IRIS - UNIRSM (Univ. degli Studi della Repubblica di San Marino)
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Quid sit metrum et quid rhythmum: Definizioni di metricità e ritmicità presso i grammatici latini antichi
No full textHigher-Order Theories for Structural Analysis of Doubly-Curved Shells with Variable Mechanical Properties
No full textThe static and dynamic behavior of several shell structures is affected by their mechanical properties. In particular, the natural linear frequencies can be subjected to a considerable variation by placing the reinforcing fibers along curvilinear paths or assuming variable thickness in the whole domain. Similarly, if a static problem is considered, the stress and strain profiles along the thickness of the structure, as well as the three-dimensional displacements, can show different trend varying the fiber orientation. Therefore, the mechanical behavior of doubly-curved structures shows more and more changes combining this kind of layers, characterized by variable properties, with classic orthotropic plies or with a soft-core. Nevertheless, the use of such laminated composite materials does not allow to employ the well-known first-order shear deformation shell theories anymore due to the strong anisotropic behavior. Hence, higher-order Equivalent Single Layer formulations and Layer-Wise models have to be introduced for this purpose. In the same way, the geometric parameters are assumed variable in each point of the domain. The study of doubly-curved shells with variable radii of curvature and variable thickness can take place once the geometric description is performed accurately by using the differential geometry principles. In order to solve numerically all these problems, the Generalized Differential Quadrature method is introduced. Several applications and numerical results are shown to exhibit the accuracy of the present technique
Il lavoro nella Costituzione
No full textL'articolo analizza l'importanza che il lavoro assume nella Costituzione italiana del 1948, come paradigma di democrazia e come strumento di dignità e realizzazione del cittadino. Dopo aver passato in rassegna le principali previsioni costituzionali relative al lavoro, ai diritti dei lavoratori e dei sindacati, l'analisi si concentra sulle attuali trasformazioni strutturali del lavoro tentando di verificare la perdurante attualità delle norme costituzionali.
MLSDQ based on RBFs for the free vibrations of laminated composite doubly-curved shells
No full textA Moving Least Squares Differential Quadrature (MLSDQ) method based on Radial Basis Functions (RBFs) is employed in this paper for solving doubly-curved shells made of composite materials. DQ method can easily approximate partial derivatives of any order by choosing proper basis functions. RBFs are functions that vary according to the radial distance from a current point and its neighborhood. The MLS method is implemented for the approximation of the shape functions used as basis functions. These shape functions depend on some weight functions that in this case are chosen as RBFs. Generally, numerical approaches based on the radial distance work very well on flat surfaces, such as plates, and on curves with constant curvature, such as spheres and cylinders, because the distance between two points can be easily measured. On the contrary, doubly-curved structures with variable radii of curvature which are defined by parametric curvilinear lines do not have a one-to-one (mutual) relationship between a curvilinear distance (defined by using curvilinear coordinates s1, s2) and the location of two points on the same surface (identified by two parameters α1, α2). Therefore, this work aims to show when it is possible to apply the MLSDQ method for solving doubly-curved laminated composite structures
Effect of agglomeration on the natural frequencies of functionally graded carbon nanotube-reinforced laminated composite doubly-curved shells
No full textThis paper aims at investigating the effect of Carbon Nanotube (CNT) agglomeration on the free vibrations of laminated composite doubly-curved shells and panels reinforced by CNTs. The great performances of doubly-curved structures are joined with the excellent mechanical properties of CNTs. Several laminations schemes and various CNT exponential distributions along the thickness of the structures are considered. Thus, it is evident that the shell dynamic behavior can be affected by many parameters which characterize the reinforcing phase. A widespread parametric study is performed in order to show the natural frequency variation. The general theoretical model for shell structures is based on the so-called Carrera Unified Formulation (CUF) which allows to consider several Higher-order Shear Deformations Theories (HSDTs). In addition, a complete characterization of the mechanical properties of CNTs is presented. The governing equations for the free vibration analysis are solved numerically by means of the well-known Generalized Differential Quadrature (GDQ) method due to its accuracy, stability and reliability features
On the mechanics of laminated doubly-curved shells subjected to point and line loads
No full textIt is well-known that the implementation of concentrated forces, such as point and line loads, represents a challenging task, especially from the computational point of view, since a strong discontinuity has to be inserted in the structural model. The present paper aims to solve the static problem of laminated composite doubly-curved shell structures subjected to concentrated loads employing the Generalized Differential Quadrature (GDQ) as numerical tool, according to what has been shown by the authors in their previous work. Its accuracy and reliability features are proven for several grid distributions when the concentrated loads are modeled through the Dirac-delta function. The theoretical framework on which this approach is based is the Unified Formulation developed by Carrera, which allows to investigate several Higher-order Shear Deformation Theories (HSDTs). The differential geometry is used to describe accurately the reference surface of various doubly-curved shell structures. The validity of the current approach is shown comparing the GDQ results with the exact and semi-analytical results available in the literature. A posteriori recovery procedure based on the three-dimensional equilibrium equations for a shell structure is introduced to compute the through-the-thickness variation of strain, stress and displacement components by means of the GDQ method
Static and Dynamic Behavior of Functionally Graded Carbon Nanotube-Reinforced Laminated Composite Doubly-Curved Shells: Higher-Order Structural Approaches
No full textShell structures are broadly employed in many engineering applications due to their efficiency in bearing with external loads, a high degree of resistance, remarkable stiffness and a high strength-to-weight ratio. These peculiar features are given by the doubly-curvature that defines their reference surface. Recently, the need of higher level performances has led to a greater use of innovative materials in shell design, such as fiber-reinforced composites, sandwiches and nanostructures. As a consequence, new materials have been introduced to design stiffer structural elements, without increasing their weight. Analogously, these advanced materials have allowed to improve their mechanical behavior, increasing for instance the safety requirements and the opposition to delamination phenomena. Advancements in manufacturing process have led to the consequent development of the so-called smart structures, which are currently the main topics of many researches and applications. A clear example of this aspect is given by the increasing use of Carbon Nanotubes (CNTs) as reinforcing phase of many composite materials. Nevertheless, since their first applications several mechanical models have been proposed to characterize these nanoparticles. A new micromechanical approach which deals with the agglomeration effect of CNTs is followed by the authors. The key point of this model consists in assuming that the distribution of CNTs in a polymer matrix is irregular. Consequently, their concentration is not uniform since they tend to concentrate in spherical shaped inclusions. The homogenization process based on the Mori-Tanaka scheme is used to compute the effective mechanical properties of such composites
Vibration analysis of variable thickness plates and shells by the Generalized Differential Quadrature method
No full textThe main purpose of this work is to perform the free vibration analysis of several laminated composite doubly-curved shells, singly-curved shells and plates, characterized by a continuous thickness variation. Variable thickness could affect the design of shell structures since it allows to tailor the stiffness features in the most stressed areas within the domain, keeping the weight constant. As a consequence, an improved dynamic behavior may be exhibited. The governing equations are solved numerically by the Generalized Differential Quadrature (GDQ) method, which has proven to be an accurate, stable and reliable numerical tool. Its accuracy is tested by means of several comparisons with analytical and semi-analytical results available in the literature, and with the solutions obtained by a three-dimensional finite element (FE) model. The theoretical approach considered in the current paper is general and allows consideration of many higher-order structural theories in a unified manner, in which the order of the kinematic expansion can be chosen arbitrarily