PSI - Issue 32

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

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system, 6—analyzing software. The software reads the 65,536-point signal every 0.001 s and performs fast Fourier transform to determine the values of fundamental frequency, second and third harmonics and their amplitudes, and wrote them to a file directly during the experiment. By changing the value of these amplitudes, the behavior of the defective structure of the material can be estimated using the nonlinear parameter of the signal β is studied by Cantrell (2001), Kumar (2011). The basis of the damage staging analysis is the registration of current specimen impedance in the presence of harmonic components: the frequency ω0, associated with forced vibration amplitude A1 and the amplitude A2 of the second harmonics with a frequency 2ω0, and higher harmonics associated with the influence of defects. The nonlinearity coefficient β is directly related to the amplitude of the second harmonic. Crack formation and growth lead to a significant increase in the nonlinearity parameter β. In this study, we did not need to know the specific value of the nonlinearity parameter, only its qualitative change in time, so it was enough to measure only the amplitude of the second harmonic. The value of amplitude was measured by an inductive displacement sensor with a signal recording frequency of 10 MHz (Fig. 3). 3. Results and discussion Fatigue loading tests were carried out on an ultrasonic Shimadzu USF-2000 resonant test machine at stress levels of 107-430 MPa and symmetric cycle R= -1. The ultrasonic testing machine allows testing materials in the loading regime of 10 9 -10 10 cycles with a frequency of 20 kHz, which significantly reduces the test time. During the experiment, the specimens were cooled by compressed air. The test results showed a ~ 42% reduction of the ultimate fracture stress after 10 9 cycle loading for the VT6 alloy specimen, which was pre-loaded by a shock wave from the stress level of 190 MPa in the initial (undeformed) state to the stress level of 110 MPa (Fig. 4).

Fig. 4. The S-N curve of VT6 alloy in the case of dynamic preloading and in the absence of preloading.

The fatigue limit reflects the sensitivity of material to stochastic impact actions characteristic of aircraft engine blades under conditions of the Foreign Object Damage is studied by Spanradet al. (2011), Oakleyet al. (2007), Chen (2005), Nowell et al. (2003), which can be used for technical inspection. This conclusion must be supported by comparative structural studies of pre-shocked and native specimens to qualify the failure precursors in blades during aircraft engine inspection.