Modeling and Testing Friction Flexible Dampers: Challenges and Peculiarities

Abstract

This paper deals with the dynamic of blades with strip dampers. The purpose is 1) to present the results of the dynamic numerical calculation, 2) to demonstrate the need for the experimental data on the blade-strip contact to be used as input to the calculation, 3) to propose a new test rig design to obtain them and 4) to test the key components of the new test rig. The forced responses of two blades coupled by a strip damper are calculated at different excitation and centrifugal force values. The dependence of the numerical results on the contact parameter values is confirmed in this significant reference case. The design of a new test rig is then proposed: both the blade frequency response function and the contact hysteresis cycles at the blade-strip contact are measured. It is shown how contact parameters can then be derived from experimental data. The main novelty of the test rig here proposed is the strip loading system, which simulates the uniform pressure distribution provided by the centrifugal force in real operating conditions. This loading system is non-contact and uses compressed air. Classical loading systems which see dead weights directly connected to the strip are assessed and their expected inadequacy is confirmed. The compressed air system is tested by measuring the pressure produced between strip and blades: pressure is uniform across the contact patch, constant in time and its mean value corresponds to realistic pressure values actually experienced by strip dampers during service.

Experimental Verification of the Dynamic Model of Turbine Blades Coupled by a Sealing Strip

“…The test rig was purposely designed and machined in order to create in the lab loading conditions as close as possible to the real ones, but with the possibility of having all the important parameters under control. The first prototype of the main components of the test rig are described in detail in [30]. Starting from the previous prototypes, the test rig was assembled with the technical features listed below.…”

“…More recently, the technical literature has shown interest in the design of a new kind of underplatform damper, which is not solid but "flexible", namely strip damper or seal damper [28][29][30]. Strip (or seal) dampers are thin flexible metallic strips mounted on turbine bladed disks with the main purpose of cooling-air sealing.…”

“…The fundamental novelty of the rig is the strip loading system, which simulates the uniform pressure distribution provided by the centrifugal force in real operating conditions. This loading system, which provides load on the strip without any contact with the strip, was designed and calibrated in [30] and here used to load the strip between two dummy blades.…”

On the Use of Ultrasound Waves to Monitor the Local Dynamics of Friction Joints

“…Friction joints are one of the main causes of uncertainty for the validation of the structural dynamic performance of built-up structures [1]. Friction joints introduce nonlinearity in the structural dynamics of the components, leading to frequency shifts, changes in damping, and increasing levels of uncertainty due to the poor repeatability in the assembly of these joints, which makes their modelling, analysis and testing particularly challenging [2][3][4]. Advanced modelling approaches have been developed over the years to account for the effect of frictional nonlinearities on the dynamic response of the system [5][6][7][8].…”

A modelling approach for the nonlinear dynamics of assembled structures undergoing fretting wear

“…From initial studies with lumped mass models [4], and thanks to the development of powerful reduction techniques [5], it has then been possible to study more realistic structures using large-scale finite-element models. In order to include the contact nonlinearity in the model, the contact interface is commonly discretized with a grid of three-dimensional (3D) node-to-node Jenkins elements [6,7] or alternatively either one or a few micro-slip elements [8,9]. One main simplification of these models is that the interfaces are smooth, and the contact parameters are simply 'tuning' factors.…”