Controlled morphing of architected liquid crystal elastomer elements: modeling and simulations

Liquid crystal elastomers (LCE) are elastomeric materials possessing a network microstructure made of chains with a preferential orientation, induced by mesogen units embedded in the material prior to polymerization. This peculiarity can be harnessed to induce deformation of an LCE element by making its network switch from the preferentially oriented nematic state to the isotropic one, as occurs for instance by rising the temperature above a transition value characteristic of the material. This mechanism can be combined with an architected arrangement of LCE elements, whose nematic orientation and transition temperature are properly differentiated among the different zones constituting the element. In this way, interesting morphing capabilities can be obtained out of an architected elastomer made of LCE portions (ALCE), leading to a morphing structure whose deformation can be activated and precisely tuned by heating up or cooling down the material. In this research, we propose some simple architected LCE elements showing the capability of producing a variety of deformed shapes. A micromechanical theoretical model for LCE is firstly illustrated and several examples of morphing of architected LCE elements, whose mechanical response is obtained through finite element (FE) numerical analyses based on the proposed micromechanical model, are illustrated and critically discussed

Mechanical behaviour of photopolymerized materials

The photopolymerization process used for the production of additively manufactured polymers employed in advanced applications, enables to obtain objects spanning a large dimensional scale thanks to the molecular size achievable by the solidification process. In fact, the photopolymerization is based on the physical-chemical network cross-linking mechanism taking place at the nanoscale. Since the starting raw material is a liquid resin that progressively becomes solid upon the irradiation by a suitable light source, the mechanical properties – and so the corresponding mechanical response of the final printed structural material – heavily depend on the degree and distribution of the polymerization induced in the material itself. In the present study, starting from the governing equations of the light-induced polymerization process, we determine the chain density formed within the solid domain. Then, the mechanical response of photopolymerized elements obtained with different photopolymerization parameters is investigated. Moreover, the microstructure optimization of polymeric elements in relation to the achievement of the desired mechanical response with the least energy spent in the polymer’s formation, is studied. Finally, some interesting considerations related to the modelling of the photopolymerization process are outlined

Smart actuation of liquid crystal elastomer elements: cross-link density-controlled response

Liquid crystalline elastomers (LCEs) exhibit some remarkable physical properties, such as the reversible large mechanical deformation induced by proper environmental stimuli of different nature, such as the thermal stimulus, allowing their use as soft actuators. The unique features displayed by LCE are originated from their anisotropic microstructure characterized by the preferential orientation of the mesogen molecules embedded in the polymer network. An open issue in the design of LCEs is how to control their actuation effectiveness: the amount of mesogens molecules, how they are linked to the network, the order degree, the cross-link density are some controllable parameters whose spatial distribution, however, in general cannot be tuned except the last one. In this paper, we develop a theoretical micromechanical-based framework to model and explore the effect of the network cross-link density on the mechanical actuation of elements made of liquid crystalline elastomer. In this context, the light-induced polymerization (photopolymerization) for obtaining the elastomers’ cross-linked network is of particular interest, being suitable for precisely tuning the cross-link density distribution within the material; this technology enables to obtain a molecular-scale architected LCEs, allowing the optimal design of the obtainable actuation. The possibility to properly set the cross-link density arrangement within the smart structural element (LCE microstructure design and optimization), represents an intriguing way to create molecular-scale engineered LCE elements having material microstructure encoded desired actuation capabilities

a phase field approach for crack modelling of elastomers

Abstract The description of a problem related to an evolving interface or a strong discontinuity requires to solve partial differential equations on a moving domain, whose evolution is unknown. Standard computational methods tackle this class of problems by adapting the discretized domain to the evolving interface, and that creates severe difficulties especially when the interface undergoes topological changes. The problem becomes even more awkward when the involved domain changes such as in mechanical problems characterized by large deformations. In this context, the phase-field approach allows us to easily reformulate the problem through the use of a continuous field variable, identifying the evolving interface (i.e. the crack in fracture problems), without the need to update the domain discretization. According to the variational theory of fracture, the crack grows by following a path that ensures that the total energy of the system is always minimized. In the present paper, we take advantage of such an approach for the description of fracture in highly deformable materials, such as the so-called elastomers. Starting from a statistical physics-based micromechanical model which employs the distribution function of the polymer's chains, we develop herein a phase-field approach to study the fracture occurring in this class of materials undergoing large deformations. Such a phase-field approach is finally applied to the solution of crack problems in elastomers

The economic impact of Italian colonial investments in Libya and in the Horn of Africa, 1920-2000

This dissertation examines the micro-level effects of Italian colonial investments in Libya, Somalia, Eritrea, and Ethiopia, and sheds light on both their short and long-term impact. It focuses on two flagship projects, launched by the dictator Benito Mussolini during the 1930s, namely the construction of a modern road network in the Horn of Africa and the settlement of Italian farmers in Libya. The contributions are twofold. First, this thesis focuses on types of colonial investment that have not been studied before, while looking at a group of colonies that have previously been neglected by the cliometrics revolution in African economic history. Thus, it enhances our understanding of the effect of colonialism in general and, on Africa, in particular. Second, by exploiting a set of quasi-natural experiments from the history of Italian colonialism to explore the micro-effect of specific policies, this thesis also contributes to the economic geography and development literatures that have looked at the determinants of agglomeration and productivity in developing countries. It is structured around three substantive chapters. The first one studies the effect of Italian road construction in the Horn of Africa on economic development and shows how locations that enjoyed a first-mover advantage in transportation thanks to the Italian road network are significantly wealthier today. The second substantive chapter assesses the effect of Italian agricultural settlement on indigenous agriculture in Libya at the end of the colonial period and pinpoints an adverse effect of Italian presence on Libyan productivity. Finally, the third substantive chapter studies the effect of the expulsion of Italian farmers from Libya after World War II and finds a reduction in agricultural commercialization in affected districts following the shock

LASER-BASED ADDITIVELY MANUFACTURED POLYMERS: A REVIEW ON PROCESSES AND MECHANICAL MODELS

Additive manufacturing (AM) is a broad definition of various techniques to produce layer-by-layer objects made of different materials. In this paper, a comprehensive review of laser-based technologies for polymers, including powder bed fusion processes (e.g. selective laser sintering, SLS) and vat photopolymerisation (e.g. stereolithography, SLA), is presented, where both the techniques employ a laser source either to melt or cure a raw polymeric material. The aim of the review is twofold: (i) to present the principal theoretical models adopted in the literature to simulate the complex physical phenomena involved in the transformation of the raw material into AM objects; (ii) to discuss the influence of process parameters on the physical final properties of the printed objects, and in turn on their mechanical performance. The models being presented simulate: the thermal problem along with the thermally activated bonding through sintering of the polymeric powder in SLS; the binding induced by the curing mechanisms of light-induced polymerisation of the liquid material in SLA. Key physical variables in AM objects, like porosity and degree of cure in SLS and SLA respectively, are discussed in relation to the manufacturing process parameters, as well as to the mechanical resistance and deformability of the objects themselves

Partial tendon tear as unusual cause of trigger finger. a case report

We report a case of post-traumatic trigger finger due to a partial longitudinal tear of the flexor digitorum superficialis. The suspect came from the clinical history and the young age of the patient. It was successfully treated with tendon flap suture and pulley A1 release

Chest pain caused by multiple exostoses of the ribs: A case report and a review of literature

Abstract The aim of this paper is to report an exceptional case of multiple internal exostoses of the ribs in a young patient affected by multiple hereditary exostoses (MHE) coming to our observation for chest pain as the only symptom of an intra-thoracic localization. A 16 years old patient with familiar history of MHE came to our observation complaining a left-sided chest pain. This pain had increased in the last months with no correlation to a traumatic event. The computed tomography (CT) scan revealed the presence of three exostoses located on the left third, fourth and sixth ribs, all protruding into the thoracic cavity, directly in contact with visceral pleura. Moreover, the apex of the one located on the sixth rib revealed to be only 12 mm away from pericardium. Patient underwent video-assisted thoracoscopy with an additional 4-cm mini toracotomy approach. At the last 1-year followup, patient was very satisfied and no signs of recurrence or major complication had occured. In conclusion, chest pain could be the only symptom of an intra-thoracic exostoses localization, possibly leading to serious complications. Thoracic localization in MHE must be suspected when patients complain chest pain. A chest CT scan is indicated to confirm exostoses and to clarify relationship with surrounding structures. Video-assisted thoracoscopic surgery can be considered a valuable option for exostoses removal, alone or in addiction to a mini-thoracotomy approach, in order to reduce thoracotomy morbidity

Mechanical characterization of additively manufactured photopolymerized polymers

Photopolymerization, based on light-induced radical polymerization, is nowadays exploited in additive manufacturing (AM) technologies enabling to achieve high dimensional quality. The mechanical properties of the obtained material are heavily dependent on the chemistry of the photopolymer and on the way the AM process is performed. Here we study, through experiments and theoretical modeling, how the mechanical properties of liquid crystal shutter (LCD) printed photopolymers depend on the printing process setup, namely UV exposure time and layer thickness. To this end, a multi-physics simulation tool considering the light diffusion, chemical kinetics, and the micro-mechanics at the network level, has been developed

Fractiles Based Sampling Procedure: a new probabilistic approach to evaluate the design resistance of a structural element

I sistemi ingegneristici, intesi come strutture o più in generale componenti meccanici o dispositivi, non sono perfetti. Una progettazione perfetta richiede che il sistema rimanga operativo e persegua tutte le sue funzioni, durante una predefinita vita utile, senza ammettere possibilità di danneggiamento o collasso. Questo approccio di progettazione è certamente qualcosa di ideale, impraticabile ed economicamente difficile da perseguire. Anche se la conoscenza tecnica non è più un fattore limitante nella progettazione, produzione, costruzione e gestione del sistema finale, il costo di sviluppo dei test, dei materiali e dell'analisi ingegneristica può superare di gran lunga le prospettive economiche di tali sistemi. Pertanto, limitazioni pratiche ed economiche delineano il perseguimento di progettazioni imperfette. Nonostante questo aspetto imprescindibile, i progettisti hanno il dovere di minimizzare la probabilità di crisi di tali sistemi. A tal proposito, specie per strutture d’elevata importanza o esistenti, analisi strutturali sempre più complesse vengono svolte[Report RTD:1016-1:2017, 2017, “Guidelines for Nonlinear Finite Element Analysis of Concrete Structures”]. La crisi di un sistema ingegneristico, in questo caso di un elemento strutturale, è legata alla presenza di incertezze che nell’analisi e nella progettazione sono da sempre presenti. Tali incertezze, possono riguardare tutte le parti di un sistema, come le caratteristiche intrinseche (geometria, armatura..), i carichi agenti, i fattori ambientali o incertezze legate ai parametri meccanici dei materiali. A questo proposito, l’approccio tradizionale (Partial Safety Factor, PSF) semplifica la progettazione assumendo che le incertezze siano di tipo deterministico, mediante l’introduzione di coefficienti di sicurezza. Tale approccio può rivelarsi in generale troppo conservativo, o in alcuni casi non affidabile; in particolare esso non fornisce informazioni su quale sia il grado di sicurezza raggiunto. Le normative forniscono poi altri approcci di tipo stocastico (Probabilistic Safety Formats). Tali approcci, nella loro forma esatta (Fully Probabilistic, FP) richiedono un elevatissimo sforzo computazionale. All’interno dei metodi stocastici le normative forniscono anche metodi probabilistici di tipo semplificato (Estimation Of Coefficient Of Variation, ECOV) che però spesso forniscono risultati non cautelativi. In questo lavoro di tesi, è stato proposto un metodo probabilistico semplificato (Fractiles Based Sampling Procedure, FBSP), alternativo a quanto proposto dalle norme, sviluppato nell’ambito della collaborazione tra l’Università di Parma e la Boku University di Vienna. Il metodo è basato sui risultati ottenuti dal Latin Hyperbolic Sampling (incardinato nel FP). Esso consente di raggiungere una resistenza di progetto, riducendo di gran lunga il numero d’analisi non lineari da condurre. In particolare, dagli N set di parametri meccanici generati dal software statistico (FReET), che andrebbero utilizzati per condurre N analisi non-lineari nell’ambito di un approccio FP, viene estratto un sotto-campione formato da soli sette set di parametri meccanici, scelti sulla base dei valori che un parametro specifico (leading parameter) assume in corrispondenza di sette frattili. Il leading parameter può essere uno solo di tutte le proprietà meccaniche che descrivono il sistema ingegneristico, ad esempio la resistenza a compressione. Attraverso lo studio di una trave a T precompressa, soggetta a taglio, sono stati proposti alcuni metodi volti alla determinazione del leading parameter. Una volta determinato tale parametro, è stata valutata la resistenza di tale trave e confrontata con le resistenze ottenute dai sopra menzionati approcci (FB, PSF e ECOV). Come verrà mostrato mediante un’analisi parametrica condotta su pannelli con differenti percentuali meccaniche d’armatura ed orientamento, la scelta di questo parametro è fortemente dipendente dal meccanismo di rottura dell’elemento strutturale analizzato, dal quale il progettista non può dunque prescindere