1,720,964 research outputs found
Development, characterisation and processing optimisation of unidirectional biodegradable continuous fibre-reinforced composites
No full textLo sviluppo di biocompositi termoplastici rinforzati con fibre continue di origine vegetale rappresenta un passo fondamentale verso soluzioni sostenibili alternative ai compositi tradizionali di origine fossile. Questa tesi si concentra sulla realizzazione e ottimizzazione di un biocomposito rinforzato con fibre continue di lino e matrice di acido polilattico, rivolgendo l’attenzione a tecniche di realizzazione innovative e all’ottimizzazione del processo produttivo. L’obiettivo è dunque superare le principali criticità legate alle performance meccaniche e alla difficoltà d’impregnazione comunemente riscontrate nella produzione di biocompositi a matrice termoplastica.
La prima fase dello studio ha riguardato la realizzazione di un dispositivo di tensionamento finalizzato alla fabbricazione di lamine composite unidirezionali. I risultati ottenuti nella direzione delle fibre sono adeguati per applicazioni strutturali, mentre sono limitate le proprietà trasversali (< 5 MPa). Questo è dovuto principalmente a un'elevata frazione volumetrica di fibra e a un’impregnazione insufficiente, attribuibile alla scarsa penetrazione della matrice termoplastica nello strato di fibra.
Per risolvere tali problematiche, è stata adottata una metodologia basata sulla tecnica del dry filament winding sviluppando un dispositivo di avvolgimento ottimizzato per questi materiali. Il processo prevedeva l’avvolgimento di un nastro continuo di lino attorno a una piastra in alluminio usata come mandrino. Questa tecnica ha consentito l’introduzione di una quantità precisa e controllata di matrice, migliorando sensibilmente l’impregnazione delle fibre. Tale modifica nella preparazione del composito ha portato a un miglioramento delle proprietà meccaniche complessive. I risultati hanno evidenziato la possibilità di ottenere un materiale strutturale nella direzione longitudinale (resistenza ~190 MPa) e determinato un miglioramento delle proprietà trasversali (resistenza ~20 MPa). La stessa differenza, in termini di ordine di grandezza, tra le proprietà longitudinali e trasversali è osservabile anche nei compositi tradizionali Il composito ha inoltre mostrato un basso contenuto di vuoti (< 5%), una buona macro-impregnazione fibra-matrice e una ridotta dispersione delle proprietà meccaniche dovuta al limitato disallineamento delle fibre.
La terza e ultima fase della ricerca ha riguardato l’ottimizzazione del processo produttivo sviluppato e descritto precedentemente. Sono stati dunque analizzati diversi parametri chiave del processo, tra cui la pressione finale, il numero di fasi di pressatura e il tempo di ciclo, con l’obiettivo di individuare le condizioni ottimali di lavorazione in grado di garantire le migliori prestazioni meccaniche del materiale. L’obiettivo era ridurre ulteriormente la presenza di vuoti, ottenere proprietà meccaniche ripetibili e migliorare l’efficienza produttiva. Gli esperimenti sono stati condotti e analizzati tramite metodologia Design of Experiments (DoE). I risultati hanno indicato che un attento controllo dei parametri di processo permette di ottenere le proprietà meccaniche desiderate e un materiale omogeneo. In particolare, il tempo di processo si è rivelato il parametro più influente per migliorare l’impregnazione e quindi ridurre il contenuto di vuoti.
In conclusione, questa ricerca contribuisce allo sviluppo di un materiale composito ecosostenibile e completamente compostabile, con proprietà meccaniche che ne permettono l’utilizzo in applicazioni strutturali proponendosi come un’alternativa promettente ai compositi tradizionali. Nonostante le proprietà trasversali siano sensibilmente inferiori rispetto a quelle longitudinali, una corretta progettazione può permettere l’utilizzo di questo materiale in numerose applicazioni quando sono presenti requisiti stringenti in termini di leggerezza e ridotto impatto ambientale.The development and production of continuous fibre reinforced thermoplastic biocomposites are essential in advancing sustainable alternatives to high performance traditional composites, which have a detrimental environmental impact. This thesis focuses on the development and realization of a flax reinforced polylactide based biocomposite through innovative fabrication techniques and process optimisation. The research aims at overcoming key challenges associated with mechanical performance and impregnation efficiency that typically are present in the production of these materials.
The first phase of this study involved the design and fabrication of a tensioning device, intended to manufacture unidirectional composite laminae from biodegradable fibres and matrices. The results were adequate for structural purposes in the longitudinal direction. However, it was found that the produced laminae exhibited poor transversal properties (< 5 MPa), mainly due to a too high fibre volume fraction and an insufficient fibre impregnation, which in turn was attributed to the inadequate penetration of the thermoplastic resin into the fibre layer.
To address these issues, a different production method was selected, utilising a dry filament winding technique. A winding machine was developed and adapted for the production of the biocomposites. This approach consisted in the winding of a continuous flax ribbon onto a flat aluminium plate used as a mandrel. The filament winding process permitted the introduction of a precise and desired amount of matrix, thus a significant improvement in fibre impregnation followed. This modification in the composite preparation determined an enhancement of the overall mechanical properties. In fact, the results showed that a structural material in the longitudinal direction (~190 MPa strength) can be obtained that has a low void content (< 5%), an adequate fibre-matrix macro-impregnation and a limited dispersion in mechanical properties due to the reduced fibre misalignment. These aspects also led to good transversal properties (~20 MPa strength). The difference found between longitudinal and transversal properties is due to the anisotropy of unidirectional materials and can be observed also in traditional composites.
The third and last phase of this research focuses on the optimisation of the manufacturing process for the filament-wound biocomposites. Several key process parameters, including ultimate pressure, number of pressure steps and cycle time were investigated to identify the optimal processing conditions, which also led to satisfactory mechanical performance of the biocomposite. The goal was to fine-tune the process to ensure void reduction, mechanical consistency and increased production efficiency. Experiments were conducted and analysed via DoE methodology, in order to understand the effects of these parameters on the impregnation quality and final material performance. The results indicated that careful control of processing parameters can lead to desired mechanical properties and a homogeneous material. In particular, processing time plays the most crucial role in achieving a better impregnation and consequently a low void content.
This research contributes to the development of an environmentally friendly and fully biodegradable composite with structural and consistent mechanical properties, which can be seen as a promising alternative to conventional composites in various applications. Despite transversal properties are much lower than those in the longitudinal direction, a proper design, that takes anisotropy into account, can be able to replace aluminium alloys in certain applications. This is possible in cases that are not too demanding in terms of mechanical performance but present a strong requirement for a lightweight material possessing a low environmental impact
A Post-Processing Method for Improving the Mechanical Properties of Fused-Filament-Fabricated 3D-Printed Parts
No full textFused-filament fabrication is one of the most popular 3D printing techniques for thermoplastic materials because it is easy to use and is low-cost. On the other hand, it has the great limit of being suitable only for developing prototypes, because the printed object generally has low mechanical properties, and this prevents its use in structural applications. To solve this issue, the scientific literature has mainly focused on the optimization of the printing parameters and on performing some post-printing treatments, e.g., annealing, but despite some results being very promising, the topic has not yet been exhaustively investigated. In this paper, a post-printing treatment was studied that was based on two subsequent stages of remelting and compaction within a mold made of a granular material. The material chosen for this study was a green composite made of poly-(lactic acid) and poly-(hydroxyalkanoate) filled with wood fibers. The density, mechanical properties in terms of tension and microscopic observations were used to evaluate the treatment effectiveness. The main results were that voids were reduced, and the quality of the interlayer welding was increased, and this improved the mechanical properties, both in terms of stiffness and strength. In particular, the initial specimens displayed remarkable anisotropy, being stronger and stiffer in the longitudinal direction. After the post-processing treatment, despite the longitudinal properties having a very limited increase, the transversal properties increased significantly until they reached the longitudinal properties, thus leading to a more isotropic material
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Appropriate Similarity Measures for Author Cocitation Analysis
Full text linkWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
Dispelling the Myths Behind First-author Citation Counts
Full text linkWe conducted a full-scale evaluative citation analysis study of scholars in the XML research field to explore just how different from each other author rankings resulting from different citation counting methods actually are, and to demonstrate the capability of emerging data and tools on the Web in supporting more realistic citation counting methods. Our results contest some common arguments for the continued
use of first-author citation counts in the evaluation of scholars, such as high correlations between author rankings by first-author citation counts and other citation
counting methods, and high costs of using more realistic citation counting methods that are not well-supported by the ISI databases. It is argued that increasingly available digital full text research papers make it possible for citation analysis studies to go beyond what the ISI databases have directly supported and to employ more
sophisticated methods
koamabayili/VECTRON-author-checklist: VECTRON author checklist
No full textWe have done our best to complete the author checklist relating to the use of animals in the hut study. Note that the objective for the hut study was to evaluate the IRS treatment applications for residual efficacy against Anopheles mosquitoes, including the local An. coluzzii mosquito population. Cows were only used to attract mosquitoes into the huts and no tests were carried out directly on the cows. The author checklist is intended for use with studies where experiments are carried out on animals, which is why we have had such difficulty in completing this for the hut study, as many of the questions do not relate to how the cows were used
Real-world data on new users of atypical antipsychotics: characterisation, prescription patterns, healthcare costs and early cardio-metabolic occurrences from a large Italian database
No full textPurpose: To describe new users of atypical antipsychotics (APs) in terms of sociodemographic characteristics, cardio-metabolic risk profile, prescription patterns, healthcare costs and cardio-metabolic events over the 24 months after treatment initiation. Methods: Atypical AP new users were selected from the ReS database and grouped into three: patients already affected by cardio-metabolic diseases (group A), patients without these clinical conditions but with predisposing conditions (group B) and patients without cardio-metabolic diseases and predisposing conditions (group C). Annual prescription patterns and healthcare costs were analysed. Subjects of groups B and C were matched with controls to compare the occurrences of cardio-metabolic events over 24 months. Results: Thirty-two thousand thirty-four new users of atypical APs were selected (median age 69). The 22.3% had cardio-metabolic diseases, 14.8% had predisposing conditions and 62.9% had none of these. The 99.3% received monotherapy. The mean annual cost per patient was €2785, and the median cost was €1108. After 24 months, a cardio-metabolic event occurred in 11.5% of group B vs. 8.7% of the controls (p < .01), and in 5.0% of group C vs. 2.1% of the controls (p < .01). Conclusion: Patients treated with atypical AP were on average old and, in a non-negligible amount, with cardio-metabolic disease or predisposing conditions. New users of atypical APs showed a significantly higher likelihood to develop a cardio-metabolic event early after treatment initiation. Keywords: Administrative data; Atypical antipsychotic; Cardio-metabolic risk; Italy; Real-world evidence
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