arXiv subjects

Fernando Fraternali

Publications and source records attributed to Fernando Fraternali.

17 recordsLinked to original sources

arXiv2022

Limit analysis of strut nets

Truss structures composed of members that work exclusively in tension or in compression appear in several problems of science and engineering, e.g., in the study of the resisting mechanisms of masonry structures, as well as in the design of spider web-inspired web structures. This work generalizes previous results on the existence of cable webs that are able to support assigned sets of nodal forces under tension. We extend such a problem to the limit analysis of compression-only 'strut nets' subjected to fixed and variable nodal loads. These systems provide discrete element models of masonry bodies, which lie inside the polygon/polyhedron with vertices at the points of application of the given forces ('underlying masonry structures'). It is assumed that fixed nodal forces are combined with variable forces growing proportionally to a scalar multiplier (load multiplier), and that the supporting strut net is subjected to kinematic constraints at given nodal positions.

physics.class-ph↗

arXiv2021

Free and forced vibrations of damped locally-resonant sandwich beams

This paper addresses the dynamics of locally-resonant sandwich beams, where multi-degree-of-freedom viscously-damped resonators are periodically distributed within the core matrix. Using an equivalent single-layer Timoshenko beam model coupled with mass-spring-dashpot subsystems representing the resonators, two solution methods are presented. The first is a direct integration method providing the exact frequency response under arbitrary loads. The second is a complex modal analysis approach obtaining exact modal impulse and frequency response functions, upon deriving appropriate orthogonality conditions for the complex modes. The challenging issue of calculating all eigenvalues, without missing anyone, is solved applying a recently-introduced contour-integral algorithm to a characteristic equation built as determinant of an exact frequency-response matrix, whose size is $4 \times 4$ regardless of the number of resonators. Numerical applications prove exactness and robustness of the proposed solutions.

physics.app-ph↗

arXiv2019

Incremental auxetic response of composite lattices under isotropic prestress

This work studies the constitutive response of two- and three-dimensional lattice materials subject to isotropic prestress. The unit cell of the examined lattices is formed by an arbitrary number of junctions attached to a junction. Analytic formulae for second-order elastic constants and elastic stiffness coefficients of stretching-dominated lattices are provided. In addition, numerical results for the incremental elastic moduli of composite lattices equipped with hard and soft rods are presented. The given results highlight that isotropically pre-tensioned lattices may exhibit marked variations of the elastic stiffness moduli in the prestressed state, over the values competing to the stress-free configuration. This study also discloses that prestressed lattices may feature incremental auxetic response, when composite architectures suitably combining hard and soft materials are employed for their fabrication.

physics.class-ph↗

arXiv2019

Mechanical modeling of superelastic tensegrity braces for earthquake-proof structures

This paper presents a novel application of tensegrity systems as lightweight and noninvasive braces for seismically resistant structures. The analyzed braces consist of D-bar systems equipped with shape memory alloy cables with superelastic response. These structures are able to markedly amplify the applied longitudinal displacements in the transverse direction, thus allowing for considerably large deformations of the SMA cables and high energy dissipation. The mechanics and the application of tensegrity braces for the reinforcement of anti-seismic structures are presented. A comparative study matches the seismic response of the analyzed systems with that of devices employing fluid viscous dampers. The results highlight the fact that D-bar units can be profitably employed as building blocks for innovative bracing systems, whose energy dissipation properties can easily surpass those of conventional anti-seismic braces.

physics.app-ph↗

arXiv2019

On the compact wave dynamics of tensegrity beams in multiple dimensions

This work presents a numerical investigation on the nonlinear wave dynamics of tensegrity beams in 1D, 2D and 3D arrangements. The simulation of impact loading on a chain of tensegrity prisms and lumped masses allows us to apply on a smaller scale recent results on the propagation of compression solitary waves in 1D tensegrity metamaterials. Novel results on the wave dynamics of 2D and 3D beams reveal - for the first time - the presence of compact compression waves in two- and three-dimensional tensegrity lattices with slender aspect ratio. The dynamics of such systems is characterized by the thermalization of the lattice nearby the impacted regions of the boundary. The portion of the absorbed energy moving along the longitudinal direction is transported by compression waves with compact support. Such waves emerge with nearly constant speed, and slight modifications of their spatial shape and amplitude, after collisions with compression waves traveling in opposite direction. The analyzed behaviors suggest the use of multidimensional tensegrity lattices for the design and additive manufacturing of novel sound focusing devices.

nlin.PS↗

arXiv2019

On a modified Becker-Doering model for two-phase materials

This work reconsiders the Becker-Doering model for nucleation under isothermal conditions in the presence of phase transitions. Based on thermodynamic principles a modified model is derived where the condensation and evaporation rates may depend on the phase parameter. The existence and uniqueness of weak solutions to the proposed model are shown and the corresponding equilibrium states are characterized in terms of response functions and constitutive variables.

math-ph↗

arXiv2018

Accordion-like metamaterials with tunable ultra-wide low-frequency band gaps

We study lightweight, elastic metamaterials consisting of tensegrity-inspired prisms, which present wide, low-frequency band gaps. For their realization, we alternate tensegrity elements with solid discs in periodic arrangements that we call "accordion-like" meta-structures. We show through analytical calculations and numerical simulations that these structures are characterized by low-frequency band gaps with strong uniform wave attenuation due to the coupling of Bragg scattering and local resonance mechanisms. This coupling helps to overcome the inherent limit of a narrow band-gap width for conventional locally-resonant metamaterials and to extend the wave attenuation to wider frequency ranges. Moreover, the band gaps can be further increased, provided a minimum structural damping is present, and tuned to desired frequencies by changing the applied prestress levels in the tensegrity structure. Results are corroborated by parametric studies showing that the band-gap frequencies are preserved for wide variations of geometric and material structural properties.

physics.app-ph↗

arXiv2018

Tuning frequency band gaps of tensegrity metamaterials with local and global prestress

This work studies the acoustic band structure of tensegrity metamaterials, and the possibility to tune the dispersion relation of such systems by playing with local and global prestress variables. Building on established results of the Bloch-Floquet theory, the paper first investigates the linearized response of chains composed of tensegrity units and lumped masses, which undergo small oscillations around an initial equilibrium state. The stiffness of the units in such a state varies with an internal self-stress induced by prestretching the cables forming the tensegrity units, and the global prestress induced by the application of compression forces to the terminal bases. The given results show that frequency band gaps of monoatomic and biatomic chains can be effectively altered by the fine tuning of local and global prestress parameters, while keeping material properties unchanged. Numerical results on the wave dynamics of chains under moderately large displacements confirm the presence of frequency band gaps of the examined systems in the elastically hardening regime. Novel engineering uses of the examined metamaterials are discussed.

nlin.PS↗

arXiv2014

Parametric Design of Minimal Mass Tensegrity Bridges Under Yielding and Buckling Constraints

This paper investigates the use of the most fundamental elements; cables for tension and bars for compression, in the search for the most efficient bridges. Stable arrangements of these elements are called tensegrity structures. We show herein the minimal mass arrangement of these basic elements to satisfy both yielding and buckling constraints. We show that the minimal mass solution for a simply-supported bridge subject to buckling constraints matches Michell's 1904 paper which treats the case of only yield constraints, even though our boundary conditions differ. The necessary and sufficient condition is given for the minimal mass bridge to lie totally above (or below) deck. Furthermore this condition depends only on material properties. If one ignores joint mass, and considers only bridges above deck level, the optimal complexity (number of elements in the bridge) tends toward infinity (producing a material continuum). If joint mass is considered then the optimal complexity is finite. The optimal (minimal mass) bridge below deck has the smallest possible complexity (and therefore cheaper to build), and under reasonable material choices, yields the smallest mass bridge.

cond-mat.mtrl-sci↗

arXiv2014

Multiscale tunability of solitary wave dynamics in tensegrity metamaterials

A new class of strongly nonlinear metamaterials based on tensegrity concepts is proposed and the solitary wave dynamics under impact loading is investigated. Such systems can be tuned into elastic hardening or elastic softening regimes by adjusting local and global prestress. In the softening regime these metamaterials are able to transform initially compression pulse into a solitary rarefaction wave followed by oscillatory tail with progressively decreasing amplitude. Interaction of a compression solitary pulse with an interface between elastically hardening and softening materials having correspondingly low-high acoustic impedances demonstrates anomalous behavior: a train of reflected compression solitary waves in the low impedance material; and a transmitted solitary rarefaction wave with oscillatory tail in high impedance material. The interaction of a rarefaction solitary wave with an interface between elastically softening and elastically hardening materials with high-low impedances also demonstrates anomalous behavior: a reflected solitary rarefaction wave with oscillatory tail in the high impedance branch; and a delayed train of transmitted compression solitary pulses in the low impedance branch. These anomalous impact transformation properties may allow for the design of ultimate impact mitigation devices without relying on energy dissipation.

cond-mat.mtrl-sci↗

arXiv2014

Recycled nylon fibers as cement mortar reinforcement

We investigate engineering applications of recycled nylon fibers obtained from waste fishing nets, focusing our attention on the use of recycled nylon fibers as tensile reinforcement of cementitious mortars. We begin by characterizing the tensile behavior of both unconditioned and alkali-cured recycled nylon fibers obtained through manual cutting of waste fishing net filaments, with the aim of assessing the resistance of such materials to chemical attacks. Special attention is also given to evaluating the workability of fresh mortar and the possible impacts of contaminants released by waste fishing nets into the environment. Next, we deal with compression and bending tests on cementitious mortars reinforced with recycled nylon fibers, and establish comparisons with the experimental behavior of the unreinforced material and with results given in existing literature. In our analysis of different weight fractions and aspect ratios of the reinforcing fibers, we observe marked increases in the tensile strength (up to +35%) and toughness (up to 13 times greater) of the nylon reinforced mortar, as compared with the unreinforced material. The presented results emphasize the high environmental and mechanical potential of recycled nylon fibers for the reinforcement of sustainable cement mortars.

physics.app-ph↗

arXiv2014

On the correspondence between 2D force networks, tensegrity structures and polyhedral stress functions

We formulate and discuss the relationship between polyhedral stress functions and internally self-equilibrated frameworks in 2D, and a two-mesh technique for the prediction of the stress field associated with such systems. We generalize classical results concerned with smooth Airy stress functions to polyhedral functions associated with arbitrary triangulations of a simply-connected domain. We also formulate a regularization technique that smoothly projects the stress function corresponding to an unstructured force network over a structured triangulation. The paper includes numerical examples dealing with a benchmark problem of plane elasticity, and the stress fields associated with tensegrity models of a cantilever beam and an elliptical membrane.

cond-mat.mtrl-sci↗

arXiv2014

Experimental investigation of the softening-stiffening response of tensegrity prisms under compressive loading

The present paper is concerned with the formulation of new assembly methods of bi-material tensegrity prisms, and the experimental characterization of the compressive response of such structures. The presented assembly techniques are easy to implement, including a string-first approach in the case of ordinary tensegrity prisms, and a base-first approach in the case of systems equipped with rigid bases. The experimental section shows that the compressive response of tensegrity prisms switches from stiffening to softening under large displacements, in dependence on the current values of suitable geometric and prestress variables. Future research lines regarding the mechanical modeling of tensegrity prisms and their use as building blocks of nonlinear periodic lattices and acoustic metamaterials are discussed.

cond-mat.mtrl-sci↗

arXiv2014

On the mechanical modeling of the extreme softening/stiffening response of axially loaded tensegrity prisms

We study the geometrically nonlinear behavior of uniformly compressed tensegrity prisms, through fully elastic and rigid--elastic models. The presented models predict a variety of mechanical behaviors in the regime of large displacements, including an extreme stiffening-type response, already known in the literature, and a newly discovered, extreme softening behavior. The latter may lead to a snap buckling event producing an axial collapse of the structure. The switching from one mechanical regime to another depends on the aspect ratio of the structure, the magnitude of the applied prestress, and the material properties of the constituent elements. We discuss potential acoustic applications of such behaviors, which are related to the design and manufacture of tensegrity lattices and innovative phononic crystals.

cond-mat.mtrl-sci↗

arXiv2011

Universal formulae for the limiting elastic energy of membrane networks

We provide universal formulae for the limiting stretching and bending energies of triangulated membrane networks endowed with nearest neighbor bond potentials and cosine-type dihedral angle potentials. The given formulae account for finite elasticity and solve some deficiencies of earlier results for Helfrich-type bending energies, due to shape-dependence and sensitivity to mesh distortion effects of the limiting elastic coefficients. We also provide the entire set of the elastic coefficients characterizing the limiting response of the examined networks, accounting for full bending-stretching coupling. We illustrate the effectiveness of the proposed formulae by way of example, on examining the special cases of cylindrical and spherical networks covered with equilateral triangles, and discussing possible strategies for the experimental characterization of selected elastic moduli.

cond-mat.mtrl-sci↗

arXiv2011

On the estimation of the curvatures and bending rigidity of membrane networks via a local maximum-entropy approach

We present a meshfree method for the curvature estimation of membrane networks based on the Local Maximum Entropy approach recently presented in (Arroyo and Ortiz, 2006). A continuum regularization of the network is carried out by balancing the maximization of the information entropy corresponding to the nodal data, with the minimization of the total width of the shape functions. The accuracy and convergence properties of the given curvature prediction procedure are assessed through numerical applications to benchmark problems, which include coarse grained molecular dynamics simulations of the fluctuations of red blood cell membranes (Marcelli et al., 2005; Hale et al., 2009). We also provide an energetic discrete-to-continuum approach to the prediction of the zero-temperature bending rigidity of membrane networks, which is based on the integration of the local curvature estimates. The Local Maximum Entropy approach is easily applicable to the continuum regularization of fluctuating membranes, and the prediction of membrane and bending elasticities of molecular dynamics models.

math-ph↗

arXiv2008

Optimal design of composite granular protectors

We employ an evolutionary algorithm to investigate the optimal design of composite protectors using one-dimensional granular chains composed of beads of various sizes, masses, and stiffnesses. We define a fitness function using the maximum force transmitted from the protector to a "wall" that represents the body to be protected and accordingly optimize the {topology} (arrangement), {size}, and {material} of the chain. We obtain optimally randomized granular protectors characterized by high-energy equipartition and the transformation of incident waves into interacting solitary pulses. We consistently observe that the pulses traveling to the wall combine to form an extended (long-wavelength), small-amplitude pulse.

cond-mat.mtrl-sci↗