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2016 Vol. 37, No. 5
Published: 2016-10-28
387
Mechanics concepts and Advances of Acoustic Metamaterials Design
Acoustic metamaterials are a special type of man-made composite materials that are characterized in the framework of continuum medium theory.They obtain anomalous dynamic material properties, such as negative dynamic mass, negative dynamic modulus, negative refraction, by careful designs of micro-scale structures.The metamaterial concept brings us the opportunity of wave propagation control, as make metamaterials potentially applied in practical engineering.This review paper aims at explaining the concept of microstructure design of elastic and acoustic metamaterials, acoustic metasurfaces, and active metamaterials, as well as the relevant research advances.The generalized concept may be of benefit to the design and material realization of wave control structures.
2016 Vol. 37 (5): 387-397 [
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398
Research progress in nanomechanics of graphene and its composites
Graphene is an allotrope of carbon in the form of a two-dimensional, atomic-scale, hexagonal lattice in which one atom forms each vertex, which is the known thinnest and strongest nanomaterial. Due to the excellent electronic, thermodynamical, optical and mechanical properties, graphene becomes a study hotspot in the fields of material science, physics, chemistry and mechanics. Derivatively, graphene composites have attracted tremendous research interest in recent years. According to the difference of mechanical behaviors, this review mainly presents and discusses the advances of graphene’s in-plane mechanical properties, out-of-plane mechanical properties, atomic revised graphene and graphene composites: the in-plane tension mechanical properties of graphene is measured via nanoindentation methods, the fracture of graphene is not completely consistent with continuum mechanical models, and the superlubricity occurs in multilayer-graphene; controllable out-of-plane displacement in graphene monolayer plays an important role in changing its physical properties, the buckling in graphene monolayer is influenced by its size and chirality, and there will be non-continuum effect in graphene-related devices with high frequency; graphene can improve the strength and toughness of composites, which is resulted through the combination of in-plane and out-of-plane interactions. Lastly, the summary and forecast of the mechanical study of graphene and graphene composites are narrated.
2016 Vol. 37 (5): 398-420 [
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428
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421
Planar locomotion of a rigid body driven by three-phase motion of two internal masses
In this paper, planar locomotion is investigated for a class of vibration-driven system with two internal masses. The internal masses, driven by three-phase motion, move on two perpendicular slots. Planar locomotion of the system is derived from the two moving internal masses. The friction between the system and supporting plane is supposed to be viscous and anisotropic. First, the mechanical system is modeled by using the second kind Lagrange’s equation. Secondly, the velocity-Verlet is employed to analyze the planar locomotion of the system. For the rectilinear locomotion, the analytical solution and the numerical simulation are in good agreement and it implies the reliability of the velocity algorithm. Thus, the planar locomotion of the system can be obtained. Moreover, we establish the relations among the internal driven parameters, the trajectory and velocity of the vibration-driven system. It follows from the numerical results that the six types of the switching trajectory can arise by choosing the driven parameters. Integrating the different switching trajectories, one can obtain any continuous-curvature paths.
2016 Vol. 37 (5): 421-437 [
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289
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438
RESEARCH ON THE TEMPERATURE OF TA2 TITANIUM ALLOY HAT-SHAPED SPECIMEN AT THE INITIATION OF ADIABATIC SHEAR LOCALIZATION
2016 Vol. 37 (5): 438-443 [
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312
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444
DISCRETE-FINITE ELEMENT ANALYSIS OF DYNAMIC BEHAVIORS OF BALLASTED RAILYWAY UNDER THE REINFORCEMENT OF GEOGRID
This paper presents a coupled discrete-finite element method for the interaction between geogrid and ballast particles. The clumps are employed to simulate the shape of ballast particles, the beam elements are used to compute the response of geogrid, a method for the transmission of dynamic parameters is proposed on the contact surface between discrete and finite element area. The reinforcement of geogrid on ballast particles is analyzed under cyclic loading. A further study on the mechanism of geogrid reinforcement is carried out by direct shear test of reinforced ballast particles. The results indicate that the lateral and vertical displacements decline obviously by the reinforcement of geogrid, the cohesive force and frictional angle of ballast material both enhance after reinforced by geogrid. Geogrid could increase the interlock of ballast particles, enhance the strength of ballast material and prevent the shear band cutting through from upper box to nether box.
2016 Vol. 37 (5): 444-455 [
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306
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456
LOCAL STRUCTURAL DERIVATIVE AND ITS APPLICATIONS
The classical derivative modelling approach describes the change rate of a certain physical variable with respect to time or space and considers to less extent the important influence of mesoscopic time-space fabric of a complex system on its physical behaviours. This report introduces the structural function and proposes the local structural derivative modeling approach to overcome the shortcoming of the traditional derivative approach. The structural function characterizes the time-space structures of system of interest and in fact is a time-space transform. By using the structural function, the structural derivative can describe causal relationship of mesoscopic time-space structure and certain physical behavior in a simple fashion and less computing costs. We can obtain the structural function by using the fundamental solution or the probability density function of statistical distribution. Two applications in this study show that the proposed structural derivative can well describe the ultraslow diffusion with the logarithmic function as its structural function in soft matters and derive the structural derivative diffusion equation of reliability using the structural function based on the probability density function of Weibull distribution.
2016 Vol. 37 (5): 456-460 [
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461
A new nonlinear finite element analysis method on the galloping of iced conductors
Based on Hamilton’s principle, this paper formulates the nonlinear galloping equations of iced conductors, which couple three translational and one torsional degrees of freedom and involve the influence of eccentric icing. A new nonlinear finite element model of iced conductors in which the adjacent conductor spans and insulator strings are represented by linear springs is established here. Taking into account the nonlinear aerodynamic forces and the geometric nonlinearity caused by large amplitude galloping, the authors adopt the Newmark-β time integration algorithm in conjunction with modified Newton-Raphson nonlinear iteration strategy to solve those equations in finite element formulation. The numerical solutions of both amplitude and frequency obtained from the present method agree well with the measured values of the galloping of a D-shaped iced conductor, which proves that the current finite element model is accurate. The present research also indicates that the galloping is a kind of low-frequency vibration which mainly moves vertically and generally occurs around the first-order vertical natural frequency. The amplitudes and frequencies of the galloping are uniquely determined by the physical parameters of the transmission line and wind loads which are irrelevant to the initial state of the movements of the iced conductors.
2016 Vol. 37 (5): 461-470 [
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315
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424
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471
Linear dynamic system identification using proper orthogonal decomposition and Padé approximants
An identification method for linear dynamical system is proposed. The Padé approximants is used to fit the curve of system's dynamic stiffness, and the coefficient matrices in the Padé polynomial are determined by the least squares method, in addition, genetic algorithms is adopted to optimize the parameters in Padé polynomial. By comparing the Padé approximants with the theory s of the system's dynamic stiffness matrices, the mass, damping and stiffness matrices can be obtained. When the order of the system is high, the POD reduced-order technology can be applied, and it means that the present method is valid for full and reduced-order model. Numerical examples illustrate the present method with good accuracy and robust.
2016 Vol. 37 (5): 471-476 [
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293
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