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Because the mechanical behaviour of composite materials doesn’t always allow the use of standard assembly methods, the mass influx of composites in aeronautics brings with it a whole new set of assembly issues, in particular for structural aircraft parts. In response to this limitation, the Capaero Company has developed and patented a new process called EB², for Expanded Bonded Bushing. This innovative process is based on the coldexpansion principle and on microencapsulated adhesive systems. West Coast Industries (WCI), mother company to Capaero’s partner WCIE, has been using cold expansion technology (cold working) for several decades. Besides the attractive mechanical performance linked to the combination of expansion and adhesive-bonding technology, another advantage of the EB² process is that it is very easily implemented.
(1) CHRISTOPHE BOIS - MAÎTRE DE CONFÉRENCE - LABORATOIRE DE GÉNIE MÉCANIQUE ET MATÉRIAUX DE BORDEAUX (2) GERMAIN BOULLIER - TOOLING ENGINEER - WEST COAST INDUSTRIES EUROPE (3) ERWANN LE GOFF - DEVELOPMENT ENGINEER - CAPAERO(published on June-July 2010 - JEC Magazine #58)
Fasteners play a key role in aeronautical structures, because they serve to assemble the different aircraft parts. They are especially numerous on the wings in the form of rivets, screw fasteners, etc. Fasteners allow transferring loads within the structure, so aircraft manufacturers must imperatively have a good grasp of such technology. Fasteners must meet many requirements for things like reliability under load (particularly in flight) or for their mounting process, which is often done manually.
Mounting fasteners can also be a challenge from a competitive point of view (e.g. time saving, ease of assembly, the process itself) during production, or later during maintenance. Since as a jumbo jet can contain fasteners in the millions, it is easy to understand how important it is to have an efficient, inventive process that is easy to implement.
Over the past few years, we have seen composite materials gradually penetrate the field of aeronautics, more recently in aircraft structural applications to replace metal. Besides their potential for weight savings, these materials also offer very attractive mechanical properties. They have unquestionable advantages. The gradual transition from all-metal to allcomposite aircraft is now under way.
The influx of composite materials in the aeronautical field has given rise to new technological problems. Because these materials behave differently than “standard” metals, particularly where damage is concerned, it was necessary to rethink fastener assembly techniques to obtain attractive mechanical properties and reasonably easy implementation. With all this in mind and with the Aquitaine region’s support, Capaero partnered with WCIE (West Coast Industries Europe) and Bordeaux’s LGM²B laboratory for mechanical engineering and materials to develop the idea for the EB² process.
Description of the EB² process
The EB² process combines the advantages of the cold expansion process and microencapsulated adhesive systems. Cold expansion (cold working) is a method that has been used for more than forty years to improve the fatigue performance of attachment holes in metal assemblies.
A spin-off of this process is the expansion assembly of bushings (used to replace nitrogen assembly), which is WCIE’s BushMax process (see Fig. 1). That is the process used by EB².
A structural bonding method by microencapsulation is used in conjunction with the expansion process. A mandrel exerts a pressure on the layer of microcapsules between the metal bushing and the composite part, squeezing them until they burst, releasing the hardener and initiating the structural adhesive’s polymerization process (see Fig. 2).
EB² and mechanical performance
One of the main objectives of the EB² process was to obtain high mechanical performance in order to use it with structural parts. Various tensile and tearing tests were carried out to compare the performance of EB² with that of other assembly processes in use.
It has already been established that adhesive bonding a metal bushing to the composite’s attachment hole considerably improves the assembly strength – especially the tensile strength – through better load transfer. Comparing the performance of EB² assemblies in both the uniaxial tensile tests and the tearing tests with that of adhesive-bonded assemblies shows that EB² improves performance even more (see Fig. 3).
The efficiency of the process depends on a number of parameters, e.g. the theoretical expansion ratio, which is the % ratio of the mandrel’s maximum diameter and the diameter of the bushing hole before assembly. This expansion ratio is a key parameter in the process, because it influences the assembly efficiency, which must be high enough to allow holding and correctly bonding the metal bushing, but also in an acceptable range that will avoid damaging or preloading the composite.
Modelling tools and experimental protocols have been developed to allow controlling the process, and specifically to adapt the process parameters to a given set of specifications (see Fig. 4).
Numerical simulations not only provide a detailed understanding of the physical phenomena and rupture modes involved, but also give access to such hard-tomeasure data as the state of residual stress following use of the mandrel or the load transfer mechanisms at the bushing/composite interface. To further these objectives, a research grant financed by Capaero, the national research and technological association ANRT and the Aquitaine Regional Council is ongoing at the LGM²B laboratory for mechanical engineering and materials.
Advantages of the EB² process
Besides the mechanical aspects, there are a number of industrial advantages to the EB² process. It can be relatively easily implemented, since the tooling required consists mainly of a manually operated hydraulic cylinder and a customized mandrel (see Fig. 5). The cylinder can be the same as the ones used in metal working.
The pre-gluing process ensures uniform glue thickness and reproducibility in terms of the process and the final assembly’s mechanical properties. The principle of microencapsulated adhesives means that the operator is at no time in contact with any toxic epoxy glue ingredients, so the bushings can be handled without need for specific safety measures or equipment to protect against chemicals, either for the operator or on the premises where the bushings are being mounted. Among other things, the technique allows storing the bushings for months to be used as needed, facilitating inventory management for materials and tooling without risk of pollution, and without degradation of the adhesive’s intrinsic properties. Perhaps the most important advantage is the fast, easy assembly afforded by the EB² process. EB² is distinctly faster than standard composite reinforcement processes, and the tooling used requires no specific technical skills.
Example of an application: CompNut (rivetless nut)
The EB² process can be used in a number of ways as a function of the desired industrial application. One such application is the CompNut, a floating fastener for which the base is mounted using the EB² process, thereby eliminating the need for rivet holes (rivetless nut). EB² can also be used to assemble hydraulic or electric connectors in order to eliminate flange openings. One advantage of this application, besides the easy implementation, is that the retaining nuts are replaceable: the base remains in place and the other fastening elements can be replaced during maintenance.