Frequency and modeshape evaluation of steam turbine blades using the metal magnetic memory method and vibration wave propagation

Identifying frequency and modeshape of vibration to which a given mechanical structure is subjected most of its operational time is a key issue in diagnostics of the technical condition of the machine. By evaluating these modal parameters, the sources of vibration, and thus the possible sources of damage can be identified. The current paper presents a new diagnostic technique based on the metal memory method (MMM), which can be used for identification of operational frequency and modeshape of steam turbine blades. The MMM method includes the measurement and analysis of blade's residual magnetic field (RMF) distribution, which reflects directions of main stresses resulting from working loads. It is demonstrated that based on the results of RMF measurement, vibration frequency and modeshape, to which the blade is subjected most of the time, can be evaluated. To calculate these modal parameters, stress waves propagating along the blade, and the resulting standing wave have been considered. Clear correlation between blade’s magnetic field distribution, stress distribution, and FEM results of frequency and modeshape calculation has been observed. It has been confirmed that the frequency and modeshape at which the blade vibrates during its normal operation is close to the blade's 10th natural frequency, which is about 1014 Hz.