PSI - Issue 32

V.A. Oborin et al. / Procedia Structural Integrity 32 (2021) 152–157 Author name / Structural Integrity Procedia 00 (2019) 000–000

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The appearance of a fatigue crack caused a major change in the amplitude of the second harmonic and the crack growth, respectively, its increase (Fig. 5).

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Amplitude, V

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9.93 9.94 9.95 9.96 9.97 9.98 9.99 10 Time, s 10 4 0

Fig. 5. The amplitude of the second harmonic (V) for shock-wave preloading specimen in the end of fatigue testing. On the basis of data measuring the amplitude of the second harmonic in real time during the fatigue tests, the number of cycles was determined, which was spent on the nucleation (first peak) and growth of the fatigue crack from bulk of specimen to its surface. Based on these data, it is possible to plot kinetic diagrams of crack initiation and growth, as was done in the works by Bannikov (2019). 4. Conclusions 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 in the flight cycle conditions, common in the practice of operating aircraft engines. A changein the fatigue strength of VT6 alloy specimens subjected to preliminary shock-wave deformation was studied in a gigacycle range of loading modes. The tests up to 10 9 cycles revealed a significant reduction in fatigue strength (up to 42% of the strength of the initial specimens). The analysis of oscillations of the specimen free end during gigacycle loading showed that the formation of fatigue cracks is accompanied by a significant increase in the amplitude of the second harmonic and the coefficient of nonlinearity. This makes it possible to predict the fatigue life of the material and detect internal defects in the precritical stage of damage accumulation. The deformation of materials loaded at high strain rates is characterized by intense nucleation and growth of structural defects both during shock-wave induced compression and in the course of stress release, when the development of spall fracture in the sample volume is possible under certain conditions. The gigacycle loads allow one to provide the unique state for impact tests due to the multiscale gigacycle damage accumulation in pre-loaded samples. These states could be used for the study of defect induced relaxation properties due to the formation of two-wave (elastic-plastic) fronts, kinetics of elastic-precursor decay, damage-failure transition stages in the spall kinetics.