PSI - Issue 35

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

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5. Conclusions Tests of the blades, carried out at a low level of stress amplitude, demonstrated a high sensitivity of the second harmonic in the spectrum of strain and acceleration vibration response at the super-harmonic resonance of order 2/1 to the presence of small crack. This sensitivity exceeds that one at the principal resonance up to two orders of magnitude. The non-linear distortions of the acceleration vibration response turned out to be several times higher than the analogous distortions of the strain response. This means that the use of acceleration response is much more effective for the vibration diagnostics of damage than the strain one. The higher harmonics method takes significantly longer to test a blade than to determine damping characteristic. For the low damping structural elements, such as the investigated blades, finding the principal and especially super harmonic resonance takes time. In addition, a sharp super-harmonic resonance causes a noticeable error of the second harmonic determination in the vibration response of damaged blade. However, the higher harmonics method does not require initial data for an intact blade. Determination of logarithmic decrement of vibration requires finding the principal resonance, excitation of vibrations of a given amplitude and recording of the damped vibration process. Its processing is almost instantaneously performed by the PC with special software to obtain the amplitude dependence of damping characteristic. Thus, the process of damage diagnostics based on the determination of damping characteristic is faster and easier to implement than the higher harmonics method. However, for the determination of crack-related change of damping characteristic, it is necessary to know its initial value for the intact blade. In general, super-harmonic resonance of the second order demonstrates a higher sensitivity to the presence of crack than the damping characteristic. The smallest considered in this work crack with an area 0.2% of the cross sectional area of the blade, which satisfies the Branch Standard-1-00304-79, causes an easily identifiable non linearity of vibration response at the super-harmonic resonance of order of 2/1. At that the change of damping characteristic slightly exceeds the error of its experimental determination. The change in the first natural frequency of the blade in this case was only 0.09%, which is insufficient for the detection of small cracks. Contemporary trends of increasing the reliability of vibration diagnostics of damage are based on the use of several methods simultaneously. 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