Mode I delamination and fibre bridging in carbon-fibre/epoxy composites with and without PVAL coating

Mode I delamination toughness, G₁, of unidirectional carbon-fibre/epoxy composites with and without a polyvinyl alcohol (PVAL) fibre coating has been measured to study the influence of the coating. It is found that G₁ increases with crack growth (or delamination) in both materials, but the maximum toughness of the PVAL coated composite is larger. In the experiments crack bridging due to fibre misalignment along the delaminated interface has been observed in both composites and, as a consequence, the measured compliance is smaller than that of an unbridged crack of the same length. Since the delamination toughness, G₁, is normally evaluated by the compliance technique, it is necessary to determine the influence of fibre bridging on compliance. A recent crack bridging theory which considers the difference in experimental and theoretical compliances has been used in the present work to determine the fibre bridging stresses. It is shown that the crack growth resistance (R) curve obtained for the delamination process is consistent with the observation of fibre bridging in the delaminated region. In addition, modifications for the simple beam theory for delamination toughness evaluation with double cantilever beams are considered and discussed.