Vibration Modes Localization in Aircraft Engines Turbine Blades

Resumo

The present work refers to the identification and analysis of structural issues in aeronautical engine
blades referring to the possible occurrence of the Phenomenon of Vibration Modes Localization. We study the
distribution of vibrational energy in turbine models in which a large number of nominally identical subsystems
(the blades) are coupled by the rotor to which they are attached. Thus, an analysis was adopted considering, first,
an ideal model, where we have characteristics, such as length, stiffness, angle of the blades etc., given as identical
for all subsystems. In this case, the vibration modes extend to the entire set of blades. Next, the model is considered
for a more real consideration, varying certain structural characteristics of the blades (in this work, the focus was
on varying the length of the blades). It is shown that, in this type of more real model, the Phenomenon of Vibration
Modes localization in Aeronautical Turbine appears and must be considered. In this phenomenon, the vibration
energy ends up being located in only part of the system, sometimes in a single blade. For these analyzes, the Matlab
language was used in a program based on the Finite Elements Method to identify the vibration modes of a cyclic
system such as turbine, generating graphs to allow a better visualization of the presence of this phenomenon and
the comparison between the models, real and ideal. This work is carried out using matrix structural analysis and
we consider real geometric characteristics of jet turbine motors for a more accurate modelling. Geometric stiffness
due to traction centrifugal forces are also taken into account, as such turbines rotate at very high frequencies. A
case study of a N2 turbine of the CFM56 engine is presented. Failure to monitor this phenomenon by manufacturers
and maintenance companies can result in serious incidents related to overload or fatigue, of one or more blades in
the system.