SNKurve

SNKurve, commonly referred to as the S-N curve or Wöhler curve, is a graphical representation of the relationship between cyclic stress amplitude and the number of cycles to failure for a material under uniaxial fatigue loading. It is widely used to assess fatigue life and compare the durability of different materials or heat treatments.

The curve is obtained through fatigue testing, where specimens are subjected to cyclic loading at controlled stress levels until they fail. The results are plotted on a log-log scale with the stress amplitude on the vertical axis and the number of cycles to failure on the horizontal axis. The resulting trend typically shows that higher stress amplitudes result in fewer cycles to failure, while lower stresses allow for more cycles before failure.

In many materials, the SNKurve exhibits two regimes. High-cycle fatigue involves stress amplitudes below the yield strength and fatigue life extending to very large numbers of cycles; the relationship in this regime is often described by Basquin’s law. Low-cycle fatigue occurs when plastic deformation accumulates, usually at higher stress amplitudes, and the life is shorter. For some ferrous materials, an endurance limit or fatigue limit may appear, indicating a certain stress level below which the material can endure an infinite number of cycles in ideal conditions. For other materials, no true endurance limit exists, and life continues to decrease with decreasing stress.

SNKurven are used in design and reliability assessments to estimate fatigue life under repetitive loading, to select suitable materials, and to evaluate the effects of factors such as surface finish, size, temperature, and mean stress. Limitations include data scatter, the influence of loading mode, multi-axial stresses, and environmental conditions, all of which can affect real-world fatigue behavior.