Enhancement of the bonding strength of adhesively-bonded composite joints at cryogenic environment by coiled carbon nanotubes

Abstract

In recent decades, fiber reinforced polymer composites have been widely used in the aerospace engineering industry to replace metallic structures. Mechanical bonding and adhesive bonding are the two main bonding methods for composite plates. The newly-developed adhesive bonding method is suggested in the current study with epoxy as the base adhesive, since this method is more beneficial to aerospace structures. One of the main challenges of applying FRP composites in aerospace structures is that they have to service in cryogenic temperature (CT) environment. Previous experiments showed that some mechanical properties of epoxy degrade at CT. It is not strong and reliable enough to be an aerospace adhesive so it is vital to improve its mechanical properties. In the current study, coiled carbon nanotubes (CCNTs) are proposed to be the nano-filler for epoxy adhesive in order to enhance its mechanical properties at CT. CCNTs, due to their special helical configuration, will not be pulled out from the epoxy matrix easily, unlike straight multi-walled carbon nanotubes (MWNTs). Mechanical tests were performed for pure epoxy, MWNT/epoxy and CCNT/epoxy adhesives at both room temperature and CT for comparison. Results show that carbon nanotube (CNT)/epoxy adhesives are stiffer than pure epoxy at CT. Since epoxy contracts at a greater extent than CNT, it eventually induces a cryogenic clamping force onto CNT which enhances the stress transfer between matrix and reinforcement. This phenomenon is also proven by finite element analysis with simulation. In the mechanical tests, the performance of CCNT/epoxy adhesive at CT is outstanding. They show the greatest Vicker's hardness value, tensile modulus and lap joint shear strength. Due to the helical configuration of CCNTs, the cryogenic clamping force is more effective in CCNT/epoxy than in MWNT/epoxy adhesive as the coils can interlock mechanically with the matrix. Hence, an excellent interfacial bonding is achieved, leading to a very effective stress transfer from epoxy to CCNTs. Two theoretical analyses are performed in this project. The first one is on the fiber pullout behavior of a CNT from a polymer matrix. A numerical model is developed to evaluate the effect of CT environment and other parameters to the stress distribution and stress transfer efficiency in CNT/polymer composites. The model also demonstrates that at CT, a greater stress is required to pull out a CCNT than a straight CNT, especially when the pitch angle of CCNT is less than 60°. Therefore, the stress transfer in CCNT/polymer composites is better than that in straight CNT/polymer composites. The second theoretical analysis is on the shear and peel stress distribution in adhesively-bonded single lap joints loaded in tension. A numerical model is applied to investigate the effects of CT environment, different kinds of adhesives and other geometric parameters, such as adhesive thickness and overlap length, to these stresses in adhesives. Owing to its outstanding performance, CCNT/epoxy nanocomposite possess a very high potential to be next-generation adhesive for aerospace structures. The investigations reported in this thesis build a good foundation for the adhesive to be further developed for commercial use.