PSI - Issue 37

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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tests of rotor blades are controlled by the pilot-engineer in the cockpit of working helicopter. To prevent flight restriction limitations the helicopter is fixed to the earth surface by chains.

a)

b)

c)

Fig. 1. Protected sensors and connector on the blade.

4. Research study

The discussed research stage aims to demonstrate the potential of OMA techniques for monitoring of rotating blades. Such demonstration means damage identification using the demonstrator’s SHM system and applying both OMA and MP techniques. The smaller size of the damage that can be detected, the more impressive the demonstration will be. So, to provide the demonstration, the sensitivity of the SHM system to the damage scale to be estimated. The different size faults seeded into the blade are used for this purpose. 5. FE modelling of the blade The task of discussed simulation is to estimate the influence of seeded faults to modal frequencies and shapes of the model. The initial part of the study was modal properties simulation of the rotor blade of the demonstrator. Mironov et.al (2021b) presents the FE model of this blade that was used as the reference. The results of modelling provide preliminary information about modal properties of the blade, including frequency and mode shapes. Modal frequencies of 1-10 computed modes of blade’s model are presented in 2 nd column of Table 1 and their modal shapes (as vectors) are used as the reference for comparison with the shapes of damaged blade’s model. For simulation and experimental verification, the round hole in the tail (foam-filled) part of composite blade was applied. Such hole imitates damages of main rotor blades that occur while take-off or landing. The actual seeded fault in the blade of the helicopter-demonstrator is illustrated on fig.2a.Two diameters of simulated fault were considered: 20 mm and 30 mm. The influence of simulated fault to each mode is estimated by normalized difference between modal parameters in “damaged” and reference states. The SHM system of the demonstrator measures deformations so, the normal stress data of FE model was used for analysis of modal shapes. The points of the blade model, where the stress computed, correspond to the pattern of deformation sensors in actual rotor blade of the demonstrator. The normalized frequency change of k th mode between reference state 0 and damaged is estimated = 0 ⁄ − 1 (4) The eigenvector that OMA techniques provide has normalized scale (-1...1) so, to compare simulated data to experimental one, the stress values are normalized to geometric sum for each k th mode ̅ = √∑ ⁄ . The MPV of k th mode (described in p.2.1) calculates total difference between normalized eigenvectors of damaged and reference 0 states (%) estimating changes in N points (DOF) of blade model = √∑ ( ̅̅̅ 0 ⁄ ̅̅̅ − 1) 2 ⁄100 (5)