PSI - Issue 68

Lars A. Lingnau et al. / Procedia Structural Integrity 68 (2025) 303–309 L. A. Lingnau et al. / Structural Integrity Procedia 00 (2025) 000–000

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Fig. 2. Total strain and rotational angle collectives for axial–torsional fatigue tests (a) without phase shift (b) and a phase shift of d = 90°.

3. Results and discussion 3.1. Phase shift-dependent fatigue behavior

To assess the impact of phase shift on fatigue performance alongside the effect of forming-induced ductile damage, synchronous and asynchronous axial-torsional load paths were compared. Fig. 3a) illustrates the material response from total strain-controlled fatigue tests, conducted with a shoulder opening angle of 2α = 30°, a total strain amplitude of ε a,t = 0.01, a stress ratio of R = -1, and an angular amplitude of θ = 10° without a phase shift and with a phase shift of d = 90°. The material response is presented in terms of maximum and minimum nominal loads and torsional moments. In all tests, a decrease in yield stress was noted in the compressive region, which can be attributed to known phenomena such as the Bauschinger effect. Cyclic softening was observed for the synchronous load path, particularly during the initial cycles, followed by a linear stress reduction. The cyclic softening mechanism, which primarily occurs in the first 15 cycles, has been identified in other studies on the low-cycle fatigue behavior of the material (Langenfeld et al. (2023); Lingnau and Walther (2023)). With regard to the phase shift of d = 90°, the number of cycles to failure was 1.25 times higher than for tests without phase shift at a shoulder opening angle of 2α = 30°.

Fig. 3. Total strain and rotational angle collectives for axial–torsional fatigue tests (a) without phase shift (b) and phase shift of d = 90°.

Fig. 3b) presents the results of constant amplitude tests on specimens with different shoulder opening angles of 2α = 30° and 2α = 90°, for total strain amplitudes of ε a,t = 0.01 and ε a,t = 0.0025, both with a phase shift of d = 90°. At a total strain amplitude of ε a,t = 0.01, the number of cycles to failure N f = 102 for a shoulder opening angle of 2α = 30°