Plan fatigue resistance quickly with selectable finishes, sizes, and loads for design. Enter strength, pick reliability, and get corrected endurance limits instantly in seconds.
| Material model | Sut (MPa) | Finish | d (mm) | Reliability | Estimated Se (MPa) |
|---|---|---|---|---|---|
| Steel | 600 | Machined | 12 | 90% | ~225 |
| Steel | 900 | Ground | 25 | 95% | ~330 |
| Aluminum | 310 | Machined | 10 | 90% | ~85 |
| Titanium | 1000 | Ground | 15 | 99% | ~330 |
| Steel | 1400 | Hot-rolled | 50 | 99.9% | ~210 |
The corrected endurance limit is estimated by multiplying a baseline endurance limit by several modifiers:
Se = Se' × ka × kb × kc × kd × ke × km
If a notch is present, the fatigue notch factor is estimated as: Kf = 1 + q (Kt − 1), and a simple alternating-stress allowance is Sa ≈ Se / Kf.
Accurate fatigue estimates help safer designs and longer lifetimes.
Many engineered parts fail under repeated stress levels far below their static strength. Fatigue design therefore focuses on stress amplitude and cycle count. For many steels, an “endurance limit” is a practical threshold: below it, failure becomes unlikely for very high cycle counts. This calculator provides a structured estimate so you can compare design options early.
A common starting point is the baseline endurance limit, Se′, derived from ultimate tensile strength (Sut). For steels, Se′ is often approximated as about 0.5·Sut up to a saturation region; this captures the trend that stronger steels generally resist fatigue better, but not indefinitely. For non‑ferrous materials, a true limit may not exist, so an equivalent long‑life fatigue strength is used.
Surface roughness behaves like a field of microscopic notches that amplifies local stress. Ground surfaces can maintain higher endurance strength than hot‑rolled or as‑forged finishes. The surface factor ka in this tool follows empirical fits that depend on Sut, reflecting that high‑strength materials are often more sensitive to surface condition.
Larger diameters tend to reduce endurance strength because a larger stressed volume increases the probability of a critical flaw and shifts the stress gradient. The size factor kb accounts for this by reducing the endurance estimate as diameter increases, especially in bending and torsion where surface stresses dominate.
Fatigue strength differs by loading type. Bending is commonly treated as the baseline case, while axial loading and torsion are reduced by load factors such as kc. In practical terms, a shaft designed for torsional cycling can require a noticeably higher Sut or improved surface finish to reach the same endurance target as a bending case.
Elevated temperature can degrade yield strength and accelerate microstructural damage mechanisms, so kd reduces the endurance estimate above room temperature. Reliability, represented by ke, addresses scatter in fatigue data. Moving from 50% to 99% reliability can reduce the allowable endurance estimate by roughly 15–20% in many design practices.
Geometric features like shoulders, holes, and keyways create stress concentration, quantified by Kt. Not all of that concentration transfers into fatigue damage; the notch sensitivity q captures material and size dependence. The fatigue notch factor Kf = 1 + q(Kt−1) is then used to reduce the corrected endurance strength for alternating stress.
Treat the output as an engineering estimate for screening and comparison. Final design should consider mean stress, load spectrum, residual stresses, corrosion, manufacturing variability, and validated S–N data. When test data exist, calibrate km or replace assumptions so the estimate matches your material and process.
For many steels, endurance limit is a long‑life threshold. For materials without a true limit, designers use fatigue strength at a specified life, commonly 107 cycles, as a comparable target.
Use the heat‑treatment and condition that matches your part, preferably from a certified material test report. If only a range is known, run best‑case and worst‑case Sut to bracket the estimate.
Select the finish that matches the final fatigue‑critical surface after machining, grinding, polishing, or forming. Coatings or shot peening can improve fatigue performance; represent these with a more favorable finish or km.
Higher reliability is used for safety‑critical parts and high production volume. Common design targets are 90–99%. If failure has severe consequences, choose 99% or higher and validate with testing.
Kt describes geometric stress amplification; q describes how strongly the material “feels” that notch in fatigue. Higher q pushes Kf closer to Kt, reducing allowable alternating stress more significantly.
Often yes at elevated temperatures because strength and microstructure can degrade. At modest temperatures near room conditions, the effect may be small. Always use material‑specific data when operating outside standard ranges.
No. It is a screening and preliminary design tool. For final validation, use component or specimen testing and appropriate standards, especially when geometry, environment, or loading history is complex.
Important Note: All the Calculators listed in this site are for educational purpose only and we do not guarentee the accuracy of results. Please do consult with other sources as well.