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/N₁bi 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.
Nije obuhvaćeno: 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.
Izvori: 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.
