Characterising the shear, stretch and in-plane bending response of a pure-unidirectional non-crimp fabric

The in-plane deformation kinematics of a pure-Unidirectional Non-Crimp Fabric (pure-UDNCF) is investigated using novel and existing experimental methods to characterise its shear, tensile, and in-plane bending responses under controlled loading. A pure-UDNCF is defined as a fabric in which stabilising tows are absent in the transverse direction relative to the primary tow orientation. The stitching threads in this fabric are made of polyamide, a highly compliant material that allows significant stretch, introducing a low-energy deformation mode that is relatively absent in biaxial engineering fabrics and quasi-UDNCFs (UDNCFs with inextensible stitching and transverse stabilising fibres). To fully characterise its forming behaviour, several novel testing methods are introduced that generate well-defined combinations of fabric shear, in-plane bending and stitch tensile strain. The total normalised axial force measured in the tests is subsequently decoupled into three contributions from shear, tensile strain in the stitch direction, and in-plane bending of the tows. An important finding is that when tested in the picture frame test, in-plane bending generates more resistance to specimen deformation than shearing of the fabric. The testing protocol and resulting data can be used to create appropriate constitutive models for pure-UDNCFs.