Deformation analysis of weft-knitted spacer fabrics under tensile loading: shape ratio vs. Poisson’s ratio

Spacer fabrics are three-dimensional textile structures valued for their unique properties, which render them suitable for a broad range of applications, including civil engineering, composite reinforcement, automotive manufacturing, medical textiles, and sportswear. Consequently, accurate prediction of their mechanical behavior is of critical importance. This study investigates the deformation behavior of various weft-knitted spacer fabrics under uniaxial tensile loading applied in both the course and wale directions. To characterize this behavior, two parameters were determined: Poisson’s ratio and the shape ratio. Poisson’s ratio is defined as the ratio of transverse strain to longitudinal strain, whereas the shape ratio quantifies the change in the fabric’s lateral surface area relative to its volume. The results indicate that, for all tested fabric structures, the deviation of the shape ratio from the 45° line is consistently smaller, by a factor ranging from 2 to 64, than that of Poisson’s ratio. This finding suggests that the shape ratio more closely satisfies the reciprocal theorem of Maxwell and provides a more reliable measure than Poisson’s ratio for describing dimensional changes in weft-knitted spacer fabrics under uniaxial tension. Therefore, owing to the porous and three-dimensional nature of spacer fabrics, their deformation under tensile loading should be assessed by considering the combined dimensional variations in width, length, and thickness, rather than by analyzing the surface and middle layers independently.