On the vibration of hybrid turbine blades with an interaction between surface discontinuities and twist angle variation

Damage related to surface fracture and changes in the twist angle of the blades are some of the most common in turbines. Modern structural health monitoring systems (SHM) are designed mainly based on vibrational signals of structures. Advanced models are necessary to create effective SHM and control systems and perform complex experiments for nonlinear dynamics of rotating structures.

For the first time, the present paper comprehensively theoretically investigates and explains the effect of the angle and depth of crack surface with blades' twist angle variation on their vibration during rotation and stationary conditions. The first-order shear deformation theory and the improved Novozhilov nonlinear shell theories are combined to consider nonlinear effects and the possibility of considering thin and thick blades with different twist angles. In addition, the blade model obtained also considers the centrifugal force effect and hybrid carbon-based composite material properties such as graphene platelets-reinforced external layers and variously oriented and dispersed carbon nanotubes-reinforced internal layers. Nonlinear coupled Euler-Largange equations of motion are discretised by the effective meshless IMLS-Ritz method. Comprehensive numerical experiments are performed to show the effects of surface crack angle and depth, variations of twist and preset angles, aspect ratio, hub radius ratio, and rotation speed on linear and nonlinear frequencies and vibrational modes of the hybrid twisted blades. In addition, results can show the resistance of the carbon-based composite material on changes in blade dynamics for surface crack presence.

The results show that the backbone curve shifts to the right as the crack length increases. In addition, it is interesting that the pre-twisted angles significantly impact the blades' mode. With increased pre-twist angles of the cracked blade, vibrational mode switching occurs. Finally, it can be seen that with an increase in rotation speed, natural frequencies are increased for the first eight modes of vibration. Taking into account modes of vibration, we can see that for higher modes, higher rotation speed introduces mode switching, which may related to crack presence. Moreover, among the four CNT patterns, the pre-twisted composite blade with FG-X distribution pattern has the highest linear and nonlinear frequencies and stiffness, followed by UD, FG-V and FG-O patterns. The presented results, new findings, and conclusions may significantly improve the planning of experimental works and the interpretation of results from SHM-designed systems for rotating blades.