Tensile and forming properties of auxetic warp-knitted spacer fabrics

Poisson's ratio (PR) is defined as the negative ratio between the lateral strain and the longitudinal strain of a material under tensile or compressive loading condition. Auxetic fabrics are those having a negative PR, which means that when their longitudinal strain is positive, their lateral strain is also positive, and vice versa. In this work, tensile and forming properties of auxetic warp-knitted spacer fabrics were investigated and compared with those of the conventional warp-knitted spacer fabrics. Both uniaxial tensile tests and hemispherical compression experiments were conducted, and the relationships of the tensile and forming properties with the auxetic effect were discussed in terms of different fabric structural parameters. The results show that the auxetic warp-knitted spacer fabrics have a prolonged low stress stage in the wale direction, indicating that they are more prone to undergo deformation along the wale direction, and fabric with a longer low stress stage has a better auxetic effect. The results also show that the formability of auxetic warp-knitted spacer fabrics is much better than that of conventional warp-knitted spacer fabrics, due to the much low forming energies required under hemispherical compression, which can be affected by many factors such as auxetic effect, fabric thickness and stiffness, yarn materials and the structure of the spacer layer. When all other parameters are similar, auxetic fabric with a better auxetic effect will have better formability. The study has provided useful information for the design and application of this type of nonconventional spacer fabrics.