Studies on three-dimensional metamaterials and tubular structures with negative Poisson’s ratio
Creators: name: REN, Xin
Publisher: name: RMIT University
Year: 2024
Subjects: classificationCode: 400505
subject: Construction materials
classificationCode: 401201
subject: Aerodynamics (excl. hypersonic aerodynamics)

description: Materials and structures with negative Poisson's ratio exhibit counter-intuitive behaviour, i.e., under uniaxial compression (tension), these materials and structures contract (expand) transversely. The materials and structures that possess this feature are also called 'auxetics' by Evans. The terminology of 'auxetic' becomes a common adjective to describe materials and structures with negative Poisson's ratio. Many desirable properties resulting from this unusual behaviour have been reported, e.g., higher shear resistance, indentation resistance, fracture resistance, improved acoustic and higher energy absorption, synclastic behaviour and variable permeability. These superior properties offer auxetics broad potential applications, e.g., smart filters, sensors, medical devices and protective equipment. However, there are still many challenging problems which impede wider applications of auxetics. First of all, most of the studied auxetic materials are two-dimensional (2D) and very few three-dimensional (3D) auxetic materials have been designed and investigated. Secondly, the base materials of the most existing auxetic metamaterials are rubber-like materials, hence these auxetic metamaterials are limited to the elastic deformation. In contrast to elastic auxetic metamaterials, metallic auxetic metamaterials exhibit some new features in mechanical properties, e.g., localisation of plastic strain, strain hardening and irreversible deformation. Furthermore, metallic metamaterials are usually stronger than those made of elastomers, which leads to superior loading resistance and energy absorption. Last but not the least, although most of the publications mention that auxetic materials possess many desirable properties, very few auxetic materials have been designed and fabricated to the practical stage. Therefore, it is worthy of carrying out more original research to explore new applications for auxetic materials.

In order to fill the research gap mentioned above and create better auxetic materials for applications, in this study, extensive numerical and experimental investigations have been conducted. Firstly, a novel methodology was proposed to generate a 3D metallic cubic auxetic material based on a cubic buckling-induced auxetic material. It was found that the base material affected the auxetic behaviour of the buckling-induced cubic auxetic materials. When the elastomer base material was replaced with a ductile metallic material, the previously observed auxetic behaviour of the buckling-induced auxetic material would disappear. Inspired by this unexpected behaviour, a new methodology of generating 3D metallic auxetic materials was developed. The effectiveness of the methodology was then proved experimentally and numerically. The mechanical properties of the designed 3D metallic auxetic materials could be easily tuned by one single parameter of pattern scale factor (PSF). In the second part of this study, a simple auxetic tubular structure which exhibited auxetic behaviour both in compression and tension was generated by using the newly proposed PSF methodology. This simple auxetic tubular structure could also be tuned by one single parameter of the PSF. When the scale of PSF reached a certain value (around 60% in this study), the designed tubular structure exhibited an approximately identical auxetic effect both in tension and compression. In the third part of this study, a simple 3D auxetic metamaterial was designed which demonstrated the unexpected 2D auxetic behaviour. By utilising the proposed PSF methodology, the 2D auxetic behaviour could be successfully transformed to 3D auxetic behaviour. Hence, the functionality of the proposed PSF methodology was further extended. In the last part of this study, the first auxetic nails were designed, fabricated and experimentally investigated. According to the experimental results, the designed auxetic nails could not demonstrate superior mechanical performance to non-auxetic nails. Therefore, it would prudent to re-examine the potential applications for auxetic materials because many existing publications in the field might have overestimated the superiorities of auxetic materials and their limitations and disadvantages have been rarely discussed.