Thermoplastic composite stiffened structures in post-buckling

The aeronautical industry has set ambitious goals to reduce its environmental impact and become sustainable. Addressing rising emissions involves strategies such as reducing structural weight, due to its direct impact on fuel consumption, and the use of new materials and manufacturing techniques. Research and development on thermoplastic composites has seen an uprise due to their mechanical properties and sustainability benefits. It enables costeffective, innovative manufacturing techniques, less manufacturing waste and recyclability. This has led to the launch of several projects utilising thermoplastics, such as TAPAS 1 and 2 in The Netherlands, and the Clean Sky 2 project STUNNING. The TAPAS projects focussed on co-consolidated structures with the butt-joint technique, while STUNNING developed and manufactured a lower half of the multifunctional fuselage demonstrator, one of the world’s largest thermoplastic structures.This thesis investigates the post-buckling and skin-stringer separation behaviour of thermoplastic composite structures, with a combined experimental and numerical methodology. Allowing structures to buckle below the ultimate load can lead to considerable weight savings, however, the failure of composite structures in post-buckling is complex and usually catastrophic. This research intends to close the knowledge gap on thermoplastic composites in post-buckling, contributing to the goal of sustainable structures allowed to operate in post-buckling....Group De Breuke

Design, analysis and testing of thermoplastic welded stiffened panels to investigate skin-stringer separation in post-buckling

Thermoplastic composite three-stringer panels with omega stiffeners and conduction welded joints are designed, analysed and tested until final failure to investigate the performance of the welded joint in post-buckling. The three-stringer panels are designed to be structurally representative of the fuselage demonstrator of the Clean Sky 2 project STUNNING. A simplified model of the fuselage keel section is analysed by finite element analysis, using the virtual crack closure technique to model skin-stringer separation of the welded joint. The post-buckling and skin-stringer separation behaviour of the fuselage section is then adopted as the reference for the design of the three-stringer panels. Two panels are then tested. The test setup utilises digital image correlation to measure the deformation of the panels, and a high-speed camera to capture the final failure mode. The panels failed in post-buckling due to the separation of the middle stringer, with unstable separation growth followed by separation of the outer stringers and then stringer fracture. The numerical analysis of the panels, with geometrical imperfections included, is able to predict the structural behaviour accurately, with only minor differences in buckling shape and separation behaviour.Aerospace Structures & Computational Mechanic

Design and analysis of thermoplastic welded stiffened panels in post-buckling

The Clean Sky 2 “SmarT mUlti-fuNctioNal and INtegrated TP fuselaGe" STUNNING project focusses on the next generation composite fuselage with emphasis on manufacturing techniques such as thermoplastic welding. Welded multi-stringer panels are investigated in this paper, with emphasis on the buckling and skin-stringer separation behavior. The multi-stringer panels are designed to approximate the structural behavior of the lower half of the MultiFunctional Fuselage Demonstrator of the STUNNING project. In particular, a section of the fuselage is analyzed using Abaqus with a dynamic implicit analysis, and the results of this analysis are used for the design of the multi-stringer panels, taking manufacturing considerations also into account. The panels have three omega stringers and a length of 500 mm. The three stringer configuration allows to study the middle stringer in pristine and damaged configuration with minimal influence of the free edges and boundary conditions. It is seen that the multi-stringer test panels show very similar buckling and skin-stringer separation behavior compared to the fuselage section. The first panels have been manufactured by project partners NLR and GKN Aerospace Fokker and will be tested at the Faculty of Aerospace Engineering at Delft University of Technology.Aerospace Structures & Computational Mechanic

Development of a Numerical Framework for Virtual Testing to Support Design of a Next Generation Thermoplastic Multifunctional Fuselage

This work summarizes the recent developments of a numerical framework to predict the mechanical behaviour of thermoplastic composites. It supports the design of a next generation thermoplastic multi-functional fuselage which uses advanced joining techniques such as thermoplastic welding to reduce both weight and cost by limiting the amount of mechanical fasteners required. At the lower end of the testing pyramid the framework is able to accurately predict typical preliminary design allowables such as laminate, open-hole and welded joints strength through a high-fidelity modelling approach. This information is then passed on to the structural level in a validated building-block approach to efficiently virtual test the compression strength of fuselage panels during post-buckling while also taking into account the influence of damages at the skin-stiffener interface.Aerospace Structures & Computational Mechanic

Skin-stringer separation in post-buckling of butt-joint stiffened thermoplastic composite panels

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.Aerospace Structures & Computational Mechanic

Analysis and testing of a thermoplastic composite stiffened panel under compression

This paper presents the numerical analysis of a thermoplastic composite stiffened panel subjected to compression load. The panel has three stringers with a non-symmetric design, with an artificial crack at the middle stringer interface and is made from a fast crystallizing polyetherketoneketone carbon composite. The finite element model includes an approximation of the geometrical imperfections which were measured using a digital image correlation system. The finite element analyses are discussed, where the crack propagation is modelled using the virtual crack closure technique. The results show that crack propagation starts rather early after buckling and the crack growth behaviour is heavily influenced by the buckling shape, which consists of three half-waves in longitudinal direction in each bay.Aerospace Structures & Computational Mechanic