1,720,988 research outputs found
Continuum versus micromechanical modeling of corneal biomechanics
No full textTwo alternative numerical models of the human cornea are used to simulate the mechanical response under the action of a physiological intraocular pressure (IOP). The first model is continuum or macromechanical, considering the stromal tissue as a bulk material with stochastic distribution of the spatial variability of reinforcing collagen fibers. The second model is discrete or micromechanical, considering the sole collagencrosslink stiffening micro-structure. The geometry of the two models is reconstructed from corneal topographer images. Simulations consider the behavior of a healthy cornea and of a keratoconus cornea. For the keratoconus the material properties of a portion of the cornea are reduced to 1/8 of the values used for the healthy tissue. It is found that, for suitable choice of the material parameters for the discrete model, in the healthy case the mechanical responses of the two models are fully comparable. In the keratoconus case, both models capture with comparable accuracy the anterior shape of the conus; in addition, the discrete model is able to describe the tissue thinning typical of the pathology. Despite the inclusion of stochastic material properties, starting from a healthy condition, continuum models of the cornea are not able to predict the thinning of a keratoconus cornea, while the inclusion of the underlying collagen microstructure allows for a proper description of pathologic mechanical behaviors
Applications of a micro-structured brittle damage model to laboratory tests on rocks
Full text linkA multiscale microstructured brittle damage model is used to describe the behavior of confined rock materials. Plane strain and triaxial tests conducted
at the laboratory scale are simulated in terms of boundary value problems. Simulations reveal good predictive qualities of the model to describe the macroscopic features of specimens at failure. The microstructures, oriented in different directions, allow the localization of the macroscopic strain along straight lines, emerging at the macroscale in the form of shear bands. The microstructured material model, characterized by recursive equidistant parallel cohesive-frictional faults, is fully defined by six elastic and inelastic material constants. The model was originally developed in a finite kinematics framework to simulate the dynamic behavior of confined brittle materials (Pandolfi et al. in J
Mech Phys Solids 54:1972–2003, 2006). In linearized form, it has been extended and used for the simulation of in-field excavations (De Bellis et al. in: Eng
Geol 215:10–24, 2016). The performance of the model in predicting the behavior of small scale rock tests in the laboratory, the object of the present study, has
never been investigated. Numerical simulations show that the model is able to capture several important features observed in rocks, in particular the reduction
of the overall stiffness for increasing deterioration of the material, fragile to ductile transition, strain localization, shear band formation, and more general size effect
A virtual element approach for micropolar continua
Full text linkIn this work we propose a novel virtual element approach for solving boundary value problems in 2D linear isotropic micropolar elasticity. Following the basic idea of the Virtual Element Method (VEM), the degrees of freedom of each material point, i.e. the displacement and rotation fields, are decomposed into both a polynomial space, either linear or quadratic, and a remaining space that is kept virtual in the formulation. Generalized consistency and stabilization terms are consistently derived. Different patch tests, properly conceived for micropolar continua, are proposed and compared to reference solutions present in literature. The obtained results are in good agreement with these solutions, confirming the capability of the proposed elements in the modelling of the expected responses. The expected applications of this methodology concern the mechanical study of microstructured materials, inherently characterized by nonlocal response, which has been widely proven to be effectively represented by micropolar continua
Electrically-tunable active metamaterials for damped elastic wave propagation control
Full text linkAn electrically-tunable metamaterial is herein designed for the active control of damped elastic waves. The periodic device is conceived including both elastic phases and a piezoelectric phase, shunted by a dissipative electric circuit whose impedance/admittance can be adjusted on demand. As a consequence, the frequency band structure of the metamaterial can be modified to meet design requirements, possibly changing over time. A significant issue is that in the presence of a dissipative circuit, the frequency spectra are obtained by solving eigen-problems with rational terms. This circumstance makes the problem particularly difficult to treat, either resorting to analytical or numerical techniques. In this context, a new derationalization strategy is proposed to overcome some limitations of standard approaches. The starting point is an infinite-dimensional rational eigen-problem, obtained by expanding in their Fourier series the periodic terms involved in the governing dynamic equations. A special derationalization is then applied to the truncated eigen-problem. The key idea is exploiting a LU factorization of the matrix collecting the rational terms. The method allows to considerably reduce the size of the problem to solve with respect to available techniques in literature. This strategy is successfully applied to the case of a three-phase metamaterial shunted by a series RLC circuit with rational admittance
Design of tunable acoustic metamaterials with periodic piezoelectric microstructure
No full textAn innovative special class of tunable periodic metamaterials is designed, suitable for realizing high-performance acoustic filters. The metamaterial is made up of a phononic crystal coupled to local resonators. Such local resonators consist of masses enclosed into piezoelectric rings, shunted by either dissipative or non-dissipative electrical circuit. By tuning the impedance/admittance of such electrical circuits, it is possible to fully adjust the constitutive properties of the shunting piezoelectric material. This feature paves the way for unconventional behaviours, well beyond the capabilities achievable with classical materials. It follows that the acoustic properties of the periodic metamaterial can be adaptively modified, in turn, opening new possibilities for the control of pass and stop bands. By exploiting a generalization of the Floquet-Bloch theory, the in-plane free wave propagation in the tunable metamaterial is investigated, by varying a certain tuning parameter, to show the efficiency of the proposed shunting piezoelectric system as a wave propagation control device. Particular attention is devoted to the determination of the in-plane constitutive equations of the shunting piezoelectric phase in the transformed Laplace space. Finally, broad design directions of tunable acoustic filters aiming to a changing performance requirement in real-time, is also provided. (C) 2020 Elsevier Ltd. All rights reserved
Modeling the degeneration of the collagen architecture in a microstructural model of the human cornea
No full textWe propose an enriched micromechanical model of the collagenous reinforcement of the eye stromal tissue. As a departure from an over-simplified model proposed a few years back, where collagen and chemical bonds were modeled as linear-elastic trusses, here we describe the chemical bonds by means of a more realistic generalized Lennard-Jones potential. In keeping with the original model, we disregard the multi-layer nature of the cornea and the continuum nature of the filling elastin matrix. The under-constrained locally orthogonal network of collagen fibrils is stabilized by crosslinks that provide the rigidity of the system and confer the ability to sustain the action of the intraocular pressure. In Ariza-Gracia et al., it has been shown that the weakening and the bulging of the cornea due to ectasia can be ascribed to the reduction of the density of the chemical bonds. The introduction of a pseudo-chemical potential supplies a more realistic model: any mechanical, enzymatic, or chemical cause of the degradation of the tissue observed in ectasia can be effectively introduced in a multi-physic potential, disregarding the adoption of phenomenological models. In numerical calculations, the high non-linearity of the model is suitably controlled by adopting a robust explicit solver based on dynamic relaxation
A numerical model of the human cornea accounting for the fiber-distributed collagen microstructure
Full text linkWe present a fiber-distributed model of the reinforcing collagen of the human cornea. The model describes the basic connections between the components of the tissue by defining an elementary block (cell) and upscaling it to the physical size of the cornea. The cell is defined by two sets of collagen fibrils running in approximately orthogonal directions, characterized by a random distribution of the spatial orientation and connected by chemical bonds of two kinds. The bonds of the first kind describe the lamellar crosslinks, forming the ribbon-like lamellae; while the bonds of the second kind describe the stacking crosslinks, piling up the lamellae to form the structure of the stroma. The spatial replication of the cell produces a truss structure with a considerable number of degrees of freedom. The statistical characterization of the collagen fibrils leads to a mechanical model that reacts to the action of the deterministic intraocular pressure with a stochastic distribution of the displacements, here characterized by their mean value and variance. The strategy to address the solution of the heavy resulting numerical problem is to use the so-called stochastic finite element improved perturbation method combined with a fully explicit solver. The results demonstrate that the variability of the mechanical properties affects in a non-negligible manner the expected response of the structure to the physiological action
Micropolar Asymptotic Homogenization for Periodic Cauchy Materials
No full textWe present a micropolar-based asymptotic homogenization approach [1,2] for the analysis of composite materials with periodic microstructure.
The up-scaling relations are inspired by those originally proposed by [3] in the framework of the computational homogenization, expressing the local displacement field as a function of a cubic polynomial kinematic map depending on first, second and third order homogeneous tensors directly related to the classical and micropolar 2D deformation modes [4,5,6]. The local displacement field is described as superposition of the macroscopic driven kinematic map and local periodic perturbation fields. These perturbation functions are inherently related to the heterogeneous nature of the composite medium and are derived from the solution of recursive cell problems. The down-scaling relations are derived from a newly proposed third order asymptotic expansion of the local displacement field in terms of the macroscopic displacement and its first, second and third order gradients. The overall micropolar elastic tensors derive from a properly conceived energy equivalence between the macroscopic point and a representative portion of the heterogeneous material at the microscopic scale.
Different applications to bi-phase orthotropic layered material to are proposed in order to exploit the capabilities of the proposed approach
Characterization of hybrid piezoelectric nanogenerators through asymptotic homogenization
Full text linkIn the framework of energy scavenging for applications in flexible/stretchable electronics, hybrid piezoelectric nanogenerators are investigated. They are made up with zinc oxide (ZnO) nanorods, embedded in a polymeric matrix, and grown on a flexible polymeric support. The ZnO nanorods are arranged in clusters, forming nearly regular distributions, so that periodic topologies can be realistically assumed. Focus is on a dynamic multi-field asymptotic homogenization approach, proposed to grasp the overall constitutive behaviour of such complex microstructures. A set of applications, both in static and dynamic regime, is proposed to explore different design paradigms, related to nanogenerators based on three working principles. Both extension and bending nanogenerators are, indeed, analysed, considering either extension along the nanorods axis, or orthogonally to it. The study of the wave propagation is, also, exploited to comprehend the main features of such piezoelectric devices in the dynamic regime
A contribution to the stability of an overhanging pipe conveying fluid
No full textWe investigate the dynamic stability of a pipe that conveys fluid, clamped or pinned at one end and with an intermediate support, thus exhibiting an overhang. The model of the pipe incorporates both Euler–Bernoulli and Bresse–Timoshenko schemes as well as transverse inertia. Material and external damping mechanisms are taken into account, while the conveyed fluid is supposed to be in fully turbulent flow. The pipe can rest on a linear elastic Winkler soil. The influence of all the physical quantities and of the overhang length on the critical velocity of the fluid front is investigated. Some numerical results are presented and discussed. © 2014, Springer-Verlag Berlin Heidelberg
- …
