1,720,959 research outputs found
Development of a quantitative measurement procedure for defect size and morphological characterization in composite materials by Shearography
Full text linkIn the last decades, new materials and solutions were studied in the field of aerospace industry in order to reach weight reduction of structures, reliability, durability and mechanical strength. In this scenario, composite materials play an important role and the study of suitable Non-Destructive-Testing (NDT) procedures is crucial. In this dissertation, the focus is put on composite materials inspection by using an optical, interferometric technique, Shearography, whose main application is defect and damage detection in composite materials. The use of Shearography is still qualitative: the user is only capable of stating if a defect is present or if it is not, by observing typical interference fringes appearing in case of defect. Shearography, in fact, measures the first derivative of the out-of-plane displacement of an observed surface, by considering variations in the phase relation between separated points of the surface itself. A fringe pattern, i.e. a variation in the phase relation between these points is detected if, and only if, a differential deformation occurs between different regions of the surface. This is what happens when a structure is damaged: the defect region of influence will be more warped with respect to the rest of the structure. The need of observing deformations implies the inspected structure to be loaded. Three kinds of load can be exploited for Shearography inspection: mechanical, vacuum and thermal load. Thermal load implies a series of effects difficult to forecast and/or simulate, such as environmental absorption, air turbulences, propagation of the radiation inside the material in cases where material properties are unknown, etc. For all these reasons, no standard procedures were given even now. In addition, Shearography performances and results are highly dependent on measurement conditions and parameters. The first influencer parameter is the shear distance, i.e. the distance between points used for evaluating phase relations, the second one is the distance between the inspected surface and the Shearocamera, affecting spatial resolution, the third one is the load to apply to the structure: the higher the load, the higher the overestimation. On the other hand, the lower the load, the lower the Signal-to-Noise Ratio (SNR). Other influencer conditions are the defect characteristics themselves, i.e. the defect depth and size, whose combination affects the needed load and the sensitivity required. Therefore, the aim of this work is to provide a deep study of Shearography technique in order to overcome its two great limits, i.e. the lack of algorithms capable of accurately determining defect size and morphology, and the lack of a repeatable procedure for thermal load. The first goal was reached by designing an algorithm that studies the phase maps involving both image analysis tools and signal processing ones. This was fundamental since it was demonstrated that analyzing the phase maps only by standard image processing methods for morphological study does not lead to good quantitative estimations neither for simple nor for complex damages. Consequently, a series of phase profiles to be trated as pure signals are extracted each time a phase map has to be processed. At this point, the defect shape is reconstructed by localizing the boundaries of each phase profile. This was obtained by the use of Wavelet Transform method for singularity estimation, in addition with another function, called Structural Intensity, that enables to select only the dominant singularities. Finally, the second goal was achieved by two different steps. At first, a synchronized and automatic test bench was designed and prototyped, making the acquisition repeatable and reducing uncertainty sources. The second step lies in the elaboration of all the acquired data corresponding to different combinations of parameters, that are matched together in order to define the optimal condition
Quantitative defect size estimation in shearography inspection by wavelet transform and shear correction
No full textRivers’ Water Level Assessment Using UAV Photogrammetry and RANSAC Method and the Analysis of Sensitivity to Uncertainty Sources
Full text linkWater-level monitoring systems are fundamental for flood warnings, disaster risk assessment and the periodical analysis of the state of reservoirs. Many advantages can be obtained by performing such investigations without the need for field measurements. In this paper, a specific method for the evaluation of the water level was developed using photogrammetry that is derived from images that were recorded by unmanned aerial vehicles (UAVs). A dense point cloud was retrieved and the plane that better fits the river water surface was found by the use of the random sample consensus (RANSAC) method. A reference point of a known altitude within the image was then exploited in order to compute the distance between it and the fitted plane, in order to monitor the altitude of the free surface of the river. This paper further aims to perform a critical analysis of the sensitivity of these photogrammetric techniques for river water level determination, starting from the effects that are highlighted by the state of the art, such as random noise that is related to the image data quality, reflections and process parameters. In this work, the influences of the plane depth and number of iterations have been investigated, showing that in correspondence to the optimal plane depth (0.5 m) the error is not affected by the number of iterations
Qualification of Additively Manufactured Aerospace Brackets: a comparison between Thermoelastic Stress Analysis and theoretical results
No full text
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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