1,720,959 research outputs found
Signal processing for experimental modal analysis
No full textThis paper is an overview of signal processing for modal analysis. The approach taken is to summarize some of the types of data that arise in modal analysis, to classify signal processing problems and to attempt to show the relevance of the latter to the former. It is intended to emphasize the diversity of approaches that may be applied and point to advanced methods that offer potential
Identifying joints from measured reflection coefficients in beam-like structures with application to a pipe support
No full textThe properties of joints in mechanical systems are notoriously uncertain causing corresponding uncertainty in the systems’ dynamic responses. A piping system is one such example where an accurate knowledge of joint properties is useful for the purposes of structure-borne sound transmission, fatigue considerations and structural health monitoring. This paper presents an inverse technique that is applicable to joint estimation in one-dimensional structures such as a pipe. Measured wave reflection coefficients are used which have several advantages over modal information. First, they characterise just the joint and adjacent pipes and are independent of the rest of the built-up system. Second, they are potentially more sensitive to the joint parameters in question than are modal parameters.The method is illustrated by means of an experimental case study featuring a straight pipe suspended by a cantilevered hanger. The stiffness and inertia of the hanger are accurately identified from measured data at frequencies significantly higher than the fundamental modes of the structur
Experimental study on a nonlinear vibration isolator based on a post-buckled inverted L-shaped beam
No full textIn this paper, the characteristics and the effectiveness of a nonlinear passive vibration isolator based on a post-buckled beam is investigated experimentally. The intended application is specifically isolation in the vertical direction where the isolator is required to be sufficiently stiff statically to bear the weight of the isolated mass. The isolator consists of two beams joined to form an inverted L-shape and the weight of the isolated mass is taken to act at the vertex. If the weight of isolated mass is larger than the buckling load of the L-shaped beam then the beam buckles in one of two modes, one of which is unstable. In this paper, the static restoring force of the unstable mode is measured and an appropriately selected coil spring is added to counteract the negative stiffness of the beam. The resulting system presents a dramatically lower stiffness to small excursions about its equilibrium position in its buckled state but maintains its static load bearing capability. Free vibration measurements are presented which show some amplitude dependency of the natural frequency for large amplitude motion. Low amplitude harmonic base excitation measurements are also conducted from which transmissibility measurements are obtained and compared with corresponding results from a Finite Element model. The fundamental resonance is about 80% lower than that achievable by a comparable linear isolator. However the performance of the prototype design is compromised by the presence of higher frequency internal resonances of the isolator, mitigation of which is the focus of ongoing wor
Shock and vibration response of museum objects during transportation
No full textMuseum objects are frequently transported by road and air when toured internationally or when moved between storage facilities which exposes them to a range of vibration and shock inputs. Many of these vulnerable objects have incipient damage such as cracks, loose joints, partial delamination or failing repairs which can be exacerbated in transit. It is the conservator's responsibility to select which objects are safe to travel, largely based on experience and professional judgement. A joint research programme between the University of Southampton and the British Museum aims to develop a scientific framework to aid and inform such judgements. A central research question, and the focus of this paper, is which methods of transport are potentially most damaging?Vibration measurements were acquired during transit by car, train, ferry and commercial airliner. A moving average kurtosis is used to identify near-Gaussian and non-Gaussian segments in a time signal. Acceleration shock response spectra (SRS) are presented for a number of impulsive events, and corresponding vibration response spectra (VRS) shown for more normally distributed random inputs. Laboratory based vibration testing was carried out on a museum artefact packed in bespoke foam packaging inside a standard wooden transport crate. The modal response, combined with SRS and VRS, enabled the response of the object and the suitability of the packaging in reducing the response to different real-life stimuli to be assessed. Actual measurements on the artefact in transit also enabled the accuracy of SRS and VRS in predicting peak and RMS acceleration levels to be quantified
Modelling piezoelectric excitation in waveguides using the semi-analytical finite element method
Get PDFIn this paper we present a wave-based technique for modelling waveguides equipped with piezoelectric actuators in which there is no need for common simplications regarding their dynamic behaviour, the interaction with the waveguide or the bonding conditions. The proposed approach is based on the semianalytical finite element (SAFE) method. We developed a new piezoelectric element and employed the analytical wave approach to model the distributed electrical excitation and scattering of the waves at discontinuities. The model was successfully verified numerically and validated against an experiment on a beam-like waveguide with emulated anechoic terminations
A nonlinear vibration isolator based on a post-buckled inverted L-shaped beam
No full textIn this paper, a nonlinear passive vibration isolator based on a post-buckled beam is proposed and its characteristics and effectiveness are investigated by FE analysis. The intended application of the isolator is specifically isolation in the vertical direction where the isolator is required to be sufficiently stiff statically to bear the weight of the isolated mass and to be soft dynamically to achieve a low natural frequency. One approach to reconcile such a conflicting requirement is to make use of a post-buckled element that maintains much of its static stiffness in its buckled state but whose dynamic stiffness dramatically reduces. In this paper, we investigate the design of a simple, compact configuration of practical isolator using a post-buckled element – an inverted L-shaped beam. The weight of the isolated mass is taken to act at the vertex causing the vertical member to buckle when the weight is sufficiently large. The static and harmonic base excited response of such a system are investigated by FE analysis and validated through experiments. The system demonstrates the achievement of high-static-low-dynamic stiffness and exhibits a correspondingly low fundamental resonance frequency. However, internal resonances of the isolator are also prominent in its frequency respons
Going Beyond Counting First Authors in Author Co-citation Analysis
Get PDFThe 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
Get PDF“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
Get PDFWe 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
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