A continuum model for needlepunched nonwoven fabrics

Abstract

Nonwoven fabrics are sheet structures created by bonding or interlocking a web (network) of fibers through mechanical, thermal or chemical processes. In general, the mechanical response of nonwoven fabrics exhibits two major characteristics. First, the mechanical response can vary significantly when the fabric is loaded along different directions, depending on the existence of a preferential orientation in the fiber arrangement and/or in the pattern of inter-fiber bonding/entanglement. Second, the mechanisms of deformation include elastic and inelastic components, accompanied by an irrecoverable evolution of the texture of the fiber network. In this work, we propose a three-dimensional, large strain continuum model for the constitutive behavior of nonwoven fabrics that accounts for the fiber network characteristics responsible for its anisotropic behavior, and captures the effects of deformation mechanisms at the micro-scale (fiber and bonds/entanglement) level. The model consists of two constitutive components: a nonlinear elastic component representing the resistances to recoverable deformation mechanisms, and a non-linear inelastic component representing the resistances to irrecoverable deformation and texture evolution. For nonwoven fabrics in which the anisotropy of fiber orientation is combined with random entanglement processes, we propose to capture the combined effects of fibers and junctions orientation distributions using a single tensorial representation of the network anisotropy (fabric ellipsoid). An orthotropic elastic constitutive model for the elastic response of nonwoven fabrics is then formulated based on this structural measure and deformation mechanisms of the network structure. The inelastic component of the model is then prescribed in terms of an evolution law for the fabric ellipsoid.

(cont.) A needlepunched web of high strength polyethylene fibers, "Dyneema Fraglight", is selected as the representative material, to be used as a test case to validate the proposed modeling approach. The model is shown to capture the macroscopic nonlinear anisotropic elastic-inelastic response of the fabric in planar deformation, as well as the underlying micromechanical deformation mechanisms, such as fiber stretch, and irrecoverable evolution of fabric texture. The proposed model can be used to predict the mechanical behavior of nonwoven fabrics and can be combined with other continuum models to aid in the design of multi-component structures. In addition, the proposed elastic formulation can be used to model different classes of anisotropic network materials, such as biological tissues, and tissue engineering scaffolds.