PSI - Issue 61

Rachid Azzi et al. / Procedia Structural Integrity 61 (2024) 241–251 Rachid Azzi and Farid Asma / Structural Integrity Procedia 00 (2023) 000 – 000

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The mode shapes obtained for the undamaged propeller blade are shown in Fig. 7 and those obtained for the damaged blade are shown in Fig. 8. The damage is inserted at position “S8” with a length of 5 mm and a “V”shape. It is seen that the changes in the mode shapes of the damaged blade compared to those of the undamaged blade are not significant. Therefore, it is difficult to detect damages by the picture of mode shape displacement.

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Fig.7. Mode shapes of intact propeller blade in the stationary frame. (a) First mode, (b) Second mode, (c) Fourth mode.

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Fig.8. Mode shapes of damaged propeller blade in the stationary frame. (a) First mode, (b) Second mode, (c) Fourth mode.

3.2. Pre-stressed modal analysis of rotating blade In the case of the rotating conditions, the propeller blade is embedded in the rigid hub and rotates around the propeller axis (X-axis). To include the rotating prestress influence on the dynamic parameters of the propeller blade, fixed support is imposed on the root surface of the blade, and a rotational velocity is applied about the X-axis as shown in Fig.9. A pre-stressed modal analysis is performed in Ansys on the model of the intact blade and the other models of the damaged blade by doing a static analysis before the modal analysis so that all the stress and displacements, which are created by the rotation, are included in the modal analysis (R. S. Mohan (2014)). The three natural frequencies and their corresponding mode shapes are obtained for damaged and undamaged propeller blades. The natural frequencies of an intact blade are compared with those of the damaged blades. Fig.10 shows the percentage frequency reduction of the first, the second, and the fourth order in function of damage length in the case of a fixed blade and the case of a rotating blade with 2500 RPM. It is seen that for the three natural frequencies, the difference between the percentage frequency reduction of a fixed blade and a rotating blade at 2500 RPM is weak. Therefore, the effect of rotational velocity on the reduction of frequencies of damaged propeller blades is not significant.