Mechanisms of interlaminar fracture toughness using non-woven veils as interleaf materials
The main objective of this research is to understand the mechanisms of interlaminar toughness using non-woven veils as the interleaf materials. The vacuum assisted resin transfer moulding (VaRTM) method was chosen for making specimens. Several types of non-woven veils were used as the toughening materials, because the non-woven veil was expected for good resin permeability. Three types of carbon fabrics, (plain, 5-harness satin, and unidirectional) and two types of resins (epoxy and vinyl ester) were selected for base materials. Firstly, the Mode-I and Mode-II interlaminar toughness tests, which are double cantilever beam (DCB) and four-point end notched flexure (4ENF) tests, were carried out to evaluate the effect of toughening by the interleaf veils. The mechanisms of the improvement by the interleaf veils were evaluated by microscopy. The adhesion between veil fibres and matrix is an important factor of the improvement of the interlaminar toughness. If veil fibres have poor adhesion to resin, these fibres would be pulled out from the matrix and work as fibre-bridging. In contrast, good adhesion of veil fibres is not necessary improvement of the interlaminar fracture toughness. Because these fibres are embedded in the matrix and interleaf veil cannot contribute to suppression of the crack propagation. The second stage of experiments was impact and compression after impact (CAI) tests. In this stage, the base materials were plain weave fabric only. Impact damage was evaluated using ultrasonic C-scan. The polyamide veils interleaved samples had superior impact and CAI resistance properties in all interleaved materials. In the final stage, correlation between each mechanical property was analysed and discussed. It was found that the relationship between each fracture toughness is affected by fabric and resin. Moreover, this work and previous literature data were compared. It can be found that the non-woven veils are effective toughening materials.