Does a polymer film due to Rayleigh-instability affect interfacial properties measured by microbond test?

The microbond (MB) test, which is primarily used to characterise the interface of fibrous composites, requires a large number of droplets to be tested and analysed in order to make a reliable conclusion about the fibre–droplet interface. The conventional method of depositing single droplets on fibre and performing the MB test can be improved by depositing multiple droplets using the Rayleigh plateau instability phenomenon (an additional film is formed between the droplets). Although the latter method has significant advantages and higher statistical reliability, the role of the additional film affecting MB test results has not been investigated. In this work, both methods are experimentally evaluated for glass and flax fibres with two different resin systems and the interfacial constants, namely critical stress for damage initiation and critical energy release rate, are validated by finite element (FE) models. The study reveals that the thickness of the additional film shows incorrect interfacial shear strength (IFSS) when determined from simple force-displacement data ((Formula presented.) 18% increase for the fibre-droplet system in this study). The FE models confirm that the damage onset at the interface occurs at a higher force with this method, but the interfacial strength constants remain the same as with the conventional method.