PSI - Issue 38

Alexander Erbe et al. / Procedia Structural Integrity 38 (2022) 192–201 Author name / Structural Integrity Procedia 00 (2021) 000 – 000

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3. Results and discussion 3.1. Influence of axis ratio and loading type

The stabilized hysteresis loops at midlife are shown in Figure 4. For visualization and confidentiality purposes, the reaction force was normalized to a constant reference value against the axial strain for Axis A and B, also normalized with a constant factor, for all four axis ratios with a test-temperature T of 400°C. Corresponding to different strain amplitudes in both axis directions, different force amplitudes are required to meet the von Mises strain range of  ε at the design point (DP). At an equal load level for each experiment it displayed in Figure 4, it can be s that the opening of the hysteresis loop is a function of the applied axis ratio. Figure 4: Comparison of hysteresis loops for axis A and B at midlife cycle (left side: Axis A, right side: Axis B) for different axis ratios at one load level  ε and T = 400 °C

Beside this, the load-drop trends have been evaluated for each experiment performed and selected for this paper under 400 °C, Figure 5. In each diagram a set of four biaxial experiments covering one common strain range loading together with the corresponding uniaxial experiment are displayed. Again, it can be concluded that, even if the von Mises equivalent strain is identical, beside the force level, also the cyclic softening behavior is clearly a function of the applied axis ratio. An assessment approach with the goal to cover and describe each axis ratio in a generalized and harmonized way is strongly needed to address this effect. Nevertheless, in this study the von Mises equivalent strain utilized is simply based on a cyclic flow curve at midlife derived from uniaxial standard specimens. This can be understood as a first step in order to discuss a possible ordering of axis-ratio in terms of crack initiation cycles. With the use of uniaxially loaded standard LCF experiments, which represent a baseline, a possible increase or reduction in lifetime of various axis-ratios realized within cruciform specimen testing can be evaluated. Results of the overall test-series performed at 400 °C are presented in Figure 6, left based on the evaluated von Mises equivalent strain ranges at the design point. The uniaxial cases are indicated with crossed markers and black and pink trend lines. Looking at the design point results it can be observed that the shortest lifetime is exhibited by the test series with an axis ratio of = 0. Here, even lower load cycles will be predicted as compared to the uniaxial reference line by using the design point equivalent strain. The highest relative lifetime is exhibited by the test series with an axis ratio of = - 1. Here, a significant lifetime increase represented by an offset of the crack initiation line can be concluded. The relative difference is approximately 26% in load cycles.