Lattice structures that possess exceptional energy absorption capabilities show promise across various fields. However, their tendency to exhibit relatively lower stiffness due to rotation or bending during impacts poses a challenge. Shear thickening fluid with the unique property of shear thickening effect can be an ideal filler to overcome the limitation of lattice structure. In this paper, composites that combined STF with new lattice structures were fabricated to investigate their dynamic response using experimental and numerical means and found that incorporating STF into these lattice structures not only preserved the inherent mechanical characteristics of the lattice structures but also led to substantial improvements on mechanical performance. The lattice structures displayed a noteworthy boost in stiffness, increasing significantly from 11.02 to 85.56 times. Additionally, there was an improvement in energy absorption, ranging from 2.78 to 5.51 times. These findings indicate that STF holds more promise in enhancing a structure's stiffness compared to its energy absorption capacity. Moreover, strain rate, weight fraction of STF, and cell size of the lattice structure were considered to further investigate their impact on the dynamic behaviour of STF-filled lattice structures. Our findings indicate that the increase in stiffness and energy absorption of the lattice structure following STF filling is correlated with an increase in strain rate and weight fraction but decreases when the cell size of the lattice structure is increased. The critical strain rate of STF-filled lattice structure was predicted, around 104 s−1, which indicates the onset of the shear-thickening effect in STF. Furthermore, it was observed that composites with higher weight fractions of STF were more responsive to changes in strain rate. Additionally, structures with initially lower mechanical properties experienced the most significant improvements after being filled with STF. These optimal outcomes provide valuable insights for the design of STF-filled lattice structures in practical applications.

Lattice structures have the potential for application in tensile conditions. However, limited studies have addressed tensile behaviours of such structures and compared them with their compressive behaviours. In this paper, re-entrant structure as the foundation was selected to develop new 3D hybrid auxetic honeycomb lattice structures, each characterized by a unique unit cell configuration that features interconnected centre auxetic honeycomb, differentiating it from 2D hybrid auxetic honeycomb structures. A series of uniaxial quasi-static tensile and compressive tests, followed by simulations were conducted on these structures to investigate and compare their energy absorption characteristics. The results show that there were significant differences between these structures in terms of their mechanical behaviours under tension and compression. Especially, the smallest value of energy absorption under tensile load was 86% higher than the largest value under compressive load. The stiffnesses of these lattice structures under tensile loading were about 84-122 times of those of the identical corresponding structures when they were compressed. Meanwhile, the progressive stretching, buckling and collapse mechanisms observed in these structures exhibited excellent stiffnesses and energy absorptions under tensile and compressive load compared to traditional 3D auxetic as the basic structure. The outer frame structure of lattice structures had a direct influence on Poisson's ratio. The work indicates that the stiffness and energy absorption of the traditional 3D auxetic structure can be enhanced by embedding auxetic and honeycomb umbrella shaped elements.

Auxetic materials have garnered significant attention due to their lightweight and excellent energy absorption capabilities. Nonetheless, they often display relatively lower stiffness when compared with conventional materials. To address this limitation and enhance their mechanical properties, researchers have explored various avenues, including designing hybrid auxetic structures by combining two or more auxetic unit cells and developing auxetic composites using multiple materials. While previous reviews extensively covered hybrid auxetic structures, discussing their classification, design methodologies, fabrication techniques, applications and mechanical behaviours, there has been a noticeable gap in the literature concerning auxetic composites with fillings. Therefore, this paper concentrates on auxetic composites with fillings, delving into their classifications, mechanical responses, and underlying mechanisms. This review article also critically examines different design factors that influence the performance of auxetic composites and compares them with conventional counterparts in terms of mechanisms and mechanical properties. Overall, auxetic composites exhibit superior mechanical characteristics compared to equivalent conventional materials. However, several challenges and limitations persist regarding the design, fabrication, and applications of auxetic composites.

Periodic lattice structures as lightweight and high-energy absorption materials have been widely used in various fields, among which re-entrant and Kelvin cell lattice structures have exhibited excellent mechanical behaviors under different loadings. Therefore, this study aims to numerically explore and compare the tensile mechanical responses of re-entrant and Kelvin cell lattice structures with the same relative density after validating with experimental tests. It has been found that the tensile behavior of the two stretching-dominated lattice structures resemble that of parent solid material but had smaller fracture stress and strain due to the lower ductility of the lattice structures. The re-entrant lattice structure displayed a better energy absorption capacity than the Kelvin cell lattice under tensile loading, i.e., the energy absorption and specific energy absorption of the re-entrant lattice were 3 times and 1.6 times, respectively, those of the Kelvin cell lattice. Meanwhile, the re-entrant lattice as expected exhibited auxetic behavior with a negative Poisson’s ratio during the whole stretching process, while the Kelvin cell had the mechanical behaviors of traditional materials with a relatively constant positive Poisson’s ratio. These results are expected to provide hints on mechanical references and guidance for their extensive applications in the future.

Shear thickness fluid (STF) is considered an ideal filler to enhance the mechanical behaviour of in-plane honeycomb due to its special rheological properties. In order to investigate the dynamic compressive behaviour of STF-filled honeycomb, a comparative study between a STF-filled honeycomb and an empty honeycomb is numerically carried out after the fluid-structure coupled model has been validated against experimental testings. It is shown that the addition of STF in the honeycomb cells results in significant improvement in the energy absorption of the STF-filled honeycomb, which in turn, effectively prevents the premature collapse of the honeycomb cell walls. Furthermore, a parametric study of STF-filled honeycomb is conducted to investigate the contributions of each component to the total energy. It is found that the honeycomb plastic deformation is the predominant mechanism of energy absorption during impact, whilst the proportion of viscous energy contribution to the total energy is significantly reduced when the honeycomb wall thickness is increased. Additionally, the mean crushing force of STF-filled honeycomb has a power law relation to the honeycomb wall thickness for each specific loading velocity. Moreover, the mean crushing force contributed by STF is proportional to loading velocity and the increase in velocity significantly improves the percentage of viscous energy to the total energy. Meanwhile, optimal honeycomb thickness and weight fraction of STF are crucial to avoid earlier plastic densification of the honeycomb during the loading process. The results of this paper provide useful insights for future design and optimization of STF-filled structures.