Damage assessment, toughness modification and potential repair techniques of variable stiffness fibre-reinforced thermoplastic composite laminates

Advancements in composite processing techniques, such as In-Situ Consolidation (ISC) for fibre-reinforced thermoplastics by Laser-Assisted Automatic Tape Placement (LATP), are enabling faster more efficient processing of advanced composite structures. In addition, automated tape laying systems allow steering of reinforcing fibres to produce variable stiffness laminates for enhanced structural performance,e.g., improved buckling, aeroelasticity, as well as allowing fibre steering around cut-outs to reduce stress concentrations. However, concerns about the vulnerability and repair of variable stiffness laminates is hindering their adoption. This thesis presents experimental work that investigated damage, toughness modifications, and potential repair techniques for conventional and variable stiffness Carbon Fibre-Reinforced Polyether Ether Ketone (CF/PEEK) laminates manufactured using LATP ISC.

Damage of LATP ISC and autoclave consolidated CF/PEEK laminates was investigated using Non-Destructive Inspection (NDI) and destructive testing techniques. NDI, in the form of C-scans and dye-penetrant X-ray imaging, indicated a 36% larger internal damage area in impacted LATP ISC laminates compared to autoclave consolidated straight-fibre laminates. Compression After Impact (CAI) testing showed equivalent levels of strength retention, albeit the LATP ISC laminates had a 50% lowerinitial level of undamaged compressive strength. Variable stiffness laminates showed more extensive internal damage due to the laminates more compliant centre. However, variable stiffness laminates still exhibited higher loads prior to the onset of buckling in both damaged and undamaged states.

Inspired by the fact that lower crystalline polymers have been shown to better sustain impact events, a unique approach to impact damage toughening was developed that involves interspersing amorphous Polyether Imide (PEI) film into a CF/PEEK laminate stacking sequence. This toughening technique produced laminates that were better able to withstand impact damage, as evidenced by the lower levels of internal damage observed, as well as the ability up to retain up to 47% more of its undamaged compressive strength at low impact energy levels.

Finally, ultrasonic welding was investigated as a potential repair method for CF/PEEK laminates. Coupon level specimens were welded in both a standard single lap joint and a novel scarf joint configuration, their bond strength was assessed using tensile testing. Results indicated that this was a potentially viable joining method for patch repairs. However further work is required to improve the welding process and fixture design to achieve more consistent weld coverage, especially when utilising the scarf joint configuration.