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While the leading edge of a lifting surface is a secondary structure, it is an essential component because it has to resist bird strike. A structural solution in carbon-fibre composite can provide the required resistance and a high level of functional integration as well. Such a solution is undoubtedly a very competitive option for new aircraft developments, as these combined advantages produce lighter elements at lower industrial cost. The joint R&D effort of EADS-CASA and IDEC (ALCOR Group) has culminated in a new leading edge concept that complies with the previously mentioned requirements. Resin Transfer Moulding (RTM) is the technology that makes this achievement possible.
(Published on April 2006 – JEC Magazine #24)
BY LUIS RUBIO, EADS-CASA., CÁDIZ, SPAIN
JOSÉ LUIS LEÓN, IDEC, ÁLAVA, SPAIN
Structural components in carbon-fibre composite are being used more and more frequently, due to their lower weight and higher specific strength compared to metal alloys. Their superior fatigue strength and corrosion resistance are significant advantages. However, carbon fibre also exhibits brittle behaviour due to the absence of plastic deformation, unlike metal, which makes its selection more problematic for the manufacture of components fulfilling impact requirements.
The leading edges of lifting surfaces are secondary structures, meaning that they are not subject to loads from ground, pressurisation or flight forces. They have only to withstand the aerodynamic pressure over their wet surface and the deformations induced by adjacent primary structures (spars and skins). This type of structure is usually manufactured in aluminium alloy, because of the good impact behaviour provided by high energy absorption through deformation. Here, however, the present development project was motivated by a new concept for an integrated leading edge in carbon fibre and specific goals in terms of weight reduction and manufacturing-cost optimisation.
To date, the only available data on leading-edge nose sizing that are widely accepted as reliable are those derived from tests performed by the R.A.E. in 1972 (Design of leading edge and intake wall structure to resist bird impact, RAE-TR72056, April, 1972) on aluminium components and adapted for titanium leading edges. There were also some general conclusions for leading edges in other materials, such as aluminium-carbon fibre hybrid. There is no equivalent model for all-carbon-fibre leading edges. No other systematic testing procedure is available to obtain general conclusions on the subject. On the other hand, different numerical analyses have been performed from dynamic simulation by means of explicit formulation based on a finite element model (FEM). Good compliance has been obtained between FEM simulation and testing results when using metallic materials. This is why the numerical results have been accepted in some cases by authorities to substantiate airworthiness certification. However, the situation is not the same for composites. Anisotropy and differences between intralaminar and interlaminar behaviours make it more difficult to obtain proper compliance between numerical simulation and real behaviour.
The aim of the present project was to initiate a process that would help to better understand the behaviour of carbon-fibre composites and their associated failure mechanisms. This should lead to the implementation of cheaper methods that do not depend on the current expensive methodology, which requires performing development and certification tests.
Selection of RTM technology
Resin Transfer Moulding (RTM) is used to obtain cost-effective structural components. This technology achieves close to 60% fibre volume and a less than 1% void content, thus guaranteeing structural laminate quality, reliability and repeatability. Adequate engineering capability and up-to-date expertise in manufacturing technologies ensure that the damage-tolerance design fulfils the qualification parameters for a flight structure.
The concept developed by the EADS CASA and IDEC joint R&D project for a leading edge in carbon fibre integrating all components into a single part has eliminated the assembly tasks associated with other types of joints.
The main advantages of the developed concept over components obtained using conventional technologies such as metals or autoclave- moulded composites are weight reduction, improved surface finishing, and lower manufacturing costs.
Process certification has driven the development of the corresponding manufacturing technology.
The main problem with using carbon-fibre composites to manufacture a leading edge is the failure mode when the leading edge is impacted by a bird. Either the leading edge fails, with total fracture and very low energy dissipation (and here, large pieces of the bird can penetrate into the component, causing major damage in the front spar web with a possible catastrophic failure), or it does not fracture at all. As a result, the brittle behaviour implies a low energy absorption capability through elasto-plastic deformation.
The low density of the material makes it a potential candidate for structural applications and, as mentioned above, the potential for functional integration associated with RTM technology is very promising for cost and product-quality optimisation.
The solution to this challenge involves a special design meeting the following two criteria:
- local reinforcement of the structure in the areas highly subject to load from a strike; - utilisation of hybrid material concepts to improve the energy absorption capability during the impact.
Meeting both criteria gives an optimised design for the leading edge:
Defining the test
The certification requirement for a leading edge is to be carried out in accordance with the FAR/JAR standard, chapter 25.631. For the purpose of this project, the JAR standard was selected. According to this standard, impact parameters are a bird strike of 4 pounds and a speed equal to 154.3m/s (300 knots). The structure must withstand the strike without catastrophic damage. In practical terms, this means the front spar of the torsion box must not be damaged.
During the project, several tests were performed in order to compare the leading edge behaviour for different configurations. The parameters taken into account were the nose material and the skin thickness. The other dimensional values were the same for all specimens; for example, specimens were 1,680mm long, including six integrated ribs plus two fixed closing ribs at both ends. An impact incidence angle of 35.56º was selected. The specimens were joined to a dummy main-box skin or to the dummy fixed ribs by means of a single row of 4.8mm screws.
The specimen/test rig joint must be locally representative of a typical wing/HTP design. This particular case concerned a portion of a CFRP front spar (made of UD tape). The joints for the L/E to skin, spar to skin, and fixed rib flanges were mounted using the suitable fasteners for a real flight structure, and not just tooling hardware. The joint of the skin to the test rig was mounted with plain steel tooling fasteners.
Although fixed leading-edge ribs may be dummies, they should be as representative as possible of a typical rib design. Main box ribs and angles, if required, may be metallic dummy parts.
The impact tests were carried out at the INTA (Instituto Nacional de Técnica Aeroespacial) facilities using high-velocity cameras. After each impact, a complete check of the specimen was done (visual and US inspection).
In parallel with this project, we also made some progress with dynamic simulation. The goal was to develop a reliable tool to facilitate the design of the component, using complex algorithms to perform the calculations. These calculations are based on explicit formulations making use of the latest techniques in FEM. The results of tests are the only way to validate the calculation model.
To date, these technologies have not yet been accepted for design purposes, nor for certification of aeronautical structures subjected to impact.
The simulations based on Euler formulation (permitting mass transfer along the model mesh) are considered as the most adequate for a reliable representation of the impact phenomenon. The main difficulty is to determine the failure modes of the structure, due to two things that must be taken into account: first, the energy absorption due to delaminating and later on, the capability of energy absorption due to transversal shear stress (fibre break). Based on the data provided by tests, the failure mode can be found in order to predict further cases.
This project has demonstrated that the new leading edge concept provides more than 15% weight reduction and cost savings compared to other current solutions. The Spanish industry’s leadership in the field of carbon fibre technology has made it possible to use and implement the knowledge and experience that was acquired previously, driving the use of new technologies capable of solving and optimising structural design concepts.
Competitiveness is synonymous with development and continuous improvement; the industry uses such improvements for better performance within the constraints of assumable costs and the dictates of market evolution. This trend is evident with the new structural applications in carbon-fibre composite materials.
The key to success for this type of development process is always close co-operation between partners. The fruitful collaboration here between EADS-CASA and IDEC was exemplary in this regard, enabling them to provide a competitive solution for what has been a major challenge.
The success of this type of project lies in mustering the necessary development effort. Without that, obtaining the results we did in this case would have been impossible. Hopefully, this strategy will help tackle major new challenges and lead further technical issues being solved.