Development of modal analysis methodologies for the identification of aerospace structures in operating conditions

Operational modal analysis differs from traditional experimental modal analysis in that it only requires information of the output responses and the modal parameters (in terms of natural frequencies damping ratios and mode shapes) are estimated under the assumption of white noise excitation. It presents several advantages including the availability of modal properties of structure in operation thus representing a closer picture of the structure and its boundary conditions (which are not that easy to realize in laboratory conditions). However, lack of input excitation force information presents several challenges as well as the proper estimation of the frequency response functions and the accurate evaluation of modal parameters in presence of harmonic components in the excitation. Several methodologies have been developed in the last years as described in this thesis and the main purpose of the research is to assess their application in aerospace such as the rotorcraft technology and the environmental testing. Solutions to the main operational modal analysis limitations are suggested and the implementation of the related algorithms allows the application on several test cases after their validation. Taking advantages of this improving in the experimental analysis capabilities, a demanding application within the rotorcraft technology is carried out. Starting from the already developed Active Pitch Link prototype (based on the Smart Spring concept), its use for vibration reduction on rotating blade is numerically and experimentally investigated, thanks to the intensive use of the operational modal analysis for the identification of the real system properties that give the necessary information for the tuning of the numerical model, that in turn suggests the operative test conditions