Mechanics Research Communications最新文献

Elastic metamaterials are man-made structures with properties that transcend naturally occurring materials. One predominant feature of elastic metamaterials is locally resonant bandgaps, i.e., frequency ranges at which wave propagation is blocked. Locally resonant bandgaps appear at relatively low frequency and arise from the existence of periodically placed mechanical local resonators. Typically, elastic metamaterials exhibit both locally resonant and Bragg-scattering bandgaps, which can generally be different in width and frequency ranges. This paper proposes two designs of active elastic metamaterials that only exhibit locally resonant bandgaps, which are infinite in number, evenly spaced in the frequency spectrum, and identical in width. The mathematical model is established using the transfer matrix method and synthesis of locally resonant bandgaps is achieved via an active elastic support with carefully designed frequency-dependent stiffness. A single unit cell of each proposed metamaterials is thoroughly studied, and its dispersion relation is derived analytically, along with the periodically repeating bandgap limits and widths. Following the dispersion analysis and bandgap parametric studies, finite arrays of the proposed metamaterials are considered, and their frequency response is calculated to verify the analytical predictions from dispersion analyses.

Nature’s morphology and optimal energetic solutions remain the key motivation for designing cellular-based lattice structures. Understanding the nonlinear dynamical behaviors that arise from different lattice topologies of such structures in the metastructure framework is crucial for their successful implementation in various novel designs and technologies related to vibration and shape control. This paper presents a study of the static and dynamic response of auxetic and honeycomb lattices with hourglass or dome-shaped metastructures. The potential tailoring of nonlinearity of such responses through various design parameters that play a vital role in shaping the dynamic properties of such structures is discussed here. The impact of cell design parameters on the resulting macroscopic behavior is assessed using both numerical simulations and experimental studies. The transition from softening to hardening nonlinear dynamic responses is reported with cell topologies ranging from the regular honeycomb to auxetic topologies that are widely used as fundamental cells of cellular materials design. The experimental study is based on the time responses measured to verify the numerical predictions. The experimental system consists of different 3D printed hourglass samples based on the auxetic and honeycomb lattices on which dynamic testing using a laser Doppler vibrometer is performed. The design strategies proposed in this paper can be integrated into a wide range of lattice-based materials for noise and vibration control applications and biomedical devices.

The coupling between material nonlinearity and dispersion/dissipation may lead to the emergence of solitary waves, which are disturbances that can propagate far away with constant velocity and fixed profile. In this paper, we present a brief review of solitary waves, concentrating on their propagation in nonlinear metamaterials with different aspects. In particular, we revisit the propagation characteristics of solitary waves in granular crystals, flexible metamaterials, multistable metamaterials, and high-dimensional metamaterials. We end the review paper with our outlook, emphasizing several potential directions that wait to be further explored and deeply understood.

Finding form is a critical step in designing tensegrity structures. On the condition that the partial node coordinates, topology, and a bar/cable attribute (the force density of bar is -1 and the force density of cable is 1.) are known, a form-finding method, which is used to find the remaining node coordinates and the force density relation between elements, is proposed in this paper. Firstly, the equilibrium conditions of the tensegrity system are analyzed, and the equilibrium equation is established. Secondly, the variables that must be solved are set and substituted into the equilibrium equation, and the target equation with the variables is built. The Levenberg-Marquardt method with a damping parameter updating strategy is introduced to solve the least squares problem by transforming the equilibrium equation problem into the least squares problem. The form-finding process is performed by solving the least squares formula. Three examples demonstrate the efficiency and accuracy of searching for self-equilibrium configurations.

Within the context of quasi-brittle fracture mechanics analyzed by finite element approaches, the present research addresses an implicit solution scheme applied to a strain-gradient continuum damage model. The implicit scheme is based onto an iterative procedure which minimizes for each loading step the increment of both the elastic energy and the damage field between two subsequent trial solutions. The performances of the proposed scheme are compared with those of a previously developed explicit scheme. Besides a better accuracy in the static response computation, it is demonstrated that the proposed approach provides more accurate fracture propagation patterns.

An important issue in conventional harvesters is that the output performance is limited to small strains and narrow bandwidth. To address these issues, a novel E-shape harvester with dynamic magnifier is devised to magnify the base vibration and enhance electric energy in low-frequency complicated environment. In this study, a novel electromechanical coupling dynamic model is established by considering the characteristics of the E-shape structure. Meanwhile, the analytical expressions of the eigenfunction and resonance frequency of the developed model are presented. The effect of system parameters on the power output are investigated and discussed. Results indicate that the electric energy can be significantly enhanced and the vibration amplitude can be magnified using properly design structural parameters. The proposed harvester with dynamic magnifier is a simple and effective approach for enhancing energy harvesting near a target operating frequency.

In this study, a Maxwell–Rayleigh-type model is investigated, describing a unidimensional lattice with a finite length, where the unit cell includes hosting and resonant masses mutually connected by elastic springs. The configuration selected is inspired by the specific engineering design to be discussed: however, the theoretical approach pursued is rather general and can be easily generalized to different scenarios. By the heuristic homogenization based on a Piola’s Ansatz, an equivalent continuum is specified: through a variational approach, resting on the minimization of the Hamilton’s action functional, a dispersion relation is deduced, revealing the existence of a band gap. Such a continuous interval of frequencies, inside which the propagation of waves is inhibited, can be tuned controlling the features of the original system. Thereafter, exploiting the same equations, the stationary elasto-dynamic response for a chain with a finite length is also deduced, corresponding to standing waves. On the basis of such results, the preliminary design of a linear chain with a finite number of cells is carried out. By the present strategy proper boundary conditions are deduced to be prescribed at the ends of the 1D lattice sample, and not over the unit cell (as it occurs in Bloch–Floquet approach with periodicity conditions). To realize the mutual elastic connections between unequal masses fitting the problem constraints, tensegrity prisms are selected, including compressed bars and tensioned cables, for which it is possible to govern the tangent axial stiffness through the cable pre-tensioning. We analyze the scenario in which the band gap coincides with the interval $1-10\mathrm{Hz}$, indicating an appropriate geometry and suitable engineering materials for the tensegrity elements.

Due to the performance requirements, structural discontinuities are inevitable in engineering structures. Free vibration of an elastically connected annular plate system with discontinuities has not yet been reported in the literature. In this study, an analytical method is developed for the free vibration of an annular plate system with discontinuities along its radius. The annular plates are connected continuously using an elastic layer. The coupled vibration equations of the annular plate system were decoupled into a series of uncoupled general “vibration” equations which are then transferred into a symplectic dual system. The general “vibration” state can then be analytically described in terms of waves utilizing the elastic wave theory. Using the analytical wave modes and satisfying the compatibility and boundary conditions at the discontinuities, a frequency equation can be obtained analytically. In numerical examples, the free vibrations of systems consist of two and three annular plates were investigated. The accuracy of the proposed method is verified by comparison with literature and finite element method (FEM).

An analytical model is formulated for 2-D periodic negative honeycomb thicken strut re-entrant lattice structures and a modified rounded corner negative honeycomb structure that shows negative Poisson’s ratios (NPR). Analytical modeling is done using Castigliano’s second theorem, where each beam is modeled using Timoshenko beam theory, considering bending, stretching, and transverse shearing. Elastic modulus and Poisson’s ratio have been formulated for both structures in the form of non-dimensional geometrical characteristics, such as length ratios, angles of re-entrant arms, shear correction factor, and the material’s Young’s modulus and Poisson’s ratios. Numerical simulations conducted in ABAQUS-CAE explicit solver validate the analytical model. The effect of the non-dimensional parameters on the qualities of the developed structure is demonstrated. It is observed that the structures with a low curvature ratio have a high fluctuation of Poisson’s ratio and Elastic constant when plotted against the other parameters. The slenderness ratio has little impact on Poisson’s ratio but significantly influences elastic modulus. It is shown that various needs can be satisfied by customizing the Poisson’s ratios and elastic constant of both forms of lattice construction over an extensive range by carefully choosing the geometrical parameters and material.