1,720,961 research outputs found
Micro-mechanical contributions to interlaminar toughness in particle-toughened CFRPs
No full textThe main objective of this thesis was to increase the understanding of interlaminar toughening mechanisms in particle-toughened interlayers within carbon/epoxy laminates. High-resolution Synchrotron Radiation Computed Tomography (SRCT)and Synchrotron Radiation Computed Laminography (SRCL) allowed the crack tip micro-mechanisms to be observed in situ under Mode I and Mode II quasi-static loading conditions. Fracture toughness tests were undertaken to establish the ranking of the ten different material systems, which were compared in terms of the micro-mechanisms observed. Two different intermediate modulus fibres were investigated, with combinations of three different particle types dispersed within the interlayers.The work showed that interlaminar failure in the materials involves a complex process zone, rather than a singular crack tip. Three distinct crack wake bridging mechanisms were identified, namely; fibre-bridging, epoxy-bridging, and particle-bridging ligaments. It was determined that an interlaminar crack path provided a high Mode I and Mode II fracture toughness. The ligament-rich and tortuous crack path appeared to provide higher energy dissipation than the comparably smooth intralaminar failure at the ply interface and associatedfibre-bridging mechanisms. Quantitatively, the work showed that a larger number of bridging ligaments in particle containing interlayers correlated to a higher Mode I fracture toughness. Provided that the particles in question could maintain an interlaminar crack path, the particle size and type had a less significant effect on the Mode II toughness.Digital Volume Correlation (DVC) was employed to quantify interlayer strains head of a Mode I crack, showing that the toughening particles can be used as effective markers to enable displacement tracking. A finite element (FE) model was used to explore key variables that were identified experimentally to have an effecton the crack path. The fibre interface strength, particle cohesive strength, density and distribution were shown to affect crack paths. The results implied that the role of the particles is to alleviate the stresses at the ply interface by de-bonding or fracturing internally, following which additional toughness may be generated via the formation of bridging ligaments as failure occurs within the interlayer. Overall, the work is intended to support material development and lead to better predictive capabilities for these materials that are increasingly used in primary aerospace structures
Modelling and quantifying Mode I interlaminar fracture in particle-toughened CFRPs
No full textFour-dimensional time-resolved Synchrotron Radiation Computed Tomography (SRCT) has been used to capture Mode I delamination propagation in particle-toughened Carbon Fibre Reinforced Polymers (CFRPs). Digital Volume Correlation (DVC) was used in order to measure ply opening displacements at the crack tip, permitting the interlayer strain ahead of the crack tip to be quantified. Estimates at which toughening particles de-bonded and/or fractured were made, giving insight into the effects of particle type and particle size on the fracture mico-mechanisms. The experiments are complemented by a 2D plane-strain finite element (FE) model, which investigated the effects of particle strength and toughness on the ply opening displacement and crack path by modelling the particles as 1D cohesive segments. Previous work has shown that Mode I crack propagation in particle-toughened interlayers involves a process zone rather than a distinct crack tip. Therefore, Augmented Finite Element Method (A-FEM) elements were used in the simulation, since the elements can account for both bifurcating and merging cracks within a single element. The nodal displacements in the simulation were compared to the DVC results, illustrating a potential path through which more complex FE simulations may be validated against experimental results in the future
Crack path simulation in a particle-toughened interlayer within a polymer composite laminate
No full textWith recent advances in computational resources and the development of arbitrary cracking methods, such as the Augmented Finite Element Method (A-FEM), more complex simulations can now be represented featuring multiple interacting cracks. It has been established that Mode I crack propagation in particle-toughened interlayers within some toughened Carbon Fibre Reinforced Polymer (CFRP) laminates involves a discontinuous process zone, rather than a distinct crack tip. This results from multiple cracks forming ahead of the main crack that subsequently coalesce, leaving behind bridging ligaments that may then provide traction across the crack flanks. An idealised two-dimensional A-FEM model is presented in this work, which represents the ‘particles’ as one-dimensional cohesive regions. The model shows that variables such as particle spacing, distribution, strength and toughness, and fibre interface strength can be tailored in order to maintain the crack path within the interlayer. This competition between crack paths is important, as a reduction in composite toughness is reported when the crack path migrates to the fibre interface. The simulations are complemented by time-resolved Synchrotron Radiation Computed Tomography (SRCT) data, which identify the chronology of the damage processes, along with the effects of particle distribution on the crack path and the formation of bridging ligaments
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
Synergetic effects of thin plies and aligned carbon nanotube interlaminar reinforcement in composite laminates
No full textThin-ply carbon fiber laminates have exhibited superior mechanical properties and damage resistance when compared to standard thickness plies and enable greater flexibility in laminate design. However, the increased ply count in thin-ply laminates also increases the number of ply-ply interfaces, thereby increasing the number of relatively weak and delamination-prone interlaminar regions. In this study, we report the first experimental realization of aligned carbon nanotube interlaminar reinforcement of thin-ply unidirectional prepreg-based carbon fiber laminates, in a hierarchical architecture termed ‘nanostitching’. We synthesize a baseline effective standard thickness laminate using multiple thin plies of the same orientation to create a ply block, and we find an ~15% improvement in the interlaminar shear strength via short beam shear (SBS) testing for thin-ply nanostitched samples when compared to the baseline. This demonstrates a synergetic strength effect of nanostitching (~5% increase) and thin-ply lamination (~10% increase). Synchrotron-based computed tomography of post mortem SBS specimens suggests a different trajectory and mode of damage accumulation in nanostitched thin-ply laminates, notably the complete suppression of delaminations in the nanostitched region. Finite element predictions of damage progression highlight the complementary nature of positive thin-ply and nanostitching effects that are consistent with an ~15% improvement in Mode I and II interlaminar fracture toughness due to the aligned carbon nanotubes at the thin-ply interfaces
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
- …
