The use of fibre reinforced composite polymeric materials for primary helicopter structures has been increasing over the last decades due to their attractive properties; high performance and lightweight inducing more fuel savings than their metallic counterparts. With respect to the joining methods; structural adhesive bonding is preferred when cost and weight are important factors. The qualification of a bonded joint requires the determination of its durability and reliability of the bond strength, but due to the possible degradation of adhesives under certain environments, the lack of a failure criterion and in view of safety consideration, adhesive joints tends to be ‘overdesign’, resulting in an increase of weight and manufacturing costs.
The present project aims to evaluate and investigate the adhesive performance of carbon fibre reinforced PPS; a reinforced high performance thermoplastic qualified in Airbus and already used on certain structural parts of the helicopter. The surface topography and chemistry were evaluated using an optical microscope, XPS and contact angle measurements before and after two surface treatments; hand sanding and grinding. Adhesive joints were manufactured to analyse its single lap shear (SLS) strength and fracture energy, GIC using three different adhesives and two environmental conditions.
Both surface treatments increased the surface energy, but while sanded samples showed an increase of the dispersive component, APP increased to a large extent the polar part. In addition, after hand sanding the morphology of the surface was modified. For most of test configurations, the specimens failed at the interface, however plasma treated samples showed higher values of lap shear strength and fracture toughness energy.
An accelerated ageing condition was simulated by the storage of the samples in a climate chamber at high temperature and humidity. The effect that these environmental conditions had on the adhesion performance depended to a large extent on the adhesive used; in this case all of the adhesive studied showed different trends on the adhesion properties; the epoxy adhesive designed to withstand high temperatures did not present any effect on the adhesive performance but a common epoxy for structural application showed a softening of the adhesive and therefore a decrease on the shear strength but an increase of the fracture toughness and with respect to polyurethane adhesive, the interface was degraded changing the failure from mixed to adhesive mode.
Finally, CF-PEEK samples were tested in single lap shear after these two surface treatments, showing better results than PPS with the same working parameters. However, some configurations still failed at the interface.