Fatigue Life Calculator

Work out fatigue life instantly with clear inputs, formula shown and shareable results.

About Fatigue Life Calculator

In the high cycle regime the S-N curve is a straight line on log-log axes, so life scales as (Se / Sa)^(1/b) about the 10⁶ cycle endurance point. With a typical exponent near 0.1 the sensitivity is extreme: a 25 percent stress increase can cut life by an order of magnitude.

Formula

Power law S-N

N = 10^6 x (Se / Sa)^(1/b)

Se: Endurance strength at 10^6 cycles
Sa: Applied stress amplitude
b: Basquin exponent

Worked example

With the default values below, Fatigue Life Calculator returns Estimated cycles to failure of 107,374. Change any input above and every figure here is recalculated in your browser.

Inputs used

Applied stress amplitudeminimum 1 MPa: 250 MPa
Endurance strength at 10⁶ cyclesminimum 1 MPa: 200 MPa
S-N curve exponent baccepts 0.01–0.5: 0.1

Results

Estimated cycles to failure: 107,374 cycles
Log of cycle life: 5.031
Life at 10 Hz loading: 3.0 h
Stress amplitude to endurance ratio: 1.250

Step by step

1Values used
2Power law S-N
3Estimated cycles to failure= 107,374 cycles
4Log of cycle life= 5.031
5Life at 10 Hz loading= 3.0 h
6Stress amplitude to endurance ratio= 1.250

Assumptions and limits

The result is derived only from the 3 inputs listed above. Anything not among them is not part of the calculation.
In particular, this calculator does not take inflation, tax, fees or charges or compounding frequency into account.
Figures are rounded for display to 3 decimal places, so totals built from several rounded parts can differ slightly from the unrounded arithmetic.
Preliminary estimate only. Fatigue assessment must be carried out by a qualified engineer using material-specific S-N data, mean stress correction, surface and size factors and stress concentration effects.

How these calculators are built and tested is described on the Methodology page. If a figure looks wrong, please report it.

Preliminary estimate only. Fatigue assessment must be carried out by a qualified engineer using material-specific S-N data, mean stress correction, surface and size factors and stress concentration effects.

Frequently Asked Questions

How is the Fatigue Life figure worked out?

N = 10^6 x (Se / Sa)^(1/b). In the high cycle regime the S-N curve is a straight line on log-log axes, so life scales as (Se / Sa)^(1/b) about the 10⁶ cycle endurance point.

Does a steel component last forever below the endurance limit?

Traditionally yes for carbon steels, but modern very-high-cycle data show failures beyond 10⁸ cycles from internal defects, and aluminium has no true endurance limit at all.

What values do I need to enter?

This calculator takes 3 inputs: Applied stress amplitude, Endurance strength at 10⁶ cycles, S-N curve exponent b. The pre-filled defaults are a realistic worked example — replace them with your own site or project figures.