Abstract

New generations of out-of-autoclave prepregs, also known as VBP (vacuum bag only) are achieving a high maturity level that would allow the composite manufacturing with autoclave quality. These materials are characterized by its capability to allow air evacuation and minimize the presence of defects before its complete polymerization. To do this, it is necessary to control the factors which govern the phenomena that occur within it, such as compression, resin flow and void growth processes. Additionally, it should be considered fibers and resins properties, and curing cycle parameters such as temperature, vacuum quality and duration. In this work we have studied comparatively conventional and OoA prepregs.To do this, several panels have been laminated using hand lay-up method and cured only using vacuum bag and without autoclave pressure. Then, the panels have been characterized by optic microscopy, ultrasonic inspection and mechanical standarized tests. The results show that out-of-autoclave prepregs are achieving similar qualities to the conventional prepregs. Having completed this stage, new research and developments should be focused on, among other issues,  guaranteeing manufacturing processes robustness and optimization of bigger structures with complex geometries.

Abstract

This study aim at understanding the void formation, transport mechanisms and porosity evolution during OoA consolidation process. The laminates were conformed by stacking and compacting of prepregs by two different technologies: manual (Hand lay-up, HLU) and automatic (Automated fiber placement, AFP). Subsequently the laminates were cured by the vacuum bag only (VBO) technique in an industrial oven at atmospheric pressure. The porosity characterization was performed using ultrasound and x-ray tomography technologies at different stages of the cure cycle. Ultrasonic inspections showed that the porosity is heterogeneously distributed, where the higher volumetric fraction of pores is located in the center of the laminate. X-ray tomography provided three-dimensional (3D) information of samples of the laminates and it allowed a quantitative and qualitative study of the pores within the tows of fibers and the porosity located between the layers of prepregs during the curing process. The prepregs designed for out of autoclave are partially impregnated in such a way to provide air evacuation channels in the fiber tows. The porosity of these channels was efficiently extracted and a volumetric fraction of pores entrapped between layers were evacuated during the curing process. The behavior of the evacuation channels and the porosity evolution for both stacking technologies were compared. An analytical model of the evolution of a pore was also developed.

Abstract

Simulation of composite structures through Finite Elements Method (FEM) is widely used in the aeronautics sector to reliably predict its performance. The incorporation of new materials with enhanced properties, and also new manufacturing processes usually require the use of such tools for validation.

This work has been focussed on studying an emerging technology such as Pultrusion for stiffeners manufacturing. For this, it has been used a demonstrator based on a common airframe structure, panels stiffened with profiles. This time, the classic stiffener manufactured using prepreg and autoclave cured has been replaced by stiffeners manufactured by Pultrusion. For that purpose, it was manufactured flat panels using prepegs and autoclave curing, for subsequently bond L shaped high resistance carbon fibre profiles manufactured by Pultrusion.

Furthermore, FEM simulations were conducted over these coupons for three different mechanical testing conditions, tensile, compression and shear, aiming to determine the capability of these methods for simulating reliably the mechanical performance of pultruded materials on such above-mentioned structures.

The results obtained in this work have been used for model parameter adjustments to obtain reasonable correlation levels, enabling the future usage of Pultrusion technology in the stiffener profiles manufacturing.

Abstract

During the last years, the benefits of using carbon reinforced thermoplastic composite materials (CRTP) have been questioned and defended (higher fatigue resistance, greater toughness, better durability, reprocesability, recyclability and so on), they are considered as a future innovative technology allowing  highly integrated structures, impacting in weight reduction and reducing energetic costs by using out of autoclave processes. Therefore, (CRTP) could be considered as a future substitute of metallic materials and composite materials with thermoset matrices for aerospace applications, automotive and other industries.

Under the frame of the OUTCOME project (Clean Sky II), the design and analysis of the Upper Skin (upper coverage of a stiffened wing) for the militar aircraft C295 is developed, a medium tactical transport aircraft of ADS.

The selected material for the work is the unidirectional composite material APC2/AS4, supplied by Solvay, with PEEK as the thermoplastic matrix and carbon fibre as the reinforcement. Flat panels, detailed elementals and subelementals have been manufactured, all of them have been widely characterized attempting to obtain the required design allowables. 

Moreover, with the goal of cost reduction, productivity increases and environmental impact control, the manufacturing selected processes have been: a)  Hot-forming in oven and/or self heated toolings for stringers manufacturing and b) Automatic lamination in one step for the co-consolidation (full-integration) of the skin with the stringers.

The main goal of this review is showing the different steps in the manufacturing of a detailed element for a later structural test. 

Abstract

The main objectives of this research work are to find new ways of reducing costs in the manufacture of composite materials, automate processes and improve energy efficiency by looking for alternatives to the use of autoclave, always meeting the quality requirements. For this, an out of autoclave curing process is studied through an innovative system of automatic and fast curing of composite parts for the aeronautic, automotive and construction sectors. The process allows the curing of prepregs of various materials and geometries. The process includes the provision of temperature by heating plate with electric resistances, pressure and vacuum adjustable through an air compressor, and cooling through circuit through mechanized plates. The main innovative feature of this system is the pressure supply with compressed air to the piece by means of a moldable and reusable silicone membrane, which allows to extend the field of study to complex elements saving the current limitation of hot plates presses, and without the need to place countermold. CFRP cured parts are manufactured in this new system, then their properties are compared with autoclave cured parts and similar properties are obtained, and auxiliary materials are considerably reduced. It can be concluded that this system improves energy efficiency, reduces costs and optimizes times.

Abstract

With the entry into service of aircrafts with a very high percentage of composite materials the necessity of efficient repairs for such structures arises. Airbus has developed an out of autoclave bonded repair process for highly loaded composite primary structures of the A350XWB. It can be used at production plants but also in the in-service operation of the aircraft. Bonded repairs eliminate the requirement of rivets in carbon fibre parts allowing flush repairs that complies with the original component functionality and requirements. The repair process includes removing the damaged plies by successive milling steps, each one equal to a single ply thickness. The repair technique consists on the application of a soft patch with specific repair materials, different from those involved in the manufacturing process and with the unique characteristic of curing at low temperature in out of autoclave conditions. The aforesaid facilitates application in plenty of circumstances and scenarios. This paper describes the coordinated investigation performed in order to provide a robust and sound out of autoclave repair system and process. The combination of feasibility trials in actual repair conditions and material characterization tests proved to be essential to understand the critical steps and to optimize the process performance.