Aluminum foam-filled circular auxetic metamaterials were designed, fabricated and investigated in this work. Firstly, the mechanical properties and deformation modes of the proposed aluminum foam-filled auxetic tubes (AFFAT) were analyzed using experimental and finite numerical methods. The load capacity of the AFFAT is larger than the sum of the load for the auxetic tube and aluminum foam. Secondly, the effect of aluminum foam’s density on the mechanical properties of AFFAT was studied. The results show that the energy contribution ratio of the interaction between the aluminum foam and auxetic tube improves with the aluminum foam’s density. Then, the effect of unit cell size on the mechanical properties of AFFAT was investigated. It can be concluded that the compression load efficiency of AFFAT with the ratio of the elliptical major axis to the minor axis increases. Finally, the study of tube type’s effect on the mechanical properties of AFFAT was carried out. Among the investigated composite structures, the AFFAT has the highest compression load efficiency. Due to their unique structural design and excellent mechanical properties, the AFFAT has great potential for applications in aerospace, vehicle crashworthiness and protective infrastructure.

An auxetic metamaterial consisting of a re-entrant honeycomb structure with hierarchical characteristics (RHS-H) is proposed. The new structure is constructed by attaching small re-entrant structural unit cells to the nodes of the traditional re-entrant structures. Not only can the overall stiffness and stability of the proposed structure be tuned during compression and tension, but a better acoustic performance is also obtained compared with traditional re-entrant honeycomb structures. Firstly, the deformation mechanism of the bandgap is numerically explored by analyzing the dispersion curve of the microstructure as well as the upper and lower bounds of the bandgap vibrational modes. Secondly, the bandgap tunability of the designed structure under uniaxial compression or tension is discussed. Finally, the transmittance of finite period size is calculated to verify the numerical results of the bandgap. Numerical simulation results show that the proposed novel RHS-H has attenuation characteristics of a tunable low-frequency plane wave through a reasonable selection of compressive strain, tensile strain and geometric parameters. The vibration damping strength of the bandgap increases under tensile strain. When the auxetic effect is enhanced, the first and second bandgaps become lower and wider. The novel metamaterial has potential applications in vibration and noise reduction and the design of acoustic devices in dynamic environments, while providing new ideas and a methodology for the real-time adjustment of bandgaps.

Two-dimensional phononic metamaterials, consisting of plates with resonant cylinders, can significantly atten-uate waves by opening a subwavelength bandgap, though their characteristic unit cell size is small. To realize the real-time adjustment of the bandgap, external excitations including mechanical load, temperature field, electric field and magnetic field could be introduced, of which applying mechanical load is the most practical way. In this work, an acoustic metamaterial plate based on the negative Poisson's ratio structure (NP-AMP) is proposed and feasible to achieve lower frequency, wider bandgap, and tunable bandgap compared with traditional ones (T-AMP). A counterpart based on the positive Poisson's ratio structure (PP-AMP) is also introduced for comparison. Studies have indicated that the newly designed structure has a lower frequency bandgap and wider bandwidth. With the increase of compression strain, the initial bandgap of PP-AMP gradually moves to a higher-frequency range. In contrast to PP-AMP, the NP-AMP exhibits lower frequency, which is beneficial for the further research of low-frequency bandgap. Moreover, the bandgap variation range can be enlarged by the enhancement of the auxetic behavior. Finally, the variation range of the NP-AMP initial bandgap frequency increased by 62%. The findings in this work will broaden the design of low-frequency broadband acoustic devices used in a dynamic environment, while providing new ideas and methodologies for real-time adjustment of bandgaps.

Auxetic materials have attracted a considerable attention due to their excellent properties, e.g., fracture resistance, shear resistance, energy dissipation, etc. However, the stiffness of auxetics tends to be much weaker than solid structure because of the existence of internal holes. Inspired by tuning the compacted point of auxetic structures to enhance their stiffness, a systematic methodology for defining a single parametric of variable stiffness scale factor (VSF) to generate auxetic unit cell with variable stiffness has been proposed and verified in this study. Two models with different VSF proportions were investigated experimentally. Different centres of rotation, heights of deformation area, and VSF percentages were analyzed to prove the effectiveness of the method numerically. The results indicate that the compacted strain can be tuned effectively using the designed VSF proportions, and the difference between the designed VSF and real VSF could be reduced by slightly changing the height of deformation area. These desirable characteristics provide a new idea for the optimal design of auxetics, with potential application in protective structures.

Auxetic metamaterials have attracted great attention due to their indentation resistance, shear resistance, synclastic behaviour, fracture toughness and energy absorption properties. As one branch of auxetics, the tubular structure with negative Poisson’s ratio has potential to be used in engineering, medical treatment, vehicle and other fields. However, studies on mechanical properties of auxetic tubular structures are limited in both tension and compression in the current literature, and auxetic tubular structures tend to exhibit low stiffness ratio due to the existence of internal holes. In this paper, a novel type of auxetic tubular structure with tuneable stiffness was developed, and finite element analysis and experimental study were carried out on the parameters of different rotation modes, the degree of advanced compaction and the height of deformation zone. The results show that the stiffness of auxetic tubes with tuneable stiffness can be turned by adjusting different proportions of compaction point, and the h value can be used to reduce the error between the designed proportion and real proportion. Auxetic behaviour of the tubes with tuneable stiffness is not significantly weakened. The auxetic tubular structures with tuneable stiffness proposed in this paper bring about an innovative design concept and have good application prospects in protection engineering.

Auxetic materials could exhibit desirable mechanical properties, e.g., fracture resistance, shear resistance and energy dissipation due to their unique deformation characteristics. However, the seismic performance of auxetics in disaster prevention and mitigation of building structures is rarely studied. In this study, a novel perforated negative Poisson's ratio core buckling-restrained brace (NP-BRB) was designed and manufactured for exploring the hysteretic performance of auxetic metamaterials under cyclic load. Experiments and verified numerical simulations were conducted to investigate the effects of porosity and section weakening rate on the seismic performance. The results show that the NP-BRB has stable hysteretic curves and low compression strength adjustment factor. In addition, the parametric analysis indicated that the energy dissipation capacity of NP-BRB with relatively large section weakening rate could be improved due to auxetic behavior when the average strain exceeds 1%. These findings are beneficial to the applications of auxetic metamaterials in damping devices for mitigating seismic effects.