Self-Evaluation Applied Mathematics 2003-2008 University of Twente

This report contains the self-study for the research assessment of the Department of Applied Mathematics (AM) of the Faculty of Electrical Engineering, Mathematics and Computer Science (EEMCS) at the University of Twente (UT). The report provides the information for the Research Assessment Committee for Applied Mathematics, dealing with mathematical sciences at the three universities of technology in the Netherlands. It describes the state of affairs pertaining to the period 1 January 2003 to 31 December 2008

Non-linear system identification in structural dynamics: advances in characterisation of non-linearities and non-linear modal analysis

Many new methods for theoretical modelling, numerical analysis and experimental testing have been developed in non-linear dynamics in recent years. Although the computational power has greatly improved our ability to predict non-linear behaviour, non-linear system identification, a central topic of this thesis, still plays a key role in obtaining and quantifying structural models from experimental data. The first part of the thesis is motivated by the industrial needs for fast and reliable detection and characterisation of structural non-linearities. For this purpose a method based on the Hilbert transform in the frequency domain is proposed. The method detects and characterises structural non-linearities from a single frequency response function and does not require a priori knowledge of the system. The second part of the thesis is driven by current research trends and advances in non-linear modal analysis and adaptive time series processing using the Hilbert-Huang transform. Firstly, the alternatives of the Hilbert transform, which is commonly used in structural dynamics for the estimation of the instantaneous frequency and amplitude despite suffering from a number of numerical issues, are compared to assess their potential for non-linear system identification. Then, a possible relation between the Hilbert-Huang transform and complex non-linear modes of mechanical systems is investigated. Based on this relation, an approach to experimental non-linear modal analysis is proposed. Since this approach integrates the Hilbert-Huang transform and non-linear modes, it allows not only to detect and characterise structural non-linearities in a non-parametric manner, but also to quantify the parameters of a selected model using extracted non-linear modes. Lastly, a new method for the identification of systems with asymmetric non-linear restoring forces is proposed. The application of all proposed methods is demonstrated on simulated and experimental data.Open Acces

Dynamic characterization of high performance materials for application to cultural heritage

Natural hazards, such as earthquakes, can compromise the integrity of the cultural heritage with potentially devastating effects. The reduction of the seismic vulnerability of the cultural heritage constitutes a question of maximum importance especially in countries where vast cultural heritage combines with a medium or high seismic risk, such as in Italy. From the second half of the last century, the scientific community edited a number of important documents and charts for the conservation, reinforcement and restoration of the cultural heritage. The aim is to mitigate the seismic vulnerability of the cultural heritage. This research focused on high performance materials for applications aimed to structural and seismic protection of cultural heritage, with a special focus on historical masonry structures. In particular, the final aim is to define a self-diagnosis strategy for fibres, yarns and ties in view of efficient, non-invasive and reversible interventions on cultural heritage buildings. In order to set up the scene, the present thesis starts by introducing the reader to the seismic protection of cultural heritage thorough an extensive review on high performance materials, strengthening techniques and systems, taking care to highlight real world applications and limitations of their use. The second step of this work concerns in the mechanical and rheological characterization of high performance material fibres. The materials investigated are essentially Kevlar® 29 (para-aramid), Carbon and Silicon Carbide. To reach this goal, an extensive experimental testing campaign was conducted on fibres and yarns in accordance with specific protocols. A further step was defining appropriate damage indices for different materials, with a special focus on Kevlar® 29. Within the same research programme, a novel testing machine was also designed in cooperation with the Laboratory of Electronic Measurements of the Politecnico di Torino. A prototype-testing machine for dynamic testing on high resistance fibres was built using recycled materials and components. A distinctive feature of this machine is that it can apply to the sample any kind of dynamic excitation (random, impulse, harmonic etc.). A second testing campaign concerned the durability of Kevlar® 29 fibres, which are known to be sensitive to long-term exposure to UV radiation. Accordingly, for this campaign, the samples were artificially damaged by using UV lamps. The analysis of the resonance profiles allowed for the extraction of parameters such as the elastic moduli, quality factors, and non-linear coefficient for a set of fibres. In particular, non-linearity parameters derived from the Krylov-Bogoliubov method demonstrated to be consistent with the damage affecting the fibres. The final chapter of the dissertation concerns a new concept for a tie endowed with self-diagnosis properties, which are obtained by integrating a low cost testing device into the tie model. The self-diagnosis properties system of existing structures has an important role in the preservation of the cultural heritage because the best therapy is preventive maintenance. Specifically, the para-aramid tie system proposed for the reinforcement of historic building constitutes a non-invasive, reversible and repeatable intervention, as required by the main guidelines on preservation of cultural heritage