Comparison of interlaminar fracture toughness in unidirectional and woven roving marine composites

(T)his paper investigates the effect of fibre lay-up and matrix toughness on mode I and mode II interlaminar fracture toughness (G(Ic) and G(IIc)) of marine composites. Unidirectional and woven roving fibres were used as reinforcements. Two vinyl ester resins with different toughness were used as matrices. Results from both modes showed toughness variation that is consistent with matrix toughness. Values of G(Ic) were not significantly influenced by fibre lay-up except at peak load points in the woven roving/brittle-matrix composite. Each peak load point, caused by interlocked bridging fibres, signified the onset of unstable crack growth. For unidirectional specimens, crack growth was stable and G(Ic) statistically more reliable than woven roving specimens, which gave fewer G(Ic) values due to frequent unstable crack growth. Mode II tests revealed that, except for crack initiation, G(IIc) was higher in woven roving composites. This was due to fibre bridging, perpendicular to the crack growth direction, which encouraged stable crack growth and increased energy absorption. Mode II R-curves were obtained for the woven roving specimens. These R-curves provide additional information useful for characterising delamination resistance. The paper concludes that composites with woven roving fibres show similar mode I delamination characteristics to the unidirectional composites; but their mode II delamination characteristics, after crack initiation, are quite different.