PSI - Issue 46

L. Lücker et al. / Procedia Structural Integrity 46 (2023) 94–98

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Lukas Lücker / Structural Integrity Procedia 00 (2021) 000–000

3. Results and discussion

3.1. Measurement-based damage characterization Within DCPD measurements, high currents are known to improve the data accuracy but simultaneously promote specimen heating. Thus, a compromise has to be chosen for reliable measurements. In preliminary tests, best results were achieved with current I = 0.2 A, with no significant temperature increase and low scattering of electrical resistance. Five specimens of each forming parameter were measured for a period of one hour. Within this time, more than 1,500 measuring cycles with one measuring point each were recorded. Fig. 4 displays the averaged results and standard deviation. Electrical resistances R = 1.47 ± 0.03 mΩ for shoulder opening angle 2α = 30° and R = 1.51 ± 0.01 mΩ for 2α = 90° were deter mined. Consequently, increasing forming-induced pre-damage can be clearly detected by increasing electrical resistance (+ 2.7%). Since strain hardening and residual stresses are expected to be comparable, the increase in electrical resistance could be related to the increase in forming-induced void size and number. Thanks to the highly sensitive and accurate measurement setup and technology, the pre-damage level could be qualitatively visualized.

Fig. 4. Electrical resistance for components with different ductile damage levels.

3.2. Three-point bending fatigue behavior The results of three-point bending fatigue testing for both conditions were plotted in form of a force-lifetime (F-N) diagram in Fig. 5. Limiting N L = 10 6 cycles were reached at a maximum force of F max = 100 N. In the HCF regime, a significant difference in the fatigue behavior cannot be determined for the two investigated ductile damage levels, although microstructure is proven to be different. Data points for shoulder opening angle 2α = 30° (less ductile damage) follow equation (1) and those for 2α = 90° (more ductile damage) equation (2). Thus, within the performed tests with one test per load level no significant effect of ductile pre-damage on bending fatigue behavior in HCF regime can be determined. For validation issues of typical fatigue scattering further tests will be performed.

Fig. 5. Force-lifetime (F-N) diagram for components with different ductile damage levels.

(1)

F max = 2,716.6 ꞏ N f

-0.248

(2)

F max = 2,138.4 ꞏ N f

-0.220