# Experimental Verification of the Dynamic Model of Turbine Blades Coupled by a Sealing Strip
## 1 Introduction  
One of the key points in the structural design of a turbine bladed disk is to avoid that the excitation frequencies of the aerodynamic forces are in the same range of the natural frequencies of the bladed disk. However, this is not always possible because of the wide range of aerodynamic frequencies and the wide range of possible resonance frequencies. Once the geometry of blades and disk have been defined, a possible solution to shift the natural frequencies and to introduce damping, is to insert friction contact surfaces between the different elements of the bladed disk. These friction contact surfaces can be between elements which are integral parts of the blades [1; 2; 3; 4; 5; 6; 7; 8; 9] such as shrouds or interlocked tip platforms, or, as an alternative, they can come from the contact with external devices such as underplatform dampers (UPDs) [10; 11; 12; 13; 14; 15; 16; 17; 18]. The UPDs are metal masses inserted between the blades, pushed against the blade platforms by the centrifugal force. They are extensively used in turbine designs, especially as \"solid dampers\" in several shapes, cylindrical, curved flat, wedge damper, etc. For this reason, in the past 15 years, the design of these solid underplatform dampers have been extensively investigated [19; 20; 21; 22; 23; 24; 25; 26; 27]. 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. As the solid dampers, they are held in place against the blade platforms by the centrifugal force. In addition to the sealing function, they can also be used to modify the stiffness and the damping of the system. Strip dampers can significantly modify the dynamic response of the turbine system. Firstly, the seal stiffens the structure and, as a result, the resonance frequencies of the bladed disk will increase. Furthermore, when relative motion takes place betweenseal and blade platforms, the friction forces dissipate vibrational energy and consequently the dynamic response of the system is damped. For all these reasons, a sound and robust turbine dynamic design should consider the seal influence on the response in terms of its frequency shift and damping contribution. Unlike other kinds of under-platform dampers, the peculiarity of seal dampers is to be flexible and very light. Their flexibility implies that they cannot be considered as a rigid body with only inertia properties, a common assumption in underplatform dampers modelling. Their low mass implies that they are not subjected to the same high centrifugal force as standard prismatic under-platform dampers. In a previous paper [29], the authors investigate the peculiar features that a sound dynamic model should have in order to effectively represent seal dampers and their friction contact with the blades' platforms. The simplifications typically adopted with rigid dampers are highlighted and abandoned. The authors analyze the choices of the model key parameters and explore the consequences of each choice on the predictions offered by the numerical simulation tool. In the present paper, the authors provide the experimental verification of the numerical model described in detail in [29]. The experiments are carried out by means of a test rig with two blades coupled with a seal damper in controlled laboratory conditions. 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. The paper presents the experimental forced response of blades with strip dampers.