The nominal stress developed at global level

Woehler diagram
Woehler diagram
Parameters for fatigue strength curves
Parameters for fatigue strength curves

The nominal stress concept (NSK) is the most commonly used method for assessing the fatigue strength of metallic structures. As a characteristic quantity of existing and endurable stresses while the nominal stress of the the component in question is assumed. If this component has significant changes in cross section, there are areas of stress concentration due to the geometric changes. Nevertheless, the nominal stresses are based on the net cross-sectional area given the fact that the spikes are usually degraded by a plastic rearrangement in the ultimate limit state for a static dimensioning continue. Under dynamic stresses, however, this is not the case, therefore, the local stress peaks due to the structural discontinuities to be considered.

On the nominal stress concept (global approach) are detected these surges in contrast to the local approach not constructed, but on the resistance side of the assessed structural details. Therefore be needed for the application of the rated stress concept component Wöhler lines based on the basis of experiments. These are named in honor of WOEHLER because he carried out the first systematic experimental studies under cyclic loading in the years around 1860.

When WOEHLER test several oscilation tests are carried out and the number of cycles N is determined up to the defined failure at different stress horizons with single nominal stress range N and predetermined duty ratio (?m = const. oder R = const.). Since the test results are subject to very large scattering, it is assumed in the analysis that the logarithms of the test results (?sN und N) can be captured by a Gaussian distribution. In general, a survival probability PÜ of 95% is set, which is bound with respect to the evaluation of the test results to a certain confidence level.

In general, the Woehler diagram is divided into the following three areas:

  • short-term strength (Low Cycle Fatigue, LCF):         area for N < 104
  • term strength:                                                          area between  N = 104 and 107
  • fatigue strength (High Cycle Fatigue, HCF):             area for N > 107

The SKI engineering company has opportunities to test welded joints and boltet connections up to the giga-cycle fatigue range (GCF), i.e. up to 109 load cycles with constant or variable amplitude.