Structural mechanics of knitted fabrics for apparel and composite materials

The importance of fabric biaxial extension, in‐plane compression, shear and bending properties, have been widely recognised by textile scientists and engineers for the evaluation of the three‐dimensional formability and drape of textile materials in apparel products and three‐dimensional preforms. In contrast to woven fabrics where bending and shear properties determine the fabric formability, knitted fabrics have very high formability as a direct result of their easy biaxial extension properties. This ability to form three‐dimensional shapes using the biaxial extensibility of knitted structures enables these knitted textile materials to be utilised for a wide variety of close fitting apparel garments and shaped composite preforms. Some representative biaxial extension curves for the plain knitted structure are described in this paper. These curves illustrate an unusual shape for the load‐extension curve of a textile material arising from the pre‐tension or pre‐stress. The pre‐stress yields an initial high tensile modulus for the structure in contrast to the very low initial modulus characteristic of apparel textiles. Accordingly, for knitted textile materials, it is shown how biaxial extension of the fabric introduces a fabric pre‐stress to maximise the three‐dimensional fabric formability especially when subjected to transverse compression by the resin or matrix in a composite material. Typical uniaxial and biaxial tensile stress–strain curves for knitted fabrics are compared.