FORMING OF THERMOPLASTIC POLYMER-BASED FIBER METAL LAMINATES AT ELEVATED TEMPERATURE

Fibre metal laminates (FMLs) as a hybrid laminated material system, made up of thin metal sheets laminated with fibre-reinforced polymer layers (FRPs), can take advantage of both material types. The material inhomogeneity also brings challenges to their in-service performance and uniform deformation during the forming process. Different kinds of pretreatment were performed on the metal surface before the fabrication of FMLs, and their following effect on the mechanical performance and delamination behavior was investigated. To assess the effect of the interfacial bonding strength on the FML compressive performance, quasi-static buckling tests were performed at varying surface treatments of the magnesium alloy sheets. It revealed that delamination can significantly reduce the buckling capability and structural stability and that the improvement of interfacial bonding strength can dramatically strengthen the FML compressive capability. To further analyze how the change in metal surface texture affects the delamination behavior for improving the metal-prepreg interfacial bonding strength, the magnesium alloy sheet surfaces were prepared through six different treatments. It was found that the bonding strength increased at increasing roughness of the metal sheets and that annealing could improve the bonding strength. To get a better understanding of the mechanisms involved in the thermoforming process of FMLs, the mechanical and tribological characteristics of the FMLs were studied. The compaction characterization of prepregs under a range of parameters typical of the thermoforming process applied to the FMLs was investigated by conducting compaction tests making use of a plate-to-plate mode testing setup. The through-thickness and transverse width of the prepregs were evaluated on the FML specimen cross-section at varying compaction force and temperature. Significant deformations were observed at lower compaction force above the prepreg polymer melting point, whereas a further increase in the compaction force led to gradually smaller through-thickness and in-plane deformations. A higher decrease in thickness and increase in width of the prepregs were detected at higher temperatures. Moreover, the metal/prepreg inter-ply friction behavior of the FMLs under a range of parameters (normal pressure, relative sliding velocity, temperature) was studied. The surface roughness of metal sheets was also varied and its influence on the inter-ply friction behaviour was evaluated. Pull-through tests with a stop-start controlling strategy were designed and carried out for identifying the inter-ply friction coefficient at varying conditions. The obtained results proved that the differences in the friction coefficient are strongly dependent on the process conditions and the metal surface roughness. The increase in the relative sliding displacement enables the transition from the hydrodynamic to the mixed lubrication mode according to the Stribeck theory, which further rises the friction coefficient. The inter-ply friction coefficient is observed to increase at decreasing temperature and normal pressure, and at increasing sliding velocity and metal surface roughness. On the basis of the experimental research on the mechanical characteristics and tribology of the FMLs during the thermoforming process, each constitute of the FMLs was modeled in Ls-Dyna to simulate the production process carried on the hat-shaped FMLs at elevated temperatures for assessing the feasibility of the modeling method to predict the forming results. The combination of 3D shell and membrane elements was developed to represent both out-plain compaction and in-plain behaviors of the prepreg. The modeling method was validated by comparing the thickness distribution of the formed FML component and the forming force between the experimental tests and numerical simulations.