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Title: Deformation and damage behaviour of calendered nonwovens : experimental and numerical analyses
Author: Cucumazzo, Vincenzo
ISNI:       0000 0004 9353 7690
Awarding Body: Loughborough University
Current Institution: Loughborough University
Date of Award: 2020
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Nonwovens are engineered materials made of fibres randomly-oriented or aligned in preferential directions, consolidated together by thermal, mechanical or chemical bonding techniques. Most applications require sufficient robustness to perform their intended function without damage during their life in service. In the last decade, various numerical models, continuous and discontinuous, that aimed to simulate deformation and damage behaviour of nonwoven materials have been proposed in literature. However, these models offer only partial solutions as they are limited to address a specific problem and neglect important design parameters. Thermally-bonded nonwovens, in particular those obtained with the hot-calendering bonding technique, are considered in this study due to their popularity. As a result of the manufacturing process, calendered nonwovens present a complex heterogeneous microstructure that comprises three mechanically distinct domains, namely fibrous matrix, bond-areas and interface regions. Variability in design parameters such as type of polymer, fabric planar density, type of fibre (mono- and bi-component), orientation distribution of fibres and bond pattern makes it difficult to have a universal model that can accommodate all these parameters. Therefore, a parametric numerical scheme is developed in this research to model and simulate deformation and damage behaviour of calendered nonwovens, while accounting for a large number of design parameters. This was achieved using a combination of experimental and numerical studies. A number of experimental techniques (uniaxial-tensile tests on fabrics in various loading directions at different strain rates, single-fibre tests, SEM and X-ray micro-computed tomography) were employed to ascertain the deformation, damage and failure mechanisms as well as the microstructural properties of the selected calendered fabrics. Results revealed that, due to the fibre re-orientation mechanism, the mechanical response of individual fibres greatly differed from that of fabrics. The mechanical response of fabrics was primarily affected by the fabric size and planar density, with the orientation distribution of fibres playing an important role in contrast to the strain rate. A balance exists between the load-bearing capacity and deformation of fabrics stretched uniaxially in various loading directions. Finally, damage was observed to originate at interface level and propagate through the fibrous matrix, causing failure of the fabric at later stages of the deformation process. The numerical framework was developed based on the experimental observations using a continuous modelling approach which allows simulation of the mechanical behaviour of large fabrics at macroscopic level. The numerical algorithm was implemented in the form of a standalone software named FabricGEN. The numerical tool enabled to compute and assess elasticplastic properties of the fabric domains along the fabric three principal directions; machine, cross and through-thickness directions. This allowed to obtain effective materials properties based on the fabric microstructure. Such information served to define the material properties in numerical models. The definition of material and geometric properties in FabricGEN allowed to generate 3D shell-based finite-element (FE) models in an automatic manner. The obtained models demonstrated good predictive capability in simulating deformation and damage behaviour under tensile loading of medium- and high-density large fabrics. In particular, the elastic-plastic response as well as the instant and location of damage were accurately predicted. Finally, a parametric study was performed to gain insight into the effect of design parameters on the mechanical response of the calendered fabrics. Due to the nature of the developed numerical scheme, this latter could also be employed to simulate the mechanical behaviour of other materials in any FE environment.
Supervisor: Not available Sponsor: EPSRC ; Nonwovens Institute ; North Carolina State University
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
Keywords: Calendered nonwovens ; Bi-component fibre ; Orientation distribution function ; Mechanical anisotropy ; Finite-element analysis ; Deformation ; Damage ; Failure