Question 1

What is the Marin equation for the endurance limit?

Accepted Answer

The Marin equation modifies the rotating-beam endurance limit Se' (about 0.5·Sut for steel) by five correction factors to estimate the endurance limit of a real part: Se = ka·kb·kc·kd·ke·Se'. ka is the surface-finish factor, kb the size factor, kc the loading factor (1 for bending, 0.85 axial, 0.59 torsion), kd the temperature factor and ke the reliability factor. This is the standard stress-life approach in Shigley's Mechanical Engineering Design, Chapter 6.

Question 2

Should I apply the notch factor Kf to the stress or to the endurance limit?

Accepted Answer

The recommended Shigley practice is to apply the fatigue stress-concentration factor Kf to the stress (both the alternating component σa and the mean component σm), not to reduce Se. Kf = 1 + q·(Kt − 1), where Kt is the theoretical (geometric) stress-concentration factor and q is the notch sensitivity from the Neuber constant √a and the notch radius r. This calculator multiplies the input stresses by Kf before applying any failure criterion.

Question 3

Which mean-stress criterion should I use — Goodman, Soderberg, Gerber or ASME?

Accepted Answer

Modified Goodman is the common, slightly conservative default. Soderberg is the most conservative because it uses the yield strength Sy on the mean-stress axis. Gerber (a parabola) and the ASME-elliptic criterion are less conservative and usually fit test data better. Conservatism ranks Soderberg > Goodman > Gerber ≈ ASME. A good workflow is to size with Goodman and report Gerber/ASME alongside; always also check first-cycle yielding with the Langer line.

Question 4

How do you handle combined bending and torsion in fatigue?

Accepted Answer

Compute a von Mises equivalent for the alternating and mean components separately: σa' = √(σa,bending² + 3·τa²) and σm' = √(σm,bending² + 3·τm²), applying Kf to the bending parts and Kfs to the torsional parts. Use loading factor kc = 1 (the von Mises combination already accounts for the load type). For first-cycle yield use the von Mises of the peak stresses: n_y = Sy / √(σmax² + 3·τmax²).

Question 5

Does this work for aluminium and other non-ferrous metals?

Accepted Answer

No — the 0.5·Sut rule and the Marin factors are calibrated for wrought steel, which has a true endurance limit. Aluminium and most non-ferrous alloys do not show a fatigue limit, so you must design to a fatigue strength at a specific number of cycles instead. For cast iron the tool uses Se' ≈ 0.4·Sut and notch sensitivity q ≈ 0.20. For non-steel materials enter the endurance/fatigue strength directly and treat the result as a finite-life estimate.

Question 6

Is the Goodman diagram interactive?

Accepted Answer

Yes. The diagram plots the alternating stress σa against the mean stress σm and draws the Goodman, Soderberg, Gerber and ASME-elliptic criterion curves, the Langer first-cycle yield line, the safe region and your load point with its load line. As you change inputs the curves and the load point update live, so you can see at a glance whether the design sits inside the safe envelope and which line governs.

Fatigue DesignCalculator

Fatigue Design
Calculator