SNcurves

SN curves, also known as Wöhler curves, are graphical representations used in fatigue analysis to relate cyclic stress amplitude to the number of cycles to failure for a material. They are typically obtained from fatigue tests in which specimens are subjected to repeated loading with controlled stress amplitudes, often under fully reversed loading (stress ratio R ≈ -1). The results are plotted on a log-log scale with stress amplitude on the vertical axis and the number of cycles to failure, N, on the horizontal axis. The curve shows that higher stress amplitudes cause earlier failure, while lower amplitudes allow more cycles before failure.

In practice, SN curves are used to estimate the life of a component or to determine allowable stresses for a target life. Materials may exhibit an endurance limit, a stress level below which the part can endure an infinite number of cycles, particularly in many ferrous alloys. Many nonferrous alloys and composites, however, do not display a true endurance limit, and fatigue life continues to increase as stress decreases without a plateau.

Interpretation of SN curves must consider mean stress effects. Real components experience nonzero mean stresses, which can be accounted for with corrections such as Goodman, Gerber, or Soderberg forms. Other factors influencing the curve include temperature, environment, surface finish, size and geometry, and notch sensitivity; there is substantial scatter in fatigue data, so design often relies on safety factors and probabilistic approaches.

SN curves are part of the broader fatigue design framework and are frequently complemented by strain-life (epsilon-N) curves for cases where plastic deformation is significant.