1,721,047 research outputs found
La formazione e l’università
No full textCome sarà l’università del futuro? Come dovrà essere per tenere il passo, accompagnare e gestire il cambiamento in un mondo dei saperi e in una società in impetuosa trasformazione? “Senza muri”. Così hanno risposto le università europee, esprimendo la loro visione in un documento, che si intitola proprio “University Without Walls”, elaborato dalla European University Association (www.eua.eu). L’università “senza muri” è quella che si apre e si impegna nella società, riuscendo a mantenere e promuovere i valori che sono la sua linfa vitale, come l'autonomia, la libertà accademica, lo spirito critico e l'inscindibilità tra didattica e ricerca (www.magna-charta.org). Con quel documento, le università europee si sono date la missione di diventare sempre più responsabili, autonome e libere, con profili istituzionali diversi, ma unite nella loro missione di apprendimento e insegnamento, ricerca, innovazione e cultura al servizio della società.
Per riassumere in tre parole, l’università senza muri dovrà essere aperta, impegnata e ibrida
Overall elastic properties of a plate containing inhomogeneities of irregular shape
No full textThe present work deals with the stiffness properties of an infinite 2D isotropic elastic system containing inhomogeneities having a circular contour. Starting from this general layout, the cases of a matrix with lenticular, perfectly circular, semi-circular, “C-shaped” and thin straight inclusions can be obtained as limit cases. Owing to the geometry of the system, reference is made to bipolar cylindrical coordinates ( ), which are linked to the Cartesian ones (x1, x2) through the conformal map [2]. The effective elastic properties of the system is analytically investigated by introducing a fourth-order compliance contribution tensor H, which represents the effect induced by the inhomogeneity on the compliance of the system according to [1], being S the compliance tensor for the homogeneous elastic matrix and e the stress field. It is remarked that the last term in eq (1) denotes the correction acting on the strain field owing to the presence of the inclusions. The system without inhomogeneities and subjected to a remote stress field is considered first. The corresponding fundamental stress field (0) within the matrix does not accomplish the BCs at the contour of the inhomogeneities. Thus, following the Jeffery approach, an auxiliary stress field deduced by a biharmonic stress function in bipolar coordinates is introduced and tensor H is then evaluated by performing proper contour integrals involving the total stress distribution along the contours of the inclusions. The study allows evaluating the effective elastic properties of a wide class of inhomogeneous materials, with particular reference to composites reinforced with natural or synthetic fibres having optimized cross sections.
References
[1] Sevostianov, I., and Kachanov, M., “Explicit cross-property correlations for anisotropic
two-phase composite materials” Journal of the Mechanics and Physics of Solids, 50, 253-282 (2002).
[2] Korn, G.A. and Korn, T.M., Mathematical handbook for scientists and engineers. Definitions, Theorems and Formulas for Reference and Review, Dover, New York (1968)
3 May 2016 - University of Bologna Rector F. Ubertini, Italian Republic, signing the guest book with Engineering Department Head R. Losito.
No full textProfessor Francesco Ubertini Rector University of Bologn
Discontinuous cohesive laws for modelling mixed-mode delamination
Full text linkThe cohesive-zone model has proven to be an effective tool to model non-linear fracture processes, such as the delamination of composite laminates in the case of large-scale bridging. According to this model, a fracture process zone – in the form of a cohesive interface – develops ahead of the crack front, where the new fracture surfaces are created and gradually separate from each other. Prior to complete separation, the fracture surfaces exchange interfacial stresses (or cohesive tractions), which depend on their relative displacements (or separations).
Commonly adopted cohesive laws consist of traction-separation relationships, which can be subdivided into two ideal stages:
i. an elastic stage, where tractions increase up to a peak value, representing the response of the undamaged cohesive interface;
ii. a softening stage, where tractions decrease to zero, representing the progressive damage leading to fracture of the interface.
Often, linearly elastic response is assumed for the elastic stage. Instead, for the softening stage, many idealised trends have been proposed in the literature: from linear, or piece-wise linear, to exponential and power laws. In a previous work, we proposed using bilinear discontinuous cohesive laws to model the response of a double cantilever beam test specimen loaded with uneven bending moments (DCB-UBM). Very good agreement between the theoretical predictions and experimental results was obtained for tests conducted under pure fracture modes I and II.
Here, we show how the abovementioned discontinuous cohesive laws can be extended to I/II mixed-mode fracture conditions under the assumption that fracture modes are uncoupled and linear mixed-mode criteria govern crack onset and propagation. As an illustrative example, the model is applied to the DCB-UBM under mixed-mode fracture conditions. The invariant J-integral is calculated along an internal (local) path and an external (global) path. Hence, theoretical predictions are obtained for the relative opening and tangential displacements at the crack tip, which qualitatively agree with experimental observations
The reinforcement of ancient timber-joints with carbon nanocomposites
No full textVolume a cura di Francesco Ubertini, Erasmo Viola, Stefano de Miranda, Giovanni Castellazz
Analytical evaluation of the stress fields in tapered box girders
Full text linkTapered beams are widely used in both civil and industrial engineering for a more efficient exploitation of material in comparison to prismatic beams. In the wind energy sector, tapered box girders are commonly used as the main structural elements of composite wind turbine blades.
In the scientific literature, it is long known that the internal distribution of stresses in tapered beams may differ significantly from that present in prismatic beams. In 1932, Bleich presented an extension of Jourawski’s formula showing that in beams of variable cross section, shear stresses arise as a result of not only shear force, but also bending moment and axial force.
Notwithstanding the above, standard design procedures do not take into account the effects of taper on the stress distribution in a beam, with consequent possible under- or over-estimation of the stress states acting in a real structure. Only recently, researchers have focused their attention on the effects of taper in beams of variable cross section with a view towards the practical design of structures. A broad review of the literature on non-prismatic beams can be found in the recent work by Balduzzi et al. (2017).
In this work, we further extend Bleich’s shear stress formula to tapered beams subjected to distributed loads. Furthermore, we specialise the obtained result to the analysis of tapered box girders. By introducing suitable assumptions on the stress field in a thin-walled cross section, from the integration of Cauchy’s equilibrium equation, we determine analytical, yet approximate, expressions for all the stress components. Thus, the contributions stemming from the effects of taper and the differences with the stress fields in prismatic beams are clearly highlighted. Comparison of the analytical predictions with the numerical results of finite element analyses obtained through the commercial software Abaqus shows an excellent agreement, thus validating the proposed approach
Recensione di Robert G. La France, Bachiacca: artist of the Medici court, Firenze, Olschki, 2008
No full textABSTRACT ITALIANO
Francesco Ubertini detto il Bachiacca ebbe scarsa fortuna critica ma questo volume di Robert G. La France colma una lacuna grave nella critica d’arte con una solida monografia in lingua inglese che prende in esame tutte le opere e i documenti di questo pittore. Stefano Colonna giudica positivamente il libro, pubblicato da Leo Olschki, limitandosi a sottolineare che alcune opere, inserite come autografe dall’autore, presentino differenti qualità.
ENGLISH ABSTRACT
Francesco Ubertini called Bachiacca has had little critical fortune, but this book by Robert G. La France fills a serious lacuna in art criticism with a solid English monograph that examines all the existing works and documents. Stefano Colonna welcomes the book published by Leo Olschki and simply points out that some of the works included by the author as autograph have a different quality
Energy harvesting from bridge vibrations with piezoelectric devices - Analysis of a case study bridge
Full text linkThe use of piezoelectric devices to harvest the energy connected to the vibrations induced on road bridges by travelling vehicles has been proposed in the recent literature, albeit with application limited to an idealised bridge modelled as a simply supported beam.
In a previous work, we started a feasibility study on the application of the above-mentioned technique to a real bridge. To this aim, we selected an existing urban bridge in Pisa as a case study and collected the available documentation about its original design. The results of a past experimental campaign provided the natural frequencies and mode shapes of the structure.
Besides, we developed a mechanical model of a laminated cantilever beam with a top piezoelectric layer and a concentrated mass on its free end. The eigenfrequencies of the cantilever beam were determined in both short- and open-circuit electrical boundary conditions. Then, the partial differential equations were deduced for the coupled electro-mechanical problem and solved in terms of the cantilever displacement and voltage output, in the case of vehicle-induced vibrations and with a finite electrical resistance connected to the device.
In this work, we develop a three-dimensional finite element model of the case study bridge. The model is first calibrated to match the results of the experimental dynamic acquisitions. Next, the dynamic response of the bridge under travelling vehicles is simulated. To this aim, the data of real cars and busses allowed to cross the bridge are considered. The outcomes of the simulations in terms of acceleration time histories are used as input for the model of the piezoelectric cantilever beam, suitably designed to feature the same first natural frequency of the case study bridge. Neglecting electrical damping, a close relation between voltage output and vehicle speed is obtained. In the practical range of vehicle speed, a maximum power output in the order of 100mW is predicted
Evaluation of the elastic stiffnesses of multi-directional laminates by bending tests
Full text linkSituations may arise in which the elastic stiffnesses of composite laminates are to be evaluated with a limited quantity of available material. Besides, tensile testing may not be possible due to inadequate geometries of the available samples with respect to the laboratory equipment, etc. In such cases, three- or four-point bending tests may result as a simple and effective alternative to tensile tests. ASTM international specifies how to evaluate the flexural properties of polymer matrix composite materials by bending tests. Shear stiffnesses of unidirectional laminates may also be determined by executing an adequate number of non-destructive three-point bending tests at different span lengths.
We extend the above procedure to multidirectional laminates, whose geometry and stacking sequence are known, but not the elastic properties of the plies. First, we measure the values of specimen compliance from experiments at different span lengths. Hence, we determine the homogenised bending and shear stiffnesses of the laminate based on Timoshenko’s beam theory. Lastly, we obtain the Young’s modulus and shear modulus of the constituting plies by reversing the usual procedure of calculating the laminate stiffnesses from ply properties through the classical theory of laminates. We apply the procedure to carbon/epoxy unidirectional laminated specimens, tested elsewhere to determine their interlaminar fracture toughness, and to multidirectional glass fibre-reinforced laminates, used in the strengthening of road bridges
La presa di Palazzo Poggi
No full textMaurizio Matteuzzi e Giorgio Tassinari raccontano la storia della presa del Rettorato dell'Università di Bologna da parte di Francesco Ubertini con i ritmi di una ballata che è istruttiva anche per chi non fa parte dell'ateneo bolognese
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