Abstract

Carbon fibre reinforced polymers are commonly used in the primary and secondary structures of aircraft, mainly due to their excellent specific mechanical properties. Currently, there is a growing interest in the use of carbon fibre and thermoplastic resin composite materials in aeronautical applications, since they offer multiple advantages compared to thermosetting equivalents.

This study presents the results of the mechanical tests carried out at the coupon and element levels for the characterization of the properties of a thermoplastic composite that will form a fuselage panel. In addition to standardized tests, tests are carried out to evaluate the behaviour of critical areas of the panel, such as the radius of the frames, the stringers, and the joint between the frame and the skin around the stringers (mouse-hole). In these latest tests, experimental procedures are developed for their execution, including the design of the necessary tools to reproduce the representative boundary conditions. As a result of this research, the allowables have been obtained for the design of the curved thermoplastic fuselage panel.

This work is developed as part of the European project DELTA, within the framework of the Clean Sky 2 JU program. The objective of this project is to develop and execute innovative experimental test procedures that allow the validation of thermoplastic composite aircraft fuselage panels, in order to apply these solutions in medium and long-range aircraft (LPA). The ultimate goal of this project will be achieved through the execution of a curved thermoplastic fuselage panel test under representative load conditions.

Abstract

Future aircraft manufacturing factories are moving towards more flexible and adaptable manufacturing systems that enable shorter manufacturing cycles, environmental friendliness, energy efficiency, and higher productivity. A global aeronautic trend to face this challenge includes the replacement of metal parts with composite counterparts with emphasis on thermoplastic composites (TPCs) for their unique ability to be recycled, reprocessed, and welded. Aiming to commit to the drastic projected increase in aircraft production rates, WELDER project, focused on the next-generation MultiFunctional Fuselage Demonstrator (MFFD), has promoted the development and investigation of highly integrated and robotized manufacturing technologies that needed to be scaled up from a laboratory to an industrial environment. To contribute to such an advance, resistance welding was investigated in this work and successfully automatized for fuselage components assembly.

Welding investigation, including process maturation, parameter optimization, and scale-up analysis was performed at laboratory using LM-PAEK/CF composites. A resistance welding Heating Element (HE), made of stainless steel metal mesh and embedded in glass fiber scrims, was first designed and manufactured using a hot press. The HE, priorly characterized by electrical analysis, was adopted, in conjunction with a lab-scale welding head, to optimize and define appropriate processing times (welding and cooling), welding power, and pressures for the welding of the composite parts. The welds were characterized by visual inspection, microscopy analysis, and mechanical tests (SLSS tests - AITM 1-0019). Once materials and processes were optimized, they were scaled up to finally perform the welding of the frame couplings into MFFD.

Abstract

Due to the huge interest of the aeronautic industry for finding new solutions in terms of processes and materials to improve performances and to reduce costs, thermoplastic composite materials appeared as an attractive solution. The high temperatures required for processing this kind of materials and the lack of knowledge in the associated manufacturing processes make their application to structural parts more complex compared to that of thermoset composite materials. The study presented here contains the design and manufacturing of a curved section of a fuselage reinforced with omega stringers and ‘Z’ curved frames. The requirements for designing were taken by considering a preliminary flat demo which was manufactured and tested. Following the manufacturing activities conducted with the flat demo, the stiffeners were obtained by hand lay-up and hot press forming, afterwards they were installed in a tooling with specific cavities and the skin was laminated on top. The lamination of the skin was conducted by using a gantry style machine from the Spanish supplier MTorres. The adhesion of the skin to the stiffeners was possible by increasing the temperature to melting point by using a diode laser. After laminating the skin, the whole demo was demolded and inspected by non-destructive testing, showing a good quality in terms of the skin consolidation and in the welding line with the stiffeners. This work was developed as part of LPA project (Large Passenger Aircraft) in the framework of the Clean Sky 2 JU program.