PSI - Issue 35

4

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

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Table 1. Mechanical properties of blades materials. Material E , GPa

 , kg/m 3

 u , MPa

 -1 , MPa

VT-3-1 EI-961

115 200

4500 7800

1000-1250 1080

480 510

It is assumed that the vibration diagnostics of blades is carried out at the stage of engine repair when the engine is completely disassembled. Two types of test setups have been used. Harmonic analysis of vibration response at non-linear resonances, which is the essence of higher harmonics method, was carried out experimentally on the TIRA electrodynamic test bench (Fig. 4). The same figure shows a scheme of a blade fixation, a measurement system of strain in the root section of the blade and acceleration of its end, as well as a system for the acceleration control of a vibrating table.

Straine gage Blade

Accelerometrs

Vibrating table Electrodynamic shaker Power input

Fig. 4. Photo and block diagram of the experimental electrodynamic test bench TIRA.

In addition to maintaining the amplitude and frequency of blade vibration, the system of vibration table acceleration control was used to prevent the possibility of pseudo-superharmonic resonance excitation. For this, the electric signal of power input to the electrodynamic shaker was distorted in such a way as to compensate its distortion by the shaker electromagnet. Thus, a harmonic excitation of the blade was formed on the vibrating table, preventing the possibility of pseudo-superharmonic resonance excitation. The absence of pseudo-superharmonic resonance was checked with the undamaged blades. Crack growth was carried out by cyclic loading of blades on the TIRA test bench at a maximum stress amplitude of  a =250 MPa in the root section. Preliminarily, a notch approximately 0.8 mm long with a tip of radius 0.1 mm was made on the outer surface of the blade. The stress concentrator was located at a distance of 10 mm from the root section. The crack length on the surface was monitored and measured by the optical microscope. The spectrum of vibration response of blades at super-harmonic resonance of order 2/1 was used as a sign of damage. The sub-harmonic resonance is less sensitive to the presence of small cracks, as it was shown by Bovsunovskii et al. (2001). The determination of blades damping characteristic (in this case – the logarithmic decrement of vibrations) was carried out with the experimental setup KD-1M, Bovsunovsky and Kratko (1998). The resonant vibrations of blades were excited by means of an electromagnetic system, which included a signal generator, a power amplifier, and an electromagnet interacting with the blade through a ferromagnetic plate fixed at its end (Fig. 5). The logarithmic decrement of vibration was determined by free damped vibrations method. The experimental technique for the determination of damping characteristic is described in detail by Bovsunovsky and Kratko (1998).