Egidio Lofrano

Identification of Uncertain Vibrating Beams through a Perturbation Approach

AbstractUncertainty characterization plays a key role in the safety assessment of many engineering structures. Nevertheless, the inverse problem for structures with uncertain parameters has received less attention than the relevant direct one, since one deals with stochastic structural system identification. This paper discusses the dynamic identification of linear structural systems with random stiffness parameters. Following a perturbation approach, recently proposed by the authors in a discrete framework, an identification technique for transversely vibrating 1-D uncertain continua is proposed. Results for a paradigmatic case, a simply supported beam, are presented and discussed.

PZT Experimental Detection of Natural Frequencies for Compressed Thin-Walled Beams

By means of PZT pickups, we experimentally measured the natural frequencies of an aluminum thin-walled open profile under centered compression, and investigated the effects of end warping constraints on free vibration and buckling loads. The specimen was mounted on a MTS testing machine, controlling the axial displacement imposed to a beam end by the hydraulic jack. The free vibration frequencies were detected for different values of the compressive force. The experimental results are compared to those provided by an in-house numerical code, which investigates the elastic stability of thin-walled beams in a dynamic setting. Indeed, the code is able to follow the paths of natural frequencies versus the applied load, accounting for the effect of cross-sectional warping on both natural frequencies and buckling loads. The experimental results show that piezoelectric pickups, like the disk adopted, can be efficiently used for the experimental modal analysis of engineering structures.

Damage Identification in a Parabolic Arch via Orthogonal Empirical Mode Decomposition

Localized damage in a parabolic arch is identified by combining a time-frequency vibration signals processing technique and modal-based properties. The approach is devised to detect and locate damaged areas in the structure from its free vibrations and relies on the comparison between information gathered for the undamaged and damaged states. The non-stationary acceleration data are decomposed by applying the orthogonal empirical mode decomposition to derive the orthogonal intrinsic mode functions. The latter provide a multi-frequency basis enabling to build “pseudo-modes” based on which a new damage index is proposed. The performance of the devised approach is shown through pseudo-experimental tests on a damaged arch.Copyright

Dynamic Identification of Classically Damped Uncertain Structures

The detection of structural damping is a crucial point in structural identification. Classic techniques usually refer to deterministic systems, since the assumption of randomness in the mechanical quantities implies non-trivial analytical difficulties in the development of both the direct and the inverse problem. In some recent works, starting from the statistics of mode-shapes and (undamped) frequencies, a perturbative approach has been introduced by the authors for the estimation of mean and variance of uncertain mass and stiffness. Here dissipative structures are considered; in detail, the method is applied for the stochastic structural identification of classically damped linear dynamic systems, dependent on a random parameter, assumed to be Gaussian. A numerical validation of the technique is then presented and discussed.

Experimental and Numerical Elastodynamic Analysis of Compressed Open Thin-walled Beams

Compressed thin-walled beams with open section are prone to torsional, or flexural-torsional, buckling. Here we present the results of several studies where, by piezoelectric pickups and a universal testing machine, we experimentally detected the natural frequencies and buckling loads of centrally compressed aluminum thin-walled beams with open cruciform section, exhibiting remarkable warping stiffness. We detected the free vibration frequencies for different values of the compressive force and for both free and (at least partially) restrained warping of the end sections. We compared the behavior of integer elements to that of analogous beams, where we introduced a localized damage, i.e., a sharp variation of a cross-section. For the integer elements, we compared the experimental results with those provided by an in-house numerical code, which investigates the elastic stability of possible non-trivial paths of thin-walled beams in a dynamic setting. The results show that, on the one hand, piezoelectric pickups can be efficiently used to extract modal parameters of structural elements; on the other hand, the numerical code proves to be robust and accurate in the determination of the buckling loads of the integer elements, in all the analyzed configurations.

Sensitivity Analysis and Improvement of a Pseudo-Modal Approach for Damage Localization

Localization of damages becomes rather challenging when the associated stiffness reduction is small in presence of structural uncertainties. This work presents a sensitivity analysis and an improvement of a novel pseudo-modal approach, recently proposed by the authors. Starting from free vibrations of the undamaged and damaged states, the method aims to maximize the damage signature embedded in the data exploiting the peculiar features of the Orthogonal Empirical Mode Decomposition technique. The role of the length of the signals and the boundary effects are here investigated; a cut-off rule useful for reducing the latter issue is also proposed.Copyright