PSI - Issue 53

Rainer Wagener et al. / Procedia Structural Integrity 53 (2024) 161–171 Author name / Structural Integrity Procedia 00 (2019) 000–000

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3. Fatigue Testing To derive the required properties for a fatigue approach according to Wagener (2020) strain-controlled fatigue tests are required. The test results are represented by a cyclic stress-strain curve and a strain-life curve. For this purpose, the local stresses and strains are obtained by the load sequence. Hence, different stress-strain behavior can be observed for constant and variable amplitude loading. Responsible for the different stress-strain behavior is the slip behavior of the material. In order to perform a sufficient fatigue approach the first should be to calculate the actual stress-strain state. Keeping in mind that the stress-strain behavior can depend on the load-time history, a test sequence is required to derive the suitable component-related material behavior under service load conditions. In the past, several load time histories have been introduced, but only the Incremental Step Test presented by Landgraf et. al. (1968) seems to be a good compromise considering the experimental effort and reproduction of the service loading conditions as discussed by Polak et al, (1977), Christ (1998) and Wagener (2007). Fatigue testing in the regimes of High Cycle Fatigue and Very High Cycle Fatigue is very time consuming. To reduce the testing time for fatigue tests in these regimes and accelerate the fatigue testing in a simple way increase of the test frequency is an appropriate option. This means to change from strain- to force-controlled testing, because it is not possible to ensure a reliable strain measuring at higher or very high frequencies, at least by the usage of clip-on strain gauges. To increase the test frequency is admissible as long as the resulting fatigue strength is not influenced by the test frequency. A strategy for accelerated fatigue testing must consider some boundary conditions, like specimen heating and corrosion effects, because these will influence the resulting fatigue strength, respectively fatigue life. To provide high frequency cyclic testing the specimen stiffness and the required displacement are two of the most important parameters. Therefore, the test frequency of the cyclic tests under axial loading will always be higher than under bending or torsional loading. A continuous Wöhler-curve from Low Cycle Fatigue regime to the Very High Cycle Fatigue regime should be strain-based, because it is not possible to derive the S-N curve in the LCF regime under force-control. On the other hand, in case of macroscopic elastic material behavior there should be no difference between force- and strain controlled fatigue test results. The concept of the Fatigue Life Curve presented by Wagener and Melz (2017) fulfills these boundary conditions, Fig. 2. Furthermore, Miner modifications can be applied to the Fatigue Life Curve to optimize the damage assessment of the cycles with small amplitudes related to the High Cycle Fatigue and Very High Cycle Fatigue regimes.

Fig. 2: Fatigue Life Curve with applied Miner modifications according to Miner (1945) original, Cortan, (1956), Haibach (1970)