Fatigue Life Calculator

Estimate fatigue life in cycles using the S-N approach with modified endurance limit. Accounts for surface finish and size effects.

Stress Amplitude (σ_a)

psi

Endurance Limit (Se')

psi

Ultimate Tensile Strength

psi

Surface Factor (ka)

Size Factor (kb)

Tab to move between fields

Cycles to Failure

33,551

Fatigue Life (@ 1000 RPM)

1 hrs

Modified Endurance Limit17,000 psi

Fatigue Safety Factor0.57

How to Use This Calculator

Enter the Stress Amplitude (σ_a) in psi at the critical location — for fully reversed loading this is half the stress range.
Enter the Endurance Limit (Se') in psi: for steel, Se' ≈ 0.5 × Sut up to about 100 ksi.
Enter the Ultimate Tensile Strength (Sut) in psi from the material datasheet.
Set the Surface Factor (ka) based on surface finish: ground ≈ 0.90, machined ≈ 0.75, hot-rolled ≈ 0.60.
Set the Size Factor (kb): 1.0 for d < 0.3", 0.85 for d = 0.3–2", 0.70–0.80 for d = 2–10".
Review Cycles to Failure and the Fatigue Safety Factor — a factor below 1.0 indicates infinite-life is not achieved at the applied stress.

How the result changes with Stress Amplitude (σ_a)

Stress Amplitude (σ_a): Alternating stress amplitude at the critical location. Half of the stress range for zero-mean cycling.
Endurance Limit (Se'): Uncorrected endurance limit. For steel: Se' ≈ 0.5 × Sut (up to ~100 ksi); for aluminum: no true endurance limit.
Ultimate Tensile Strength: Ultimate tensile strength (Sut) of the material. Used to define the S-N curve at 10³ cycles.
Surface Factor (ka): Surface finish modification factor. Ground ≈ 0.9; machined ≈ 0.7-0.8; hot-rolled ≈ 0.5-0.7; as-forged ≈ 0.3-0.5.
Size Factor (kb): Size modification factor. d < 0.3" → 1.0; 0.3-2" → 0.85; 2-10" → 0.70-0.80.

Identify Input Parameters
4 parameters
Stress Amplitude (σ_a) = 30000, Endurance Limit (Se') = 25000, Ultimate Tensile Strength = 60000, Surface Factor (ka) = 0.8 = 5 input(s) provided
Calculate Cycles to Failure
33551 = 33551
Calculate Fatigue Life
1 = 1
Calculate Modified Endurance Limit
Modified Endurance Limit
17000 = 17000
Calculate Fatigue Safety Factor
Fatigue Safety Factor
0.57 = 0.57