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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of the conventional strain-life curve, which are used in the case of applying the compatibility conditions, are affected by test results with pure macroscopic elastic stress-strain behaviour. The knee point in the elastic strain-life curve is neglected, Figure 6. In the case of cyclic test series with the conventional sample size, the stress-strain curve derived by the usage of the compatibility conditions could be used to validate the model and the properties of the strain-life curve. 3.2. Extrapolation up to the Very High Cycle Fatigue regime In times of considering service life extensions, the cyclic material behaviour, especially the course of the Wöhler curve, has to be known in the correct manner in order to estimate the fatigue life. Fatigue testing with more than 1 ∙ 10 6 cycles is expensive and time consuming. Therefore, the Fatigue Life Curve offers another advantage, due to the definition of the knee point by the stress-strain behaviour and the suggestion of using the slope b 1 of the elastic strain line of the first regime as well as the slop b 3 of the elastic strain line of the third regime. Figure 7 shows test results of EN AW6060 T4, which were not included in the database to determine the Fatigue Life Curve. These test results have been achieved carrying out strain-controlled tests in the Low Cycle Fatigue and High Cycle Fatigue regime as well as high frequency stress-controlled fatigue tests in the High Cycle Fatigue and Very High Cycle Fatigue regimes.

Figure 7: Fatigue Life Curve of EN-AW6060 T4

The properties of the Fatigue Life Curve have been derived by the evaluation of the strain-controlled test results only, without knowing the results of the stress-controlled fatigue tests. The stress-controlled test results are used in order to validate the course of the SN-curve in the third regime. The classification of the test results to the different regimes was done by the stress-strain behaviour according to the definition of the Fatigue Life Curve. The evaluation of the stress-controlled fatigue tests leads to a slope of k 2 = 18. Due to the different formulation of load- and strain-life curves, the norm of slope of the elastic strain-life line is the reciprocal value of load Wöhler curve, eq. 6. � � � � � � (6) Considering that b 1 = -0.0555, it seems to be possible to cut down the number of time-consuming and expensive fatigue tests up to the Very High Cycle Fatigue regime. More than ever regarding the statistical robustness of a small sample size as normally used to determine a Wöhler-curve, using the slope of the Low Cyclic Fatigue regime as the slope for the third regime is a proper estimation for the course of the Wöhler-curve up to a very high number of cycles to crack initiation. Finally, the cyclic stress-strain curve derived by the compatibility conditions represents the test results.