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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The combination of stress- and strain-controlled fatigue tests reduces the required experimental test effort and increases the output quality, because measurement uncertainties will be reduced by the change of the control mode depending on the load level. Furthermore, and beside the Fatigue Life Curve, the cyclic stress-strain behavior can be investigated. 4. Cyclic stress-strain behavior and fatigue life The first step in the derivation of Representative Structural Elements is to analyze the cyclic material behavior. For this purpose, specimens were cut out from WAAM walls made of AA2319 aluminum wire. All side faces were polished so that there is no longer any influence of the rough surface on the fatigue strength. Furthermore, the build to-force direction has been varied to get a first impression of the anisotropic behavior. After carrying out the fatigue tests with constant amplitudes under strain- and force-control with a strain respectively stress ratio of R = -1, the stress-strain curve according to Ramberg and Osgood (1943) and the Fatigue Life Curve according to Wagener and Melz (2017) are determined, Fig. 3. In order to derive the first knee-point the algorithm presented by Wagener and Maciolek et al. (2019) is used. This algorithm is based on the compatibility conditions to decide whether a result should be linked to the first or second and third regime. Thereby the definitions of the cyclic stress-strain behavior and their link to the different regimes is exploited. For comparison purposes only, the conventional strain-life curve according to Basquin (1910), Coffin (1954), Manson (1965) and Morrow (1965) is calculated and added to Fig. 3. In order to derive the cyclic stress-strain curves the compatibility conditions are used.

Fig. 3: Cyclic stress-strain chart and strain-life chart of aluminium AA2319 specimens manufactured by WAAM for different build-to-force orientations.

In order to extent the experimentally validity range the results of the force-controlled tests are transferred to the strain-life domain by the use of the cyclic stress-strain curve. As discussed above the control mode should not influence the stress-strain behavior in case of macroscopic elastic material behavior. In case of an influence of the test frequency on the fatigue life this domain transfer can be executed as it is suggested by Fischer (2017). The statistical evaluation of the test results derived with polished specimens shows no significant influence of the build-to-force orientation. That means it is not necessary to consider anisotropic material behavior at least in case of core material. Thus, it is also sufficient to use a common stress-strain curve and a Fatigue Life Curve to describe the cyclic material behavior for the fatigue approach.