Documented curve segment required
S–N Log-Log Interpolation
Interpolate cycles between two documented points from the same constant-amplitude S–N curve. This tool does not create fatigue data from UTS, infer an endurance limit, extrapolate, or approve a design.
Interpolated point on this curve segment
For two positive points (N₁,S₁) and (N₂,S₂) on one straight segment in log-log coordinates, this calculator uses b=ln(S₂/S₁)/ln(N₂/N₁), a=S₁/N₁b, and N=(S/a)1/b. Equivalently, ln N is linearly interpolated between ln N₁ and ln N₂. Cycle count is dimensionless; a has the selected stress unit. Using MPa or ksi consistently gives the same N and b.
The calculator accepts only N₁<N₂, S₁>S₂ and S₂≤S≤S₁. Values outside the segment are rejected because extrapolation can cross a knee, runout region, different failure mechanism, or a regime where a different model is required. Equality with either endpoint returns that endpoint; no “infinite life” is inferred.
ISO 1099:2017, edition 3 is Published but at stage 90.92 (to be revised); ISO/FDIS 1099, edition 4 is under development. Its scope is axial, constant-amplitude, force-controlled fatigue testing of metallic specimens at ambient temperature and it obtains stress-versus-cycles information at specified stress ratios. It does not make a UTS-based generic curve or turn this interpolation into an ISO conformity result.
ISO 12107:2012, edition 2 is Published and addresses statistical planning and analysis of metallic fatigue data; an edition 3 work item is under development. The licensed standard is required for its detailed procedures. This page neither performs the specified statistical analysis nor assigns a survival probability.
NASA material describes a Basquin curve as a straight line in log-log form defined by fitted coefficients, and NASA’s reliability handbook describes its constants as empirical and dependent on material and environment. A NIST fatigue study illustrates that fitted coefficients can carry substantial uncertainty and that mean stress changes the lifetime relation. Those sources support the restrictions here; they do not supply universal coefficients.
Not included: mean-stress correction, variable-amplitude cycle counting or Palmgren–Miner damage, low-cycle strain-life, multiaxial fatigue, crack-growth/fracture mechanics, size/surface/notch/environment corrections, runout treatment, confidence bounds, safety factors, inspection intervals, or design acceptance. Use a qualified fatigue analysis and applicable design code for consequential decisions.
Sources: NASA/NESC S–N and spectral-fatigue paper; NASA fatigue reliability handbook; NIST fatigue-coefficient study; ISO catalogue records linked above. Status checked 12 July 2026.
Nikolai Shelkovenko
Nikolai Shelkovenko is a vibration analysis engineer and the founder and CEO of Vibromera. For more than 15 years he has balanced rotating equipment in the field rather than on a test bench: mulchers, industrial fans, crushers, centrifuges, shafts and spindles. That work is what the Balanset instruments grew out of — they were designed as a tool a specialist can carry to the machine and use alone, on site, not as laboratory equipment. Vibromera was founded in 2017 and has been based in Porto, Portugal, since 2023. Development, assembly and support of the Balanset line all happen here. The flagship instrument is the Balanset-1A, a portable analyser for single- and two-plane balancing and for vibration diagnostics. Nikolai is personally involved in customer support, in working through difficult balancing cases and in the development of the software. He works with customers worldwide, in any language.
