## 5 Conclusions  
The aim of this paper is to highlight the challenges and peculiarities that modelling flexible friction dampers entails. Flexible dampers, also known as \"strips\", are a relatively new research topic in the field and very limited literature on their influence on the dynamic behavior of turbines is available. One key parameter needed for a robust blade/damper design is the maximum frequency shift induced by the presence of the strip damper between adjacent blades. While extensive experimental investigation and industrial practices are available for rigid dampers, flexible dampers are still understudied. This paper builds on the authors' previous experience with rigid dampers: in detail, a state-of-the-art validated numerical code for the prediction of the dynamic behavior of bladed disks coupled by rigid dampers has been purposely modified to accommodate flexible dampers as well. The upgrade included:  
* introduction of the FE model of the strip damper (rigid/solid dampers are modeled using a rigid body assumption);
* contact parameters determined for lower values of contact pressure are used (strip dampers, given their limited mass generate a moderate centrifugal force). The authors' effort did not stop to the simple application of this new and upgraded numerical tool. Rather, for each modeling step, several possible modeling choices were explored and a sensitivity analysis for key parameters performed. The outcome of this analysis is a set of guidelines to help the designer identify the most critical points when modelling flexible dampers. The main outcomes of this analysis are outlined below. 1. Mesh size, provided it meets a minimum quality standard, does not critically influence the response. 2. The flexibility of the strip generates complex kinematics at the contact. In detail, portions of the contact patch display higher relative displacements. As a result, only a high density of contact points will ensure an adequate representation of the contact conditions. The minimum density which ensures an adequate representation of the contact patch can be found through a convergence study in the low excitation (i.e., linear regime). \begin{table}
\begin{tabular}{l l} \hline \hline
**Key parameters** & **Guidelines** \\ \hline
**Mesh size** & Regular mesh on the blade and strip contact surfaces, grid size approximately 1 mm. \\ \hline
**Master contact** & 4.5 nodes/cm along the contact long edge, 7 nodes/cm along the contact short edge. \\
**nodes density** & A convergence study in linear regime (springs between contact nodes), see Figure 6b, \\  & is suggested to ensure an accurate choice if different modes (e.g., torsional) are investigated. \\ \hline
**Contact stiffness values** & To be determined experimentally. In the present case \(k_{ng}\) = 1000 N/\(\upmu\)m and \(k_{tg}\) = 233 N/\(\upmu\)m \\  & were assumed from experiments performed on rigid dampers having the same geometry, \\  & material and contact pressure as the strip object of this investigation. \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Guidelines in a nutshell. Contact stiffness values can strongly influence the response, depending on the relative displacement at the contact (i.e., produced by different EO excitation). In detail, the influence will be at its maximum if the contact spring is parallel to the blade-strip relative displacement, while contact springs whose direction is orthogonal to the relative blade-strip relative displacement will have a negligible effect on the predicted response. 4 Multiple combinations of contact parameters can lead to the same predicted response (i.e., multiple solutions to the contact parameter problem). Some of the challenges outlined in the paper (e.g., point 4.) will be entirely solved only with the help of dedicated experimental investigation. However, being aware of which parameters play a key role under which conditions is paramount in designing and performing meaningful experimental investigations.