This article presents an experimental and geometrical study of a novel kind of plied yarn structure with negative Poisson's ratio (NPR). The deformation mechanism of the yarn structure formed with two stiff yarns and two soft yarns to achieve NPR behavior was firstly introduced. Based on the proposed yarn structure, four kinds of yarn samples were then fabricated with two kinds of stiff yarns and two types of soft yarns and tested under the axial extension to verify their auxetic or NPR effect. A geometric analysis was finally conducted to theoretically calculate the Poisson's ratio of the yarn structure and compared with the experimental results. The study shows that the proposed yarn structure has obvious NPR effect and the geometrical analysis can well predict its Poisson's ratio in the higher axial strain range. It is expected that this study could promote further development of auxetic textiles with unusual behavior.

This paper presents an innovative three-dimensional (3D) fabric structure for composite reinforcement. Different from most conventional 3D fabric structures, the new structure displays a negative Poisson’s ratio (NPR) effect under compression. Based on a manufacturing process developed by combining both non-weaving and knitting technologies, four NPR 3D fabric samples with different warp yarn diameters were first manufactured manually. Then, their Poisson’s ratio (PR) values under compression along the fabric thickness direction were experimentally evaluated. A geometrical model was also proposed for the theoretical calculation of PR values of these fabrics and was compared with experimental data. The good agreements were obtained between the calculation and experiment. The results show that all the 3D fabrics display NPR effect under compression, which results in a unique feature that allows the structure to concentrate itself under the compressive load to better resist the load. This special feature makes this innovative 3D fabric structure very attractive for many potential applications such as automobile, aerospace, defense and sports equipment, where impact protection can be a highly desirable property.

This paper reports the finite element analysis of an innovative 3D auxetic textile structure consisting of three yarn systems (weft, warp and stitch yarns). Different from conventional 3D textile structures, the proposed structure exhibits an auxetic behaviour under compression and can be used as a reinforcement to manufacture auxetic composites. The geometry of the structure is first described. Then a 3D finite element model is established using ANSYS software and validated by the experimental results. The deformation process of the structure at different compression strains is demonstrated, and the validated finite element model is finally used to simulate the auxetic behaviour of the structure with different structural parameters and yarn properties. The results show that the auxetic behaviour of the proposed structure increases with increasing compression strain, and all the structural parameters and yarn properties have significant effects on the auxetic behaviour of the structure. It is expected that the study could provide a better understanding of 3D auxetic textile structures and could promote their application in auxetic composites.