Microbond testing of the interface in glass fibre vinylester composites

The microbond test was used to investigate the interface strength (IFSS) in various glass fibre-vinylester (VE) composite systems and a number of issues related to the sample preparation were identified. Cure schedules which produce well reacted VE polymers on the macroscale did not result in cured microdroplets. Hence the microbond test could not be carried out on samples with the same cure history as macroscale composites. Testable microdroplet samples could only be obtained when resin cure was carried out under an inert atmosphere. Higher IFSS values were obtained by raising the final temperature of the cure schedule. Glass fibres with a full sizing gave significantly higher apparent IFSS values compared to bare fibres or fibre coated with only silane coupling agent. It was discovered that the measured IFSS of VE compatible glass fibres was approximately doubled when fibres were mounted using epoxy glue instead of cyanoacrylate glue. This phenomenon appears to be related to the deposition of cyanoacrylate vapours onto the surface of the fibres during sample preparation. It is concluded that great care must be taken in ensuring that effects observed using the microbond test are evidence of real material characteristics and not artefacts of sample preparation.