1,720,954 research outputs found
Scratch hardness as a quasi-intrinsic parameter to measure the scratch resistance of polymers
Full text linkIn this work four different polymers (acrylonitrile-butadiene-styrene, high-impact polystyrene, rubber-toughened polybutylene terephthalate, linear low-density polyethylene) were characterized in terms of their bulk (modulus and yield stress) and surface (scratch hardness) mechanical properties. The intrinsic time-dependence of the materials was addressed by performing DMA and compression tests at varying testing speed/frequency, exploiting time-temperature superposition and Eyring’s model to obtain data at strain rates compatible with scratch experiments. The latter were performed by applying different loading histories (constant depth or load) and indenters. Scratch hardness was determined using Pelletier’s model; it was demonstrated that such a parameter provides a reliable and almost intrinsic (i.e. loading history independent) evaluation of scratch resistance, seen as the resistance the material opposes to indenter penetration. The material compressive yield stress (evaluated at the strain rate relevant to scratch phenomena) was found to be the key controlling factor in determining scratch hardness. It can therefore be taken as a measure of the mechanical scratch resistance when evaluating the possible effects of variables such as material composition, crystallinity, physical ageing... Its relation with other aspects of the scratch phenomenon (in particular deformation recovery) was also explored, accounting for the specific deformation regime imposed by the indenter (transitioning from elastic to predominantly plastic)
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
A study of the scratch behaviour of ductile polymers
No full textLAUREA MAGISTRALEOggigiorno i polimeri sono sempre più utilizzati in una vasta gamma di campi d’applicazione, sostituendo molto spesso altri materiali come i metalli, grazie al loro basso costo, leggerezza, versatilità e facilità di produzione. Tuttavia, sono generalmente caratterizzati da minori proprietà meccaniche rispetto ai materiali metallici e ceramici; questo limite porta ad una maggiore sensibilità dei polimeri, in particolare di quelli duttili, ai fenomeni tribologici, come il graffio. I danni superficiali rappresentano uno svantaggio non solo da un punto di vista estetico, ma anche perché possono influenzare negativamente la funzionalità e l’integrità strutturale del componente plastico.
Lo studio del comportamento al graffio dei polimeri duttili risulta perciò di grande rilevanza per tutte quelle applicazioni che richiedono un’elevata qualità superficiale, su tutti il settore dell’automotive, ma anche l’industria ottica e di data storage. È dunque importante studiare questo fenomeno e la sua relazione con le proprietà meccaniche dei polimeri in modo da migliorarne la risposta al graffio.
Solitamente l’analisi del graffio è svolta tramite prove di graffio basate sullo scorrimento di un indentatore con una specifica geometria sulla superficie del materiale, in condizioni di prova controllate. Tuttavia, la descrizione di questo fenomeno, nel caso dei polimeri duttili, è estremamente complicata a causa della contemporanea dipendenza da vari parametri e dal comportamento viscoelastico-viscoplastico dei materiali coinvolti, poiché sia la temperatura che la velocità di deformazione hanno un’elevata influenza sull’aspetto finale del graffio. Infatti, durante il graffio dei polimeri, l’area di contatto tra la punta e il campione è determinata dall’accumulo di materiale (pile-up) di fronte alla punta e dal recupero elastico nella regione retrostante. Di conseguenza, non è ancora stato trovato un modello in grado di descrivere in modo esaustivo e univoco il comportamento al graffio dei polimeri duttili.
L’obiettivo di questo lavoro di tesi è verificare l’affidabilità di un modello recentemente proposto da Pelletier, il quale tiene conto, nel calcolo dell’area di contatto, sia del fenomeno di pile-up che del recupero elastico. Questo approccio si basa su un parametro noto come “fattore reologico”, già precedentemente introdotto in letteratura e dipendente dal rapporto tra il modulo di Young e lo sforzo a snervamento del materiale. La dipendenza dalla velocità di deformazione è implicitamente inclusa nell’espressione del fattore reologico, poiché sia i valori di E che di σy sono controllati da ε ̇. Questo modello permette di determinare un valore di durezza al graffio Hs, il quale fornisce un’indicazione della resistenza al graffio del polimero, e di definire due quantità in grado di descrivere il fenomeno di pile-up ed il recupero elastico, che sono rispettivamente il rapporto di forma e il parametro angolare.
Nella prima parte di questo studio è stata analizzata la risposta al graffio di quattro polimeri duttili con diverse viscoelasticità e rigidezza, eseguendo tre serie di prove a varie condizioni di carico. Gli obiettivi di questa analisi sono stati, da un lato, lo studio dell’effetto sul graffio dei principali parametri di influenza, e dall’altro il confronto dei dati sperimentali con i risultati ottenuti tramite il modello di Pelletier, in modo da valutare la sua affidabilità.
Tuttavia, i risultati ottenuti si sono dimostrati in disaccordo con i dati sperimentali acquisiti durante le prove di graffio. L’inadeguatezza del fattore reologico, e di conseguenza del modello di Pelletier, nella descrizione del comportamento al graffio dei polimeri duttili, è principalmente dovuta al fatto che non tiene conto né del carattere viscoelastico del materiale, né delle sue caratteristiche di scorrimento. Ad ogni modo, questo approccio permette di quantificare la resistenza opposta dal polimero durante la penetrazione dell’indentatore, definendo un valore della durezza al graffio. Inoltre, è stata riscontrata una correlazione lineare tra Hs e lo sforzo a snervamento del materiale.
Nell’ultima parte di questo studio si è tentato di effettuare un confronto tra i comportamenti a graffio e frattura dei polimeri duttili esaminati, con l’obiettivo di individuare possibili analogie. Questa correlazione è giustificata dal fatto che entrambi i processi sono dominati da sforzi in compressione e sono fortemente influenzati dal comportamento post-snervamento. I dati di frattura analizzati provengono da un’attività di round-robin effettuata dalla commissione ESIS TC4 sugli stessi quattro polimeri duttili esaminati nella prima parte tramite prove di graffio. In particolare, il parametro ms, che rappresenta l’avanzamento della cricca per unità di spostamento plastico, è sembrato essere un punto di partenza interessante per la correlazione con il processo di graffio. Infatti si è osservato che i valori di ms e delle profondità residue misurate durante le prove di graffio seguono un simile andamento, suggerendo così che il contributo plastico influenza in modo simile il comportamento dei polimeri esaminati nei due distinti processi. Tuttavia, non è stata riscontrata nessuna relazione tra la durezza al graffio e il parametro ms – e per cui tra il graffio e la frattura.
Il presente lavoro di tesi è diviso in cinque capitoli principali: il primo è dedicato all’introduzione delle caratteristiche fondamentali, degli aspetti critici e dei modelli teorici presenti in letteratura, riferiti sia al graffio che alla frattura. Segue poi una descrizione della procedura di analisi adottata durante la fase sperimentale e degli strumenti utilizzati. I risultati sono stati riportati e discussi nella terza sezione, mentre nel quarto capitolo sono stati confrontati con i dati di frattura. L’ultima parte è dedicata alle conclusioni finali.Polymers are increasingly being used in a wide range of application fields, often substituting other materials such as metals, thanks to their low cost, lightness, versatility and ease of production. Nevertheless, they are usually characterized by lower mechanical properties with respect to metallic or ceramic materials; this limitation leads to a higher sensitivity of polymers, in particular the ductile ones, to tribological phenomena, such as scratching. Superficial damages represent an issue not only from an aesthetic point of view, but also because they can negatively affect the functionality and structural integrity of the plastic component.
The study of the scratch behaviour of ductile polymers is therefore of great relevance for all those applications demanding a high surface quality, above all the automotive sector, but also data storage and optical industries. Hence, the importance of investigate this phenomenon and its relationship with mechanical properties, in order to improve polymer performance to scratch.
Usually, scratch analyses are carried out by means of scratch tests, by sliding an indenter with a specific geometry on the material surface, under controlled testing conditions. However, the description of this phenomenon in case of ductile polymers is extremely complex due to the dependence on many different parameters and on the viscoelastic-viscoplastic behaviour of the involved material, since both temperature and strain rate highly influence the final scratch damage. Indeed, during the polymers’ scratch, the accumulation of material (pile-up) in front of the indenter and the elastic recovery at the rear side, significantly affect the true contact area between the tip and the sample. Consequently, it hasn’t been found yet a model able to describe exhaustively and univocally the scratch behaviour of ductile polymers.
The aim of this thesis work is to verify the reliability of a model recently proposed by Pelletier, which accounts for both the pile-up phenomenon and the elastic recovery in the evaluation of the true contact area. This approach is based on a parameter called “rheological factor”, already introduced in literature and dependent on the ratio between the Young’s modulus and the yield stress of the material. The dependence on the strain rate is implicitly included in the rheological factor expression, since both E and σy values are controlled by ε ̇. This model allows to determine a scratch hardness value Hs, which gives an indication of the polymer’s scratch resistance, and to define two quantities that describe the pile-up phenomenon and the elastic recovery, which are the shape ratio and the angular parameter, respectively.
In the first part of this study, the scratch performance of four ductile polymers with different viscoelasticity and stiffness has been assessed, performing three sets of scratch tests at various testing conditions. The purposes were, on the one hand, to evaluate the effect of the principal influence parameters, and on the other to compare the experimental data with the Pelletier’s model results, in order to evaluate its reliability.
However, results obtained turned out in disagreement with experimental data acquired in scratch tests. The unsuitability of the rheological factor, and consequently of the Pelletier’s model, in the description of the scratch behaviour of ductile polymers, is mainly ascribed to the fact that both the viscoelastic character and the flow characteristics of the material are neglected. Nevertheless, this approach allows to quantify the resistance opposed by the polymer during the penetration of the indenter defining a value of scratch hardness. Moreover, a linear correlation between Hs and the material yield stress has been assessed.
In the last part of this study a comparison between the scratch and fracture behaviour has been attempted, in order to look for possible analogies. This correlation is justified by the fact that both processes are dominated by compressive stresses and highly influenced by post-yielding behaviour. The fracture data analysed came from a round-robin activity performed by ESIS TC4 on the same four ductile polymers examined in the first part by means of scratch tests. Specifically, the ms parameter, which represents the crack growth per unit of plastic displacement, appeared to be an interesting starting point for the correlation with scratch process. Indeed, it has been observed that values of ms and residual depths measured during the scratch test follow a comparable ranking, suggesting that the plastic contribution affects in a similar way the behaviour of the examined polymers in the two distinct processes. However, no significant relationship between scratch hardness and ms parameter – and therefore between scratch and fracture processes – has been found.
The present thesis work is divided in five main chapters: the first one is devoted to the introduction of the principal features, critical aspects and theoretical models present in literature, related to both the scratch and fracture processes. Then follows a description of the analysis procedure adopted in the experimental part and the instruments used. In the third section scratch results are reported and discussed, and then compared with fracture data in the fourth chapter. The last part is dedicated to final conclusions
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
Author-wise bibliometric analysis based on entropy.
No full textAuthor-wise bibliometric analysis based on entropy.</p
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