Finite Element models for the dynamic analysis of composite and sandwich structures

Abstract

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