PSI - Issue 33

S. Schoenborn et al. / Procedia Structural Integrity 33 (2021) 757–764 S. Schoenborn, T. Melz, J. Baumgartner, C. Bleicher / Structural Integrity Procedia 00 (2019) 000–000

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3. Test results The S/N curve of the hourglass specimen tested in air has a flat slope k = 24.3 for the LCF and HCF regime. At the knee point at N k = 1.5 ∙ 10 6 cycles the S/N curve runs flatter with an experience-based slope of k* = 45 (5% fatigue strength decrease per decade) assumed for this material, covering the few results determined [16], Fig. 3.

Fig. 3. S/N curve for 1.4521 (X2CrMoTi18-2) in air (filled triangles) and results in air and pressurized hydrogen for the hourglass specimens K t ≈ 1.0. With decreasing test frequency, a slight tendency towards lower endurable number of cycles was observed for the test in air. Due to the flat course of the S/N curve, the frequency influence has no significant impact on the degree of scatter which is quite low with T σ = 1: 1.10. In addition, the test results for the hourglass specimen carried out in pressurized hydrogen conditions lay within the scatter band of the S/N curve determined in air. Only for the high load level of σ a,n = 400 MPa a slight tendency towards shorter lifetimes is visible, Fig. 3. For the S/N curve of the notched specimens in air, the knee point shifts in comparison to the hourglass specimen with increasing notch factor to higher number of cycles. For K t ≈ 2.0, the knee point is located at N k = 2 ∙ 10 6 cycles and for K t ≈ 2.6 at N k = 2.5 ∙ 10 6 cycles. Furthermore, a steeper slope of the S/N curve for the LCF and HCF regime could also be determined with increasing notch factor. This leads to a slope of the S/N curve of k = 10.7 for the mildly notched specimens (K t ≈ 2.0) and of k = 8.6 for the sharply notched specimens (K t ≈ 2.7). A significant influence of the test frequency on the fatigue strength results could not be determined for the notched specimens, Fig. 4 and Fig. 5.

Fig. 4. S/N curve for 1.4521 in air (without results at 0.1 Hz) and in pressurized hydrogen for the mildly notched specimens K t ≈ 2.0.