PSI - Issue 35

A. Bovsunovsky et al. / Procedia Structural Integrity 35 (2022) 74–81 Author name / Structural Integrity Procedia 00 (2019) 000 – 000

80

7

with a crack: VT-3-1 (2 c =9.7 mm) - black line; EI-961 (2 c =7.5 mm) - red line.

As can be seen from Fig. 7, the second harmonic of the spectrum of acceleration vibration response at super harmonic resonance for the EI-961 alloy blade is slightly higher than that one for the VT-3-1 alloy blade, despite the fact that the crack in it is smaller. The damping level plays a role here: the lower the damping in the system, the more sensitive the method of higher harmonics to the presence of a crack. Due to the sharpness of the resonance peak, it is extremely difficult to tune the excitation system precisely to super-harmonic resonance, which is the main reason for the error of experiment in this case. A slight deviation of the excitation frequency from the super-harmonic resonance leads to a sharp decrease in the non-linearity of vibration response in terms of second harmonic of the spectrum of vibration response. That is why the numerical predictions for the second harmonic turned out to be 1.6 times higher for the VT-3-1 alloy blade and 2.3 times – for the EI-961 alloy blade than the experimental data. At the same time, for the qualitative diagnostics of damage, that is, for the identification of crack without the need to determine accurately its parameters, this method has demonstrated high efficiency. 4. Damping The amplitude dependences of logarithmic decrement of vibration for undamaged blades and for two levels of damage (see Table 2) are shown in Fig. 8. As can be seen, even the first level of damage leads to the increase of damping characteristic by 21-28% in the range of stress amplitudes  a =5-20 MPa. Such an increase of damping could be revealed with the used experimental technic since its relative error did not exceed 15%, Bovsunovsky and Kratko (1998). The second level of damage in the same range of stress amplitudes causes the increased damping characteristic of VT-3-1 and EI-961 alloy blades by 2.2-1.7 and 2.4-2.2 times, respectively. As it was shown by Bovsunovsky (2004), the sensitivity of damping characteristic to the presence of crack depends significantly on the initial level of damping of the structure in the undamaged state: the lower this level, the higher the sensitivity of damping characteristic to the presence of damage such as a fatigue crack. In our case, the initial level of blades damping characteristic was low, since they were made of low-damping alloys, which led to the evident increase of damping characteristic even in the case of small crack.

 , %

 , %

0.20

0.16

0.16

0.12

0.12

0.08

0.08

0.04

0.04

5

10 15 20 25

5

10

15

20

(a)  a , MPa Fig. 8. Amplitude dependences of the logarithmic decrement of vibrations for the alloy VT-3-1 (a) and EI-961 (b) blades: undamaged state - black curves; first level of damage - blue curves; second level of damage - red curves.  a , MPa (b)

Thus, the change of damping characteristic of aircraft gas turbine engines blades can respond reliably to the occurrence of fatigue crack, the area of which is only 0.2% of the cross-sectional area of the blade.