Dipartimento di Ingegneria Meccanica, Energetica e Gestionale

The use of lightweight multi-layered materials is dramatically changing the design process and criteria in many engineering fields. The transportation industry, for example, is facing major challenges in order to replace traditional materials while keeping at least the same level of passengers’ comfort and safety. In particular, the Noise, Vibration and Harshness (NVH) performances are affected by the novel combination of high stiffness and low density. If the aeronautic industry still heavily relies on testing to assess designs’ validity, such an approach cannot be applied to the automotive industry for the development costs would be too high. It is therefore necessary to identify CAE tools capable of giving realistic, reliable and cost-effective predictions of multi-layered structures’ behaviour under dynamic loadings. An often overlooked problem is that of damping which is generally higher in composite and sandwich structure but rarely it is also efficiently exploited, so that in most cases the classic approach of applying NVH treatments is followed. However, this procedure has a detrimental effect on the attained weight saving and on the global dynamic performance of lightweight structures, therefore leading to unsatisfactory results. Moreover, the variability of mechanical properties due to the low repeatability of some manufacturing processes can also have an impact on the global behaviour of the as-manufactured component. An early integration of damping prediction and an estimate of possible stiffness variations due to the manufacturing can actually lead to better designs in less time. In this thesis these challenges are tackled from the Computer Aided Engineering (CAE) point of view, thanks to the introduction of a novel finite element for the prediction of the damped response of generic multi-layered structures and the proposition of a CAMCAE approach to introduce manufacturing simulations at an early stage in the design and analysis process. In the first chapters, different analytical and numerical approaches for the modelling of multi-layered structures are presented and used for the development of a 1D finite element. The results of the mono-dimensional analysis show that zigzag theories are a cost-effective and accurate alternative to solid finite element models, motivating the development of a 2D element for the analysis of plates and shells. With respect to previous investigations on zigzag theories, the current study focus on their use for modal parameters prediction, i.e. eigenfrequencies, mode shapes and damping. It will be shown that compared to classic models, the zigzag elements are able to predict the dynamic response, damped and undamped, of beam, plates and shells with the same accuracy of 3D models but at a much lower computational cost. In the last chapter, the available homogenisation methods for the analysis of long fibres composites are reviewed and compared to more refined models based on manufacturing simulation algorithms. Results show that changes in manufacturing parameters lead to substantially different results. The goal is to show that CAM/FE coupling is possible already at an early design stage and that manufacturing simulations can be used as a mean to further optimise the performance of composite structures. As a final stage, an example of coupling between zigzag theories and manufacturing simulations is presented. Despite some limitations, the proposed methods increase the accuracy of the analysis and gives a better understanding of lightweight multi-layered structures. Further research could focus on the use of the developed zigzag elements for fatigue analysis and delamination modelling as well as detailed modelling of drop-off regions in the framework of CAM tools improvements

The nite element method is the reference technique in the simulation of metal forming and provides excellent results with both Eulerian and Lagrangian implementations. The latter approach is more natural and direct but the large deformations involved in such processes require remeshing-rezoning algorithms that increase the computational times and a ect the quality of the results. For this reasons alternative techniques based on modi ed FEM formulations or meshless approximants are worth to be investigated. In particular, in this project, the nite element method with nodal integration and the maximum entropy meshfree approximants are studied and applied to the simulation of selected problems in metal forming and orthogonal cutting. Il metodo degli elementi niti e attualmente la tecnica di riferimento nella simulazione di processi di formatura e fornisce ottimi risultati sia con formulazioni Euleriane che Lagranzgiane. Quest'ultimo approccio e pi u naturale e diretto ma le grosse deformazioni che entrano in gioco in questi processi richiedono l'applicazione di algoritmi di remeshing-rezoning che aumentano i tempi computazionali e peggiorano la qualit a dei risultati. Per questi motivi assume particolare interesse lo sviluppo di tecniche alternative, basate su formulazioni FEM modi cate o su approssimazioni senza maglia (meshless). In particolare, in questo lavoro, sono studiati il metodo degli elementi niti con integrazione nodale e le tecniche meshless di tipo maximum entropy e, inoltre, sono applicati alla simulazione di processi di formatura e taglio

Dipartimento di Ingegneria Meccanica, Energetica e Gestionale - Tesi di Dottorato

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