Study of auxetic composite with isotropic poisson’s ratio by random inclusions

Abstract

Materials with negative Poisson's Ratio (NPR), also known as auxetic materials, are of great interests due to their excellent properties including extremely high indentation resistance, fracture resistance and energy absorption etc. With these enhanced properties, auxetic materials are prominent for the application of making sports equipment, protection equipment and others. As a type of auxetic materials, auxetic composites have attracted great attention because they can be tailor-made to achieving different properties by using different combinations of materials and structures. In recent three decades, both two dimensional and three dimensional auxetic composites have been developed by using different auxetic reinforcements and matrixes, special structure designs or special lay-up angles. However, most of them have anisotropic NPR behaviour, which limits their practical applications when isotropic behaviour is required. Although the auxetic composites with isotropic properties have been theoretically discussed in many works, there are no auxetic composites with isotropic properties that have been reported in the literature. This research developed a new type of three-dimensional (3D) auxetic composites with nearly isotropic NPR by randomly embedding rigid inclusions into a soft matrix. Different auxetic structures were reviewed and studied. The reentrant equilateral triangle was selected to make the inclusions. Four types of reentrant triangle inclusions with different structural parameters including re-entrant angle, prism length and rib thickness were first made from two types of carbon fibre prepregs, and then embedded into a silicon rubber matrix with different inclusion numbers to fabricate 3D composites having different volume fractions of inclusions. The inclusions were manufactured using a special three-stage methods from the carbon fibre prepreg as the carbon fibre was too stiff to be bent to form complicated structures directly. The fabrication method of the 3D auxetic composite embedded with randomly distributed inclusions was developed based on conventional techniques but a new approach. Both the experimental and finite element (FE) analyses were conducted to investigate the deformation behaviours of all fabricated samples under quasi-static compression and the effects of different structural parameters and carbon fibre prepregs on the auxetic performance. The compression tests were conducted using Instron 5566 universal testing machine, and the FE models were constructed and analysed using SOLIDWORKS and ABAQUS respectively. The FE simulated and experimental results were found to have good agreement. The results obtained from both the experiment and FE simulation showed that the NPR of the composites can be realised by using the inclusions with suitable structural parameters and properties. The lowest NPR of -0.19 was obtained by via embedding the 6mm length inclusions made of one-layer 3K twill weave carbon fibre prepreg with an inclusion number of 800. In addition, the FE simulated results indicated that the number of inclusions of the composite must exceed 300 to achieve the auxetic effect. The deformation process of all composites in both experiments and simulation were recorded and compared to understand the deformation mechanism of the composite. Furthermore, the composites were subjected to drop weight low-velocity impact test to study the impact resistance, deformation behaviours and energy absorption performances of the composites. The low-velocity impact results indicated that the embedding of inclusions can seriously influence the deformation behaviour, impact resistance and energy absorption of the composites. The embedding of inclusions could result in better energy absorption of the composite. However, the composites also became easier to be damaged with more inclusions. The auxetic composite samples were found to exhibit better energy absorption than non-auxetic composite samples. In order to improve the measurement of the Poisson's ratio from the compression experiments, a programme based on the image processing technique was developed. The algorithm for processing the image and calculating the Poisson's ratio was verified by comparing the Poisson's ratio obtained from the image processing programme with that obtained from conventional manual measurement. This research provides a new approach for the development of 3D composites with isotropic NPR for the practical applications of auxetic materials.