Nonlinear characteristics and a new fractional constitutive model for warp-knitted NCF composites under normal loading conditions

Abstract

This paper presents a new fractional constitutive model to reveal the nonlinear and viscoelastic properties of fabric composites under normal loading conditions. The uniaxial tensile tests for warp-knitted non-crimp fabric (NCF) composites under different strain rates were carried out, and their nonlinear characteristics and quasi-yield behaviors under normal loading conditions were investigated. A new nonlinear constitutive model was then proposed by employing the fractional calculus operator theory, and the proposed models were verified by the experimental results and implemented in the simulations based on UMAT subroutine in FEA code ABAQUS. Results show that the warp-knitted NCF composites' filling yarns would produce greater yarn geometric deformation than the warp yarns due to their yarn waviness. The variations of initial elastic modulus under increasing strain rate show rising tendencies, which uncovers the materials’ strain-rate effect. Owing to the expressiveness of deformation mechanisms offered by model parameters, the proposed model could effectively present the nonlinear and viscoelastic characteristics of the fabric materials under normal loading conditions. In addition, The UMAT subroutine shows a good applicability, and the FE model with the Equation Constraint could simulate the uniform stress states accurately.

Keywords

• Warp-knitted non-crimp fabric composite
• Quasi-yield behaviors
• Fractional constitutive model
• Viscoelastic
• Nonlinear characteristics
• UMAT