Calculator Inputs
Example Data Table
| Case | Sut (MPa) | Finish | Load | Diameter (mm) | Temperature (°C) | Reliability | Misc. Factor |
|---|---|---|---|---|---|---|---|
| Shaft A | 900 | Machined | Bending | 25 | 20 | 99% | 1.00 |
| Axle B | 1050 | Ground | Torsion | 40 | 60 | 95% | 0.95 |
| Rod C | 760 | Hot-rolled | Axial | 18 | 30 | 90% | 1.00 |
Use these values as a starting point, then replace them with your own design data.
Formula Used
Corrected endurance limit: Se = ka × kb × kc × kd × ke × k_misc × Se′
Automatic steel specimen estimate: Se′ = 0.5 × Sut, capped at 700 MPa or 100 ksi.
Surface factor: calculated from finish-specific coefficients using ka = a × Sut^b.
Size factor: based on equivalent diameter. For axial loading, this calculator sets kb = 1.0.
Load factor: uses common design values for bending, axial, and torsion.
Temperature factor: may be estimated automatically from temperature or entered manually.
Reliability factor: selected from preset levels or entered manually.
Stress check: the optional safety factor uses the modified Goodman relation 1 / (σa/Se + σm/Sut).
How to Use This Calculator
- Select metric or imperial units first.
- Enter the ultimate tensile strength of the material.
- Choose automatic steel estimation or manually enter the specimen endurance limit.
- Select the surface finish and load type.
- Choose a section option and provide the related geometry.
- Set temperature and reliability factors using automatic or custom mode.
- Enter any miscellaneous factor that reflects extra conditions.
- Optionally add alternating and mean stress for a Goodman safety check.
- Press the calculate button to display results above the form.
- Use the export buttons to save the results as CSV or PDF.
Frequently Asked Questions
1) What does the endurance limit represent?
It is the estimated stress amplitude a part may withstand for very high cycle counts without fatigue failure. This page calculates a corrected component value using common Marin-style modifiers.
2) Is the automatic Se′ formula valid for every material?
No. The automatic base estimate is mainly intended for steel-style fatigue design. For test-based values or other materials, switch to the manual Se′ option and enter the measured specimen endurance limit.
3) Why does surface finish reduce endurance limit?
Rougher surfaces contain deeper micro-notches that raise local stress concentration and accelerate crack initiation. That is why forged and hot-rolled parts usually receive lower surface factors than ground parts.
4) Why is there a size factor?
Larger stressed volumes have a greater chance of containing defects and unfavorable microstructure. The size factor helps translate polished specimen behavior into a more realistic component estimate.
5) When should I use the mean stress check?
Use it when the load cycle is not fully reversed. A positive mean stress lowers the allowable alternating stress, while a negative mean stress can improve fatigue performance.
6) What is the difference between Goodman and Gerber on the graph?
Goodman is a straight-line design relation and is often used conservatively. Gerber is a parabolic relation that may fit ductile behavior better, but many designers still prefer Goodman for safety.
7) Should I trust the automatic temperature factor at extreme temperatures?
Use caution. Extreme temperature behavior depends strongly on material, environment, and heat treatment. When accurate test data exists, enter a custom temperature factor instead of relying on a broad estimate.
8) Can I export the results for reports?
Yes. The calculator includes CSV export for tabular data and PDF export for the visible result section, including the chart and summary tables.