1,720,958 research outputs found
Noncontact fatigue crack detection using non-linear frequency mixing of surface acoustic waves
No full textThe presence of fatigue cracks poses a significant challenge to the early detection of structural defects, prior to the formation of macro-cracks. This study investigates the feasibility of using acoustic nonlinearity parameters for the early evaluation and localization of fatigue damage in aluminum, employing a fully non-contact method for both excitation and sensing. The method is based on the sample excitation of two narrowband surface acoustic waves (SAWs) by two laser beams at two frequencies f(a) and f(b)(f(a) > f(b)). A laser-based surface acoustic wave (SAW) technique, utilizing micro-lens arrays, was used to generate two distinct, counter-propagating narrowband SAWs with wavelengths of 300 mu m and 500 mu m, corresponding to frequencies of 9.84 MHz and 5.86 MHz, respectively. The nonlinear interaction of these collinearly propagating counter-directed waves was monitored using a Laser Doppler Vibrometer across various fatigue levels. Subsequently, the interaction of these collinearly counter-propagating SAWs was employed to assess fatigue cracks at various damage levels. The presence of micro-cracks significantly increased material nonlinearity, leading to higher amplitudes of the quadratic components (eg., 2f(a), 2f(b), f(a) +/- f(a)) observed in the mixed acoustic signal. The results of this study demonstrate the feasibility of an early fatigue crack detection and achieving high spatial resolution for crack localization.
미세결함 검사를 위한 접촉/비접촉식 비선형 초음파 램브 웨이브 믹싱 기술 개발
No full text학위논문(박사) - 한국과학기술원 : 건설및환경공학과, 2022.8,[viii, 123 p. :]Early detection of damage in structures is important to ensure their structural safety. Nonlinear ultrasonic techniques are widely used to measure the quadratic nonlinearity that represents the third-order elastic constants of materials for damage detection. In addition, there are ongoing efforts to exploit both the third- and fourth-order elastic constants that describe the cubic nonlinearity to detect damage. This dissertation develops a nonlinear ultrasonic Lamb mixing technique to measure cubic nonlinearity and to detect and localize micro-damage. The proposed wave mixing technique generates three Lamb waves in the target structure and measures the nonlinear mixed waves produced by nonlinear cross-mixing of the Lamb waves.
A theoretical model is developed to describe the generation of nonlinear mixed waves in a nonlinear elastic and homogeneous plate. In addition, a theoretical relationship between the nonlinear mixed components and the fatigue crack size is derived from the Paris-Erdogan and Nazarov-Sutin theories. Experiments were then conducted on aluminum plate specimens with micro-damage to validate the effectiveness of the proposed technique. The position of the micro-damage within the target structure was identified by spatially scanning the mixing zone. In particular, the proposed technique is more sensitive to micro-damage than existing nonlinear ultrasonic techniques like two-wave mixing and third harmonic production.
The amplitude of these mixed components weaken in a noisy environment, necessitating noise elimination for a reliable crack detection. To overcome this, a novel hybrid method that incorporates a deep learning (DL) model with higher-order spectral analysis is proposed in this dissertation. The DL model based on long short-term memory (LSTM) takes an original ultrasonic time signal and outputs a reconstructed ultrasonic signal after noise reduction. Finally, random noise in the reconstructed signal is eliminated by trispectrum (TS)-based higher-order spectral analysis.
In this dissertation, a non-contact nonlinear Lamb wave mixing technique based on laser line-array excitation was developed for microcrack detection in plate-like structures. Specifically, a pulsed laser with a line-array pattern (LAP) source was created to generate two narrowband Lamb waves with distinctive frequenciesthen, a laser Doppler vibrometer (LDV) was used to measure the corresponding ultrasonic responses. The performance of the developed laser ultrasonic system was experimentally validated by applying it to aluminum specimens with microcracks. In this dissertation, the effects of the number of LAP on the wave mixing zone size, as well as the amplitude of the mixed components, are discussed. The obtained results indicate that the proposed system can locate and detect microcrack in a plate by scanning the wave mixing zone.한국과학기술원 :건설및환경공학과
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
koamabayili/VECTRON-author-checklist: VECTRON author checklist
No full textWe have done our best to complete the author checklist relating to the use of animals in the hut study. Note that the objective for the hut study was to evaluate the IRS treatment applications for residual efficacy against Anopheles mosquitoes, including the local An. coluzzii mosquito population. Cows were only used to attract mosquitoes into the huts and no tests were carried out directly on the cows. The author checklist is intended for use with studies where experiments are carried out on animals, which is why we have had such difficulty in completing this for the hut study, as many of the questions do not relate to how the cows were used
Development of a nonlinear ultrasonic three-wave mixing technique for fatigue crack detection
No full text- …
