A novel test rig for the validation of nonlinear friction contact parameters of turbine blade root joints

The assembly of components into a large-scale engineering system naturally leads to the presence of joints with frictional interfaces. The degree of agreement between numerical models and their experimental counterparts decreases when assemblies based in this kind of interfaces are studied due to the nonlinear dynamic behaviour that joints introduce. This is, for example, the case in turbine blade root joints. The main cause for these deviations are the friction-related nonlinear damping and stiffness effects influencing the dynamic behaviour of the assembly. The experimental measurement of these damping effects poses a challenge due to the presence of the excitation rig itself, which can introduces significant parasitic damping in the system. A free decay measurement is consequently the ideal way to extract the nonlinear behaviour, however, the exciter must be initially in physical contact with the test fixture in order to reach the high excitation amplitudes that lead to macro-slip friction in the fixture joints. The test setup proposed in this paper is developed for a beam on which two blade root designs have been machined at both ends (dog bone). This beam is fitted between two clamps equipped with dovetail roots and pulled into tension to simulate rotational centrifugal loading, thus creating a blade root contact joint at either end of the beam. The novel excitation method excites the beam harmonically with a rigidly connected shaker to macro-slip deflection amplitudes before decoupling from the beam to release it into free decay. This test procedure allows the contactless measurement of the variation in vibrational decay in the beam and the subsequent extraction of the resulting nonlinear frictional behaviour associated with the joints.