130,373 research outputs found
Experimental Characterization and Finite Element Modelling of a thermoplastic composite lamina subjected to large shear deformation
No full textIn this work, the behaviour of a woven lamina for thermoplastic composite material is experimentally and numerically investigated. A thermoplastic composite made of hot-pressed polypropylene (PP) woven laminas is the object of the study. The laminas are composed by tapes which are made up of co-extruded core and skins. Experimental tests were carried out to determine the properties of the lamina. The experimental data are used to develop numerical models for the simulation of the experimental test. Three modelling techniques are proposed: the first one implements a macro-mechanical material model, the second one uses a meso-mechanical approach, and the third one discretizes the geometry at the yarn level. The global response of the meso-mechanical material model showed the best agreement with the experimental test, even if it does not represent the optimal solution from the computational-cost point of view
Experimental crushing analysis of thermoplastic and hybrid composites
No full textIn this work the mechanical properties of composites specimens made of thermosetting and thermoplastic
materials are compared. The behaviour of a traditional carbon/epoxy laminate is compared with those of a fully
polypropylene composite and of a hybrid composite. The hybrid composite is made by stacking different layers
of thermoplastic and thermosetting material. The influence of different production processes, for the hybrid
composite, are also investigated. The capability of energy absorption are investigated studying the crushing
behaviour of impact attenuators. The impact attenuators made of carbon fibre reinforced plastic exhibit the
highest load capability with the highest energy absorption. However, the failure mode is brittle with debris
formation. Conversely, a failure mode of plastic crushing is observed in the structures made of fully polypropylene.
The experimental tests on the specimen made of hybrid laminates proved a strong influence of the
production process of the components on the mechanical properties. In the hybrid laminates, the different
chemical properties of the base materials lead to a pre-existing delamination state in the final. The results
highlight that curing the thermoplastic material before the compaction with the thermosetting laminates is the
best solution to guarantee the highest impact resistance
Methodology for parameter identification on a thermoplastic composite crash absorber by the Sequential Response Surface Method and Efficient Global Optimization
No full textNumerical simulations for crashworthiness require the definition of material properties that are not always predictable with standard experimental tests. This paper deals with the numerical optimization of a thermoplastic composite material model. The component is a vehicle impact attenuator made of an innovative All-PP (PolyPropylene) composite material. The peculiar failure mechanism of this material makes the numerical simulation of the collapse a difficult challenge to achieve with a trial-and-error calibration of the material card. Therefore, an optimization procedure is proposed to determine the material parameters. The optimization is implemented in LS-OPT, where the mean square error between the experimental and numerical load-displacement curves is the objective function to be minimized. Two test cases are considered: (1) optimization of the material card based on the full load-displacement curve from the experimental tests and (2) optimization of relevant parameters of a numerical trigger added to control initial contact instabilities between the impacting rigid wall and the component in the numerical simulations. The optimization strategies Sequential Response Surface Method (SRSM) and Efficient Global Optimization (EGO) are used. The results show that the presented methodology allows characterizing the studied composite material and thus obtaining a more efficient numerical model
Indagine teorica e di campo per la stima delle portate nella sezione di chiusura del bacino sperimentale "Fiumarella di Corleto"
No full textVengono presentati i risultati ottenuti da un’indagine di campo per l’individuazione della scala di efflusso relativa alle portate ordinarie e di magra nella sezione di chiusura del bacino sperimentale Fiumarella di Corleto (Fiume Agri, Basilicata). Nonostante la complessita' del campo di moto della corrente, si dimostra come si possa ragionevolmente far riferimento alla formula degli stramazzi a larga soglia, a meno di un coefficiente di contrazione, e come i risultati sperimentali risultino in buon accordo con quelli teorici ottenuti dalla risoluzione delle equazioni del De Saint-Venant nell’ipotesi di moto bidimensionale
Spazi di soglia
No full textL’osservazione diretta degli spazi del costruito ha condotto ad una valutazione sulla possibilità di riconnessione fra spazi pubblici e privati.
I criteri di individuazione per gli elementi di interesse sono:
- Qualità delle corti
- Collegamento fra corti e strada
- Stato di conservazione delle corti
- Misura degli spazi qualità dei corpi scala
- Variabili e invariabili all’interno delle corti
- Organizzazione interna del costruito
- Porzione di cielo visibile dalle corti
Dall’osservazione di tali elementi è derivata una proposta di riqualificazione generale grazie alla riconnessione degli spazi di soglia individuati con:
- Possibilità di creazione di una rete di spazi con qualità riconoscibile a scala urbana
- Incrementare la salubrità delle condizioni di vita nei punti di alta concentrazione di pozzi di luce
-Connessione fra corti contigue
-Individuazione di corti predisposte ad accoglierefunzioni pubbliche
MeSH term explosion and author rank improve expert recommendations
No full textInformation overload is an often-cited phenomenon that reduces the productivity, efficiency and efficacy of scientists. One challenge for scientists is to find appropriate collaborators in their research. The literature describes various solutions to the problem of expertise location, but most current approaches do not appear to be very suitable for expert recommendations in biomedical research. In this study, we present the development and initial evaluation of a vector space model-based algorithm to calculate researcher similarity using four inputs: 1) MeSH terms of publications; 2) MeSH terms and author rank; 3) exploded MeSH terms; and 4) exploded MeSH terms and author rank. We developed and evaluated the algorithm using a data set of 17,525 authors and their 22,542 papers. On average, our algorithms correctly predicted 2.5 of the top 5/10 coauthors of individual scientists. Exploded MeSH and author rank outperformed all other algorithms in accuracy, followed closely by MeSH and author rank. Our results show that the accuracy of MeSH term-based matching can be enhanced with other metadata such as author rank
Going Beyond Counting First Authors in Author Co-citation Analysis
No full textThe 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
Material parameter optimization of flax/epoxy composite laminates under low-velocity impact
No full textSince natural fibers have great potential as an alternative to synthetic fibers when the components are impacted at low energies, their mechanical properties under different types of loads need to be investigated. This can be accomplished by using finite element analysis, which is based on the definition of numerical models that reproduce the objects of the physical phenomenon under study. In defining these models, many parameters of the material cards are determined by experimental tests. However, experiments are time-consuming and costly, and it is not always possible to perform all the necessary tests to determine the values for all unknown parameters. For this purpose, the trial-and-error method is usually used. In this work, we present an optimization procedure for predicting the behavior of flax/epoxy composite laminates under low-velocity impact, using the LS-DYNA solver for numerical simulation. The study aims at identifying the values of relevant parameters that allow for predicting the experimental force–displacement trend as accurately as possible and reproducing the damage mechanisms numerically. Each step of the optimization flow is performed with the external tool LS-OPT, using dynamic time warping as a similarity measure to efficiently handle noise. For this purpose, we use the Efficient Global Optimization algorithm, a strategy based on surrogate modeling techniques. We address a multi-target scenario, i.e., we consider several energy levels simultaneously, aiming to find an optimal parameter configuration that is less sensitive to variations in impact energy. The results obtained not only demonstrate the potential of surrogate-based optimization to identify material parameters, but also provide a characterization of the studied composite configuration in view of future applications
"Closing the R&D Gap, Evaluating the Sources of R&D Spending"
No full textBoth spending and tax policies have been implemented in the United States with the goal of stimulating private sector research and development (R&D). Karier questions whether current R&D policy, especially the research and experimentation tax credit, can contribute to closing the gap between nondefense expenditures on R&D in the United States and such expenditures in other countries, such as Japan and Germany. He also explores possible changes to our current R&D policy to make it more effective.
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