1,720,958 research outputs found
Structural response of CFRP materials subjected to simulated lightning strikes
No full textThe buckling response of Carbon Fibre Reinforced Polymer (CFRP) panels damaged by simulated lightning strikes is evaluated. Five panels constructed with carbon/epoxy material were subjected to an electrical current with 10/350µs waveform that simulates an idealized lightning strike. The buckling experiments were conducted in a newly designed modified buckling test, herein referred to as Compression After Lightning Strike (CALS), which is able to accommodate large composite panels and allows the full extent of the lightning damage to be included in the compression test. Stereo Digital Image Correlation (DIC) was used on both sides of the plate to obtain the strain and displacements that occur during the CALS test and hence determine how the lightning strike affects the mechanical performance of the structure. The experimental results were benchmarked and compared against a shell post-buckling finite element model (FEM). The results reported show good agreement between the experiment and the model predictions
Characterisation of lightning strike induced damage in CFRP laminates and components for wind turbine blades
No full textTo meet worldwide increases in energy demand Wind Turbine (WT) manufacturers are producing longer blades to generate more energy. These blades contain Carbon Fibre Reinforced Polymers (CFRP) in the load carrying structures to lightweight the blade. The introduction of the CFRP composites has presented new challenges in protecting the structure from lightning. The semiconductive nature of CFRP leads to an additional path to ground for the current in the structure and the anisotropic nature of the material’s thermal and electrical properties leads to large amounts of resistive heating especially in the through-thickness direction where the electrical conductivity is the lowest. The aim of this PhD is to devise a new means of assessing the damage and resulting structural behaviour caused by a lightning strike. A modelling framework is developed and validated against high fidelity experimental data that can be used by design engineers to understand the consequences of various lightning damage scenarios and the effectiveness of lightning protection methods. The framework is validated against a representative scale WT sparcap test component in the form of a large panel subjected to compression. The novel damage model is a thermal-electrical Joule heating model which simulates the resistive heating in a UD laminate with electric field dependent material properties to account for electric breakdown. The damage prediction is then exported into a structural Finite Element Model (FEM) by assuming the damaged elements have different material properties. The structural behaviour under compression loading is the main design driver for long slender WT blades. Therefore, the structural model simulates the behaviour of a damaged laminate in a non-linear post-buckling FEM. To validate and inform the damage model and the FEM two different types of tests were conducted. The first type of test simulated the lightning strikes and comprised of direct strike and conducted current tests. The effect of conducting current along the fibre direction showed a deleterious effect on the compressive and shear properties of the material. Initial direct strike tests were used to vary the typical lightning parameters to determine the largest influence on damage among peak current, specific energy, or charge. The last direct strike test is conducted on a representative WT sparcap panel. All damaged panels were evaluated using visual inspection, a new thermography technique, and X-ray computed tomography (CT). The newly developed damage model was validated using the experimental observations with the damage area predictions within 15% of the visual observation and the damage depth within 5% of the CT scans. Hence, the electric field dependency was successfully implemented in the model. The second test type was a structural test that incorporated the development of a new testing methodology named the compression after lightning strike (CALS) test. Large representative sparcap panel specimens, with and without lightning damage were tested to failure in the CALS rig and Digital Image Correlation (DIC) was used to determine the resulting surface displacements and strains. The structural model closely predicted the compressive behaviour and failure loads identified by the DIC. The resulting structural model calculated the first ply failure stresses from the LaRC failure criteria which were within 8% of experimental values, which provided a successful validation of the modelling framework
Lighting protection of carbon fibre reinforced plastic for wind turbines blades
No full textWind turbines are a widely used renewable energy source for the generation of electric power. The wind turbine industry is introducing longer blades to increase the power output a single turbine. To achieve this the blades have incorporated Carbon Fibre Reinforced Polymer (CFRP) materials. CFRP materials provide a strong, stiff and lightweight material which allows longer blades to be manufacture. However, longer blades have the higher potential for lightning strikes and the CFRP materials have a susceptibility to damage caused by lightning current. Wind turbine blades struck by lightning can incur damage which cost the industry in damages to the turbine, loss of revenue due to shut down of wind turbine and repair or replacement cost. The protection of these blades from lightning is of the utmost importance to reduce the overall operating cost of the wind turbine system. This project aims to develop lightning protection solutions to help mitigate damage and keep wind turbines operational. There are five goals to achieve this aim: 1. develop and validate a coupled thermo-electrical model of CFRP laminates subjected to lightning strikes; 2. develop, implement and validate models for damage and failure prediction of CFRP materials subjected to lightning strike; 3. validate the damage models by conducting experiments on CFRP coupons and sub-structural components subjected to lightning strike; 4. develop predictive models residual strength on CFRP materials post lightning strike; 5. validate the residual strength models by conducting panel compression experiments. The damage models are used to develop lightning protection concept and solutions. The structural models are used to predict remaining strength capability (residual strength, and stiffness) of the CFRP panels
Hybrid multiscale modelling to predict lightning damage on CFRP materials
No full textCarbon Fiber Reinforced Polymers (CFRPs) materials are increasingly being used in the wind turbine industry to reduce weight in turbine blades. However, CFRPs have a particular issue when dealing with lightning strikes because of their anisotropic material properties; in particular, their electrical and thermal conductivities. These issues cause significant damage when exposed to large electric currents. This paper presents a time dependent coupled thermal-electric joule heating model to predict the thermal damage of a CFRP panel when subjected to a lightning strike. The approach of this model uses volume fractions to determine the resulting fiber and resin damage separately. The damage prediction is calculated by a set of pyrolysis equations. The pyrolysis damage alters the materials properties of the elements. The electric current applied to the panel is the 10/350 standard waveform which corresponds to the waveform used to test wind turbines according to the IEC61400 section 24 Ed 1.0
Damage prediction of CFRP materials subjected to lightning strike
No full textThis paper presents a coupled thermal-electric model to predict the thermal damage of a Carbon Fiber Reinforced Polymer (CFRP) material when subjected to a lightning strike. A Finite Element Model (FEM) is used to predict the heat response of the CFRP material by solving the Joule heating governing equations. Joule heating, also known as resistive heating, is the heating of the material when subjected to an electrical current. Solutions to the Joule heating model are developed using a time dependent simulation with the 10/350μs standard waveform used to test wind turbine blades in accordance to IEC61400 section 24 Ed 1.0. The time dependent model implements damage criteria and is able to identify damaged elements. The damage criteria are based on a combination of material decomposition by pyrolysis described by the Arrhenius equation. The COMSOL software engine was used to derive the results from the Joule heating model. An integrated MATLAB script was run during the simulation to determine the amount of damage that each element is subjected to during a lightning strike event. The final result is a damage map of the CFRP panel subjected to a lightning discharge. The damage model is validated through lightning discharge experiments. Two samples with unidirectional fibers were made by vacuum assisted liquid resin infusion to mimic the sparcaps of a wind turbine blade located near the wind blade tip region. The samples were tested using the arc entry test of IEC 61400-24 Ed 1.0 with simulated first return stroke electric current components (10/350μs) with magnitudes of 30 kA and 60 kA unipolar waveforms. The resulting damages were inspected by use of X-ray Computed Tomography (CT) to determine the total damaged volume. The CT scans used an imaging segmentation algorithm to systematically determine the location and type of the damage done to the CFRP. The resulting CT scans are compared to the damage model
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
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