Effective complex conductivity of skeletal muscle in the radio-frequency range

This work deals with the dielectric properties of biological tissues comprising tubular cells, such as skeletal muscle. This issue is of significance to many applications for noninvasive diagnosis and treatment, such as electrical impedance tomography, body composition, dialysis, radio-frequency hyperthermia and ablation. The dielectric properties of tissues vary as a function of frequency. Experiments show indeed three dispersions, α, β and γ, mainly attributed to different relaxation processes: ionic diffusion, interfacial polarization and dipolar orientation, respectively. The β dispersion, considered herein, takes place in the radio-frequency range and principally arises from the capacitive charging of cell membranes, known as Maxwell-Wagner effect. Different phenomenological relaxation models are available in the literature, as well as equivalent-circuit models, which however pose the problem of parameter identification. In the present work, a micromechanical approach is used, which enables to derive the effective dielectric properties of the tissue from the properties of the constituent phases and to take into account microstructural details

Analysis of the vibration localization phenomenon in imperfect rings

The modal analysis of imperfect rings vibrating in their own plane is considered in this paper. The imperfections are modeled as generic perturbations, depending on the angular variable, of the linear mass density and the bending stiffness of the ring. The Euler-Bernoulli theory is used to develop the dynamical model of the ring, and a perturbation expansion of the solution is performed in order to find out the modal split eigenfrequencies and the relevant perturbed modal shapes. Finally, some case-study problems are considered and the analytical results obtained by using the proposed approach are compared to results obtained by employing a finite-element model of the imperfect ring

Collapse capacity of masonry domes under horizontal loads: A static limit analysis approach

A static limit analysis approach is proposed for assessing the collapse capacity of axisymmetric masonry domes subject to horizontal forces. The problem formulation is based on the sound theoretical framework provided by the classical statics of shells. After introducing the shell stress tensors on the dome mid-surface, integral equilibrium equations are enforced for its typical part. Heyman's assumptions of infinite compressive and vanishing tensile strengths are made, with cohesionless friction behavior governing the shear strength, to characterize the admissible stress states in the dome. An original computational strategy is developed to address the resulting static limit analysis problem, involving the introduction of a mesh on the dome mid-surface, the interpolation of the physical components of the shell stress tensors on the element boundaries, and the imposition of equilibrium and admissibility conditions respectively for the elements and at the nodes of the mesh. The descending discrete convex optimization problem is solved by standard and effective optimization tools, automatically providing collapse multiplier of horizontal forces, incipient collapse mechanism and expected crack pattern. Convergence analysis, validation with experimental results available in the literature, and parametric analyses with respect to geometric parameters and friction coefficient, are presented for spherical and ellipsoidal masonry domes, proving the reliability of the proposed approach for estimating the pseudo-static seismic resistance of masonry domes.Comment: 29 pages, 11 figure

A finite difference method for the static limit analysis of masonry domes under seismic loads

The static limit analysis of axially symmetric masonry domes subject to pseudo-static seismic forces is addressed. The stress state in the dome is represented by the shell stress resultants (normal-force tensor, bending-moment tensor, and shear-force vector) on the dome mid-surface. The classical differential equilibrium equations of shells are resorted to for imposing the equilibrium of the dome. Heyman's assumptions of infinite compressive and vanishing tensile strength, alongside with cohesive-frictional shear response, are adopted for imposing the admissibility of the stress state. A finite difference method is proposed for the numerical discretization of the problem, based on the use of two staggered rectangular grids in the parameter space generating the dome mid-surface. The resulting discrete static limit analysis problem results to be a second-order cone programming problem, to be effectively solved by available convex optimization softwares. In addition to a convergence analysis, numerical simulations are presented, dealing with the parametric analysis of the collapse capacity under seismic forces of spherical and ogival domes with parameterized geometry. In particular, the influence that the shear response of masonry material and the distribution of horizontal forces along the height of the dome have on the collapse capacity is explored. The obtained results, that are new in the literature, show the computational merit of the proposed method, and quantitatively shed light on the seismic resistance of masonry domes

Mechanical response of multistable tensegrity-like lattice chains

Recent developments in the quality and accuracy of additive manufacturing have drawn particular attention to metamaterials characterised by a multistable response to achieve exceptional mechanical properties. This work focuses on the design, fabrication, testing, and simulation of tensegrity-like lattice chains accomplishing a multistable behaviour. The chains are composed of chiral tensegrity-like units featuring a highly nonlinear bistable response with compression-twisting coupling. Different chains are designed by exploiting the chirality of the units and realised by the inverted stereolithography technique. Their mechanical response is experimentally characterised, demonstrating the attainment of the desired multistable behaviour. A predictive semi-analytical model is derived to reconstruct the multistable energy landscape and force-vs.-displacement curve of the whole chain. The presented chains may constitute a flexible platform for programmable materials, potentially extending to modular chains also based on other types of tensegrity-like units

Effect of the matrix subsystem on hydrostatic parameters of a novel 1-3-type piezo-composite

The influence of the aspect ratio and volume fraction of ferroelectric ceramic inclusions in a 0-3 matrix on the hydrostatic parameters of a three-component 1-3-type composite is studied to demonstrate the important role of the elastic properties of the two-component matrix on the composite performance. Differences in the elastic properties of the 0-3 matrix and single-crystal rods lead to a considerable dependence of the hydrostatic response of the composite on the anisotropy of the matrix elastic properties. The performance of a 1-0-3 0.92Pb(Zn1/3Nb2/3)O3-0.08PbTiO3 SC/modified PbTiO3 ceramic/polyurethane composite suggests that this composite system is of interest for hydroacoustic applications due to its high hydrostatic piezoelectric coefficients dh∗ ≈; (400-500); pC/N and gh∗ ∼ 0.1 V.mN, squared figure of merit dh∗gh∗ ≈ (30-40). 10-12Pa-1, and electromechanical coupling factor kh∗ ≈; 0.5-0.6

Modeling And Design Of Periodic Lattices With Tensegrity Architecture And Highly Nonlinear Response

In recent years, the nonlinear response of tensegrity systems has attracted increasing attention in the study of mechanical metamaterials. It has been shown in the literature that geometry and prestress of an elastic tensegrity structure can be designed to obtain different behaviors: stiffening, softening, and snap-through behavior in statics; propagation of solitary waves in dynamics. However, the realization of tensegrity systems is challenging, because of their prestressed state and the presence of tension-only cable members. A design method for periodic lattices with null prestress and no cables is here proposed, in which the repeating unit is at, or close to, a tensegrity configuration, maintaining the nonlinear types of response aforementioned. These structures can be realized by conventional additive manufacturing techniques, while the static and dynamic response can be predicted by means of stick-and-spring models

Design of piezoelectric lattice metamaterials

Piezoelectric lattice metamaterials are considered. A computationally-effective homogenisation method is developed based on the recent solution to the Saint-Venant problem for general anisotropic piezoelectric cylinders. A publicly available repository of unit cell topologies is used to identify piezoelectric metamaterials with optimal figures of merit

Numerical and experimental characterization of a piezoelectric actuator for microfluidic cell sorting

Piezoelectric actuators offer great opportunities for precise and low-cost control of fluids at the microscale. Microfluidic systems with integrated piezoelectric actuators find application as droplet generators, micropumps, and microsorters. To accelerate device design and optimization, modeling and simulation approaches represent an attractive tool, but there are challenges arising from the multiphysics nature of the problem. Simple, potentially real-time approaches to experimentally characterize the fluid response to piezoelectric actuation are also highly desirable. In this work, we propose a strategy for the numerical and experimental characterization of a piezoelectric microfluidic cell sorter. Specifically, we present a 3D coupled multiphysics finite-element model of the system and an easy image-based approach for flow monitoring. Sinusoidal and pulse actuation are considered as case studies to test the proposed methodology. The results demonstrate the validity of the approach as well as the suitability of the system for cell sorting applications