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Delamination between self-assembled monolayers
No full textSelf-assembled monolayers are adsorbed onto substrates without any external guidance. They are highly ordered and closely packed, and modify the physical and chemical properties of the surface onto which they are adsorbed. For this reason, self-assembled monolayers are of critical importance in many areas such as MEMs or biomedical applications. They have been extensively studied for the past twenty-five years. Nevertheless, the delamination between two self-assembled monolayers had not been studied before. The following thesis presents in the first chapter the process used to create self-assembled monolayers terminated by carboxyl groups or amine groups on Si(111). The fracture experiments were designed to be performed in an Ultra High Vacuum environment with a four-point bending configuration. The design and the construction of the loading device, including the Ultra High Vacuum chamber, are described in the second section. Finally, the analysis of the delamination between self-assembled monolayers using a semi-analytical method and a finite element method is presented. SAMs were fabricated but did not show strong enough adhesion forces to be used for fracture experiments thus mica was used to test the loading device. Future research will be done using stronger binding techniques such as covalent binding.Aerospace Engineerin
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
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Failure of laterally crushed aluminum tubes under combined bending and tension
Full text linktextThis thesis is concerned with the accurate numerical simulation of localized deformation that can develop into necking and failure, induced by combined bending and tension in aluminum alloy shell structures. The study is motivated by the need to establish the onset and evolution of such failures in imploding underwater cylindrical aluminum alloy shell structures. However, failure under combined bending and tension is also of concern in sheet metal forming. Such localized zones of deformation are shown to develop under controlled conditions in specially designed crushing experiments of Al-6061-T6 cylindrical shells. In these experiments shells of finite length and radially constrained ends are crushed laterally by rigid punches. The crushing, which is conducted under displacement control, causes the shell to develop bending and stretching stresses that lead to arcs of localized wall thinning to appear near the radially constrained locations. The local wall thinning develops into depressions with a width of the order of the shell wall thickness. As crushing progresses the depressions deepen, increase their span, become neck-like and develop inclined failures. The crushing was terminated when the first of four such depressions ruptured. After unloading, the shell was sliced along the principal plane of crushing and the most deformed cross sections of the necks were measured using an optical microscope.
The crushing experiments were simulated numerically using solid FE models. The material was modeled as a finitely deforming elastic-plastic solid that hardens isotropically using three constitutive models: the first is based on the von Mises yield function, the second on the non-quadratic isotropic Hosford yield function and the third on the anisotropic Yld04-3D yield function. The models were calibrated to the same stress-strain response and to data from a set of radial biaxial experiments conducted on the same alloy tubes. The overall structural response was reproduced well by all models. Apparently such global responses smear out local differences introduced by the shape of the yield function adopted. However, differences between the three constitutive models were observed in the evolution of localization in the depressions. For the von Mises yield function, the localized deformation was significantly milder than in the experiments. The isotropic Hosford yield function produced necks that were closer to the experimental ones, while Yld04-3D produced results that were very close to the measurements. Clearly, and in concert with other applications, the adoption of a non-quadratic yield function is necessary for reproduction of localization and other challenging deformation histories in Al alloys. The addition of anisotropy in such models improves further the predictions. The results also demonstrated that accurate simulation of the evolution of the depressions in the presence of normal contact stresses requires the use of solid elements. Localization is clearly a three-dimensional phenomenon and shell elements reproduce most of the structural response well, but not the depressions and their evolution that eventually cause failure.Aerospace Engineerin
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
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Transition from intimate contact to monolayer lubricated contact
Full text linkOver the years, nonwear friction with single asperity contact has been examined through experiments using the Surface Force Apparatus (SFA) and the Atomic Force Microscope (AFM). The contact radii in SFA and AFM friction experiments ranged in the order of tens of [mu]m (>10⁵ m) and several nm (< 10⁻⁸ m), respectively. In spite of the fact that the contact radii in these experiments differ by several orders of magnitude, the data from both experiments obey Bowden and Tabor's friction law F = [tau]A , where F is the friction force, [tau] is the frictional shear strength and A is the real contact area. However, there is a crucial difference between the results obtained with the two instruments. The shear strength from the SFA experiments in dry environment is in the tens of MPa, while the shear strength from the AFM measurement is several hundreds of MPa. In the intervening mesoscales, with contact radii ranging from 10⁻⁸ < a < 10⁻⁵, the frictional shear strength must be dependent on contact area in order to link these two extremes. Some models based on dislocation motions have recently been developed to bridge the gap (Hurtado and Kim, 1999a; b). Hitherto, no systematic mesoscale friction experiments to bridge the shear strengths obtained from AFM and SFA have been provided. In addition, this is precisely the range in which MEMS and potential NEMS devices are expected to operate. Therefore, apart from the fundamental challenges involved in resolving the scale dependence of friction, there is a strong technological motivation for studying friction at this scale. In the present work, this transition in shear strength is bridged using a newlydeveloped Mesoscale Friction Tester (MFT) over a wide range of contact radii and relative humidity levels. Since a nonwear and single asperity contact is of interest, novel procedures to fabricate tungsten probes with subnanometer (<0.3nm) surface roughness are initiated. In order to choose an appropriate contact mechanics theory in an ambient environment to obtain the true contact area, a modified Tabor parameter for JKR-DMT transition for capillary force dominant contact is employed. Results from friction experiments show that the transition in shear strength occurred over contact radii of only 20~30nm in both ambient and dry environments. It is hypothesized that shear strengths in the tens of MPa resulted from contact separated by a monolayer of interfacial molecules and shear strengths in hundreds of MPa resulted from intimate contact (no interfacial molecules inside the contact zone). It was the interfacial condition inside the contact zone that governed the transition. Furthermore, there is no continuous spectrum of shear strength, but a "quantized" behavior. A continuum analysis based on Lifshitz theory, which related the shear strength to the estimated strength of van der Waals bonds is proposed to explain the quantized shear strengths obtained from current experiments and both previous AFM and SFA friction experiments.Engineering MechanicsAerospace Engineerin
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
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Investigation of high strain rate behavior of metallic specimens using electromagnetic inductive loading
Full text linktextAerospace EngineeringThe aim of this thesis is to explore the high strain rate behavior of metallic specimens using electromagnetic inductive loading as the means to inflict the required high strain rate deformation on laboratory scale specimens, allowing for controlled, repeatable experiments to be performed. Three separate experiments were designed and performed, using helical and spiral coils as the sources of radial and unidirectional loading.
The first experiment evaluated the effect of applying a polymer coating on 30.5 mm diameter, Al 6061- O tube samples, in two lengths, 18 and 36 mm. The expanding tube experiment was used to apply a radial loading on the specimens and record the event. Several optical techniques were then used to evaluate the behavior of the samples. Coatings of polyurea and polycarbonate were used. It was observed that the polycarbonate coating seemed to have a more profound effect on the behavior of the metal, by applying a larger restraining pressure on the tube surface during the expansion process, and thereby modifying the stress state of the specimen.
The second experiment looked to design an experimental arrangement to test the plane strain, high strain rate behavior of Al 6061-O tubes of different lengths. A 112 mm long solenoid was designed and manufactured, and testing was performed on 30.5 mm diameter Al 6061-O tubes in lengths of 50, 70 and 90 mm. It was observed that the coil behaved similar to shorter ones at low voltages and that the longer the specimen used, the more its deformation path approached a plane strain condition.
Finally, a third experiment was performed to develop an experiment to accelerate a plate to high linear velocities, as a means to evaluate the use of a flat spiral coil as the driver for future experiments based upon electromagnetic inductive loading. A prototype coil was manufactured and installed into a converted expanding tube experimental setup. Three samples were tested in several sizes, and materials: aluminum and steel. Speeds in the range of 45 to 251 m/s were obtained, validating the apparatus as a viable method to provide a unidirectional loading.Aerospace Engineerin
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