Damage accumulation and failure in stochastic fibrous materials

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Deogekar, Sai
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Electronic thesis
Mechanical engineering
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Damage accumulation and failure in random fiber networks is of importance in a variety of applications, from design of synthetic materials, such as paper and non-wovens, to accidental tearing of biological tissues. In this work we study these processes using three-dimensional models of athermal, crosslinked fiber networks, focusing attention on the modes of failure and on the relationship between network strength and network structural parameters. We consider network failure at small and large strains associated with the rupture of inter-fiber bonds. It is observed that the strength increases linearly with the bond number density, with the average distance between the bonds, and with the bond strength. Rendering the bond strength stochastic causes a reduction of the network strength. However, heterogeneity retards damage localization and increases the stretch at peak stress, therefore promoting ductility. Network strength, in general, is found to be independent of fiber material properties and fiber tortuosity. Random fiber networks, due to their inherent structural heterogeneity exhibit size effect in their strengths and we find that network strength follows Weibull statistics. We characterize the behavior and strength of random networks composed of fibers with non-circular cross-sections. Such fibers are characterized by two bending modes along different axes. For such networks, the torsional stiffness of the fibers controls the relative contribution of the two bending modes to the network stiffness at small strains. The presence of an additional bending mode does not affect the network deformation at large strains and the fiber cross-section, in general, does not affect the network strength. Using the structure-property relationships established in this work, we design a new class of materials, called the Non-Convex Voronoi networks. The Non-Convex Voronoi networks are more compliant and exhibit higher strength, rendering such networks of interest in a variety of applications, such as artificial tendons and ligaments, protective clothing etc. Finally, we also analyze network failure under multiaxial loading conditions and attempt to develop suitable failure criterion to predict network failure under generalized loading conditions, when inter-fiber bond breakage is the primary failure mechanism. The results established in this thesis can be used to design fiber networks of a specified strength and, in general, enhance the understanding of mechanical behavior of fibrous materials.
December 2019
School of Engineering
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Rensselaer Polytechnic Institute, Troy, NY
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