Modélisation multi-échelle des tissus secs : Application à l'impact

Abstract : The current thesis work focused on the development of a predictive numerical model of dry fabrics under high velocity impact.A mature bibliography exists on the subject. The impact phenomenon can be essentially resumed as an energy transfer between the colliding object and the fabric layers. The correct prediction of the fabric ballistic performance by a numerical model is related to the correct representation of the fabric energy evolution and its failure dynamic. Different numerical strategies have been proposed to model a fabric under ballistic impact. Mesoscopic numerical models resulted to be the most popular since they provide a realistic representation of the phenomenon for a reasonable computational cost. This is possible thanks to the main assumption of treating yarns as continuous media.In order to represent a discrete fiber bundle as a continuum an appropriate constitutive behavior have to be formulated. The universally adopted constitutive law accurately describes yarns longitudinal properties but it is limited in the representation of their transverse mechanical behavior. Recent studies have demonstrated how this last point is intrinsically related to fabrics failure and multilayer textiles response, then its correct representation becomes a critical point for an accurate model. The goal of the current work has been to provide a new constitutive model which overcome the limitation of the classic linear elastic approach while keeping unaltered its advantages, i.e. low computational costs and accurate description of yarn longitudinal behavior.The first step of this dissertation was to quantify the yarn cross section effects over textile ballistic properties and the phenomena related to this aspect. In order to provide an answer, two microscopic numerical studies of a single Kevlar yarn transversely impacted have been presented. Results showed how yarn transverse mechanical behavior has a role in failure initialization, while its energetic contribution to the global energy balance is not negligible during the first phases of an impact. The importance of a correct representation of the yarn transverse behavior for a predictive fabric numerical model was then confirmed.Starting from the previous microscopic observations, a consistent yarn continuum model for impact applications has been proposed. An hyperelastic formulation previously developed for static applications has been extended to impact analyses and a novel multiscale approach for the determination of all the material parameters has been introduced. The validation of the hyperelastic approach has been performed comparing the results with those obtained at the microscale. Compared to the classical approach, the introduced constitutive law is actually able to reproduce the evolution of the yarn cross section during the impact while keeping a correct representation of the yarn longitudinal properties. Moreover, the proposed formulation provides new physical measurement to exploit the physic behind the impact and new possibilities in terms of failure modelisation.In the final part of the dissertation, the proposed yarn continuum model is introduced at the fabric level. Results confirmed the observation performed at the yarn level. The proposed hyperelastic approach is able to correctly represent the impact dynamic and fabric energies trends. Moreover, it provides more stability and a better representation of the fabric failure compared to linear elastic approach. The proposed hyperelastic constitutive law and the linear elastic one can be adopted for different portion of the same yarn without occurring into model instabilities and providing accurate results.The yarn mesoscopic model developed in the current work offers new possibilities in terms of failure modelisation and post processing tools. These could be used to develop more accurate fabric model and exploit the phenomena behind fabrics and yarns failure mechanic.
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Pietro del Sorbo. Modélisation multi-échelle des tissus secs : Application à l'impact. Mécanique [physics.med-ph]. Ecole nationale supérieure d'arts et métiers - ENSAM, 2019. Français. ⟨NNT : 2019ENAM0003⟩. ⟨tel-02187127⟩

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