PSI - Issue 52

Valery Shlyannikov et al. / Procedia Structural Integrity 52 (2024) 214–223 V.Shlyannikov, A.Sulamanidze, D.Kosov/ Structural Integrity Procedia 00 (2023) 000 – 000

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The variations in the experimental CGR diagrams of nickel alloy XH73H for isothermal (T=400 ˚ C, 650 ˚ C) creep-fatigue interaction which consists of the dwell time of 60 s and loading/unloading time, for each trapezoidal loading cycle as well as in-phase and out-of-phase thermo-mechanical fatigue conditions are shown in Fig.7b in terms of the elastic SIFs. The TMF crack growth tests were with either IP or OOP thermal cycling at different stress levels, and temperatures of 400°C – 650°C over a cycle comprising 30 s of loading (heating)/unloading (cooling) periods in triangular waveforms. Recall that the maximum effective von Mises stress σ eqv values occurred at time point t = 30 s, which corresponds to temperature T = 650°C for IP and T = 400°C for OPP cycles. Therefore, it would be logical to compare the results for the OOP TMF loading with the characteristics of crack growth at the temperature of isothermal tests T=400 ˚ C. A comparison of fatigue fracture diagrams in Fig.7b shows that for the isothermal high temperature test of T=650˚C the CGR during creep – fatigue interaction exceeds by approximately one order and more of magnitude with respect to the in-phase TMF loading with maximal cycle temperature of T=650˚C at the same applied maximal nominal stress level of σ =80MPa. On the contrary to the IP TMF results, the behavior of the cyclic fracture diagram for out-of-phase TMF conditions with m inimal cycle temperature of T=400˚C shows higher CGR than crack growth rate under creep-fatigue interaction at isothermal test temperature T=400˚C with approximately the same nominal stress value. In the tests considered, a significant difference was noted in the crack growth rate between the IP TMF and the OOP TMF tests for the same temperature range (Fig. 7b). Figure 7c illustrates CGR versus the elastic SIF for the SENT specimen of nickel alloy XH73H for isothermal pure fatigue (T=23˚ C, 400 ˚ C, 650 ˚ C) and in-phase and out-of-phase thermo-mechanical fatigue conditions at elevated temperature range of 400°C – 650°C. Both conventional pure fatigue tests and either IP or OOP thermal cycling were carried out under sinusoidal harmonic loading at a frequency of 0.014-0.017 Hz in triangular waveforms as can be seen in Fig. 2. By considering the experimental data shown in Fig. 7c, it can be found that assessments of the influence of the temperature distribution profile (isothermal or thermo-cycling) on the CGR in terms of the elastic stress intensity factors K 1 yield some obvious results. It was observed that for the high temperature test of T=650˚C as the elastic SIF increased, the CGR during the isothermal harmonic loading gradually increased with respect to the in-phase TMF conditions at the same applied maximal nominal stress level of σ =80MPa. When phase angles 180° for the OOP TMF loading was applied, the contribution of out-of-phase cyclic loading decreased the CGR by approximately one order and more of magnitude compared with the IP TMF cycling. Nevertheless, the crack growth rate for OOP TMF with a minimum cycle temperature of T=400°C is significantly higher compared to isothermal pure fatigue deformation at the same temperature and nominal stresses σ =200MPa. The effect of the increase in test temperature on the crack growth rate for classical isothermal harmonic fatigue is conventional and coincides with the known literature data. 5. Conclusions As a result of the polycrystalline XH73M nickel-based alloy high temperature tests performed, it was found that from the crack growth acceleration point of view, the following order of arrangement of fatigue fracture diagrams is formed: isothermal creep-fatigue interaction, isothermal pure fatigue, non-isothermal in-phase thermo-mechanical fatigue and non-isothermal out-of-phase thermo-mechanical fatigue. Thus, these the fatigue cyclic fracture diagrams provide some general understanding of the competitive effects of the interaction between the isothermal and non isothermal temperature distributions and cyclic mechanical loads. Assessments of the theses effects on the crack growth rate characteristics will depend on the cyclic thermo-mechanical loading history and the material properties at elevated temperatures. Acknowledgements The authors gratefully acknowledge the financial support of the Russian Science Foundation under the Project 23-19-00158.