A novel meta-lattice sandwich structure for dynamic load mitigation

In this article, a novel meta-lattice sandwich structure is proposed and designed for impulsive wave attenuation and dynamic load mitigation. This original meta-lattice truss core sandwich structure has a similar configuration as a normal lattice sandwich structure, except that its truss bars are composed of meta-lattice truss unit cells. The design philosophy of locally resonant elastic metamaterials is integrated into the meta-lattice truss unit cell whereby a relatively heavier metal core (the resonator) is coated with a soft material layer (rubber coat), which is then connected to an outer shell. Based on this unique construction, several frequency band gaps are created by the locally resonant behavior of the specially designed resonators, in which stress waves within the stopping band gaps are not able to propagate through the material. Analytical spring-mass model is employed to predict the frequency band gaps, whereas numerical finite element simulation is utilized to model the continuum structure under impulsive loadings. The impact response, wave attenuation, and stress distribution contours between normal sandwich structure and meta-lattice sandwich structure are compared and analyzed. The mechanisms of wave mitigation and energy absorption by the internal resonators are thoroughly investigated. Results evidently show that the proposed meta-lattice sandwich structure has a more superior ability for impact mitigation and higher kinetic energy absorption capability due to the locally resonant behavior of the internal resonators.

The Effects of Cubic Stiffness Nonlinearity on the Attenuation Bandwidth of 1D Elasto-Dynamic Metamaterials

Metamaterials demonstrate unique frequency dependent responses due to the presence of internal resonators; hence, it can be used to filter, absorb, cloak, or otherwise manipulate waves in unique ways. However, its applicability is normally limited to a very narrow frequency range (bandwidth) due to a dependency on linear resonance. The applications of these linear metamaterials are limited when used under the broadband excitation spectra that are common in real life applications. This paper numerically investigates the effect of introducing the two main classes of Duffing type cubic nonlinearities, namely monostable and bistable, on the attenuation bandwidth of an elasto-dynamic metamaterial. From the analysis, it is found that the attenuation bandwidth of a bistable nonlinear system is two to three times wider than that of an equivalent linear system; whereas, in case of a monostable system the bandwidth is remained same. In both cases, the attenuation bandwidth shifts towards the higher end of the frequency spectra and for higher nonlinearity and excitation amplitude, second transmission zone completely vanishes.

Metamaterial Application in Stress Wave Mitigation Generated by Impulsive Force

The idea to use metamaterials to mitigate mechanical waves is recent and constitutes a technology under development. These materials have a special design, presenting characteristics not found in nature. The interesting feature is a negative effective mass density. This property is achieved by creating in the structure masses linked by springs which act as internal resonators and, as a result, it is observed that metamaterials act as mechanical filters, preventing or reducing the intensity of propagation of mechanical waves that travel in the structure, when the frequency of propagation is close to the resonance frequencies of the internal resonators. An internal combustion generates a blast wave which acts on a structure as an impulsive effort. This is a basic phenomenon in the shooting of an armament leading to this research that target to investigate the possible application of metamaterials to improve recoil mechanism technology. A recoil mechanism moderates the firing loads on the supporting structure by prolonging the time of resistance to the propellant gas forces. Depending on application of the armament, recoil can be very undesirable. Firstly, carriage mount where the armament is fixed will suffer premature wear. Secondly and more critical, if the armament is mounted onto a vehicle, its dynamics during shooting is completely affected and an accident can be caused when shooting occurs during a critical situation, like a curvilinear path for example. It is intended to use numerical simulations and experimental validation to verify the behavior of the designed metamaterial under a controlled impulse input. Finite Element Method (FEM) is used to simulate wave propagation through a common material and then through a special designed metamaterial to evaluate how this kind of pulse will be affected by internal resonators. After the simulations, a prototype adequate to validate numerical results will be investigated on a test bench. In a further development the impulse input will be adapted to real measured blast efforts.

Vibration isolation characteristics of finite periodic tetra-chiral lattice coating filled with internal resonators

Owing to their locally resonant mechanism, internal resonators are usually used to provide band gaps in low-frequency region for many types of periodic structures. In this study, internal resonators are used to improve the vibration attenuation ability of finite periodic tetra-chiral coating, enabling high reduction of the radiated sound power by a vibrating stiffened plate. Based on the Bloch theorem and finite element method, the band gap characteristics of tetra-chiral unit cells filled with and without internal resonators are analysed and compared to reveal the relationship between band gaps and vibration modes of such tetra-chiral unit cells. The rotational vibration of internal resonators can effectively strengthen the vibration attenuation ability of tetra-chiral lattice and extend the effective frequency range of vibration attenuation. Two tetra-chiral lattices with and without internal resonators are respectively designed and their vibration transmissibilities are measured using the hammering method. The experimental results confirm the vibration isolation effect of the internal resonators on the finite periodic tetra-chiral lattice. The tetra-chiral lattice as an acoustic coating is applied to a stiffened plate, and analysis results indicate that the internal resonators can obviously enhance the vibration attenuation ability of tetra-chiral lattice coating in the frequency range of the band gap corresponding to the rotating vibration mode of internal resonators. When the soft rubber with the internal resonators in tetra-chiral layers has gradient elastic modulus, the vibration attenuation ability and noise reduction of the tetra-chiral lattice coating are basically enhanced in the frequency range of the corresponding band gaps of tetra-chiral unit cells.

Negative Effective Mass Density of One-Dimensional Hierarchical Metacomposite

A theoretical model of one-dimensional (1D) hierarchical metacomposite with internal resonators was proposed to generate negative effective mass over specific frequency ranges. Different from the single-resonator microstructure, the current hierarchical metamaterial with multilevel resonators was constructed by a series of springs and rigid bodies. The general formula of the current hierarchical metamaterial model was induced to reveal the relationship between the effective mass and the forcing frequency. It is found that the hierarchical metamaterial with multilevel resonators generates multifrequency band gaps with negative effective masses. The number of the band gaps equals to the order of the hierarchy. The total bandwidth for the negative effective mass increases with the hierarchy, meanwhile increasing the mass ratio can also obviously increase the bandwidth generating negative effective mass.