Fatigue tests are typically illustrated using a generally represented by Wöhler curve which shows the relationship between Stresses to Number of cycles to failure (hence SN relation). When plotted on a log-log scale1, the Wöhler curve generally appears as a straight line, with Stress values2 (σmax or σamp) in Y-axis and Number of cycles to failure (Nf) in X-axis.
To establish an SN relationship, fatigue tests must be performed at several different stress levels, not including repetitions3, which are conducted at the same stress levels. Therefore a simple Design Of Experiments (DOE) is required for every fatigue test request (example request document).
Example test request: Determine the SN relationship for a Syensqo grade polymer, using ISO 527-1BA dry as molded (DAM) samples, tested at ambient temperature (23°C), with a frequency of 5 Hz, and a loading ratio of R = 0.14. Tests should be conducted at five different stress levels (80%, 70%, ..., 45% of σUTS), with four repetitions at each level, except for the lowest stress level (45% of σUTS), where only two repetitions are sufficient.
Foot-notes:
1. Some data are also expressed in semi log scale (Y in linear & X in log), a common practice to make the Stresses scale visible.
2. Stresses can be represented as both maximum stress (σmax) or stress amplitude (σamp).
3. Repetitions between 2-5 (less repetitions at low stress levels to save time and cost) are chosen to account for variability, which are inherent for any material (both plastics and metals).
4. Loading ratio of R 0.1 is generally chosen as default loading case. Please be aware that a part can experience loading cases much wider than mere R0.1, (R=1 is Creep loading).