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Laura Balea
Abstract
Mechanical behaviour of weft knitted fabric composites manufactured by resin injection
Laura Balea - 11 April 2011
Knitted fabrics are of great interest for reinforcing composites and particularly for producing complex and 3D shape parts due to their high drapability and their automation capability. Nevertheless the looped structure of the reinforcement confers to the composite poor in plane mechanical properties. Improving these properties requires an optimization of the fabric architecture. The aim of this thesis is to study the mechanical behaviour of weft-knitted reinforced composites and to relate it with the fabric architecture, the fibre type and the dry preform behaviour. In a first step, tensile behaviour of the dry knitted fabrics has been studied according to their architecture and the fibre type (E-glass, basalt and carbon), focusing first on the basic knitted fabrics (1x1 rib and plain), and then by inserting inlay yarns in the course direction of the plain knit fabric. In a second step, composite plates, reinforced by these knitted preforms were elaborated by resin injection and subjected to tensile tests. Digital Image Correlation has been used to highlight the deformation heterogeneity due to knitted reinforcement and explain the difference of behaviour depending on loading direction. Damages have been studied by the fracture pattern observation. In a last step, the semi-analytical modelling of the plain-knit fabric behaviour, based on the theory of elasticity, gives a geometrical model of the plain stitch for any deformed state of the fabric. The fibre orientation calculated has been used in a homogenization model for estimating the elastic properties of the resulting composite. Theoretical results are in good agreement with those obtained experimentally. This model allows to take into account the presence of the inlay yarns and also an initial extension of the reinforcement before composite elaboration and thus to define an optimized architecture of the knitted fabric to improve the properties of the composite.
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