21 research outputs found
A framework to investigate instabilities of homogeneous andcomposite dielectric elastomer actuators
No full textPredictive models for Dielectric Elastomer Actuators require the nonlinear solid mechanics theory of soft dielectrics.
This is certainly true for homogeneous systems, but also for devices made of composite materials,
where the insertion of stiff conductive particles in the soft matrix may help to improve the overall actuation
performance. In this note, we present a theoretical framework to investigate a wide range of instabilities in both
homogeneous and composite-manufactured actuators: pull-in/electromechanical instability, buckling-like modes
and band-localization failure, that can be analyzed taking into account all the geometric and electromechanical
properties of the device such as i) nonlinearities associated with large strains and the employed material model;
ii) initial prestretch applied to the system; iii) dependency of the permittivity on the deformation (electrostriction).
In particular, we focus on the general expression which gives the condition for pull-in instability, also
valid for anisotropic composite soft dielectrics. In the second part, we show that in a layered composite an
electromechanical/snap through instability can be designed and possibly exploited to conceive release-actuated
systems
The role of material behavior in the performances of electroactive polymer energy harvesters
No full textElectroactive polymer energy harvesters are promising devices for the conversion of mechanical work to electrical energy. The performances of these devices are strongly dependent on the mechanical response of the polymeric material and on the type of electromechanical cycle, and these are limited by the occurrence of dielectric breakdown, compression induced wrinkling and electromechanical instability (pull-in). To identify the optimal electromechanical cycle that complies with all of these limitations, we set-up and solve a constraint optimization problem and we critically discuss the influence of material behavior of the polymer in the optimal performances of the energy harvesting device. Finally, we show that if the rate-independent dissipative behavior of the polymer (Mullins effect) is neglected, the optimization procedure may lead to quite unsatisfactory predictions: by making reference to explicit experimental data from literature we show that an optimal harvesting cycle deduced by neglecting the Mullins effect is far from being optimal when this is taken in consideration
The role of electrostriction on the stability of dielectric elastomer actuators
No full textAbstractIn the field of soft dielectric elastomers, the notion ‘electrostriction’ indicates the dependency of the permittivity on strain. The present paper is aimed at investigating the effects of electrostriction onto the stability behaviour of homogeneous electrically activated dielectric elastomer actuators. In particular, three objectives are pursued and achieved: (i) the description of the phenomenon within the general nonlinear theory of electroelasticity; (ii) the application of the recently proposed theory of bifurcation for electroelastic bodies in order to determine its role on the onset of electromechanical and diffuse-mode instabilities in prestressed or prestretched dielectric layers; (iii) the analysis of band-localization instability in homogeneous dielectric elastomers. Results for a typical soft acrylic elastomer show that electrostriction is responsible for an enhancement towards diffuse-mode instability, while it represents a crucial property – necessarily to be taken into account – in order to provide a solution to the problem of electromechanical band-localization, that can be interpreted as a possible reason of electric breakdown. A comparison between the buckling stresses of a mechanical compressed slab and the electrically activated counterpart concludes the paper
First Record of Anthonomus eugenii (Coleoptera: Curculionidae) in Italy
No full textAnthonomus eugenii Cano (Coleoptera: Curculionidae) was observed for the first time in Italy and the Mediterranean Region attacking sweet pepper (Capsicum annum L.; Solanaceae) in greenhouses and in fields in the coastal area of the Lazio Region of Italy. The incursion, detected in Oct 2013, was a heavy infestation of sweet pepper buds and not fully developed fruits, and causing their premature abscission. The outbreak area was delineated by the coast on the west, and hills on the south and east
Wild olive seed weevil in South Africa, Anchonocranus oleae Marshall (Coleoptera: Curculionidae), a rediscovery after a century
No full textThe wild olive seed weevil, Anchonocranus oleae Marshall, 1912 was not found after its description and the study of its biology by Silvestri in 1915. Recently, A. oleae larvae were found alive inside the kernels of wild olives (O. europaea subsp. cuspidata) collected from trees between 2009 and 2012 in the Western Cape, South Africa. No weevil larvae were found in cultivated olives collected during the same period. Only two adult weevils emerged from olives collected on the tree and on the ground. Over the sampling period 20 adults were collected directly on wild olive trees at different sites in the Western Cape and one in the Eastern Cape. All adults were shown to be conspecific with the holotype of A. oleae preserved in the Natural History Museum, London. Females and males have fully developed metathoracic wings, although they were never seen flying in limited field and laboratory observations. Adults in the laboratory immediately fed on wild olive pulp, but refused to feed on cultivated olives, even in the absence of alternative food. Oviposition sites remain visible even when the fruit pulp has dried out. The extremely low rate of adult emergence from wild olives can explain its absence in records of recent surveys. Barcoding COI sequences of young and mature larvae matched reference sequences obtained from A. oleae adults. The clear preference of A. oleae for wild olive fruit and its extremely low occurrence in cultivated olives suggest its possible use in classical biological control programs for invasive wild olive
Instability of Dielectric Elastomer Actuators
No full textDielectric elastomers (DEs) are an important class of materials, currently employed in the design and realization of electrically-driven, highly deformable actuators and devices, which find application in several fields of technology and engineering, including aerospace, biomedical and mechanical engineering. Subject to a voltage, a membrane of a soft dielectric elastomer coated by compliant electrodes reduces its thickness and expands its area, possibly deforming in-plane well beyond 100%: this principle is exploited to conceive transducers for a broad range of applications, including soft robots, adaptive optics, Braille displays and energy harvesters. Soft dielectrics undergo finite strains, and their modelling requires a formulation based on the Mechanics of Solids at large deformations. A major problem that limits the widespread diffusion of such devices in everyday technology is the high voltage required to activate large strains, because of the low dielectric permittivity of typical materials (acrylic elastomers or silicones), in the order of few unities, which governs the electromechanical coupling.
Composite materials (reinforcing a soft matrix with stiff and high-permittivity particles) provide a way to overcome these limitations, as suggested by some experiments. In addition, composites can display failure modes and instabilities not displayed by homogeneous specimens that must be thoroughly investigated. Commonly, instability phenomena are seen as a serious drawback, that should be predicted and avoided. However, in some cases they can be used to activate snap-through actuation, as in the case of buckling-like or highly-deformable balloon-like actuators.
Soft dielectric elastomers display electrostrictive properties (permittivity depending on the deformation) and we show how to take into account such a phenomenon within the theory of electroelasticity. Original results regard the investigation of diffuse modes (buckling like instabilities etc.), surface mode instabilities and localized modes. New (analytical) solutions for band-localization instability are provided and then it has been investigated how such instabilities are related to electrostriction. Regarding DE composites, the goal is to evaluate in detail the behaviour of two-phase rank-1 laminates in terms of different types of actuation, geometric and mechanical properties of phases, applied boundary conditions, and instabilities phenomena, in order to establish precise ranges in which the performance enhancement is effective with respect to the homogeneous counterpart
Torneuma (Torneuma) vastum Stuben 2007, stat. rev.
No full textTorneuma (Torneuma) vastum Stüben, 2007 stat. rev. Material examined. Sardinia: Carbonia-Iglesias prov., Iglesias, Case Marganai, 725 m, UTM 32S 463890 4355925, 1.IV.2002, L. Fancello leg., 3 ex (CCO, CNBFVR). Notes. Three specimens of this taxon, recently described by Stüben (2007) as a subspecies of T. curtulum F. Solari, 1937 from two rather close localities of eastern Sardinia (Escalapiano, and Mt. Narba east of Muravera) were found at Case Marganai, near Iglesias. The known distribution of Torneuma curtulum F. Solari, 1937 is, according to our personal records (Fig. 1), much more widespread than that given by Stüben (2007), who only knew this weevil from Domusnovas, Flumentorgius and Fluminimaggiore, all localities in western Sardinia close to the present one. A specimen of T. curtulum was collected at Mamenga, just a short distance from Case Marganai (where the above-mentioned 3 specimens were found), so the two taxa can be considered as sympatric. This implies that Torneuma vastum Stüben, 2007 (stat. rev.) is a self-standing species, and not a subspecies of T. curtulum. For the subtle characters which differentiate T. curtulum from T. vastum see Stüben (2007), and for their distribution see Fig. 1. Among the inaccuracies of the cited paper by Stüben (2007) are the descriptions of subspecies of the same species the distributional areas of which largely overlap, a circumstance that clearly weakens the even explicit provisional taxonomy proposed by this author. In the future we will publish additional changes affecting the taxonomy of the Torneumatini, which are beyond the purpose of the present paper.Published as part of Colonnelli, Enzo & Osella, Giuseppe, 2009, New data on some Curculionoidea (Coleoptera: Anthribidae, Apionidae, Curculionidae) from Sardinia *, pp. 421-426 in Zootaxa 2318 on page 42
CARTA A MENECEU SOBRE A FELICIDADE ΕΠΙΣΤΟΛΗ ΜΕΝΟΙΚΕΙ ΠΕΡΙ ΤΗΣ ΕΥΔΑΙΜΟΝΙΑΣ
No full textThe present work consists in the translation of the work of the Greek philosopher Epicurus of Samos (341 BC - 270 BC), letter to Meneceus, preserved in the work of Diogenes Laertius “Life and Doctrine of Eminent Philosophers” (Βίοι καὶ γνῶμαι τῶν ἐν φιλοσοφιαὶ), chapter X, accompanied by an introduction in which the author; life and work were discussed, and a development in chapters, in which we tried to present some characteristics concerning the letter genre, so that certain particularities presente in this corpus could help us in the analyses, especially in the syntax. After defining the genre, we tried to discuss some main concepts about Epicurean philosophy, in order to support us in the philosophical passages of the translated text. Ultimately, the translation intention was to bring the two languages closer together, as much as possible, without causing semantic damage to the translated language.O presente trabalho consiste na tradução da obra do filósofo grego Epicuro de Samos (341 a.C. - 270 a.C), Carta a Meneceu, preservada na obra de Diógenes Laércio “Vida e Doutrina dos Filósofos Eminentes” (Βίοι καὶ γνῶμαι τῶν ἐν φιλοσοφιαὶ), capítulo X, acompanhada de uma introdução na qual se discorreu sobre a vida e a obra do autor, e de um desenvolvimento em capítulos, nos quais procurou-se apresentar algumas características concernentes ao gênero carta, para que certas particularidades presentes no corpus pudessem nos auxiliar nas análises, sobretudo nas sintáticas. Após a definição do gênero, intentou-se discorrer sobre alguns conceitos principais a respeito da filosofia epicurista, a fim de nos amparar nas passagens filosóficas do texto traduzido. Por fim, o intuito tradutório foi o de aproximar, o quanto possível, as duas línguas, sem que houvesse prejuízo semântico na língua vertida
