ka) and size factor (kb) scale that lab-derived limit down to something more representative of your actual part's finish and dimensions.

It anchors the S-N curve at two standard reference points — 90% of ultimate tensile strength at 10³ cycles, and the modified endurance limit at 10⁶ cycles — both well-established conventions in fatigue analysis. Solving the two-point system for Basquin's σₐ = a·Nᵇ gives the constants a and b, which the calculator then inverts to solve for cycles to failure at your actual stress amplitude.

That's a fixed reference cycling speed chosen as a convenient conversion (1,000 RPM = 60,000 cycles/minute), not a measurement or assumption about your actual part. If your component cycles at a different frequency, rescale the reported hours yourself using your real cycling rate — the Cycles to Failure number itself doesn't depend on this assumption.