1,720,974 research outputs found
Nondestructive consolidation assessment of historical camorcanna ceilings by scanning laser Doppler vibrometry
No full textA wavefront track approach to defect detection in composites by scanning laser Doppler vibrometry
Full text linkComposite laminates are becoming increasingly popular in a large variety of applications due to their favourable mechanical properties. However, laminates production processes can lead to various defects in the final material. The most common type is related to thickness variations, e.g. delaminations between layers, which can compromise the mechanical strength of the structure. Therefore, there is a great interest in developing non-destructive and non-contact quality control techniques for composite material assessment to minimize process costs. An interesting approach is the use of laser Doppler vibrometry combined with signal analysis based on Lamb waves propagation. In this work, we used an impulsive force given by a piezoelectric disk to the specimen and a laser Doppler vibrometer acquiring the points velocity over time along a scanning grid on the surface. The specimen is a fiberglass reinforced flat panel with seven different orientated layers which presents a delamination of about 22 mm. The maximum thickness-frequency product achieved in this analysis has been 0.2 MHz mm. In contrast to state-of-the-art methods for identifying thickness variation based on local estimation of the principal wave number, the proposed algorithm makes use of a tracking filter of the wave front of the propagating A0 mode waves, returning a final image in polar coordinates. The final information given by the algorithm provides the position of the delamination and, hence, can be used as a pass/failure test. State-of-the-art methods are also able to identify the shape of the defect but pay the price of a higher computational cost by using at least 4D matrix processing unlike our method which only uses 3D matrices
A compact 3D bandgap multi-material metamaterial design for vibration testing
No full textThe use of metamaterials is an established approach in the field of vibration isolation, leveraging their exceptional tunability over a vast range of frequencies. A key challenge in designing lattice structures is the trade-off between geometrical compactness and wideness of the bandgap, i.e. the frequency range in which wave propagation is not allowed. A possible solution to reduce the size of the structure while maintaining or improving its vibrational performances, is to incorporate different materials in the metamaterial design. In this paper, we present a compact metamaterial, whose design is driven by a multi-material approach, that exhibits extraordinary properties in terms of wide frequency bandgap and lower frequency threshold. The vibrational performance of the proposed metamaterial is validated both numerically and experimentally for a unit cell with a side length of 3 cm. The results show a bandgap opening frequency of 1257 Hz and a low pass filter behavior in the range 1.2..
An innovative wide and low-frequency bandgap metastructure for vibration isolation
No full textEngineering the architecture of materials is a new and very promising approach to obtain vibration isolation properties. The biggest challenge for lattice structures exhibiting vibration isolation properties is the trade-off between compactness and wide and low-frequency bandgaps, i.e., frequency ranges where the propagation of elastic or acoustic waves is prohibited. Here, we, both numerically and experimentally, propose and demonstrate a new design concept for compact metamaterials exhibiting extraordinary properties in terms of wide and low frequency bandgap and structural characteristics. With its 4 cm side length unit cell, its bandgap opening frequency of 1478 Hz, its band-stop filter behavior in the range 1.48-15.24 kHz, and its structural characteristics, the proposed 1 x 1 x 3 metastructure represents great progress in the field of vibration isolation and a very promising solution for hand-held vibration probes applications that were unattainable so far through conventional materials
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
Time stability of the dynamic behaviour of a 3D-printed airless wheel by stereolithography
No full textCook's distance-based regression to enhance clustering of metal additive manufactured components through a resonance testing system
No full textResonance Testing (RT) represents a simple and fast pass/fail solution particularly suited for inspecting small mass-produced hard components without requiring radiation, scanning, or immersion in liquids, chemicals, abrasives, or other consumables. However, the variability introduced by the Additive Manufacturing (AM) process can mask the presence of defective parts. A set of 220 AM cylindrical Ti6Al4V specimens was inspected with an Acoustic Resonance Testing (ART) system to cluster compliant parts and specimens with a seeded defect, present in 18 of them with a threadlike shape. The dataset analysed consists of specimens build plate positions and response spectrum peak frequencies, captured by a set of detection windows through Simcenter Anovis software. To increase the performance of a clustering algorithm like HDBSCAN the systematic variability of resonance frequencies along the build plate positions has been compensated in the pre-processing step through a Cook’s distance-based linear regression
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