Modal characteristics of a flexible dual-rotor coupling system with blade crack

Abstract

Rotating blade is highly susceptible to cracking in harsh conditions, which seriously affects the dynamic characteristics of the power equipment. Existing research focuses primarily on the blade crack in a single blade or bladed disk, ignoring the flexibility and coupling factors of the shaft. This paper aims to present a theoretical framework for modeling a whole flexible dual-rotor system and investigate the effects of blade crack on the modal characteristics of the coupling system. The finite element and assumed modes (FE-AM) hybrid method is adopted to model the dual-rotor system. The crack-caused stiffness loss is determined based on the released strain energy. The accuracy of the developed model is verified by comparing it with the finite model and experimental results. After that, the influences of the rotating speed and crack parameters on the coupling system are systematically investigated. The results indicate that frequency veering and mode shift occur with increasing rotating speed due to the rotational effects affecting each flexible component differently. Blade bending and shaft torsion are coupled, and each affects the mode predominated by the other. The blade crack causes mode localization and mistuned distribution phenomenon in the disk. The shaft’s bending and torsion modes are coupled into the coupling modes associated with blade bending due to the blade crack as well. It is also demonstrated that the crack-caused disk’s localized mode is gradually transformed from the 2-nodal diameter mode. Furthermore, the natural frequencies associated with the disk’s low-nodal diameter and the mode shapes associated with the high-nodal diameter are sensitive to the blade crack. The proposed model can provide a theoretical foundation for fault diagnosis of blade crack in the rotating machine.