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Abstract: The objective of this paper is the definition and verification of an iterative procedure for the detailed monitoring of the dynamic response of a structural system based on the experimental modal model of the system on ground and a limited number of measurements in the operating condition. The method is formulated starting from a linear model, but it can be applied to weakly non-linear system, because of the iterative nature of the algorithm. It does not require the definition of a numerical model, but an experimental representation in the modal coordinates, achievable with a preliminary modal survey with an arbitrary large amount of experimental points. The proposed method is presented as well as its implementation and several numerical test cases in order to validate its formulation. Experimental analyses are performed on a uniform beam and on a full-scale aeronautical composite component with promising results. [Copyright &y& Elsevier]

Abstract: This paper is focused on an assessment of the state-of-the-art of operational modal analysis (OMA) methodologies in estimating modal parameters from output responses on helicopter structures. For this purpose, a ground vibration test was performed on a real helicopter airframe. In the following stages, several OMA techniques were applied to the measured data and compared with the results from typical input–output approach. The results presented are part of a more general research activity carried out in the Group of Aeronautical Research and Technology in Europe (GARTEUR) Action Group 19, helicopter technical activity, whose overall objective is the improvement of the structural dynamic finite element models using in-flight test data. The structure considered is a medium-size helicopter, a time-expired Lynx Mk7 (XZ649) airframe. In order to have a comprehensive analysis, the behaviour of both frequency- and time-domain-based OMA techniques are considered for the modal parameter estimates. An accuracy index and the reliability of the OMA methods with respect to the standard EMA procedures, together with the evaluation of the influence of the experimental setup on the estimate of the modal parameters, will be presented in the paper. [Copyright &y& Elsevier]

Output‐only analysis considers the system excited either by operative or by natural forces, in both cases the input loading which excites the structure is considered, at least in a limited frequency band, as a white noise. Because it is not possible to find directly the spectra, it is necessary to pass through the correlation functions so as to apply the Wiener–Khintchine theorem in order to find the power spectral densities (PSDs). In the past, the modal parameters have been essentially derived by approaches that manipulated the functions mentioned above. In this paper, the modal parameters will be directly estimated from the PSDs, in the frequency domain, and from the correlation functions in the time domain. A particular attention is devoted to the problems regarding the damping ratio overestimation. This effect due to the limited time window in the correlation function estimate is highly present in the low‐frequency modes and could bring to large estimation errors. Experimental examples, carried out both on cantilever beams and on an helicopter blade, are presented. They show the problems related to the techniques using the PSDs, or the correlation functions and the possibility to overcome the overestimation of the damping factors due to the triangular window, also known as Bartlett window. [ABSTRACT FROM AUTHOR]