Dynamic Identfication of Friction Damping in the Joints of an Engine-Like Bladed-Disks

The trend towards “greener” turbomachines stands in direct conflict with mechanical integrity aspects as lighter and more highly loaded components are more likely to vibrate and high cycles fatigue failures could occur. In aircraft engines the blade resonant vibration amplitude is normally reduced by increasing the structural damping. An extremely effective method to increase the structural damping is to dissipate energy between contact surfaces, for example in the dovetail attachment or between tip shrouds. The energy is dissipated in the contacts where a friction force and a relative displacement, called slip, arise. Both fluid dynamics and structural dynamics methods have become important tools for estimating specific phenomena in turbomachinery aeromechanics such as determination of the maximum amplitude magnifications, flutter margins determination, excitation levels at different engine orders and so on. However, it is not fully clear what would be the accuracy expected while using these methods for predicting the overall fatigue failure risk. The main challenge to validate the assessment from numerical code is to overcome the difficulties in determining the model parameters such as the excitation force level, the aerodynamic damping and the structural damping. A special test rig, Fig. 1, have been develop in order to measure the structural damping due to the friction joints in an engine-like bladed disk at different excitation levels and engine orders. The test rig is made of two co-axial rotors; the first rotor is the engine-like bladed disk under investigation, made with 146 aluminum blades, the second rotor carries the excitation system, Fig.2, made with 24 permanent magnets. The rotors rotate with different rotational speeds and their difference can be regulated so that the excited eigenfrequency of the rotor under test is consistent with its rotational speed. A non-intrusive method, namely Blade Tip Timing (BTT), has been use to determine the dynamic behaviour of the bladed rotating rotor. The processing of the data, Fig. 3, allows the determination of a few contact parameters and especially the structural damping for different nodal diameter.