Mechanisms and design
Auxetic materials are those that exhibit a negative Poisson’s ratio (NPR). When stretched, auxetics become thicker perpendicular to the width-wise directions, while they become thinner in one or several transverse directions when subjected to uniaxial compression. Since their discovery in the late twentieth century, they have been increasingly studied and applied in a wide range of industrial applications owing to their extraordinary mechanical and bulk-control properties.
For the past decades, most studies on auxetic materials have focused on elucidating their properties, responses, and underlying mechanisms with keen attention to their auxetic behaviors under different loading conditions. Nevertheless, despite the progress in the study of auxetic behaviors of perforated structures, more are yet to be uncovered. Consequently, most of the existing research on different auxetic perforated materials focused on their theoretical tensile behaviors with few on their mechanical and mechanism performance, such as Poisson’s ratio, specific Young’s modulus, and absorption energy, under compression.
With the increasing application of three-dimensional (3D) printing technology in the fabrication of different structures, carbon fibre reinforced perforated structures have been successfully printed. Despite their potential practical applications, their use remains rare due to the limited knowledge of their compressive mechanisms and behaviors. To this note, Dr. Yuan Chen from The University of Sidney investigated the compressive responses and mechanisms of auxetic perforated structures with negative Poisson’s ratio. Their aim of to accurately predict the auxetic effects and design an enhanced lightweight auxetic structure. The work is currently published in the research journal for composites, Composite Structures.
In their approach, the perforated structures were made of 3D-printed pure nylon polymer (NP) and short carbon fiber reinforced nylon polymer (SCF/NP). A theoretical model for predicting the Poisson’s ratio of the auxetic structures considering all the four stages: initial, rotating, critical and expansion, was proposed and validated. The authors also investigated the different factors affecting the mechanical performance and auxetic behaviors of the structures, such as material type and perforation geometry. The study results were validated through comparisons drawn between the responses and performances of conventional and auxetic perforated structures under compression. Importantly, an improved novel lightweight auxetic structure was designed and developed with significant improvement in comparison to the commonly available auxetic structures.
Results showed that the structures made of SCF/NP exhibited enhanced stiffness than that made of pure nylon polymer and could achieve a negative Poisson’s ratio of up to – 1, compared to the nominal values (0.2 – 0.4) for the conventional structure. Moreover, the mechanical properties of the structures highly dependent on the material type and the perforation geometry. An increase in the perforation gap resulted in a decrease in the nominal stress and an increase in the average Poisson’s ratio, effective strain, and relative density. Additionally, with improved specific energy absorption, Young’s modulus, effective strain, SCF/NP specimen generally performed better than pure nylon specimen. Furthermore, the designed structure significantly outperformed the two-common engineering auxetic structures in terms of auxetic behaviors, Young’s modulus, and specific absorption energy.
In summary, Dr. Yuan Chen innovatively demonstrated that the overall performance of SCF/NP is superior to NP for auxetics. Moreover, his designed and 3D-printed enhanced auxetic perforated structures significantly outperformed the commonly used engineering auxetics. Advances in Engineering believes that this study paves a foundation to using perforated structures for high-performance auxetics in different applications.
Chen, Y., & He, Q. (2020). 3D-printed short carbon fibre reinforced perforated structures with negative Poisson’s ratios: Mechanisms and design. Composite Structures, 236, 111859.