Endurance strength refers to the maximum load on a material without considerable signs of fatigue or even defects.
The endurance strength is determined in a fatigue test. During this test, components may break if the load is below their tensile strength (fatigue crack or failure, fig. 1a and b).
For example, the endurance strength of flake graphite cast iron lies at approx. 40 to 50 % of its tensile strength. The endurance strength depends on the load and is proportional to the tensile strength.
Fig. 2 illustrates the essential parameters influencing the endurance strength of components in cast condition from nodular graphite cast iron.
Materials with higher strengths respond more sensitively to irregular component surfaces (casting defects, notches, rough surfaces, etc. See fig. 3).
According to W Bauer, defects with a local increase in tension that exceeds the tension supported by the sample geometry also promote fatigue failures from the defect at surface points that are less loaded. Naturally, defects at or near of the edges considerably reduce the service life, even if they are minor.
Stress exceeding the endurance strength leads to considerable damage and only a certain number of loading cycles are endured until fraction (fatigue limit). This interaction is illustrated in the Wöhler curve.