1,721,063 research outputs found
Stereo-camera calibration for large-scale DIC measurements with active phase targets and planar mirrors
Full text linkWe propose an easy-to-implement yet accurate calibration method for large-scale 3D measurements that makes use of a regular-sized phase target and two planar mirrors. Being insensitive to severe defocus, the phase target is placed as to span a large depth within the field of view (FOV) of each camera for accurate intrinsic calibration. Extrinsic calibration is achieved by placing the phase target in the FOV of a short-range virtual stereo-system generated by the mirrors. Results from 3D shape and deformation measurements demonstrate that the proposed method is capable to operate within a working volume of 3 m × 2 m × 1.8 m with an error < 0.1% of the FOV thus opening to new possibilities for large-scale measurements in mechanical and civil engineering applications
Effect of the number of projections in X-ray CT imaging on image quality and digital volume correlation measurement
No full textDigital volume correlation (DVC) quantifies internal 3D displacement and strain fields by correlating the volume images of a tested object acquired at different states. When using X-ray CT-based DVC, the number of projections is a key parameter that affects the acquisition time and quality of reconstructed volumetric images, and therefore the precision and temporal resolutions of DVC measurement. More projections result in high-quality volume images for DVC calculation but longer acquisition time and pronounced thermal drifts of the X-ray source. Few projections lead to quicker acquisition and fewer thermal drifts, but may degrade image quality and thus induce larger DVC measurement errors. Selecting an appropriate number of projections during CT imaging is therefore of practical significance for DVC measurement. To solve this dilemma, the effect of the number of projections on DVC measurements with X-ray CT is experimentally investigated in this work. First, numerically simulated speckle volume images with different numbers of projections were reconstructed by using FDK (Feldkamp) al-gorithm, and the influence of the number of projections on DVC measurement was analyzed. Then, real rescan and compression experiments performed on a copper foam sample were carried out to further study the effect of projection number on DVC measurements. Both simulation and real experiments show that more projections result in longer imaging time but higher quality volume image and DVC measurement. DVC measurement errors decrease with the increase of projections at different decline rates. Therefore, an appropriate number of pro-jections can be specified based on the results according to the requirements of DVC measurement precision and temporal resolution. For the specific X-ray CT device used in this real compression experiment, 36 ~ 60 pro-jections are suggested to balance measurement precision and temporal resolution, and more than 720 projections are necessary for pursuing higher accuracy
Mapping internal deformation fields in 3D printed porous structure with digital volume correlation
No full textAs a new manufacturing technology, 3D printing is gradually attracting more interest in both the academic community and the industrial field. To realize its full potential in engineering applications, it is necessary to characterize the mechanical behaviors and properties of 3D printed materials and structures under external loading. Although surface deformation of 3D printed materials has been extensively investigated, the 3D internal deformation measurement and further mechanical analysis using digital volume correlation (DVC) gain less attention in the field of 3D printing. In this work, step-wise uniaxial compression tests of a 3D printed porous structure were first performed in combination with in-situ X-ray computed tomography (CT) imaging. Then, the global and local evolutions of internal full-field deformation at different compressive stages were evaluated via DVC technique. Finally, a multiscale experimental framework was presented to correlate mechanical properties with geometric structure of the 3D printed porous structure. On this basis, the failure patterns along with the possible damage mechanism were predicted and discussed
Chambered structures from the Ediacaran Dengying Formation, Yunnan, China: comparison with the Cryogenian analogues and their microbial interpretation
No full textAbstractEnigmatic chambered structures have been reported forming reef frames in Cryogenian interglacial carbonates, prior to the commonly acknowledged microbial-metazoan reefs at the terminal Ediacaran, and interpreted as fossils of possible sponge-grade organisms. A better constraint on the affinity of these structures is partly hindered by few analogues in other time periods. This study describes similar structures from peritidal dolostones of the Ediacaran Denying Formation from Yunnan, China. Samples were investigated using optical microscopy and three-dimensional (3-D) reconstruction based on grinding tomography. The Dengying chambered structures are comparable with Cryogenian structures in basic construction, but are not frame building, and show variations in overall shape and inhabiting facies. Two-dimensional (2-D) cross-sections show that thin, homogeneous micritic laminae are the basic building blocks of the chamber walls. Thick walls represent parallel accretion of these laminae, and thin walls developed from the angular growth of a single lamina or merging of multiple laminae. In 3-D space, the laminae primarily correspond to continuous surfaces which sometimes contain sub-circular holes, while a few represent filamentous elements connected to the surfaces. The morphological features and growth pattern of the Dengying chambered structures indicate that they are likely to be calcified microbial constructions rather than skeletal remains of basic metazoans. However, aside from the Cryogenian and Dengying examples, comparable chambered constructions with laminae-based architecture are yet unknown from other fossil or extant microbialites. Further work investigating related structures is needed to determine the microbial consortia and controlling environmental factors that produced these chambered structures.</jats:p
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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