Two curved thermoplastic composite multi-stringer panels with roller boundary conditions are analysed and tested to investigate the buckling and failure behaviour. The panels are made of AS4D/PEKK-FC thermoplastic composite, have five stringers with an angled cap on the side and are joined to the skin with the short-fibre reinforced butt-joint technique. The panels have a roller attached to each loading edge, approximating simply-supported boundary conditions to apply compression and bending. One panel has an initial damage representing a barely visible impact damage in one of the stringer butt-joints, and one panel is in pristine condition. Finite element analyses are performed to predict the structural behaviour, and different approximations of the roller boundary conditions are compared. The analyses include material damage initiation and evolution. The out-of-plane displacement of the panels is measured by digital image correlation, and failure is captured with high-speed cameras. The panels fail in a sudden manner when the cap separates from the web, followed by web failure and skin–stringer separation in the butt-joint. The numerical analysis predicts the overall structural behaviour but cannot capture well the sudden panel collapse due to material damage.

Two aeronautical thermoplastic composite stiffened panels are analysed and tested to investigate the buckling behaviour, the skin-stringer separation and the final failure mode. The panels are made of fast crystallising polyetherketoneketone carbon composite, have three stringers with an angled cap on one side, and are joined to the skin by a short-fibre reinforced butt-joint. The panels contain an initial damage in the middle skin-stringer interface representing barely visible impact damage. Finite element analysis using the virtual crack closure technique are conducted before the test to predict the structural behaviour. During the tests, the deformation of the panels is measured by digital image correlation, the damage propagation is recorded by GoPro cameras and the final failure is captured by high speed cameras. The panels show an initial three half-wave buckling shape in each bay, with damage propagation starting shortly after buckling. A combination of relatively stable and unstable damage propagation is observed until final failure, when the middle stringer separates completely and the panels fail in an unstable manner. The test results are compared to the numerical prediction, which shows great agreement for both the buckling and failure behaviour.

The capability of joining two thermoplastic composite parts by welding is a key technology to reduce the weight and cost of assembled parts and enables high volume manufacturing of future aeronautical structures made of thermoplastic composite materials. However, there is not much experimental understanding of the mechanisms involving welded joint failure, and the computational tools available for the simulation of thermoset composites have not yet been completely assessed for thermoplastic materials. In this work, a numerical and experimental evaluation is performed to investigate the strength and failure behavior of conduction welded thermoplastic composite joints. A welded single lap shear joint is designed, manufactured, tested and analyzed proposing two distinct modeling approaches. A simplified modeling strategy which only accounts for damage at the weld is compared to a high-fidelity model which can take into account the physical failure mechanisms at the lamina level. The high-fidelity modeling methodology is able to predict the experimental failure mode of the investigated welded joints with high accuracy and is used to gain new insights into the key-variables that influence the strength of thermoplastic welded joints. It is also found that the joint strength is highly influenced by the failure mechanisms not only of the welded interface but also of the surrounding plies.