1,721,084 research outputs found
Chemically Modified Graphene Films with Tunable Negative Poisson’s Ratios
No full textIncludes supplementary materialGraphene-derived macroscopic assemblies feature hierarchical nano- and microstructures that provide numerous routes for surface and interfacial functionalization achieving unconventional material properties. We report that the microstructural hierarchy of pristine chemically modified graphene films, featuring wrinkles, delamination of close-packed laminates, their ordered and disordered stacks, renders remarkable negative Poisson’s ratios ranging from −0.25 to −0.55. The mechanism proposed is validated by the experimental characterization and theoretical analysis. Based on the understanding of microstructural origins, pre-strech is applied to endow chemically modified graphene films with controlled negative Poisson’s ratios. Modulating the wavy textures of the inter-connected network of close-packed laminates in the chemically modified graphene films also yields finely-tuned negative Poisson’s ratios. These findings offer the key insights into rational design of films constructed from two-dimensional materials with negative Poisson’s ratios and mechanomutable performance. © 2019, The Author(s).National Natural Science Foundation of China (51673108 and 51433005) and National Key R&D Program of China (2016YFA0200202); National Natural Science Foundation of China through Grants 11825203 and 11472150; National Science Foundation CMMI-1636306, CMMI-1661246, and CMMI-1726435Erik Jonsson School of Engineering and Computer Scienc
A Multiscale Model to Study the Enhancement in the Compressive Strength of Multi-Walled CNT Sheet Overwrapped Carbon Fiber Composites
No full textDue to copyright restrictions full text access from Treasures at UT Dallas is restricted to current UTD affiliates (use the provided Link to Article).The high tensile strength of polymer matrix composites is derived primarily from the high strength of the carbon fibers embedded in the polymer matrix. However, their compressive strength is generally much lower due to the fact that under compression, the fibers tend to fail through micro-buckling well before compressive fracture occurs. In this work, we consider multi-walled carbon nanotube (MWNT) sheets wrapped around carbon fiber at room temperature to improve fiber/matrix interfacial properties which, in turn, influences compressive strength of the composite. To investigate the effect of the wrapping of MWNT sheet on composite strength, Molecular Dynamics simulations were performed on an atomistic model of the interface region between the epoxy, carbon fiber and the scrolled MWNT sheets. The compressive strength of the unidirectional composite was computed using a novel hierarchical multi-scale model comprising of the rule of mixtures at the microscale, and the modified Argon's formula for composites at the macroscale. Model predictions were benchmarked through comparison with experimental data for different volume fractions of MWNT sheet. ©2019 Elsevier LtdThis research was supported by the Low Density Materials Program at AFOSR, Grant No. FA9550-14-1-0227 and NSF CMMI-1636308.School of Natural Sciences and MathematicsErik Jonsson School of Engineering and Computer ScienceAlan G. MacDiarmid NanoTech Institut
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
Mechanical Characterization and Modeling of Composite Materials During Compression and Actuation
No full textComposites have a wide range of applications due to their excellent mechanical properties.
In order to investigate the mechanisms of the recently developed composites, it is critical
to characterize and model the performance of such composites for load-bearing applications.
The high tensile strength of polymer matrix composites is mainly derived from the high
strength of the carbon fibers embedded in the matrix. Fibers typically have high strength
in tension. However, their compressive strengths are generally much lower than tensile
strengths due to weak fiber/matrix interfacial shear strength. A new approach was recently
developed to overwrap an individual carbon fiber with carbon nanotube (CNT) sheet, which
is subsequently impregnated into a polymer matrix to enhance the interfacial shear strength
and therefore increase the compressive strength without degrading the tensile properties of
the carbon fibers. A theoretical model is established to identify the appropriate thickness
of the interphase region formed by CNTs embedded in matrix. Fibers are modeled as an
anisotropic elastic material, and the interphase region and matrix are considered as isotropic.
A micro-buckling problem is considered to take into account of the unidirectional composite
under elastic micro-buckling. The formulated problem is solved numerically and the results
are compared with finite element simulations for verification. It is determined that the
critical load at the onset of buckling is lower in an anisotropic carbon fiber composite than in an otherwise isotropic composites due to lower transverse properties in anisotropic fibers.
An optimal thickness for the CNT and matrix is determined and this finding will provide a
guidance in the manufacture of composites using aligned CNTs as fillers in the interphase
region.
The other type of composite is made of CNT yarn and polymer, which can be used as artificial
muscles. The topology of this type of artificial muscle is polymer coated on a twisted or coiled
CNT core, which can provide higher performance than the muscles made of guest-filled,
twisted and coiled CNT yarns. However, the mechanisms of torsion and tension of the two
artificial muscles are unclear. A theoretical model considering the torque balance between
the polymer and yarn, both before and after twisting actuation is established to predict
the torsional stroke. The theory captures the two primary mechanistic contributions to the
torsional actuation of a polymer coated CNT yarn artificial muscle, where both polymer
swelling and softening combine to partially release elastically stored torsional energy in the
core yarn. This theory shows that while a low polymer thickness to core diameter ratio limits
the capability of the polymer to compress the core and maintain the initially inserted twist
before actuation, a very high polymer thickness to core diameter ratio provides less release
of such inserted twist after the polymer coated CNT yarns is actuated. Consequently, there
is an optimum polymer thickness to core diameter ratio that maximizes torsional stroke.
We next theoretically predict the stress dependence of tensile stroke and contractile work
capacity for coiled polymer coated CNT yarns and polymer infiltrated CNT yarns for isobaric
actuation.
Another theoretical model established is for nylon artificial muscle. The torsional nylon
artificial muscle is fabricated by twisting nylon wire into coil spring. When subjected to
temperature fluctuations, nylon muscle fiber can sustain 300 000 heating-cooling cycles rotation
to spin a magnet rotor in three phase coil. In this process, the rotor’s mechanical
energy transfers to electrical energy. The theoretical model captures the entire process by considering all the torques acted on the magnet, hence temperature, electrical field will be
incorporated. Transfer efficiency and parameters effect on kinetic energy are studied. Numerical
results show that the kinetic energy increases with the artificial muscle diameter,
which is consistent with experimental observation. Although the kinetic energy increase
with the increase of magnet mass and radius, the torsional speed decreases as the magnet
mass and radius become smaller. Hence there exist an optimum magnet mass and radius to
maximize kinetic energy.National Science Foundation (Grant #CMMI-1636306, CMMI-1661246, and FAIN-172643
Mechanical Properties Characterization of Sheet Metal Forming materials--Anisotropy of Grade 1 Titanium and Stainless Steel 316
No full textSheet metal is formed through a process of metal working which produces thin sheets of metal from thicker plates. The sheet metal materials considered in this work are formed through a process of cold rolling. This cold rolling process increases the anisotropic characteristics of the sheet metal product. In this work, the utilization of three uniaxial tensile tests to characterize the sheet metal materials in the rolling, transverse and diagonal directions are discussed. The use of digital image correlation (DIC) to determine the strains experienced by the materials before and after yield by observing the necking regions during the conventional tensile tests is introduced.
Due to increased competition in the heat exchanger manufacturing industry, many companies are turning to finite element simulations of the heat exchanger plate forming process in order to reduce product development costs and time to market for new and redesigned products. The accuracy and reliability of the simulation results are largely dependent on how closely the material model used in the simulation matches the behavior of actual material, therefore, it is important to test and understand the behavior of the actual materials used in the fabrication of the products.
In this project, an accurate and verified tensile testing simulation is developed. It is suitable to model the forming process of Tranter heat exchanger plates using Tranter provided commercially pure Titanium and stainless steel 316 sheet metals. Tensile tests of two kinds of the sheet metals have been conducted and the full-field deformations are obtained using digital image correlation (DIC) techniques to provide constitutive data for elastic-plastic material models. Anisotropy material models are created for both sheet materials. Simulations of finite element method utilizing the developed material models produced results which closely matched the full-field deformation results obtained using DIC
Characterization and Modeling of Mechanical Behavior of Polymers and Composites at Small Scales by Nanoindentation
No full textPolymer has been used extensively for the final MEMS structures or devices. In order to ensure the design reliability, it is critical to precisely determine the mechanical properties of the polymer-based electronic packaging materials at microscale. In recent years, nanoindentation technique is gradually becoming an effective technique for determining the local mechanical properties at the microscale and nanoscale. In this study, the mechanical properties of SU-8, a photoresist material of great interest to MEMS community, were measured under both micropillar compression and nanoindentation on a film on a substrate by nanoindentation. Measurement results in literature characterizing the mechanical behavior of SU-8, by elastic-plastic analysis of nanoindentation data, have shown to provide incorrect results. In this study, an appropriate viscoelastic analysis of nanoindentation load-displacement data was conducted, the time-average Young’s modulus at a given strain rate was determined to be near 3.6 GPa, which agrees with the reported values in literature obtained from tension and bending, and also correlates reasonably well with data from microcompression. This work indicates that viscoelastic analysis is necessary to extract the valid mechanical properties at nano/microscales for SU-8.
The same viscoelastic analysis approach has also been applied to characterize the mechanical properties of a molding compound on a packaged integrated circuit (IC) by spherical nanoindentation using a 50 μm radius diamond tip. The molding compound is a heterogeneous material, consisting of assorted diameters of glass beads embedded in an epoxy. Statistical analysis was conducted to determine the representative volume element (RVE) size for a nanoindentation grid. Nanoindentation was made on the RVE to determine the effective viscoelastic properties. The relaxation functions were converted to temperature-dependent Young’s modulus at a given strain rate at several elevated temperatures. The spatial distribution of the Young’s modulus at a given strain rate was also determined using nanoindentation with a Berkovich tip.
In addition to the application on MEMS structure, nanoindentation technique has also been extended to characterize the fiber reinforced polymer matrix composites, which have found increasing applications in such areas as aerospace, automotive, wind farms, offshore drilling, sports, and construction. In this study, Fiber push-in nanoindentation was conducted on a unidirectional carbon fiber reinforced bismaleimide resin composite (IM7/BMI) subjected to environmental degradation to determine the interfacial shear strength. It was found that thermal oxidation at 245oC in air leads to a significant reduction in interfacial shear strength of the IM7/BMI unidirectional composite. Moisture-saturated specimens under thermal shock showed a significant reduction in interfacial shear strength as well.
It is thus encouraged to increase the interfacial strength of fiber reinforced polymer matrix composites. In this study, we propose using multiwall carbon nanotube sheet to spiral-wrap around an individual carbon fiber for enhancement of mechanical properties of the fiber/matrix interphase that directly influences the fiber/matrix debond strength and compressive strength of the composite. Different methods were used in experiments to characterize the interfacial shear strengths. All experimental results show consistently a significant improvement in interfacial shear strength by using MWNT scrolled carbon fibers in a composite
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
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