Using the refined theory of dry bundles, we analyzed the curvature (length diversity) of yarns in the fabric composite for obtaining the maximum effect of pseudo-ductility (constant yield stresses during the deformation). Because of length diversity, yarns destruction happens consequently, starting with the straight yarns and ending with the most curved ones. The polymer matrix ensures a monolithic structure and high damping of the oscillations due to the intensification of matrix shears during tension/compression of the composite. To simulate an ideal elastoplastic stress-strain diagram of the composite, we formulated the law of changes in fiber lengths, the mechanical properties of which were considered to be the same. To study the technological possibilities of the local curvatures of yarns in fabric preforms, we performed experiments using the indenting method and arrays of conical and flat needles. We showed that conical needles allow us to obtain simultaneous curvature of warp and weft yarns to the same extent. The needle spacing controls the ratio of straight and curved yarns; so, by changing the spacing we can bring this ratio closer to the value required for getting the needed length of the yield plateau. In the case of flat needles with various orientation to the warp yarns, we can obtain anisotropic yarns curvature (of only one yarns family, for example). For experimental deformation of the fabrics with locally curved yarns we used aramid fabrics of plain weave and carbon fabrics of twill weave.
In this work, there was developed a three-axis CNC felting machine with one barbed needle. Felting is a kind of needle-punch technology for joining fabric stack by broken and pulled fibres. The density of needle hits is a valuable factor to make the required shear strength of polymeric composite materials in the specified places. Using the glass-fibre reinforced plastic (GFRP) with plain weave felted fabric, a noticeable decrease in strength with an increase in the needle hits density from 0 to 90 cm−2 is shown. For a laminated GFRP made from a stack of fabrics, it has been shown that the shear strength of the single-lap joint and the interlaminar shear fracture toughness GIIC significantly increase with an increase in the needle hits density. This is important for modern aircraft shells exposed to random low-velocity impacts during ground or flight operations.
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