Abstract
The combination of fiber-reinforced polymer composites and lightweight alloys has emerged as a lightweight solution for the transportation sector, mainly due to the optimal specific strength and stiffness associated with these materials. The possibility of joining metals and composites is an important topic for the cost-effectiveness of hybrid structures in mass production. Friction Spot Joining (FSpJ) is an alternative solid-state joining technique for hybrid structures. This technology has demonstrated its potential as a joining solution for metal-composite structures by attaining high mechanical and durability performances in previous investigations. Nevertheless, the industrial transferability of such new technology requires further assessment regarding the mechanical integrity and corrosion behavior of the joints.
Therefore, this thesis is dedicated to understand the damage evolution at the interface of AA2024-T3/CF-PPS friction spot joints. For this purpose, finite element modelling was applied and the bonding zones of the joints were discretized using the traction-separation law. It was demonstrated that the damage in friction spot joints initiates at the AZ (adhesion zone) and then propagates as a symmetric linear front from the edges towards the center of the joined area. Nevertheless, as the damage advances inside the PDZ (plastically deformed zone), its propagation became an asymmetrical linear front that evolves preferably from the free edge of the composite part due to the higher peeling stresses in this region (asymmetrical secondary bending of the structure took place due to differential stiffness of materials). Based on the findings of this study, modifications were proposed to the failure theory previously stated for friction spot joints.
In addition, the fatigue damage tolerance of the joints was evaluated under mixed-mode I/II loading. The AZ presented low crack growth resistance (GI/II = 0.85 ± 0.01 J m-2), while the PDZ demonstrated to be the most damage tolerant zone of the joints (GI/II max = 274 ± 1 J m-2). The fatigue crack growth of the friction spot joints was dictated by the bonding zones and occurred in three well-defined stages: initiation, linear region, and unstable crack growth. Steady crack growth rates were found for AZ (0.10 ± 0.03 mm/cycle) and PDZ (0.006 ± 0.001 mm/cycle). This shows that the main bonding zones of these joints have defined properties. Thereby, the mechanical behavior of the joints can be tailored by their zones. Moreover, the friction spot joints generally present inferior and more stable fatigue crack growth rates when compared to adhesive bonded joints.
The impact resistance of the joints was investigated using the drop weight test. Four levels of impact energy were tested: 2 J, 4 J, 6 J, and 8 J. The joints were aluminum-side and composite-side impacted to provide a preliminary design guideline regarding the impact damage tolerance of such hybrid joints. This study showed that a friction spot joint absorbs up to 103 kJ m-2 of the joined area, while the literature reports energy absorption up to 48 kJ m-2 for bonded joints. Shear after impact (ShAI) test was employed to evaluate the residual strength of the joints. The impact energy introduced from the aluminum-side was mostly absorbed into the global plastic deformation of the aluminum part, thereby promoting the detachment of the joint interface. Otherwise, the impact energy introduced from the composite-side was mostly absorbed into the creation/extension of internal damage through the plies of the composite. Thus, the impact energy was only partially transferred to the interface of the joint in case of composite-side impact. Consequently, these joints presented higher residual strength than the aluminum-side impacted joints.
Further, the corrosion behavior of the joints was investigated during six weeks of salt spray exposure. The process-related changes in the microstructure, precipitation state, and local mechanical performance of the aluminum part were investigated and correlated with the corrosion development on the top surface of the joints. Regarding the corrosion at the interface of the joints, four stages were identified and correlated with the global strength degradation of the joints: I – Water and NaCl migration and the consequent plasticization of the composite (-24% of ULSF); II – corrosion protection of the PDZ by the polymer layer in the AZ (-28% of ULSF); III – detachment of the polymer layer in the AZ and corrosion inside the PDZ (-44% of ULSF); and IV – generalized corrosion in the PDZ causing the final strength degradation of the joints.
Finally, as a first step for the upscaling of the FSpJ technology, a fuselage sub-component was constructed using FSpJ in combination with other friction-based technologies. A reduction of 20% in weight was reached in comparison with the full-metallic and bolted design, thereby successfully demonstrating the potential of FSpJ as a joining solution for hybrid aircraft structures in the future.
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