Flexible Composites - Strength, Deformation and Fracture Proecesses in Woven and D.O.S. Reinforcement Materials
Abstract: In this thesis the tensile strength, deformation, damage resistance and tear propagation for flexible composites are analysed. There are two essential requirements common to all flexible composites; namely, high tearing strength and good matrix-to-fabric adhesion. The interaction of fabric and matrix is complex. The most critical variables are those of yarn and fabric structure, the mechanical properties of the matrix, the adhesion of fibre-matrix interface and the penetration of the matrix into spun and multifilament yarns. Coated fabrics are an important class of flexible composites and mechanical properties and testing methods for such materials are emphasised in the thesis. The effect of used yarns and fabric structures has been analysed. Two distinct failure and crack-propagation mechanisms in flexible composites have been analysed: ductile and brittle failure. Four different ways for the crack to propagate have been identified for coated D.O.S.-fabrics (D.O.S. = Direct Oriented Structures). Tongue tear test methods are sufficient for evaluation of tear strength, not only for coated woven materials, but also for coated D.O.S. materials, if a higher rate of extension, use of capstan clamps and detailed expression of the test result is permitted in the test methods. There is a need to introduce new analysis methods for evaluating the mechanical properties of all the novel kinds of flexible materials that have appeared on the market during recent decades. Examples are D.O.S. materials for reinforcement and use of co-mingled yarns as a matrix material. New test methods are introduced for evaluating damage resistance and crack propagation. The rule of mixture (ROM) can be used as a first approximation of the tensile behaviour of textile-based flexible composites if the thermo-mechanical history of the fibre and matrix properties is taken into account. The ROM is usually used for rigid composites, but can also be a tool for evaluating the tensile properties of semi-flexible composites, which are defined as an intermediate between the rigid and flexible composites with at least some feature of the flexible composites incorporated, providing non-catastrophic large-strain behaviour. The mechanical properties of D.O.S. materials can be similar to those of semi-flexible composites. Studies on joint strength of PVC-coated polyester or nylon fabrics with different levels of adhesion, including peel and lap joints, have been carried out. Examples of applications are given, such as the use of decorative textiles in composites for appearance as well as for improved impact resistance.
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