A new tunable elastic metamaterial structure for manipulating band gaps/wave propagation

A one-dimensional mechanical lattice system with local resonators is proposed as an elastic metamaterial model,which shows negative mass and negative modulus under specific frequency ranges.The proposed representative units,consisting of accurately arranged rigid components,can generate controllable translational resonance and achieve negative mass and negative modulus by adjusting the local structural parameters.A shape memory polymer is adopted as a spring component,whose Young’s modulus is obviously affected by temperature,and the proposed metamaterials’tunable ability is achieved by adjusting temperature.The effect of the shape memory polymer’s stiffness variation on the band gaps is investigated detailedly,and the special phenomenon of intersecting dispersion curves is discussed,which can be designed and controlled by adjusting temperature.The dispersion relationship of the continuum metamaterial model affected by temperature is obtained,which shows great tunable ability to manipulate wave propagation.

Uncovering the creep deformation mechanism of rock-forming minerals using nanoindentation

The creep phenomenon of rocks is quite complex and the creep mechanisms are far from being well understood.Although laboratory creep tests have been carried out to determine the creep deformation of various rocks,these tests are expensive and time-consuming.Nanoindentation creep tests,as an alternative method,can be performed to investigate the mechanical and viscoelastic properties of granite samples.In this study,the reduced Young’s modulus,hardness,fracture toughness,creep strain rate,stress exponent,activation volume and maximum creep displacement of common rock-forming minerals of granite were calculated from nanoindentation results.It was found that the hardness decreases with the increase of holding time and the initial decrease in hardness was swift,and then it decreased slowly.The stress exponent values obtained were in the range from 4.5 to 22.9,which indicates that dislocation climb is the creep deformation mechanism.In addition,fracture toughness of granite’s rock-forming minerals was calculated using energy-based method and homogenization method was adopted to upscale the micro-scale mechanical properties to macro-scale mechanical properties.Last but not least,both three-element Voigt model and Burgers model fit the nanoindentation creep curves well.This study is beneficial to the understanding of the long-term mechanical properties of rock samples from a microscale perspective,which is of great significance to the understanding of localized deformation processes of rocks.

Bandgap characteristics of the two-dimensional missing rib lattice structure

In this paper,the bandgap characteristics of a missing rib lattice structure composed of beam elements are investigated by using the Floquet-Bloch theorem.The tuning of the width and position of the bandgap is achieved by changing the local structural parameters,i.e.,the rotation angle,the short beam length,and the beam thickness.In order to expand the regulation of the bandgap,the influence of the material parameters of the crossed long beams inside the structure on the bandgap is analyzed.The results show that the mass density and stiffness of the structure have significant effects on the bandgap,while Poisson’s ratio has no effect on the bandgap.By analyzing the first ten bands of the reference unit cell,it can be found that the missing rib lattice structure generates multiple local resonance bandgaps for vibration reduction,and these bandgap widths are wider.The modal analysis reveals that the formation of the bandgap is due to the dipole resonance of the lattice structure,and this dipole resonance originates from the coupling of the bending deformation of the beam elements.In the band structure,the vibrational mode of the 9th band with a negative slope corresponds to a rotational resonance,which is different from that with the conventional negative slope formed by the coupling of two resonance modes.This study can provide a theoretical reference for the design of simple and lightweight elastic metamaterials,as well as for the regulation of bandgaps and the suppression of elastic waves.

Investigation on the Velocity-Dependent Adhesion Hysteresis via Molecular Dynamics Simulation

Adhesion plays an important role in miniaturized devices and technologies,which depends not only on indentation depth but also on the history of contact making and breaking,giving rise to adhesion hysteresis.In the present work,adhesion hysteresis has been investigated via molecular dynamics simulations on approaching and retracting a rigid tip to and from a substrate.The results show that hysteresis in the force-displacement curve that depends on approaching and retraction velocities arises under both elastic and plastic deformation.The underlying mechanisms have been analyzed.The implications of the results in friction have been discussed briefly.