PSI - Issue 37

247 7

Aleksey Mironov et al. / Procedia Structural Integrity 37 (2022) 241–249 Aleksey Mironov, Pavel Doronkin / Structural Integrity Procedia 00 (2019) 000 – 000

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Fig.2. Experimental verification of MP technique: illustration of seeded fault ref1 (a), diagram of MPVI for different states of the blade (b).

Totally four test series were conducted: two series of healthy blade in reference state ( ref1, ref2 ) and two ones with seeded faults. Four test series provide data of four testing states of the blade. The testing procedure for each state includes five “flight cycles” of the operating helicopter demonstrator. To approximate blade excitation to the basic assumption of OMA the “flight cycle” includes the specific consequence of engines operation modes. The signal data is collected while testing and transmitted wirelessly to the workstation for accumulation. Special chains anchor the helicopter to the ground, so the “flight cycle” is performed without taking - off as 3 minutes operation of helicopter’s power plant. The procedure of vibration data development includes consequence of steps to comply the requirements of the OMA techniques and the MP concept. The geometrical modal model is created using the ARTeMIS software to develop data collected from the blade sensors. To build OMA model the geometric characteristics of the actual blade are used. The data sets as deformation signals from each “flight cycle” of the testing state are assembled for further development. ARTeMIS consequently develops the data set of each “flight cycle” computing the parameters (frequency, damping and eigenvector) of each detected mode. The number of detected modes may vary between tests because of random externalities. After eigenvectors of all five “flying cycles” of each blade state are computed, the similarity between modal shapes is estimated using MAC. For further development only the similar (MAC>0.95) and sustainable modes are used. The modes detected by ARTeMIS in all considered states are reckoned sustainable. The eigenvectors of similar modes are averaged to compute the resulting eigenvectors of sustainable modes of each blade state. The resulting eigenvectors of sustainable modes of each blade state are used to calculate the MPVI values for each blade state. Fig.2b illustrates MPVI values computed for four states of the operating blade of the demonstrator. The typical reference state was calculated for MPVI estimation by averaging the eigenvectors of similar modes of states ref1 and ref2 . Compared to typical reference state the MPVI of both reference states have the same level 0.8% (fig.2b). As the blade’s state between ref1 and ref2 remains unchanged, this MPVI level indicates influence of external factors and errors of measurement system. Therefore, this MPVI level may be considered as the resolution level of the SHM system for the rotating blade. The smaller fault ( def20 ) practically does not exceed this threshold (0.82%), so the 20mm hole in this part of composite blade is undetectable by the SHM system. However, the larger hole (30mm) is clearly detected as MPVI value (1.88%) of this state twice exceeds the reference level.