Adhesively bonded repair of fibre reinforced composites : the effect of substrate moisture sorption characteristics

• Main Author: Bond, David Anthony
• Published: University of Surrey 1996
• Subjects: 620.118; Composites
• Online Access: http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.320910

A detailed review of moisture sorption kinetics and equilibria within polymers and fibre reinforced composites (FRC) provides an indication of the mechanisms which may affect moisture transport in FRCs. Three reviewed models are investigated for their applicability to the sorption behaviour of a FRC. The first considers only classical Fickian diffusive transport, while the other two include additional parameters to account for dual sorption phenomena or matrix relaxation respectively. An edge diffusion correction technique is developed to correct experimental 3D sorption data to equivalent 1D data for ease of modelling. The sorption data used to assess the suitability of the models is used also to examine the effects of lay-up, environmental conditions, multiple sorption cycling and long term exposure on the sorption behaviour of the a FRC. Each model is numerically assessed for its ability to represent the experimental sorption data. The more complex models provide good representation of the slightly anomalous FRC absorption data while the Fickian model is acceptable for desorption. The effect of absorbed moisture on the mechanical performance of the FRC is assessed and found to be especially detrimental to the matrix dominated properties. The glass transition temperature of the FRC is also found to be reduced by absorbed moisture. The effects of post-join (atmospheric) and pre-join (substrate) moisture on the durability and strength of bonded joints with FRC substrates are assessed using a scarf joint. One months exposure to high environmental humidites after bonding is found to reduce the strength of the joints significantly. A similar loss in performance is also observed if the FRC has a high substrate pre-join moisture content. Pre-drying of moist substrates is found to be beneficial in regaining the performance of the joint. Joint strength is found to be directly related to the quantity of moisture desorbed from the substrates during joint/adhesive cure. A model for the drying process is subsequently developed and found capable of estimating the ideal drying conditions for a given substrate moisture content.