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Damage Detection of a large space structure using a model obtained by Modal Coupling

In the first part of the work, the model of a large space structure is created, starting from the finite element models of its components. The presented results refer to an experiment carried out in 1992 at the Politecnico di Milano, but the same method has been used with great success in many other instances in which building the entire full scale model of a structure was not feasible. The matter at hand fits into the broader cathegory of substructuring, and a brief overview of other methods is also given. Having chosen to implement modal coupling through the use of fictitious masses, the numerical version of the boundary masses used in the experiments, the technique is explained in detail. Before entering the specifics, and the results, it is appropriate to remind that the connection between the substructures, or each substructure and the fictitious mass, is always modeled as isostatic – this is not the only way. Then, examples are provided in order to give the taste of the power of performing modal coupling with this architecture. In this sense, a parametric study is carried out on the fictitious mass “shape”, in order to show the benefits coming from the use of this device, and on the size of the whole process, depending on which the results can differ.
The second part of the work deals with a problem which is becoming more and more important as structures increase in size, and operational times increase: damage detection. This can be undertaken resorting to different techniques, lately non-destructive are improving the level of performance. Dynamic tests, although, can still be a valuable help, and we will concentrate on these. The hypothesis we make is that the damage is located in a precise area of the structure, and the algorithm we want to use is a least-squares approximation based on two different indicators. The first kind of indicators we use are the changes in the natural frequencies values brought about by the damaging of the structure, while the second group are the transfer functions values close to the resonances that show more evident differences with respect to the ones obtained from the undamaged structure. The obtained results highlight the difficulty of the analysis we set out for. First, using the model devised in the first part constitutes a problem, in that even if the approximation is very accurate, the differences brought about by the damages are not always distinctively significant. Second, depending on the parameters we use for damage detection, the results can differ quite significantly, because the damage introduced alters the particular parameter in a specific way.

Mostra/Nascondi contenuto.
17 1. INTRODUCTION 1. Purposes The purpose of the present work is to start from the results of an experiment of Modal Coupling and to get to a model of an assembled structure that can be used conveniently for damage detection. In the phases of the design of a space structure, both an experimental part and a numerical part are included. This work does not have the aim of representing the whole numerical analysis, as that would require a much more thorough work of quantification, and a very precise specification concerning the operative life of the mission the structure has to accomplish, and a clear statement of the performances to be achieved. This work has the objective of investigating the possibility of detecting a problem in the functioning of a specific part of the structure using a model devised through Modal Coupling with fictitious masses. This type of substructuring architecture is introduced as a tool to create a numerical model that can be used as an alternative to the finite element model. The advantage of Modal Coupling in practical terms is that it does not require to model (or to setup for experiments) the whole structure. After the finite element models of the components have been created, and updating, if necessary, has been performed using experimental results in order to correct discrepancies between numerical results and measurements, then the routine of Modal Coupling makes it possible to have a precise model of the assembled structure. The fundamental point is that the expenses that a full scale entire structure model, along with the excitation and measuring devices bring about are not always easy to withstand, and can actually be prohibitive at times. This is why it’s so important to be able to have a tool that can give good results in creating an assembly of components, once these are known, and provided that sufficient data about the interfaces among them are given. The idea behind substructuring is to be able to reach conclusions concerning the properties of the assembled structures, only working with the models of its components.

Tesi di Laurea Magistrale

Facoltà: Ingegneria

Autore: Camillo Vitale Contatta »

Composta da 119 pagine.

 

Questa tesi ha raggiunto 26 click dal 05/02/2014.

Disponibile in PDF, la consultazione è esclusivamente in formato digitale.

 

 

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