Abstract

Carbon fibre reinforced polymers are widely used in primary and secondary aircraft structures, mainly due to their excellent fatigue endurance and specific strength. Nowadays, improvements related to their manufacturing costs, impact resistance and fracture toughness are pursued through the investigation of new matrix-fibre combinations and the application of cutting-edge manufacturing technologies. This work presents the results of the coupons test campaign that has been conducted to assess the mechanical properties of different advanced carbon fibre reinforced polymers, being compared with those obtained for a composite material traditionally used in aero-structures. The effect of the introduction of innovative elements such as reinforcing-matrix Carbon Nanotubes or acoustic damping veil is evaluated. Moreover, the influence of ageing is assessed through the evaluation of the mechanical properties after specimen environmental conditioning. From the execution of this test campaign, a comprehensive knowledge base of mechanical properties has been generated, enabling the comparative analysis with reference composite and down selection of the materials that will be applied in the regional aircraft fuselage concept developed in Clean Sky 2 JU. The study shown here belongs to the scope of SHERLOC project. The purpose of the project is to perform a down selection of the most advanced composite materials, manufacturing processes and Structural Health Monitoring (SHM) systems that contribute moving towards a Condition-Based Maintenance.

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

This work focuses on analyzing the uniaxial response of carbon fiber reinforced polymer matrix laminates with the configurations [±45]_2S under tension and [±45]_4S under compression. These present non-linear stress-strain evolutions that allow them to withstand large deformations before losing their load-carrying capacity. Both responses are characterized by a first linear stage, followed by a plateau in which the strain grows without increasing the stress level, and finally by a re-stiffening phase. But there are quantitative differences that lead to different failure patterns, which are explained with the help of the state of stress in the plane of maximum shear stress. For a better understanding of the process, tensile load-unload-reload tests are performed to verify if the energy recovered in each cycle could be related to the loss of the apparent stiffness. During the first two stages of the mechanical response, the laminate suffers a progressive damage with a reduction in stiffness related to the dissipated energy, but this pattern is not repeated in the last stage of strain hardening. Based on the experience of other authors, it is assumed that the re-stiffening stage follows a different pattern due to possible microstructural changes in the matrix during the plateau. These are promoted by a narrowing process under tensile loads and a local widening of the test zone under compression. The dimensional changes perpendicular to the load direction are observed thanks to the strain fields obtained by Digital Image Correlation.

Abstract

Continuous Fibre Reinforced Thermoplastic Composites (CFRTPCs) are becoming alternative materials to replace the conventional thermosetting polymers and metals due to excellent mechanical performance, recycling and potential used in lightweight structures. Fused deposition modelling (FDM) is a promising additive manufacturing technology and an alternative of conventional processes for the fabrication of CFRTPCs due to its ability to build functional parts having complex geometries. The mechanical properties of a built part depend on several process parameters. The aim of this study is to characterize the effect of build orientation, layer thickness and fibre volume content on the mechanical performance of 3D printed continuous fibre reinforced composites components manufactured by a desktop 3D printer. Tensile and three-point bending tests are carried out to determine the mechanical response of the printed specimens. SEM images of fractured surfaces are evaluated to determine the effects of process parameters on failure modes.

It is observed that the effect of layer thickness of nylon samples on the mechanical performance is marginally significant. In addition, continuous fibre reinforced samples show higher strength and stiffness values than unreinforced ones. The results show that carbon fibre reinforced composites exhibit the best mechanical performance with higher stiffness and flat samples exhibit higher values of strength and stiffness than on-edge samples. Additionally, the results show that strength and stiffness increase as fibre volume content increases in most cases but, conversely, the level of increment in mechanical performance is moderate with continued rise in fibre content, particularly in the case of Kevlar® and glass fibres, due to weak bonding between the fibre/nylon layers as well as the presence of increased levels of defects.