Adhesion enhancement of PEI–PVDF interfaces in multifunctional bondlines with surface toughening based disbond arrest feature for robust and sensed composite joints

Structural adhesive bonds are a desirable joining method for lightweight composite structures, but they are currently not certifiable as the sole joining method for primary joints in aircraft structures. The multifunctional bondline approach, which includes a disbond arrest technology and strain monitoring, aims to solve this problem. The multifunctional bondline consists of, in addition to the adhesive bondline, a thin film Polyetherimide (PEI) based sensor and a Poly(vinylidene fluoride) (PVDF) layer toughening the fiber reinforced plastic surface. Previous studies have shown that adhesive failures in the PEI-PVDF interface impair the functionality of the toughening layer. This study, therefore, analyzes methods to increase the interfacial adhesion between PEI and PVDF to eliminate this weak point of the multifunctional bondline. Using physical vacuum ultraviolet (VUV), Corona, and mechanical Peel Ply methods, the adhesion enhancement is analyzed using Double Cantilever Beam and End Notched Flexure tests to determine critical energy release rate (cERR) under Mode I and II. Under Mode II, for Corona and VUV pretreatments, only the standard deviation of cERR could be reduced. However, under Mode I, the cERR value could be increased by 80 % with VUV pretreatment. Results of Energy Dispersive X-ray Spectroscopy (EDX) analysis suggest that the crack path is transferred from the PEI-PVDF interface to the PVDF film because fluoride elements were detected on the specimen half covered with PEI. This shows that the adhesive failure observed in previous studies was caused by a weak PEI-PVDF interface resulting from insufficient surface pretreatment. Peel Ply structured PEI surface decreases interfacial adhesion under Modes I and II. The significantly improved interfacial strength resulting from VUV pretreatment can pave the way for a robust multifunctional bondline. Avoiding the PEI-PVDF interface from being the weakest link in the structural bond enables the PVDF disbond-arrest function and the PEI sensor function to be retained, allowing the full potential of the base materials to be utilized.