Matthias Geistbeck

Bonding of CFRP primary aerospace structures – discussion of the certification boundary conditions and related technology fields addressing the needs for development

Today, the application of bonding technology for primary aerospace structures is limited due to the certification regulations. State of the art is an additional application of fasteners which is limiting the benefits of the application of composite bonded joints. This article provides an overview of the current state of the art of certification compliance within the context of bonded CFRP primary aerospace structures. Beginning with the boundary conditions and the background of the certification regulations, the current framework for the development of composite bonded joints is described including the experiences within the aerospace industry as well as the physical principles that are leading to initial damages and uncertainties within a composite bonded joint. Also, the role and interaction of different technology streams such as surface preparation, process safety, material and adhesive development, and design and sizing concepts to enable a secured roadmap to certification is discussed. The resulting needs for current and future technology development and their interaction within the different disciplines is described. Also, solution approaches to build a roadmap to certification of CFRP-bonded joints are highlighted.

Automated welding of thermoplastic joining element on CFRP

In aircrafts today a multitude of joining elements are mounted. The program of semifinished products includes cage nuts, stud bolts and cable supports. They are used both for applications with low requirements, like the attachment of heat insulation, and for the fixation of complex component assembly to the aircraft structure. The manual adhesive bonding process of the supports is connected with a high manufacturing effort. Several specific work steps are required,shown in Figure 1 and the curing time of 8 to 12 h constitutes a significant productivity limiting factor. With the aim to come to a more efficient fastener installation that also shows potential for structural applications the possibility of a welding application of fasteners was investigated. Carbon f ibre reinforced poly mers (CFRPs) since many years are an important material in aerospace. The predominant CFRP material is built of carbon fibres with a thermoset epoxy matrix. Two relevant manufacturing processes can be distinguished, the dominant prepreg technology with autoclave curing and the vacuum assisted resin infusion (VAP) technology. For future applications thermosets will predominate, although in the meantime also parts with thermoplastic matrix are used. The investigations described aimed at the modification of the thermoplastic CFRP with a thermoplastic surface layer to allow for the welding of standard joining elements. In a first step a thermoplastic film had to be applied to the thermoset CFRP surface. It had to be ensured that the interface between the thermoplastic film and the base epoxy CFRP does not constitute the weak point in the later assemblage. The thermoplastic counterpart could then be welded on the functionalised surface [1]. For this an appropriate welding process was required that allows the processing of aerospace high performance thermoplastics without a thermal degradation of the epoxy matrix within the CFRP.