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Adhesion and friction in steady and unsteady viscoelastic contacts
Full text linkQuesta tesi presenta un’innovativa formulazione di natura energetica per lo studio della meccanica del contatto adesivo di materiali viscoelastici. La formulazione proposta si basa sul formalismo del principio dei lavori virtuali: variazioni virtuali del dominio di contatto implicano il bilancio tra il lavoro delle forze (esterne) adesive e il lavoro delle tensioni interne. Quest'ultimo non può essere semplicemente derivato come variazione di energia potenziale: il comportamento intrinsecamente non-conservativo dei materiali viscoelastici deve essere adeguatamente considerato. Di conseguenza, gli aspetti matematici e fisici della formulazione energetica si discostano significativamente da equivalenti casi puramente elastici. A partire dall'assunzione range di interazione adesiva infinitamente corto, l'approccio proposto generalizza, di fatto, il criterio di frattura di Griffith ai materiali non-conservativi. L'equazione di chiusura del problema di contatto stazionario o non-stazionario è derivata imponendo il bilancio energetico al contorno dell'area di contatto, sfruttando formulazioni di tipo Boundary-Element basate sulle opportune funzioni di Green. Dunque, la presenza di dissipazione viscoelastica nell'intero volume del materiale è modellata correttamente. Di conseguenza, sono superate le limitazioni di molti studi precedenti che hanno approcciato il contatto adesivo viscoelastico assumendo che le perdite viscoelastiche siano localizzate al contorno dell’area di contatto e si annullino nella maggior parte del volume del sistema. La teoria proposta fornisce risultati supportati da evidenze sperimentali nel campo e offre approfondimenti sui meccanismi fisici responsabili di fenomeni osservati sperimentalmente. La prima parte della tesi si concentra sul contatto tra superfici rugose con moto relativo laterale in regime stazionario. A seconda del regime di velocità relativa, le performance adesive del sistema, in termini di forza di distacco, tenacità e dimensione dell'area di contatto, risultano notevolmente incrementata rispetto ai corrispondenti casi puramente elastici, in accordo con esistenti evidenze sperimentali. Questo fenomeno è attribuibile alle perdite viscoelastiche localizzate al trailing ege del contatto, dove l’energy release rate è incrementato. Tale fenomeno influisce significativamente anche sull’attrito: il coefficiente di attrito, dipendente dalla velocità, risulta notevolmente incrementato rispetto a condizioni di contatto prive di adesione. A basse velocità, questo comportamento dipende dalle perdite viscoelastiche localizzate al bordo del contatto. A velocità intermedie, riflette una complessa interazione tra viscoelasticità macroscopica, viscoelasticità locale, e adesione. I risultati evidenziano che le perdite viscoelastiche locali e macroscopiche non possono essere separate linearmente. Questo è un risultato chiave, confermato da studi sperimentali esistenti. La formulazione energetica è successivamente estesa a generiche condizioni non-stazionarie e applicata per analizzare il moto dinamico di indentazione e retrazione di una sfera rigida in contatto adesivo con un semi-spazio viscoelastico. In questo caso, oltre a prevedere correttamente gli effetti delle perdite viscoelastiche locali, la teoria identifica un differente fondamentale meccanismo, osservato anche sperimentalmente, responsabile dell'incremento dell'adesione effettiva indotto dalla viscoelasticità. In particolare, quando la retrazione del rigido inizia da uno stato completamente rilassato, l'aumento della forza di distacco dipende da una sorta di "stato congelato", innescato dalla risposta elastica (di alta frequenza) del materiale, durante il quale l'area di contatto rimane quasi costante. I diversi meccanismi fisici responsabili dell'incremento della forza adesiva effettiva in condizioni non stazionarie sono analizzati in dettaglio per differenti storie di carico temporali, sfruttando un nuovo approccio sviluppato per calcolare correttamente l’energy release rate per materiali viscoelastici in condizioni non-stazionarie generali. I risultati indicano chiaramente che trascurare la risposta viscoelastica macroscopica del materiale e assumere che la dissipazione sia localmente confinata potrebbe generare errori significativi. Nell'ultimo capitolo, la teoria oggetto della tesi è applicata allo studio della meccanica della frattura in solidi viscoelastici. Nel momento in cui le condizioni di propagazione stazionaria di una cricca sono assunte, l'approccio fornisce risultati perfettamente in accordo con studi precedenti. Se questa ipotesi è rilassata, la teoria è in grado di predire correttamente complessi fenomeni non stazionari, come la cosiddetta delayed fracture: quando un solido viscoelastico è sottoposto ad un certo carico, applicato in maniera discontinua, la sua frattura può avvenire dopo un certo tempo di ritardo, il cui ordine di grandezza corrisponde a quello del tempo di rilassamento del materiale. Nel complesso, la formulazione energetica proposta è di interesse in diverse applicazioni ingegneristiche, in cui gli effetti dell'interazione tra viscoelasticità e adesione sul comportamento meccanico del contatto devono essere adeguatamente controllati e progettati, come ad esempio adesivi strutturali, adesivi sensibili alla pressione, rivestimenti protettivi, adesivi ispirati a sistemi biologici, applicazioni ortopediche, sistemi micro-elettro-meccanici, micro-manipolazione e micro-assemblaggio.This thesis presents a novel general energy approach for adhesive contact mechanics of viscoelastic materials. The proposed formulation relies on the virtual work formalism: virtual variations of the contact domain must imply the precise balance between the work of the (external) adhesive forces and the work of internal stresses. Importantly, the latter cannot be simply derived as the variation of a potential energy: the intrinsic non-conservative behavior of viscoelastic materials must be properly considered. For this reason, the mathematical and physical aspects of the energy formulation significantly deviate from equivalent elastic cases. Moving from the assumption of infinitely short-range adhesive forces, the proposed approach, in fact, generalizes the Griffith's fracture criterion to hysteretic materials. The closure equation of the steady or unsteady contact problem is derived by enforcing the energy balance at the boundary of the contact area and exploiting boundary formulations based on the Green's function approach. This allows to correctly model the viscoelastic dissipation involving the entire volume of the material, thus overcoming limitations of many previous studies that approached viscoelastic adhesive contacts by assuming that viscoelastic losses are localized at the contact edges and vanish in the bulk of the material. The proposed theory provides results in solid agreement with experimental evidence and insights into the underlying physical mechanisms responsible for the experimentally observed phenomena. The first part of the thesis focuses on steady-state sliding contacts between rough surfaces. Depending on the sliding velocity, the effective adhesive strength of the system in terms of pull-off force, toughness, and contact area size is found highly enhanced compared to corresponding purely elastic cases, in agreement with experimental evidence. This is ascribable to viscoelastic losses localized at the contact trailing edge, where the energy release rate is increased. This phenomenon also highly affects the frictional response: the velocity-dependent friction coefficient is found significantly increased compared to corresponding adhesiveless conditions. At low velocity values, this behavior depends on local small-scale viscoelastic losses. At intermediate velocity, it reflects a complex interplay between bulk viscoelasticity, small-scale viscoelasticity, and adhesion. Importantly, summing up independent estimations of small-scale and large-scale viscoelastic losses does not provide a correct estimation of the friction coefficient. This is a key result, confirmed by existing experimental studies. The proposed energy formulation is then extended to general unsteady conditions and applied to analyze the dynamic approach-retraction motion of a rigid sphere in adhesive contact with a viscoelastic half-space. In this case, besides correctly predicting the effects of local viscoelastic losses, the proposed theory identifies a different fundamental mechanism, also experimentally observed, responsible for adhesion enhancement. Specifically, when the retraction of the indenter begins from a fully relaxed state, the enhancement of the pull-off force depends on a sort of "frozen" state, triggered by the material's glassy response, during which the contact area is almost constant. The different physical mechanisms responsible for the increase of adhesion strength in unsteady conditions are investigated in detail for different loading time-histories and by exploiting a novel approach to correctly calculate the energy release rate for viscoelastic materials under general unsteady conditions. Results clearly indicate that neglecting the viscoelastic response of the bulk material while modeling adhesive contacts might lead to significative errors. In the last part of the thesis, the proposed theory is applied to investigate crack propagation and healing in viscoelastic solids. When steady-state conditions are assumed, the approach provides results in perfect agreement with previous studies. If this assumption is relaxed, the theory is able to correctly tackle complex unsteady phenomena, as the so called delayed-fracture: under a given applied load, the fracture of a viscoelastic solid may occur after a certain delay-time, whose order of magnitude corresponds to that of the material’s relaxation time. Overall, the proposed energy formulation might be of interest in several engineering application, in which the effects of the interplay between viscoelasticity and adhesion on the contact behavior must be properly controlled and designed, such as, for instance, structural adhesives, pressure-sensitive adhesives, protective coatings, bio-inspired adhesives, orthopedic applications, micro-electro-mechanical systems, micro-manipulations and micro-assembly
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
Author-wise bibliometric analysis based on entropy.
No full textAuthor-wise bibliometric analysis based on entropy.</p
Author Under Sail The Imagination of Jack London, 1893-1902
No full textIn Author Under Sail, Jay Williams offers the first complete literary biography of Jack London as a professional writer engaged in the labor of writing. It examines the authorial imagination in London's work, the use of imagination in both his fiction and nonfiction, and the ways he defined imagination in the creative process in his business dealings with his publishers, editors, and agents. In this first volume of a two-volume biography, Williams traverses the years 1893 to 1902, from London's "Story of a Typhoon" to The People of the Abyss. The Jack London who emerges in the pages of Author Under Sail is a writer whose partnership with publishers, most notably his productive alliance with George Brett of Macmillan, was one of the most formative in American literary history. London pioneered many author models during the heyday of realism and naturalism, blurring the boundaries of these popular genres by focusing on absorption and theatricality and the representation of the seen and unseen. London created an impassioned, sincere, and extremely personal realism unlike that of other American writers of the time. Author Under Sail is a literary tour de force that reveals the full range of London as writer, creative citizen, and entrepreneur at the same time it sheds light on the maverick side of machine-age literature.Intro -- Title Page -- Copyright Page -- Dedication -- Contents -- Acknowledgments -- Introduction -- 1. Spirit Truth -- 2. From Absorption to Theatricality and Back Again -- 3. "I Will Build a New Present" -- 4. Sons as Authors -- 5. Fathers as Publishers -- 6. The Daughter as Author -- 7. Lovers as Authors -- 8. At Sea with the Family -- 9. Yellow News, Yellow Stories -- 10. The Return Home -- Notes -- Bibliography -- Index -- About Jay WilliamsIn Author Under Sail, Jay Williams offers the first complete literary biography of Jack London as a professional writer engaged in the labor of writing. It examines the authorial imagination in London's work, the use of imagination in both his fiction and nonfiction, and the ways he defined imagination in the creative process in his business dealings with his publishers, editors, and agents. In this first volume of a two-volume biography, Williams traverses the years 1893 to 1902, from London's "Story of a Typhoon" to The People of the Abyss. The Jack London who emerges in the pages of Author Under Sail is a writer whose partnership with publishers, most notably his productive alliance with George Brett of Macmillan, was one of the most formative in American literary history. London pioneered many author models during the heyday of realism and naturalism, blurring the boundaries of these popular genres by focusing on absorption and theatricality and the representation of the seen and unseen. London created an impassioned, sincere, and extremely personal realism unlike that of other American writers of the time. Author Under Sail is a literary tour de force that reveals the full range of London as writer, creative citizen, and entrepreneur at the same time it sheds light on the maverick side of machine-age literature.Description based on publisher supplied metadata and other sources.Electronic reproduction. Ann Arbor, Michigan : ProQuest Ebook Central, YYYY. Available via World Wide Web. Access may be limited to ProQuest Ebook Central affiliated libraries
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