Mechanical behaviour of composite spandrels in unreinforced masonry buildings

La presente tesi tratta il comportamento meccanico delle fasce di piano in muratura composite. Con tale termine ci si riferisce alle fasce di piano che hanno al di sotto un elemento portante in conglomerato cementizio armato, come ad esempio cordoli o solai. Assieme ai maschi murari, le fasce di piano costituiscono gli elementi portanti di una parete in muratura. Tuttavia, in caso di analisi sismica di un edificio in muratura, l’effetto fornito da tali elementi è trascurato e si considera solamente il contributo dei maschi murari. Ciò è dovuto anche alla scarsa conoscenza che ancora oggi si possiede sul loro comportamento meccanico. Per questo motivo diversi gruppi di ricerca tutt’ora sono impegnati in tale studio. In particolare, il lavoro di questa tesi, s’inserisce nel più ampio progetto di ricerca condotto dalla professoressa Katrin Beyer, direttrice del Laboratorio di Ingegneria Sismica e Dinamica Strutturale del Politecnico di Losanna (Svizzera).

Computer aided design and finite element modeling to predict bulk mechanical properties of bone scaffolds using triply periodic minimal surfaces (progettazione assistita al calcolatore ed analisi con il metodo degli elementi finiti per predire il comportamento meccanico di scaffolds ossei creati con l'uso di superfici minime periodiche in tre direzioni)

The aim of Tissue Engineering is to develop biological substitutes that will restore lost morphological and functional features of diseased or damaged portions of organs. Recently computer-aided technology has received considerable attention in the area of tissue engineering and the advance of additive manufacture (AM) techniques has significantly improved control over the pore network architecture of tissue engineering scaffolds. To regenerate tissues more efficiently, an ideal scaffold should have appropriate porosity and pore structure. More sophisticated porous configurations with higher architectures of the pore network and scaffolding structures that mimic the intricate architecture and complexity of native organs and tissues are then required. This study adopts a macro-structural shape design approach to the production of open porous materials (Titanium foams), which utilizes spatial periodicity as a simple way to generate the models. From among various pore architectures which have been studied, this work simulated pore structure by triply-periodic minimal surfaces (TPMS) for the construction of tissue engineering scaffolds. TPMS are shown to be a versatile source of biomorphic scaffold design. A set of tissue scaffolds using the TPMS-based unit cell libraries was designed. TPMS-based Titanium foams were meant to be printed three dimensional with the relative predicted geometry, microstructure and consequently mechanical properties. Trough a finite element analysis (FEA) the mechanical properties of the designed scaffolds were determined in compression and analyzed in terms of their porosity and assemblies of unit cells. The purpose of this work was to investigate the mechanical performance of TPMS models trying to understand the best compromise between mechanical and geometrical requirements of the scaffolds. The intention was to predict the structural modulus in open porous materials via structural design of interconnected three-dimensional lattices, hence optimising geometrical properties. With the aid of FEA results, it is expected that the effective mechanical properties for the TPMS-based scaffold units can be used to design optimized scaffolds for tissue engineering applications. Regardless of the influence of fabrication method, it is desirable to calculate scaffold properties so that the effect of these properties on tissue regeneration may be better understood.

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.

On the characterization of the mechanical behaviour of bonded steel-CFRP adhesive joints

The use of adhesively bonded carbon fiber reinforced polymers (CFRP) is well established to repair metallic structural elements in the aerospace industry for more than three decades. Despite a few exceptions, this technology has yet not been exploited for the steel construction industry where there is a great need to rehabilitate old metallic bridges. For instance, in Europe more than 30% of the railway bridge stock operated for more than 100 years. These bridges are made of old mild steel or puddle iron that exhibits poor behaviour due to the quality of the material itself and degradation caused by the long-term loading or environmental effects. The modest results for Steel/CFRP joints obtained may be due to the type of adhesive used. In fact, most of the previous studies utilized brittle adhesives specially developed for concrete structures. Recent ductile adhesives that made for the automotive industry for metallic joints should be more appropriate. In this study, an experimental investigation on the behaviour of CFRP/steel adhesively bonded joints is presented. A comparison between brittle adhesives and ductile adhesives is conducted. The results show that the ductile adhesives achieve much higher performance than the brittle ones. The brittle adhesives provide more stiffness to the adhesive joint. In the specimens with the ductile adhesives, the failure pattern started by yielding the steel bars first then the adhesive joint which is promising since it can facilitate the design significantly if the steel yielding can be used as a design criterion. The main disadvantage of ductile adhesives is they are usually more expensive than brittle ones. In order to solve this issue, bi-adhesive joints, in which the joint is mainly made of (low cost) brittle adhesive and ductile adhesive in the stress concentration region, are proposed. The results revealed very high improvement up to the yielding strength of the steel bars and with a balanced stiffness.

Evaluation of the microstructural pattern and mechanical behaviour of human metastatic vertebrae

La spina dorsale è uno dei principali siti di sviluppo di metastasi ossee. Queste alterano sia la composizione strutturale che il comportamento meccanico delle vertebre metastatiche, riducendone la resistenza meccanica ed aumentandone il rischio di rottura. Questo studio ha valutato la composizione microstrutturale ed il comportamento meccanico a rottura in specifiche regioni all’interno di vertebre metastatiche. 11 segmenti vertebrali da cadavere, costituiti da una vertebra sana ed una con metastasi (litica, mista o blastica), sono stati testati con carichi graduali di compressione e scansionati con microCT. Le deformazioni interne sono state misurate tramite un algoritmo globale di Digital Volume Correlation (DVC). I risultati dall’analisi microstrutturale hanno mostrato l’ influenza sulla microstruttura delle diverse tipologie di metastasi in corrispondenza della lesione, mentre le caratteristiche microstrutturali nelle regioni intorno alla lesione sono risultate simili a quelle delle vertebre sane. L’analisi delle deformazioni ha inoltre permesso di valutare l’ effetto delle diverse tipologie di metastasi nel compromettere la stabilità spinale. Le vertebre con metastasi litiche hanno raggiunto deformazioni maggiori in corrispondenza della lesione, regione meccanicamente più debole e con una microstruttura maggiormente compromessa a causa della metastasi. Le vertebre con metastasi blastiche hanno raggiunto deformazioni minori nella lesione, regione che ha mostrato una maggiore resistenza meccanica ai carichi, e deformazioni maggiori nelle zone più lontane. Le vertebre con metastasi miste hanno mostrato un comportamento meccanico non univoco, legato alla predominanza di una lesione sull’altra. Infatti, la posizione e la proporzione tra le due lesioni sembra influenzare il comportamento meccanico. I risultati di questo studio, una volta generalizzati, potrebbero portare alla spiegazione delle cause di instabilità meccanica nelle vertebre metastatiche.

Mechanical characterization of 3D bioprinted meniscus

Le lesioni del menisco sono le più comuni nella società di oggi: si verificano per un trauma meccanico o per cambiamenti degenerativi nella composizione dei tessuti. In caso di rottura o danneggiamento si interviene mediante riparazione del menisco, menisectomia parziale o totale, o allotrapianto, ma tali tecniche portano a degenerazione della cartilagine articolare, aumento dello stress sull'articolazione tibiale e infiammazione. Gli impianti di sostituzione presenti in commercio non riescono a ricreare il tessuto naturale del ginocchio o a prevenire malattie degenerative della cartilagine; si cerca quindi di creare un menisco meccanicamente e chimicamente simile a quello nativo. In questo studio è realizzato, tramite stampante 3D, uno scaffold di alginato e nanocellulosa, con condrociti umani al suo interno. Le cellule sono opportunamente coltivate, raggruppate a formare sferoidi di diverse concentrazioni (5000 e 10000 cellule/sferoide) e inserite all'interno di scaffold caratterizzati rispettivamente da 4000 e 2000 sferoidi/ml di inchiostro. Le loro proprietà meccaniche, insieme a quelle del campione costituito dal solo bio-inchiostro, sono caratterizzate mediante nanoindentazione. Un'analisi statistica (0.05% di significatività), ha appurato una differenza nelle proprietà meccaniche dei campioni con diverse concentrazioni di sferoidi, e tra questi e il campione senza sferoidi. Il modulo elastico e la durezza riscontrati (kPa): E=23.97±13.05, H=3.15±1.23 nel controllo negativo, E=35.34±7.28, H=4.37±0.79 nel campione da 2000 sferoidi/ml e E=49.28±9.75, H=5.44±0.87 nel campione da 4000 sferoidi/ml. In conclusione, la formazione di agglomerati di cellule e la loro introduzione all'interno di uno scaffold è possibile ed è un buon metodo per controllare il numero di cellule inserite, in termini di sferoidi. Gli organoidi contribuiscono al modulo elastico e alla durezza del campione, determinando un incremento e una maggiore equità nelle proprietà meccaniche.

Design, fabrication and mechanical characterization of a structural node made using Wire and Arc Additive Manufacturing (WAAM) technology

Additive Manufacturing (AM), also known as “3D printing”, is a recent production technique that allows the creation of three-dimensional elements by depositing multiple layers of material. This technology is widely used in various industrial sectors, such as automotive, aerospace and aviation. With AM, it is possible to produce particularly complex elements for which traditional techniques cannot be used. These technologies are not yet widespread in the civil engineering sector, which is slowly changing thanks to the advantages of AM, such as the possibility of realizing elements without geometric restrictions, with less material usage and a higher efficiency, in particular employing Wire-and-Arc Additive Manufacturing (WAAM) technology. Buildings that benefit most from AM are all those structures designed using form-finding and free-form techniques. These include gridshells, where joints are the most critical and difficult elements to design, as the overall behaviour of the structure depends on them. It must also be considered that, during the design, the engineer must try to minimize the structure's own weight. Self-weight reductions can be achieved by Topological Optimization (TO) of the joint itself, which generates complex geometries that could not be made using traditional techniques. To sum up, weight reductions through TO combined with AM allow for several potential benefits, including economic ones. In this thesis, the roof of the British Museum is considered as a case study, analysing the gridshell structure of which a joint will be chosen to be designed and manufactured, using TO and WAAM techniques. Then, the designed joint will be studied in order to understand its structural behaviour in terms of stiffness and strength. Finally, a printing test will be performed to assess the production feasibility using WAAM technology. The computational design and fabrication stages were carried out at Technische Universität Braunschweig in Germany.