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Effect of nonwoven veil architectures on interlaminar fracture toughness of interleaved composites
[Thesis]. Manchester, UK: The University of Manchester; 2015.
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Abstract
This thesis addresses the influence of veil architecture on interlaminar fracture toughness (IFT) of interleaved unidirectional (UD) carbon fibre-epoxy composites with the aim to provide insights. Two nonwoven veils sets formed from polyphenylene sulfide (PPS) fibres with different diameters, with a range of increasing areal density, and a sample of polyetheretherketone (PEEK) fibres, with comparable fibre diameter, are characterised gravimetrically and by tensile tests (long and zero span). Consequently, the anisotropy and maximum stress transfer efficiency (MSTE) parameters are shown by these veils. Subsequently, the veils are interleaved within UD composites and assessed for mode I and mode II IFT. In both modes the veils show a strong dependence on areal density before a plateau at high areal densities, although the lower diameter fibres showed higher IFT values. Interpretation of the results reveal that the difference is attributable to the coverage of veils and thus, to the fraction of fibres in the propagation of crack. However, the effect of fibres is quite evident through the fibre bridging mechanism in the propagation of cracks, more significantly in mode I than in mode II. Moreover, in mode I and mode II a linkage of MSTE of veils with low data variability in IFT is observed. With regard to the anisotropy, this is notably significant only for the PEEK sample, though a statistical analysis supports that the IFT values from both types of fibres are consistent. A comparison of data revealed a slight dependence of the ratio mode II/mode I on areal density only for the larger diameter PPS fibre and the anisotropy of PEEK sample has a strong influence on this ratio. In both modes, however, data presented by this study are consistent with data provided by previous work. Subsequently, mass distribution of veil handsheets is assessed for both modes of IFT into UD composites, revealing no significant dependence of mass distribution on mode I IFT, whereas for mode II this dependence is significant due to the effect a variety of fractional open area size and the floculatted fibres. Fractographic observations via SEM (Scanning Electro Microscope) from representative interleaved composites are analysed and discussed.