•A novel auxetic chiral lattice composite is proposed by designing joints of the reinforcement phase.•The mechanical properties and deformation characteristics of four specimens are investigated ...under uniaxial compression.•The novel auxetic chiral lattice composite exhibits an enhanced stiffness and SEA capacity.•The effects of geometrical parameters and volume fraction of the reinforcement phase on the novel auxetic composites are investigated.
Auxetic materials and structures have many potential applications due to their counter-intuitive deformation behavior and desirable mechanical properties. However, auxetic structures have some drawbacks, such as relatively low stiffness and stability. To improve their mechanical performance, one common method is to fill soft materials in auxetic frames, i.e., auxetic two-phase composites. In this paper, a novel method to further enhance the mechanical properties of auxetic composites has been proposed, which is implemented by designing joints of frames. Mechanical properties and deformation characteristics of these novel composites and their conventional counterparts are investigated experimentally and numerically. The results of finite element analysis and experiments exhibit a good agreement. The energy absorption capacity and auxeticity of the proposed composites could be enhanced by optimizing the design of joints. Subsequently, parametrical studies are conducted to quantify the effects of the geometrical parameters and the volume fraction of the frame.
Four metallic metamaterials with tailorable mechanical properties are designed using bi-material star-shaped re-entrant planar lattice structures, which do not involve pins, adhesive, welding or ...pressure-fit joints and can be fabricated through laser-based additive manufacturing. Three length parameters, one angle parameter and three material combinations are used as adjustable design parameters to explore structure-property relations. For each of the four designed metamaterials, the effects of the design parameters on the Poisson’s ratio (PR), coefficient of thermal expansion (CTE), Young’s modulus and relative density are systematically investigated using unit cell-based finite element simulations that incorporate periodic boundary conditions. It is found that the bi-material lattice structures can be tailored to obtain 3-D printable metallic metamaterials with positive, near-zero or negative PR and CTE together with an uncompromised Young’s modulus. In particular, it is shown that metamaterial # 1 can exhibit both a negative PR and a non-positive CTE simultaneously. These metallic metamaterials can find applications in structures or devices such as antennas and precision instruments to reduce thermomechanical stresses and extend service lives.
•A novel auxetic composite is proposed by filling re-entrant honeycombs with slow recovery foam.•The mechanical properties and auxetic effect of four types of specimens under uniaxial compression are ...investigated.•The proposed auxetic composite exhibits an enhanced stiffness and a large auxetic effect.•The effects of geometrical parameters and loading rates on the Poisson’s ratio and energy absorption capacity are investigated.
Auxetic materials have excellent mechanical properties, e.g., indentation resistance, shear resistance, fracture toughness and energy absorption. However, the stiffness of auxetics is normally lower than that of solid structures due to the existence of voids. In this study, to improve the mechanical properties of re-entrant honeycombs, a buffer material called slow recovery foam is filled into re-entrant honeycombs. The mechanical properties and deformation patterns of slow recovery foam-filled re-entrant honeycombs are investigated numerically and experimentally. Parametric studies are conducted to investigate the effects of geometrical parameters on the Poisson’s ratio and energy absorption capacity. The results show that filling foam into re-entrant honeycombs will prevent lateral buckling of the structure. Compared with foam-filled hexagonal honeycombs, foam-filled re-entrant honeycombs have higher stiffness. With the increase of the strain rates, the stiffness and energy absorption capacity of slow recovery foam-filled re-entrant honeycombs will increase. With the increase of cell wall thickness and the decrease of cell angle, the energy absorption capacity of slow recovery foam-filled re-entrant honeycombs will also increase. The results indicate that slow recovery foam-filled re-entrant honeycombs are promising in the field of protective engineering.
•An analytical model was established and analyzed for the first time.•The design was verified with experiments and finite element analysis.•The tailorability of mechanical properties was demonstrated ...for the structure.•Manufacturing induced mechanical property issues were discussed.
In this work, an analytical model of a 3D re-entrant honeycomb auxetic cellular structure has been established based on both a large deflection beam model and a Timoshenko beam model. Analytical solutions for the modulus, Poisson’s ratios and yield strength of the cellular structure in all principal directions were obtained, which indicate a wide range of mechanical property control via geometrical designs. The results were compared with experimentation and finite element analysis, and it was verified that the analytical model provides a convenient and relatively accurate method in the prediction of the performance for the auxetic cellular structures once the manufacturing related factors are adequately incorporated into the model. It was also found that the model provides less accurate predictions when higher-order coupling effects such as warp locking becomes significant under lower structural symmetry.
An important feature that drives the auxetic behaviour of the star-shaped auxetic structures is the hinge-functional connection at the vertex connections. This feature poses a great challenge for ...manufacturing and may lead to significant stress concentrations. To overcome these problems, we introduced smoothed petal-shaped auxetic structures, where the hinges are replaced by smoothed connections. To accommodate the curved features of the petal-shaped auxetics, a parametrisation modelling scheme using multiple NURBS patches is proposed. Next, an integrated shape design frame work using isogeometric analysis is adopted to improve the structural performance. To ensure a minimum thickness for each member, a geometry sizing constraint is imposed via piece-wise bounding polynomials. This geometry sizing constraint, in the context of isogeometric shape optimization, is particularly interesting due to the non-interpolatory nature of NURBS basis. The effective Poisson ratio is used directly as the objective function, and an adjoint sensitivity analysis is carried out. The optimized designs – smoothed petal auxetic structures – are shown to achieve low negative Poisson’s ratios, while the difficulties of manufacturing the hinges are avoided. For the case with six petals, an in-plane isotropy is achieved.
Both auxetic structures and hierarchical honeycombs are marked with lightweight and excellent mechanical properties. Here, we combine the characteristics of auxetic structures and hierarchical ...honeycombs, and propose two re-entrant hierarchical honeycombs constructed by replacing the cell walls of re-entrant honeycombs with regular hexagon substructure (RHH) and equilateral triangle substructure (RHT). The honeycombs are subjected to in-plane impact in order to investigate the crashworthiness by using the commercial software LS-DYNA. The plateau stress of RHH and RHT in x and y directions are derived by a two-scale method. The results from numerical simulation indicate that the specific energy absorption of RHT and RHH is improved by up to 292% and 105%. RHT and RHH improve the mean crushing force value by 298%, 108% respectively compared with the classic re-entrant honeycomb (RH) under quasi-static loading at stress plateau region. The RHT and RHH still have the characteristic of negative Poisson’s ratio. Additionally, the parametric studies are further carried out to investigate the effects of impact velocities and relative densities on crashworthiness. All the findings of this study indicate that the proposed two hierarchical honeycombs exhibit an improved crushing performance, and RHT provides the highest energy absorption capacity among all specimens.
This paper presents an investigation on the nonlinear flexural vibrations of carbon nanotube-reinforced composite (CNTRC) laminated cylindrical shells with negative Poisson’s ratios in thermal ...environments. The material properties of the CNTRCs are temperature-dependent and the functionally graded (FG) in a piece-wise pattern in the thickness direction of the shell. An extended Voigt (rule of mixture) model is employed to estimate the CNTRC material properties. The motion equations for the nonlinear flexural vibration of FG-CNTRC laminated cylindrical shells are based on the Reddy’s third order shear deformation theory and the von Kármán-type kinematic nonlinearity, and the effects of thermal environmental conditions are included. The nonlinear vibration solutions for the FG-CNTRC laminated cylindrical shells can be obtained by applying a singular perturbation technique along with a two-step perturbation approach. The effects of material property gradient, the temperature variation, shell geometric parameter, stacking sequence as well as the end conditions on the vibration characteristics of CNTRC laminated cylindrical shells are discussed in detail through a parametric study. The results show that negative Poisson’s ratio has a significant effect on the linear and nonlinear vibration characteristics of CNTRC laminated cylindrical shells.
•Both piece-wise FG configurations and NPRs are considered.•A multi-scale approach is proposed for nonlinear vibration of auxetic CNTRC shells.•NPR has a significant effect on the nonlinear vibration behavior of FG-CNTRC shells.
Sandwich panels composed of auxetic cellular cores and metal facets are presented for blast resistance applications. The performance of this hybrid composite structure under impulsive loading is ...numerically studied, taking into account the rate-dependent effects. The Johnson–Cook law is used to model the behaviours of composite materials at high strain rates. Parametric analyses are performed to evaluate the performances of different designs of composite panels and compared with equivalent monolithic panels of identical areal masses in terms of deformations and dissipated plastic energy of the metal facets and auxetic crushable cores. Various design parameters are considered, including the auxetic unit cell effective Poisson’s ratio, material properties, thickness of facet, and diameter of the unit cell truss member. To reduce the computational time, a quarter of the panel is modelled with shell elements for the facets and beam elements for the core. In blast events, auxetic composite panels are found to effectively absorb double the amount of impulsive energy via plastic deformation, and reduce up to 70% of the back facet’s maximum velocity when compared with monolithic ones. The maximum back facet displacement is also noticeably reduced by up to 30% due to the densification and plastic deformation of the auxetic cores.
•A newly developed meta steel with a uniform elongation of 55.3% can greatly delay necking initiation.•The newly developed meta steel has high ductility with a minor size effect.•A necking lag effect ...was observed for mild steel.•A unified post-necking modification method capable of simulating full-range plasticity of steels proposed.•A unified model to describe full-range monotonic plasticity and ductile fracture of steels proposed.
This paper investigated mechanical behaviors and monotonic plasticity of a new type of high strength and high ductility meta steel with negative Poisson’s ratio effect (termed as NPR steel in this study). NPR steel can postpone necking initiation, so it has a high uniform elongation of 55.3% and a high elongation till fracture of 61.7% in this study. Classical plasticity theory postulates that metals have a constant Poisson’s ratio of 0.5 under plastic straining, but Poisson’s ratio of NPR steel decreases from around 0.5 to 0.44 at the plastic stage. In this paper, experimental and numerical studies on the new material were conducted, and four types of conventional engineering steels were also investigated for comparison. For each type of steel, coupons of three different gauge lengths were tested to examine the size effect. Full-range true stress – true strain relationships of the five types of steels under monotonic tension were obtained by a unified post-necking modification method newly proposed in this study, and numerical results compared well with the corresponding experimental ones. Finally, a unified plasticity model and ductile fracture parameters were presented for the steels.
It is generally acknowledged that the indentation resistance or hardness of auxetic materials is higher than that of their conventional counterparts under elastic deformation. However, this property ...of the auxetic material may not always be superior to that of the non-auxetic materials when the deformation is relatively large with plasticity considered. In this study, we come up with an index to quantitatively depict the indentation resistance of the hexagonal honeycombs under large deformation. The indentation resistance of both the auxetic and non-auxetic hexagonal honeycombs is compared and discussed. Results show that in the premise of honeycombs possessing the same relative density, the indentation resistance of auxetic hexagonal honeycombs is not always higher than that of the non-auxetic honeycombs. This phenomenon is verified by the numerical simulations. Further analysis shows that there is a critical value of the absolute value of Poisson’s ratio, which is determined by the cell-wall length ratio, to estimate the higher indentation resistance between the auxetic and non-auxetic hexagonal honeycombs. The influence of indentation velocity is also analyzed based on numerical simulations. This present work is supposed to shed light on the design and evaluation of the indentation resistance for both auxetic and conventional honeycombs.
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