PSI - Issue 42

Lukas Lücker et al. / Procedia Structural Integrity 42 (2022) 368–373 Lukas Lücker / Structural Integrity Procedia 00 (2019) 000 – 000

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2.3. Intermittent fatigue tests Intermitting fatigue tests were carried out on a Shimadzu type EHF-LV-020 servo-hydraulic fatigue test system at load ratio R  = -1 and frequency f = 10 Hz. Actually, the developed resistance measurement setup does not allow simultaneous measurement during fatigue testing yet. Therefore, the specimen was removed in defined fatigue states for measurements in the test setup. Before measurement, the specimens were cooled down to room temperature in order to be able to neglect the influence of temperature on electrical measurements. After the measurements, they were reinstalled in the fatigue testing system and further cyclically loaded. In this study the specimens were measured after 25%, 50% and 75% of their fatigue lifetime. 3. Results and discussion 3.1. Measurement-based forming-induced pre-damage assessment

In Fig. 3 the electrical resistance R is plotted for the shoulder opening angles 2 α of 30° (green) and 90° (blue). For each shoulder opening angle four specimens in initial state before cyclic loading (N = 0) were investigated, each over a measurement time of 10 min. Even for this longer measurement time it was possible to realize a low scatter of only 5 n  , which is less than 0.1%, and caused by external influencing parameters, like temperature variations. These measurement results are each plotted in form of narrow bars with scatter bands in Fig. 3. The differences within the 30° and 90° states are caused by differences in pore distributions. In addition, the measurements for four specimens per shoulder opening angle were averaged and the associated standard deviation was calculated. These results are shown as wider (transparent) bars with scatter bands for 30° and 90°, respectively. Despite small differences within the states, there is a clear tendency that forming-induced pre-damage increases

Fig. 3. Electrical resistance for specimens with shoulder opening angles of 30° and 90°.

leads to electrical resistance increases. The mean resistance value at 30° is 475.9 ± 1.6 µ  and at 90° 490.0 ± 2.5 µ  . Consequently, increasing forming-induced pre-damage can clearly be detected by increasing electrical resistance of + 2.9%. Thus, it is possible to qualitative characterize the forming-induced ductile damage (pore volume) by means of DCPD. 3.2. Measurement-based fatigue damage assessment In order to measure fatigue damage by means of electrical resistance, fatigue tests were carried out with the specimens tested in Fig. 3. At 25%, 50% and 75% of the fatigue lifetime, the specimens were demounted and analyzed using the testing setup for resistance measurement. Exemplary results for a specimen formed with 30° and 90° are plotted in Fig. 4. For comparison reasons, the change in electrical resistance based on absolute values in Fig. 3 is displayed. The scatter bands of each measuring point visualize the temperature-dependent scatter of 10 min. measurement.