1,721,049 research outputs found
Mitigation of seismic waves: Metabarriers and metafoundations bench tested
No full textThe article analyses two potential metamaterial designs, the metafoundation and the metabarrier, capable to attenuate seismic waves impact on buildings or structural components in a frequency band between 3.5 and 8 Hz. The metafoundation serves the dual purpose of reducing the seismic response and supporting the superstructure. Conversely the metabarrier surrounds and shields the structure from incoming waves. The two solutions are based on a cell layout of local resonators whose dynamic properties are tuned using finite element simulations combined with Bloch periodicity boundary conditions. To enlarge the attenuation band, a graded design where the resonant frequency of each cell varies spatially is employed. If appropriately enlarged or reduced, the metamaterial designs could attenuate lower frequency seismic waves or groundborne vibrations respectively. A sensitivity analysis over various design parameters including size, number of resonators, soil type and source directivity, carried out by computing full 3D numerical simulations in time domain for horizontal shear waves is proposed. Overall, the metamaterial solutions discussed here can reduce the spectral amplification of the superstructure by approx. 15–70% depending on several parameters such as the metastructure size and the properties of the soil. Pitfalls and advantages of each configuration are discussed in detail. The role of damping, crucial to avoid multiple resonant coupling, and the analogies between graded metamaterials and tuned mass dampers is also investigated
Physics-informed surrogate modeling for a damaged rotating shaft
No full textIn online structural health monitoring frameworks, surrogate modeling approaches aim to reproduce the dynamics of an underlying high-fidelity model for facilitating fast simulations. Conventional machine learning techniques based on purely data-driven approaches, i.e., neural networks (NN), often behave as a black box delivering predictions that may lack physical consistency. In an effort to overcome such limitations, we propose an NN mapping constrained by known dynamic equations provided by a physics-based model of the operating system. Incorporating the system matrices into the loss function steers the learning process towards more physically reliable predictions while maintaining accuracy. Specifically, a surrogate model relying on physics-informed neural networks (PINN), is developed to enable accurate and fast ballistic impact damage identification on a helicopter transmission. At first, a physics-based model simulates the damage-induced vibration loads and eventually the system response. The PINN model receives the system response as input and predicts the external load and model parameters, eventually assessing the impact damage extent
Parameter identifiability through information theory
Full text linkIn this paper, we address the problem of assessing the identifiability of model parameters in a mechanical system, i.e., whether unknown parameters can be estimated given a set of measurements collected through sensor networks. Practical identifiability can arise due to either a lack of sensitivity or a joint effect of the parameters on the measurements. Information theory can be used to detect the sources of non-identifiability, with the purpose of establishing an efficient sensor network design. Mutual Information between the parameter and the measured outputs, and Conditional Mutual Information of each parameter couple, conditioned on the measurements, are considered. Adoption of these indices is overviewed for practically assessing the identifiability of the mechanical properties of a non-linear structural model
On Introducing Imperfection in the Non-Linear Analysis of Buckling of Thin Shell Structures
No full textThis master thesis details the investigation of the effect of geometrical imperfection on thin shell structures using general FEM software packages. The author proposes a finite element based method for the analysis and design of thin shell structures, and describes the implementation of such a procedure on four different FEM packages. The procedure involves the assessment of imperfection sensitivity of a design, and imposing geometrical imperfection in the shape of the first buckling mode prior to a geometrically non-linear analysis. Starting with thin metallic cylinders, by incorporating imperfection to the surface, the author shows that ANSYS is capable of reproducing resulting similar to Koiter's asymptotic theory and to experimental data. The author further demonstrates the robustness and easy-to-use nature of the imperfection procedure by implementing it on four simple yet realistic structures with both symmetric and asymmetric load cases. For extremely imperfection sensitive structures such as an axially loaded cylinder. The author then introduces four different types of imperfection that may be imposed in place of the first buckling mode, and gauges the effectiveness of each with a modified knock-down factor. By varying the vertical curvature, it is discovered that an axisymmetric imperfection shape governs the ultimate buckling capacity of any near-cylindrical shells. An theory is being developed that explains the effect of the axisymmetric shape. Change in Gaussian curvature is calculated as the end of every load step in the FEM analysis. By plotting the change in Gaussian curvature, the onset of buckling can be readily defined. Finally, physical non-linearities are introduced to the FEM models to gauge the effect of yielding and cracking on a steel cylinder, and a reinforced concrete (RC) cooling tower respectively. It is found that metal buckles within the elastic range, and yielding eliminates post-buckling capacity without altering the ultimate capacity. With RC cracking, it is discovered that instability occurs soon after cracks develop at the buckles of the imperfection shape, therefore reducing the capacity by as much as 8 times from that of the elastic model.Structural MechanicsStructural EngineeringCivil Engineering and Geoscience
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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