Fatigue S-N Curve Data
Basquin constants and endurance limits for common aerospace materials at R = −1. Apply modifying factors before use.
Basquin Constants (R = −1, polished specimen)
The Basquin relationship relates stress amplitude to cycles to failure in the finite-life region: σ_a = σ_f' · (2Nf)^b
| Material | σ_f' (MPa) | b | Endurance at 10⁷ (MPa) | Notes |
|---|---|---|---|---|
| 2024-T3 Al (polished) | 724 | −0.097 | 138 | No true endurance limit |
| 7075-T6 Al (polished) | 1170 | −0.126 | 159 | Notch sensitive |
| Ti-6Al-4V (STA) | 2030 | −0.104 | 550 | True endurance limit ~450 MPa |
| 4340 steel (1240 MPa) | 2720 | −0.085 | 620 | True endurance limit ~500 MPa |
| A286 CRES (aged) | 1930 | −0.100 | 450 | Good fatigue resistance |
| CFRP [0/±45/90]s | 1760 | −0.140 | 250 | Fibre-dominated lay-up |
Modifying Factors
The endurance limit is NOT a material constant — it is degraded by surface roughness, residual tensile stress, environment, size effect and stress concentration. Apply modifying factors before using tabulated values.
| Factor | Symbol | Typical Range | Effect |
|---|---|---|---|
| Surface finish | k_a | 0.55-1.0 | Ground > machined > hot-rolled > forged |
| Size | k_b | 0.75-1.0 | Decreases with specimen diameter |
| Load type | k_c | 0.7-1.0 | Bending > axial > torsion |
| Temperature | k_d | 0.8-1.0 | Degrades above ~150°C for Al |
| Reliability | k_e | 0.87-0.62 | 90%-99.99% survival probability |
Notes and Limitations
- Values are for un-notched polished specimens at R = −1
- Ferrous metals and titanium exhibit a true endurance limit; aluminium does not
- For aluminium, use the 10⁷ cycle strength as a pseudo endurance limit
- Values are indicative — for design, use MMPDS or the certified material data for the specific grade and temper
Related Knowledge
See How to Perform an S-N Fatigue Assessment for the practical workflow. See the Fatigue & Damage Tolerance Knowledge category for S-N theory.