Non Contact Measurement System with Electromagnets for Vibration Tests on Bladed Disks

In vibration testing one of the main concerns in the measurement of the forced response of vibrating structures is the coupling between the test article and the exciter. When the structural behaviour of the structure is pretty linear a hammer test allows getting a first idea of the resonant frequencies and mode shapes without perturbing the dynamic response. Unfortunately, real applications of structures whose behaviour is linear are rare. Usually, structures are made of components assembled together by means of joints whose behaviour may result highly nonlinear. Depending on the amount of excitation, joints can dramatically change the dynamic behaviour of the whole system and the modelling of this type of constraint is therefore crucial for a correct design. For this reason, tests are performed to characterize the contact parameters of joints focusing the attention in the material used in the application or extending the study by considering the whole components coupled by a particular geometry of joint. The characterization of joints in dynamics requires the knowledge of the amplitude of the exciting force which is usually set as a harmonic waveform whose frequency varies within a range of interest in the so called stepped-sine test which overcomes two limits of the hammer test. In fact, the stepped-sine test controls the force amplitude at each value of frequency in a given range by means of a feedback control which activates the measurement only when the target force is reached. Moreover a realistic value of force amplitude must be reached in order to explore the nonlinear behaviour of the structure. For this reason electromagnetic shakers are used by connecting the exciter to the test article by means of a stinger. Different strategies are used to detune the dynamics of the test article itself from the dynamics of the exciter , they are based on the design of the stinger or on the constraint of the shaker. Unfortunately, these strategies are not useful in particular applications where a small perturbation may introduce large changes in the dynamics of structures or in general it may interfere with the measurement of the physical phenomena. In turbomachinery, this is the case when the nonlinear friction damping of the blade root attachment to the disk is studied. The amount of damping introduced by the contact is very small, nonetheless it is still of the same order of magnitude of the structural damping of the blade and it can be increased by a proper design of the blade root geometry (dovetail, fir-tree, flat, crowned). A contact excitation of the blade like the one required by the electromagnetic shaker would perturb the measurement of the

Hali tarjima qilinmagan