Abstract

The evaluation of damping, obtained as a consequence of the frictional contact between shrouded blades of a low-pressure turbine, is of major interest in the aeronautical industry. The main purpose of the shrouds is to keep the tightness of the primary air stream, but it also produces a nonlinear damping which makes it possible to limit the blades vibration amplitude during classical service conditions of a plane turbojet, like take-off or cruise. One of the possible technological choices to introduce this nonlinear damping consist of having a geometrical design of the low-pressure turbine blades which includes a pre-torsion angle, and will produce a static loading of the blades after the assembly, when the shrouds are brought into contact. In dynamic conditions, the contact between the shrouds can evolve to different statuses like bonding, sliding or even separation, which obviously leads to a strongly nonlinear behavior and depends on certain parameters like levels of excitation force, friction coefficient and static loading.

In order to reproduce as accurately as possible the static loading of the blades due to the pre-torsion angle, an experimental test bench has been designed and manufactured. The main goal is to evaluate the nonlinear damping through dynamic responses. The materials used and the manufacturing process are mentioned. The major novelties of the proposed test bench in comparison to the ones found in the literature are firstly the presence of three test blades, secondly the application of a static loading thanks to the blades assembly with the pre-torsion angle, and thirdly the realistic Z-shape shroud geometry.

Several experimental results are presented in this article: some modal analyses of the blades in free and embedded conditions to validate the numerical model and to estimate the blades disparity. Then each blade has been calibrated by means of strain gauges to estimate the applied torsion torque, to control precisely the blades position and the repeatability of the assembly process. Finally, the blades are tested under a dynamic excitation, using an electro-dynamic shaker and a laser Doppler vibrometer. The aim of this experiment is to observe the nonlinear damping on the blades frequency responses, and its dependence to different varying parameters like the excitation force, the static loading, etc.

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