The influence of the nonwoven veil architectures on interlaminar fracture toughness of interleaved composites

Nonwoven veils with a range of weights formed from polyphenylenesulfide (PPS) fibres with different diameter were interleaved within unidirectional carbon fibre epoxy composites and their mode I and mode II interlaminar fracture toughness (IFT) measured. In modes I and II the IFT increases with the areal density of the veil up to a plateau; at a given areal density, the mode I IFT is greater for thin fibres than for thicker fibres. For the PPS veils, we observe no significant influence of nonwoven anisotropy on IFT; though some dependence is observed for a highly anisotropic PEEK veil. Interpretation of the results using theory describing nonwoven architectures reveals that for both modes the IFT depends on the mean coverage of the veil and hence on the fraction of the propagating crack front that contains no fibres. Results for composites formed from a veil of polyetheretherketone (PEEK) fibres exhibit behaviours consistent with those observed for PPS.