JEC COMPOSITES MAGAZINE - Issue #69 - November/December 2011 - 40

joining

Different 3D reinforcement methods have been extensively studied over the past years to address delamination and disbond issues and to improve the damage tolerance of CFRP structures and/or joints. The goal of the 3D reinforcement is to establish a mechanical link between the different plies of the CFRP laminate (as a structural reinforcement) or between two adhesive-bonded CFRP structures (as a joining reinforcement). Techniques such as stitching, tufting and Z-pinning are examples of such reinforcement methods. In stitching, a needle is used to perforate a fabric preform and insert a high-tensile-strength yarn such as glass, carbon or aramid. Studies have shown an improvement in resistance to delamination as well as Mode I and Mode II toughness (Ref. 1-3). Impact behaviour improvements have also been investigated (Ref. 4). Tufting is a very similar technique to stitching, the difference being that access to only one side of the laminate/structure is necessary since the needle penetrates and retracts through the same pathway. The thread forms a loop and stays in place due to the friction with the fabric preform. Like stitching, tufting improves resistance to delamination and disbonds (Ref. 5-6) and also enhances the bending and compression properties of sandwich panels (Ref. 7). Z-pinning is a 3D reinforcement method suited only for pre-impregnated (also known as prepreg) laminates. It consists in inserting small rods into a laminate through the use of a carrier foam and an ultrasonic gun. These rods can be made from pultruded CFRP, glass fibre composite or a titanium alloy. This technique also improves the delamination resistance of composites (Ref. 5, Ref. 8), their toughness (Ref. 9) and their flexural properties (Ref. 10). All of these 3D reinforcement methods, despite being valuable tools for enhancing the mechanical performance of CFRP, also have detrimental effects in the laminates such as fibre kinking, fibre misalignment, broken fibres and resin pockets (Ref. 10-12). Joining of CFRP structures in aerospace currently involves fasteners or the combination of structural adhesive bonding with fasteners. The low weight penalty and the homogeneous stress distribution associated with adhesives are the main advantages of this joining method. The performance of adhesive-bonded joints strongly depends on surface preparation, overlap length, fibre orientation and joint-end configurations (Ref. 13). However, due to the uncontained failure behaviour demonstrated by bonded joints, “safety fasteners” are required for bonded CFRP structures
Fig. 2: Co-bonding manufacturing process for RHEA joints

in aerospace, which significantly increases weight and assembly costs. Moreover, the notch sensitivity and the matrix bearing behaviour encountered after drilling a hole and installing a fastener into a CFRP part significantly reduce mechanical performance. Taking into account these different factors, it clearly appears that the investigation of a novel joining technology capable of minimizing localized stress concentrations and reducing weight while encompassing damage tolerance characteristics is not only desirable but necessary. In order to fully explore the advantages of the efficient load transfer associated to structural adhesive joints combined with damage tolerance characteristics, Airbus, EADS Innovation Works and Fraunhofer ICT-A have been investigating RHEA joints as an innovative solution for future-generation rivetless CFRP assembly concepts. This integrative joining technology achieves a 3D interlocking of CFRP laminates through the use of thin spiked metal sheets between co-bonded structures. The resulting combination of adhesive and mechanical load transfers allows more progressive and predictable failure mechanisms. The concept involves the introduction of a spiked metal sheet reinforcement (Fig. 1) into a CRFP prepreg laminate during its curing cycle. The metal reinforcements are produced by laser cutting and subsequent bending of the spiked geometry with a special Fig. 1: RHEA reinforcement tooling.

The RHEA joint approach

jec composites magazine / No69 November - December 2011

Ideally, these sheets would be produced in one single manufacturing step through metal sheet forming (stamping). During the curing process, the autoclave pressure and temperature lead to the insertion of the spikes into the prepreg. Once this first curing cycle is completed, the 3D reinforced cured part can be joined with another uncured part or with another 3D reinforced cured part through the use of an additional uncured doubler. This co-bonding manufacturing process is shown schematically in Fig. 2.

JEC COMPOSITES MAGAZINE - Issue #69 - November/December 2011

Table of Contents for the Digital Edition of JEC COMPOSITES MAGAZINE - Issue #69 - November/December 2011