PSI - Issue 18

Rainer Wagener et al. / Procedia Structural Integrity 18 (2019) 490–500 Author name / Structural Integrity Procedia 00 (2019) 000–000

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that reason, the test result has to be shifted along the cyclic flow curve to a fictitious strain amplitude and the empirical standard deviation stress direction has to be evaluated. This procedure has to be done within every iteration loop, Figure 9.

Figure 9: Empirical standard deviation s log(sa) depending on the number of considered test results of one test series

In order to classify the test result to the first regime, the number of specimens has to be chosen, which is related to the lowest empirical standard deviation. Due to the standard deviation, the first seven test results belong to the first regime of the Fatigue Life Curve. The results of the further tests could be classified to the second or third regime. Furthermore, this solution agrees with the optical one quite well. 4. Conclusions Digital twins offer new possibilities in order to optimise the fatigue approach. Firstly, information about the manufacturing process is available. Hence, the local material behaviour can be approximated and considered within a fatigue approach of cyclically loaded components. Secondly, digital twins gather information about the service loading. Therefore, the fatigue approach can be optimised, for each component, considering the individual service loading conditions. At the end, the fatigue approach could be performed online. The knowledge about the local material behaviour enforces the use of the local strain-based fatigue method. Therefore, a continuous strain-life curve and a relevant stress-strain behaviour are required. In the case of the stress-stain behaviour, the results of an Incremental Step Test with a maximum amplitude in the range of the expected maximum loading should be used in order to consider influences of the glide character. The strain-life relation can be represented using the Fatigue Life Curve, which is the first continuous Wöhler-curve from the Low Cycle Fatigue regime up to the Very High Cycle Fatigue regime.

Nomenclature b

fatigue strength exponent fatigue ductility exponent

c