1,721,038 research outputs found
Damage detection in a PA 66,6/Glass woven fabric composite material using ultrasonic techniques towards durability prediction of automotive parts
No full textCes travaux de thèse portent sur l'étude expérimentale approfondie d'un composite à base polyamide 66/6 renforcé par des fibres de verres tissés suivant un motif sergé 2/2. L’objectif est de proposer des solutions de Contrôle Non Destructif (CND) basées sur les ultrasons afin de détecter différents niveaux d’endommagement induis. Pour cela, une étude approfondis des mécanismes d’endommagement apparaissant lors de sollicitations en traction suivant l’axe des fibres et hors axes est réalisé. Le cas d’impact induis par poids tombant est également étudiés. En effet, ces différents cas de sollicitions entraînent l’apparition de différent mécanismes d’endommagement. Ces derniers, ainsi que leur ordre d’apparition, sont caractérisés par Microscopie Electronique à Balayage (MEB) et tomographie à rayons X principalement. L’évaluation de la réduction du module élastique pré et post chargement ainsi que la fraction volumique de vide montrent une évolution de l’endommagement plus importante lors de chargement en traction hors axes des fibres que lors de chargement suivant l’axe. Lors des essais d’impact par poids tombant différents niveaux d’énergie sont considérés en restant proche du domaine des BVID en vue d’éprouver la sensibilité des méthodes de CND. Deux méthodes de CND par ultrasons étudiées durant ce projet peuvent être mises en avant. Premièrement, par mesure de la vitesse de propagation des ondes dans plusieurs directions du composites, le tenseur de rigidité est estimé dans tous ces cas de sollicitation mécanique pour différents niveaux d’endommagement. Des indicateurs d’endommagement basés sur ces mesures montrent une évolution de l’état d’endommagement similaire à celle discutée précédemment. Deuxièmement, une étude de la détection de l’endommagement par ondes guidées est menée. Aucun changement des modes transmit n’est visible lors de l’augmentation de l’état d’endommagement. L’évolution de l’énergie du signal transmis est alors proposée et validée comme indicateur d’endommagement efficace pour des chargements en traction mais pas pour l’impact. La mesure du décalage temporel à en revanche permis une localisation et une quantification de l’endommagement induit par impact.The present study is focused on the experimental study of a polyamide 66/6 based composite reinforced by a 2/2 twill weave glass fabric. The aim is to propose Non Destructive Evaluation (NDE) methods based on ultrasound that can efficiently distinguish different damage state. In order to do so, an investigation of the damage mechanisms induced by different type of mechanical solicitations. Tension along and off the axis of the fibers was considered as well as the case of drop weight impact. Those solicitations were shown to induce different damage mechanisms. The latter were characterized by means of Scanning Electronic Microscopy (SEM) and X-Ray tomography mostly. The decreasing of the elastic modulus and the void volume fraction evolution were shown to be more significant for the samples loaded in tension off-axis. During the drop weight impact tests, the energies were considered in order to remain close to the Barely Visible Impact Damage (BVID) regime in order to experience the capability of the ultrasound based NDE methods. Two NDE methods investigated during this study deserve to be highlighted. Firstly, the stiffness tensor was estimated by means of phase velocities measurements in different propagation direction. Damage indicators based on results from this method were proposed. They were found to give results similar with the one from the evaluation of damage discussed earlier on. Secondly, a study of the damage detection using guided waves was performed. No mode conversion effect was observed from this investigation. Consequently, the signal energy was proposed as damage indicator and was found to be suitable to detect damage induced by tension but not by impact. The measure of time shift allowed obtaining a localization and evaluation of the damage induced by impact
Operando detection and quantification of damage in the automotive components made of polyamide based composite reinforced with continuous fibres submitted to the fatigue-pre and/or post-impact by advanced ultrasonic techniques
No full textLa détection et la caractérisation des endommagements dans des composantes automobiles fabriquées en matériaux composites restent un souci majeur que les constructeurs automobiles cherchent à confronter. Le projet de thèse s'inscrit dans cette thématique et a comme but l'analyse qualitative et quantitative des endommagements induits par différentes sollicitations dans un composite à matrice polyamide 66/6 renforcé de fibres de verre tissées sergé 2/2. Pour cela, des essais d’impact à faible vitesse à différents niveaux d’énergie, des essais de fatigue en traction-traction ainsi que des essais d’impact post-fatigue, sont réalisés. Une méthodologie expérimentale qui repose sur l'utilisation des méthodes de contrôle non-destructif est établit et a pour but l’évaluation de l’endommagement induit par différents chargements. Différents outils sont utilisés dans cette étude, à savoir : la microscopie optique, la microscopie électronique, la tomographie aux rayons X, les ondes guidées (les ondes de Lamb), l'imagerie C-Scan par propagation des ondes de volumes et d'autres outils ultrasonores. En effet, les mécanismes d’endommagement liés aux multiples sollicitations sont observés et leurs degrés de propagation en fonction du type et du niveau de chargement sont examinés. Les propriétés résiduelles sont estimées par le biais de la vitesse de propagation des ondes de volume. En outre, une investigation basée sur la propagation des ondes de Lamb est menée. A l'issu de cette étude, des indicateurs d'endommagement, permettant de quantifier l'état d'endommagement induit dans chaque échantillon inspecté, sont proposés. Finalement, des essais de traction quasi-statique sont réalisés pour la prédiction des propriétés résiduelles post-chargement.The detection and characterization of damage in automotive components made of composite materials remains a major concern that automotive manufacturers are seeking to address. The aim of this thesis project is to qualitatively and quantitatively analyze the damage induced by different solicitations in a polyamide 66/6 based composite reinforced with 2/2 woven glass fibers intended for automotive applications. For this purpose, low-velocity impact tests at different energy levels, tensile-tensile fatigue tests as well as impact post-fatigue tests are conducted. An experimental methodology relying on the use of non-destructive testing methods (NDT) is established and aims at the assessment of the damage induced by different types of loads. Several tools are used in this study, namely: optical microscopy, X-ray tomography, guided waves (Lamb waves), C-Scan imaging and other ultrasonic tools. The damage mechanisms associated with the multiple loading are observed and their degrees of propagation as a function of the type and level of loading are examined. The stiffness matrix components are estimated by means of the velocity of propagation of the bulk waves. Furthermore, a Lamb waves-based investigation is conducted. By estimating the velocity of propagation of the waves, velocity profiles as a function of the angle of emission/reception are obtained. As a result of this study, several damage indicators are proposed to quantify the damage state induced in each inspected specimen. A good correlation between all experimental results is found indicating the efficiency of the developed Lamb waves-based method. Finally, quasi-static tensile tests are carried out to estimate the post-loading residual properties
Ultrasone beeldvorming van microschade gebruik makend van technieken gebaseerd op tijdsomkering en niet-lineaire elastische golfspectroscopie.
No full textDuring the last decade, Nonlinear techniques based on Time Reversed Acoustics (NLTRA) have proven their efficiency and extreme sensitivity for Nondestructive Testing (NDT) of materials containing damage with a nonlinear behavior. NLTRA-techniques combine Nonlinear Elastic Wave Spectroscopy (NEWS), which is a collective term for techniques aiming to detect various kinds of nonlinear behavior in response signals, with the principle of Time Reversal (TR). This combination results in more efficient and sensitive techniques to localize and image incipient damage in the form of microcracks, delaminations, etc.Despite the success of NLTRA, these techniques share an important shortcoming. In a purely experimental application, only near-surface defects can be localized, since it is required to position a detector near the defect region. Detection of embedded defects is therefore only possible in a hybrid experimental/numerical approach. The main obstacle is to achieve a reliable numerical model, with a high accuracy on the material parameters such that the propagation of multiple reverberating signals that are typical for NLTRA can be calculated without losing the focus quality in the TR-process. Apart from the density and the geometry, it is critical to know the velocities and/or velocity distribution of the sample under consideration.In view of future hybrid ultrasonic NDT applications, we first developed a new automatic procedure for the determination of the (isotropic) longitudinal and shear material velocities, based on traditional TR-principles, and the signal dilatation and contraction method developed by Scalerandi et al. The method requires a single recorded signal as input, together with a (crude) estimation of the longitudinal velocity. Density and geometry are assumed to be known exactly, as well as the position of excitation and recording. Iteration of this TR based procedure allows to determine the longitudinal and shear velocities of the sample material with an accuracy better than 0.5%. Results are presented for a 3D numerical 'blind' test case and for two applications involving experimentally recorded data.Further, a NLTRA-technique was investigated that consists of the following two steps. In the first step, TR is used to focus high levels of energy in a small area of a medium in order to activate possible defects, and in a second postprocessing step, information regarding the nonlinear contribution is extracted from the reponse signals. Numerical simulations are reported showing the potential of a combination consisting of dual energy reciprocal time reversal and nonlinearity filtering using the scaling subtraction method. The method is applied to the detection of planar near-surface defects parallel to the surface in a 2D domain. The results are evaluated for sweep excitation at different frequency ranges; for point-like receivers as well as extended transducers, and for in-plane as well as out-of-plane focusing. The observable nonlinear response at the surface is linked to an effective nonlinearitywithin the medium based on the defect geometry and the distribution of the local stresses.The final problem that is considered in this thesis deals with the difficulty of imaging multiple masked scatterers. Inherent limitations of the traditional time reversal process in the case of multiple sources or scatterers make it difficult to distinguish the scattering sources individually. The selective source reduction (SSR) method employs a subtraction technique to selectively suppress in amplitude (and ideally eliminate) a time reversed focal signal that is masking another focus. In previous work, Scalerandi et al. and Anderson et al. successfully applied the SSR method to identify masked primary sources in a fully linear medium. Here, we extend the capabilities of the SSR method to deal with scattering caused by embedded defects. We call this new method SSR-NLTRA (Selective Source Reduction based on Nonlinear Time Reversed Acoustics). In the extended approach, the contribution of all primary and linear sources is first eliminated by means of the scaling subtraction method. Subsequently, the SSR(-TRA) method is applied to the remaining nonlinear content of the signals. We show by means of 2D wave propagation simulations that the new method can be applied iteratively to successfully image multiple masked nonlinear defects.status: Publishe
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
Interaction of ultrasound with periodic structures and composite structures
No full textLa recherche présentée dans cette thèse comporte deux parties : l’interaction des ondes ultrasonores avec des structures périodiques et également l’interaction avec des matériaux composites. La recherche sur l’interaction des ultrasons avec des structures périodiques comporte deux aspects : l’étude du phénomène physique de la diffraction des ultrasons d’une part. Les chapitres II et III représentent l’enquête des deux phénomènes physiques de la diffraction d’ultrasons, c’est-à-dire l’anomalie dites « anomalie de Wood » et le phénomène de diffraction subharmonique. Dans le chapitre IV, le principe de la diffraction de Bragg est appliquée à la caractérisation non destructive de la périodicité des structures périodiques par ondes ultrasonores générées et se propageant dans l’air, tandis qu’une caractérisation géométrique plus générale de surface périodique à l’aide d’une technique ultrasonore à haute fréquence par la technique de imagerie C-scan est présentée au chapitre V. Dans cette thèse, les travaux de recherches sur l’interaction des ultrasons avec des structures composites se concentrent sur la caractérisation des défauts d’impacts des plaques composites. Ce travail comporte deux parties : l’analyse spectrale des défauts d’impact et la comparaison des différentes techniques de caractérisation des défauts d’impact. La première partie de l’ouvrage tente de faire face à la situation où les signaux reçus pour la caractérisation des défauts sont corrompus par le bruit causé par le système d’acquisition. Alors que dans la seconde partie, différentes configurations de test, différents capteurs et différentes méthodes de traitement de données sont comparées, afin d’optimiser la caractérisation des défautsThe research presented in this thesis contains two parts : the interaction of ultrasound with periodic structures and the interaction of ultrasound with composite materials. The investigation in the interaction of ultrasound with periodic structures is made in two aspects : study of the physical phenomenon of ultrasound diffraction and nondestructive characterization of the coorugated structures using ultrasonic techniques. Chapter II and III represent the investigation of two physical phenomena of ultrasound diffraction, i.e. acoustic Wood anomaly and the so-called sub-harmonic diffraction, respectively. In chapter IV the principle of Bragg diffraction is applied to the nondestructive characterization of the periodicity of periodic structures using air-coupled ultrasonic technique; while a more general geometrical characterization of periodic surface using high frequency ultrasonic C-scan imaging technique is presented in chapter V. In this thesis the research work of the interaction of ultrasound with composite structures focuses on the characterization of impact defects of composite plates. This work contains two parts : the spectral analysis of impact defects and the comparison of different techniques in impact defects characterization. The first part of the work tries to deal with the situation where the received signals for defect characterization are corrupted by the avoidable high-level noise caused by the data acquisition system. While in the second part different testing configurations, different transducers and different data processing methods are compared in order to optimize the defect characterizatio
Mobile robot-based Ultrasonic mechanical structure detection on a ship hull
No full textStructural health monitoring(SHM) is essential for modern industrial development, in particular in the aspect of maintaining the integrity of huge mechanical structures such as storage tanks or shells of transportation vessels. In such context, the inspection of marine transportation vessels presents an even more challenging problem due to its extremely corrosive working environment and the uncertainty of their working schedule, which prevents it from regular, long-term inspection. How to tackle this problem to achieve fast, light, preliminary health inspections have become a paramount challenge in the field of SHM.
The overall objective of this project is to be able to develop a robust signal processing method for information extraction and evaluation to achieve inner mechanical structure detection and visual reproduction. Such a method could be further developed with better precision and robustness to inspect mechanical defects which are considered minor reflectors compared to the detection object in question here.M.S
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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