PSI - Issue 35

Martin Ferreira Fernandes et al. / Procedia Structural Integrity 35 (2022) 141–149 Martin Ferreira Fernandes et al. / Structural Integrity Procedia 00 (2021) 000 – 000

144

4

3. Results and discussion 3.1. Dwell-fatigue tests

The dwell-fatigue tests demand a long duration when the test is conducted until fracture and depend on the dwell period of the tests. For example, achieving 10 4 , 10 5 , and 10 6 cycles for a dwell period of 10 s (plus 2 seconds during load/unload) consumes about 33, 333, and 3333 hours (i.e., 1.4, 13.9 and 138.9 days) for only one test, respectively. Therefore, it is only feasible to perform dwell-fatigue tests at laboratories until fracture at high-stress levels. The load hold duration influences the cumulated plastic strain and, consequently, the dwell-fatigue damage. As the dwell period increases, more slip systems contribute to the cumulated plastic strain (Lavogiez et al., 2020). However, the dwell fatigue damage and the number of cycles to failure approach a stable value for titanium alloys after 120 s, according to Wang et al. 2015. The dwell period in the literature used in laboratories for aeronautical applications ranges from a few seconds to several minutes (Goswami and Hänninen, 2001). In the present work, the dwell period of 10 s was investigated. The selection of a dwell period of 10 s is in accordance with the recent literature (Xi et al., 2020). Table 1 shows the dwell-fatigue test results with a dwell period of 10 seconds during each cycle. The tests were performed at maximum stress values of 1000, 975, and 950 MPa, corresponding to 1.0, 0.975, and 0.95 of the yield strength. At each stress level, three dwell-fatigue tests were performed, and one fatigue test with a triangular waveform as a reference evaluating the dwell-life debit. The fatigue tests are presented as the lines with a zero dwell period in Table 1 since the triangular waveform is equivalent to a trapezoidal waveform with zero seconds in the maximum stress. Table 1. Dwell-fatigue test results for load hold of 10 s. (MPa) ⁄ Dwell period (s) (cycles) Dwell-life debit 1000 1 10 510 24.1 10 1821 6.8 10 1278 9.6 0 12299 - Average 10 1203 10.2 975 0.975 10 677 34.6 10 2391 9.8 10 3933 6.0 0 23415 - Average 10 2334 10.0 950 0.95 10 16485 5.1 10 20309 4.1 10 18630 4.5 0 83281 - Average 10 18475 4.5 Fig. 3a shows the S-N curve with the dwell-fatigue data of Table 1. Equations 2 and 3 show the experimental regression equations of the dwell-fatigue and fatigue tests, respectively. The life prediction equations for the dwell fatigue loading with load holds of 10 s and the fatigue loading are given by Equations 4 and 5, respectively. =1081.68-30.31 log (2) =1234.23-58.12 log (3) log =35.69-0.033 (4) log =21.24-0.017 (5)