Experimental investigation on the damping effectiveness of blade root joints

The mechanical design of turbine blades aims at verifying the structural integrity against High Cycle Fatigue damage. For this reason Forced Response are simulated in order to evaluate the distribution of variable stress in the blade. One of the main parameter affecting the level of the response is the structural and the mechanical damping. When resonant excitation can not be avoided in service the second source of damping is enhanced by the design of friction joints which can limit the amount of oscillation due to the dissipation of energy caused by the slip of contact surfaces. One of the typical feature in a blade that is a source of damping is the blade root attached to the slot of the rim of the disk. Despite the relatively low value of the damping introduced by the blade root joint, it can reach the same order of magnitude of the structural damping. In order to increase its effectiveness, different geometries of blade root joints are usually adopted (fir tree, dovetail) and optimized according to the operating conditions of the bladed disk. In this paper a test campaign is presented in order to collect a database of measurements of the damping introduced by two different geometry of blade root joints: dovetail joint and the crowned joint. The test rig is first described together with the excitation system and the measurement system. Due to the low amount of damping that must be measured, both systems are non-contact in order to avoid the introduction of any spurious damping. The excitation system is provided by a an electromagnet facing the dummy blade carrying the real geometry of the root attachment. Commissioning of the exciter and of the measurement system is described and discussed in order to define the fastest and most reliable Forced Response measurement, including issues like the heating of the dummy blade and the calibration of the electromagnetic exciter. The test campaign shows the effectiveness of the two geometries at a different level of pulling loads simulating the centrifugal force and different amplitude of the exciting force.