Development and testing of a Fourier trasform based experimental system for Dynamic Mechanical Thermal Analysis tests

Altough nowadays DMTA is one of the most used techniques to characterize polymers thermo-mechanical behaviour, it is only effective for small amplitude oscillatory tests and limited to a single frequency analysis (linear regime). In this thesis work a Fourier transform based experimental system has proven to give hint on structural and chemical changes in specimens during large amplitude oscillatory tests exploiting multi frequency spectral analysis turning out in a more sensitive tool than classical linear approach. The test campaign has been focused on three test typologies: Strain sweep tests, Damage investigation and temperature sweep tests.

Dynamic mechanical-thermal, microstructural and mechanical analysis of ultra-light polymer-metal composites: influence of forming

A really particular and innovative metal-polymer sandwich material is Hybrix. Hybrix is a product developed and manufactured by Lamera AB, Gothenburg, Sweden. This innovative hybrid material is composed by two relatively thin metal layers if compared to the core thickness. The most used metals are aluminum and stainless steel and are separated by a core of nylon fibres oriented perpendicularly to the metal plates. The core is then completed by adhesive layers applied at the PA66-metal interface that once cured maintain the nylon fibres in position. This special material is very light and formable. Moreover Hybrix, depending on the specific metal which is used, can achieve a good corrosion resistance and it can be cut and punched easily. Hybrix architecture itself provides extremely good bending stiffness, damping properties, insulation capability, etc., which again, of course, change in magnitude depending in the metal alloy which is used, its thickness and core thickness. For these reasons nowadays it shows potential for all the applications which have the above mentioned characteristic as a requirement. Finally Hybrix can be processed with tools used in regular metal sheet industry and can be handled as solid metal sheets. In this master thesis project, pre-formed parts of Hybrix were studied and characterized. Previous work on Hybrix was focused on analyze its market potential and different adhesive to be used in the core. All the tests were carried out on flat unformed specimens. However, in order to have a complete description of this material also the effect of the forming process must be taken into account. Thus the main activities of the present master thesis are the following: Dynamic Mechanical-Thermal Analysis (DMTA) on unformed Hybrix samples of different thickness and on pre-strained Hybrix samples, pure epoxy adhesive samples analysis and finally moisture effects evaluation on Hybrix composite structure.

Analisi sperimentale degli effetti di temperature elevate sul comportamento strutturale dei rinforzi in FRP

In questa tesi si è voluta porre l’attenzione sulla suscettibilità alle alte temperature delle resine che li compongono. Lo studio del comportamento alle alte temperature delle resine utilizzate per l’applicazione dei materiali compositi è risultato un campo di studio ancora non completamente sviluppato, nel quale c’è ancora necessità di ricerche per meglio chiarire alcuni aspetti del comportamento. L’analisi di questi materiali si sviluppa partendo dal contesto storico, e procedendo successivamente ad una accurata classificazione delle varie tipologie di materiali compositi soffermandosi sull’ utilizzo nel campo civile degli FRP (Fiber Reinforced Polymer) e mettendone in risalto le proprietà meccaniche. Considerata l’influenza che il comportamento delle resine riveste nel comportamento alle alte temperature dei materiali compositi si è, per questi elementi, eseguita una classificazione in base alle loro proprietà fisico-chimiche e ne sono state esaminate le principali proprietà meccaniche e termiche quali il modulo elastico, la tensione di rottura, la temperatura di transizione vetrosa e il fenomeno del creep. Sono state successivamente eseguite delle prove sperimentali, effettuate presso il Laboratorio Resistenza Materiali e presso il Laboratorio del Dipartimento di Chimica Applicata e Scienza dei Materiali, su dei provini confezionati con otto differenti resine epossidiche. Per valutarne il comportamento alle alte temperature, le indagini sperimentali hanno valutato dapprima le temperature di transizione vetrosa delle resine in questione e, in seguito, le loro caratteristiche meccaniche. Dalla correlazione dei dati rilevati si sono cercati possibili legami tra le caratteristiche meccaniche e le proprietà termiche delle resine. Si sono infine valutati gli aspetti dell’applicazione degli FRP che possano influire sul comportamento del materiale composito soggetto alle alte temperature valutando delle possibili precauzioni che possano essere considerate in fase progettuale.

Misura di adesione di film polimerici su tappi di sughero

L’industria enologica oggigiorno propone numerosi modi per sigillare le bottiglie in alternativa ai tappi di sughero: tappi sintetici, tappi a vite, tappi a corona. Tuttavia si conosce la preferenza del consumatore medio per il sughero. I motivi sono molteplici: il sughero restituisce un'immagine di qualità del vino, esprime la tradizione ed è un materiale vivo e naturale, così come il vino. La naturalità del sughero però comporta dei rischi. Ad esempio si stima in 10 miliardi di dollari a livello mondiale la perdita economica dovuta al "sentore di tappo" o "cork taint". Recenti ricerche attribuiscono la responsabilità di tale difetto a composti chimici che migrano dal sughero al vino. Innovazione e tradizione si sono incontrate nell'ideazione della "tecnologia a membrana" o "membrane technology". Essa consiste nell'isolare le teste dei tappi con delle pellicole multistrato per non far diffondere nel vino alcun tipo di agente contaminante. In questo studio si espone in dettaglio un metodo per misurare l'adesione di film multistrato polimerici – strato esterno in polietilene – su tappi di sughero monopezzo. I tappi già rivestiti sono stati forniti da un'azienda del settore senza alcuna specifica di fabbricazione. In particolare la fase di trazione è stata ideata in analogia alla prova meccanica nota come pull-off test, usata nell'ambito dell'ingegneria edile. La bontà dell'adesione è indagata dal punto di vista della fisica dei materiali. Per le misurazioni si è utilizzato il Dynamic Mechanical Analyser (DMA), modello Q800, che si trova presso il laboratorio CIRI-MAM ospitato dal Dipartimento di Fisica e Astronomia di Bologna.

Substructuring approache in state space models for dynamic system parameters identification

In the recent decade, the request for structural health monitoring expertise increased exponentially in the United States. The aging issues that most of the transportation structures are experiencing can put in serious jeopardy the economic system of a region as well as of a country. At the same time, the monitoring of structures is a central topic of discussion in Europe, where the preservation of historical buildings has been addressed over the last four centuries. More recently, various concerns arose about security performance of civil structures after tragic events such the 9/11 or the 2011 Japan earthquake: engineers looks for a design able to resist exceptional loadings due to earthquakes, hurricanes and terrorist attacks. After events of such a kind, the assessment of the remaining life of the structure is at least as important as the initial performance design. Consequently, it appears very clear that the introduction of reliable and accessible damage assessment techniques is crucial for the localization of issues and for a correct and immediate rehabilitation. The System Identification is a branch of the more general Control Theory. In Civil Engineering, this field addresses the techniques needed to find mechanical characteristics as the stiffness or the mass starting from the signals captured by sensors. The objective of the Dynamic Structural Identification (DSI) is to define, starting from experimental measurements, the modal fundamental parameters of a generic structure in order to characterize, via a mathematical model, the dynamic behavior. The knowledge of these parameters is helpful in the Model Updating procedure, that permits to define corrected theoretical models through experimental validation. The main aim of this technique is to minimize the differences between the theoretical model results and in situ measurements of dynamic data. Therefore, the new model becomes a very effective control practice when it comes to rehabilitation of structures or damage assessment. The instrumentation of a whole structure is an unfeasible procedure sometimes because of the high cost involved or, sometimes, because it’s not possible to physically reach each point of the structure. Therefore, numerous scholars have been trying to address this problem. In general two are the main involved methods. Since the limited number of sensors, in a first case, it’s possible to gather time histories only for some locations, then to move the instruments to another location and replay the procedure. Otherwise, if the number of sensors is enough and the structure does not present a complicate geometry, it’s usually sufficient to detect only the principal first modes. This two problems are well presented in the works of Balsamo [1] for the application to a simple system and Jun [2] for the analysis of system with a limited number of sensors. Once the system identification has been carried, it is possible to access the actual system characteristics. A frequent practice is to create an updated FEM model and assess whether the structure fulfills or not the requested functions. Once again the objective of this work is to present a general methodology to analyze big structure using a limited number of instrumentation and at the same time, obtaining the most information about an identified structure without recalling methodologies of difficult interpretation. A general framework of the state space identification procedure via OKID/ERA algorithm is developed and implemented in Matlab. Then, some simple examples are proposed to highlight the principal characteristics and advantage of this methodology. A new algebraic manipulation for a prolific use of substructuring results is developed and implemented.

Updating of Finite Element Models using static and dynamic optical strains with application to damage assessment

In the recent years, vibration-based structural damage identification has been subject of significant research in structural engineering. The basic idea of vibration-based methods is that damage induces mechanical properties changes that cause anomalies in the dynamic response of the structure, which measures allow to localize damage and its extension. Vibration measured data, such as frequencies and mode shapes, can be used in the Finite Element Model Updating in order to adjust structural parameters sensible at damage (e.g. Young’s Modulus). The novel aspect of this thesis is the introduction into the objective function of accurate measures of strains mode shapes, evaluated through FBG sensors. After a review of the relevant literature, the case of study, i.e. an irregular prestressed concrete beam destined for roofing of industrial structures, will be presented. The mathematical model was built through FE models, studying static and dynamic behaviour of the element. Another analytical model was developed, based on the ‘Ritz method’, in order to investigate the possible interaction between the RC beam and the steel supporting table used for testing. Experimental data, recorded through the contemporary use of different measurement techniques (optical fibers, accelerometers, LVDTs) were compared whit theoretical data, allowing to detect the best model, for which have been outlined the settings for the updating procedure.