R. Freeman, F. Smith, TWI Limited, Cambridge, United Kingdom

Summary:

Fibre reinforced polymers used by themselves offer high strength and low weight, but their use in engineering invariably demands that they are joined to other materials, usually metals. Poor mechanical performance, low fatigue resistance, and a deep-seated distrust of the joint’s repeatability and integrity have, until now, coerced designers into over-conservatism regarding design and weight savings.

With the advent of Comeld™, the world of composite to metal joining could change dramatically. Using a novel metal pre-treatment process, called Surfi-Sculpt™ (invented and patented by TWI), it allows composite materials to be joined to metals with excellent load/displacement results. Surfi-Sculpt™ involves texturing of a metallic surface utilising a power beam technique e.g. electron beam, before the treated surface is joined to a composite material using standard techniques such as autoclave when prepreg materials are used, and vacuum infusion for dry fabrics.

The tailoring of the joints allows for load to be transferred through the thickness of the joint reducing stress concentrations at the bond line. In addition the joint can be designed such that the failure mode is predetermined. For example in a composite to metal joint, the metal can be made to undergo plastic deformation, which can be identified before ultimate failure. This is preferable to composite failure, which would be sudden and catastrophic.

This presentation will explain the pre-treatment of stainless steels, aluminium and titanium alloys, and the method of joining the pre-treated metals to glass and carbon fibre reinforced composite materials. Data demonstrating the mechanical performance of the composite to metal joints will also be presented. The results offer great encouragement to the aerospace industry, for future designs of aircraft structures.