PSI - Issue 41

Aleksandr Inozemtsev et al. / Procedia Structural Integrity 41 (2022) 510–517 Author name / Structural Integrity Procedia 00 (2019) 000 – 000

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Table 3. The values of the Hurst exponent H and scales L pz and l sc at various levels of fatigue longevity. Sample number Deformation, %  , MPa   , cycles l sc , mkm L pz , mkm H

1 2 3 4 5 6

14 1.5

200 150 150 145 140 130

2,11·10 +7 3,57·10 +6 3,87·10 +7 3,09·10 +7 6,89·10 +7 1,35·10 +8

0.5±0.2 1.3±0.3 0.5±0.2 0.7±0.2 1.5±0.7 0.7±0.3

11.0±2.4 22.2±1.9 22.2±4.5 19.7±3.2 17.8±2.6 12.3±2.2

0.33±0.04 0.55±0.04 0.50±0.02 0.51±0.01 0.48±0.03 0.42±0.02

11 14 13 13

A comparative analysis of scaling characteristics of the specimens subject to cyclic loading in the high cycle and gigacycle fatigue regimes shows a the existence of a range of spatial scales (0.5-22.2 mkm), where the Hurst exponent remains constant for dynamically loaded specimens. The scale invariants and corresponding lengths were used to formulate the generalized Paris law governing the crack growth in damaged materials by Naimark (2021).

4. Conclusion In the present work, an experimental methodology for estimating the ultra-high cycle lifetime has been developed with reference to the situation of accidental high-speed collision of solid particles with fan blades and subsequent fatigue failure under flight cycle conditions, which are the events of frequent occurrence in the practice of aircraft engine operation. The estimation made allowed us to conclude that the fatigue limit of preloaded VT6 titanium alloy decreases from the stress level of 460 MPa to 130 MPa, which corresponds to the critical number of cycles ~10 8 .The deformation of materials loaded at high strain rates is characterized by the intense nucleation and growth of structural defects both during dynamic preloading and at the stage of stress release, when the development of spall fracture in the specimen is still possible under certain conditions. The gigacycle loading regime can provide unique conditions for dynamic tests due to the multiscale gigacycle damage accumulation in pre-loaded samples. The fatigue limit reflects the ability of preloaded material to withstand stochastic impacts experienced by aircraft engine blades under conditions of the Foreign Object Damage, which can be used as a criterion for the assessment of endurance of aircraft engines and its parts during service inspections. This conclusion must be supported by comparative structural studies of preloaded and original specimens to identify the failure precursors during blade inspection.

Acknowledgements This research was supported by the Russian Science Foundation (grant n. 21-79-30041 ). References

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