KEVOS® MAKEOS · Strength
Fatigue Life from the S-N Curve (Steel)
How many cycles will this steel part survive at this alternating stress?
Estimated cycles to failure for a fully reversed (or equivalent) alternating stress from a Basquin S-N line between 10³ cycles and the corrected endurance limit at 10⁶ cycles.
Open the calculatorRuns in your browser. No account needed.
How this tool works
Inputs, units and where to find them
| Input | Unit | Typical source |
|---|---|---|
| Alternating stress (fully reversed equivalent) | MPa | Alternating stress amplitude, including Kf; convert mean stress effects first (e.g. with the Goodman relation). |
| Tensile strength | MPa | Ultimate tensile strength. |
| Corrected endurance limit | MPa | Endurance limit corrected for surface, size, load, temperature and reliability. |
| Fatigue strength fraction at 10³ cycles | Fraction of Su reached at 1000 cycles (about 0.9 for steels below 500 MPa, falling for stronger steels). | |
| Required life | cycles | Number of cycles the part must survive. |
Method and formulas
- Line through (10³, f Su) and (10⁶, Se) on log axes: σ = a N^b.
- a = (f Su)² ÷ Se; b = −(1/3) log₁₀(f Su ÷ Se).
- N = (σa ÷ a)^(1/b); below Se the life is taken as unlimited.
- Example: 300 MPa alternating in 700 MPa steel gives about 255,980 cycles.
Limitations
- Approximate S-N estimate for steels; scatter is large, so apply a life factor.
- Above about 10³ cycles only; low-cycle fatigue needs strain-life methods.
- Example values are illustrations. Replace them with values for your actual task.
Typical uses
- Estimate the life of a part loaded above its endurance limit
- Find the stress allowed for a required number of cycles
- Compare design options for limited-life parts
Who it is for
- Design engineers
- Structural and mechanical engineers
- Engineering students
What you get
- Estimated cycles to failure
- Stress allowed for the required life
- Basquin constants
Interpreting the result
- Fatigue life is very sensitive to stress: 10% less stress can mean several times more life.
References
- Machinery’s Handbook, 29th edition (Industrial Press, 2012), printed pp. 200–211, 358. Topic reference; no source tables or recommended settings are reproduced.
WORKOS tools are screening and decision-support utilities. Verify important financial, engineering, safety, legal or operational decisions using appropriate professional standards and your organisation’s approved data.
