Fatigue Life Calculator

The Load That Never Breaks It Once Can Still Break It Eventually

A stress well below a material's yield strength feels safe applied once. Applied millions of times, it isn't necessarily — fatigue failure happens through the slow growth of microscopic cracks under repeated loading, and it can take down a part that never came close to yielding on any single cycle. Basquin's equation models how many cycles a given stress amplitude can survive before that crack growth ends in fracture.

The Formula

σ_a is the applied stress amplitude, σ_f' is the material's fatigue strength coefficient, b is the fatigue strength exponent (a negative value, typically determined experimentally per material), and N is the predicted number of cycles to failure.

Worked Examples

Cutting stress amplitude in half in this example increased predicted life by roughly a factor of 2,200 — the negative exponent b makes fatigue life extraordinarily sensitive to how close operating stress runs to the material's fatigue strength.

Where This Calculation Matters

• Rotating and reciprocating component design — shafts, connecting rods, and springs all see cyclic loading and need a fatigue check beyond a simple static stress check.
• Setting inspection and replacement intervals — comparing predicted cycles to failure against expected operating cycles to build a maintenance schedule with margin.
• Root-cause analysis of unexpected failures — a part that fails well before its predicted static strength would suggest often turns out to be a fatigue failure under cyclic load.

How to Use This Calculator

1. Enter Stress Amplitude (MPa) — the alternating stress the part experiences per cycle.
2. Enter Fatigue Strength Coefficient (MPa) for the material.
3. Enter Fatigue Strength Exponent (b) as a negative decimal, per the material's fatigue data.
4. Select Calculate to get the predicted number of cycles to failure.

Related Calculations

Establish the stress amplitude itself first with the Stress Strain Calculator.