title: Quasi-static and dynamic behaviour of composite structures based on glass fibre reinforced PEKK
Creators: name: Almtteri, Nassier
Publisher: name: University of Liverpool Repository
Year: 2024

description: ABSTRACT The aim of this thesis is to investigate the quasi-static and impact response of the newly developed composite material and the related fibre metal laminates (FMLs) that have potential to satisfy the requirements of the new generation of aircrafts such as the damage tolerance and high service temperatures. Initially, this thesis presents the findings of a research study to develop high temperature thermoplastic composites. Here, woven S-glass fibre (GF) reinforced poly-ether-ketone-ketone (PEKK) thermoplastic prepreg materials are manufactured using a dry powder prepregging method. Prior to impact testing and modelling, the properties of the composites are evaluated by conducting a series of quasi-static tests, including tension, bending and in-plane shear, at room and elevated temperatures. Quasi-static mechanical testing has shown that the optimum weight fraction of PEKK, wf, is approximately 0.4, with the properties remaining constant or dropping slightly at higher values of wf in comparison to the optimum one. The high temperature tensile tests of GF/PEKK composites have shown that there is no obvious reduction of its tensile strength under heating up to 100 oC. The perforation resistance of the target is also peaked at wf = 0.4. As expected, the energy required to perforate the targets increases with indenter diameter. Subsequent tests show that panels impacted with partially flat indenter exhibit the highest perforation resistance among the other projectiles shapes investigated. The perforation response of the titanium- and aluminium-based FMLs, with various stacking configurations, are also investigated under quasi-static and dynamic loading rates. Initial attention is focused on assessing the effect of the laser surface treatment on the residual strength of the titanium alloy and its bonding strength with the adhesive film. The tests show that the laser parameter of 4.54 J/cm2 seems an optimum parameter which gives an excellent bonding strength between the titanium foils and the PEKK film. In contrast, these treatments do not show any significant drop in the residual tensile strength of the titanium alloy. However, there is a reduction of around 35% in both tensile strength and yield strength of the aluminium alloy produced due to the processing temperature cycle. A comparison of the quasi-static and dynamic perforation responses of the FMLs have demonstrated the rate-sensitivity of these laminates at which the perforation energy increases when the loading rates pass from the quasi-static to dynamic. After testing, the FMLs specimens are sectioned to highlight the failure modes under both test conditions. The cross-sections indicate that the impact energy on these FMLs is absorbed through (1) plastic deformation and tearing of the metals, (2) delamination between the composite plies and metal layers and (3) fibre fracture. The finite element models using ABAQUS/Explicit have been developed to predict the response of the glass fibre reinforced PEKK composites to impact by projectiles based on different diameters and shapes. The outputs of these FE models are validated against the corresponding experimental force-displacement traces and failure modes. A good agreement between the predicted and measured results, in terms of the initial stiffness, maximum force and displacement and perforation mechanisms, is obtained. The validated FE models are then used to predict the perforation resistance of the fibre metal laminates with various stacking configurations under low velocity impact loading. Here, prior to the onset of the damage, the fibre reinforced composites is modelled as an orthotropic elastic material with 2D Hashin’s failure criteria. The titanium and aluminium alloys are modelled as isotropic elasto-plastic materials with strain hardening. Ductile and shear damage criteria are used to model the damage initiation of the metal layers. Again, the experimental results, including the load-displacement traces, perforation energies and failure modes are successfully predicted using the FE models developed. Furthermore, the validated models have been exploited to predict the perforation response of the FMLs under different loading conditions.