Adhesive Bonding of Composite Structures—Progress to Date and Some Remaining Challenges

The purpose of this paper is to describe and explain some highlights in the history of adhesive bonding of fibrous composite structures. Successes are described, along with opportunities to do better in future. The importance of making simple details is stressed, in the context of making high-strength composite parts free from wrinkles in the fibers. A case is made that secondary bonding is frequently less expensive than co-curing of complex “single-step” parts, despite the consistent projections that the opposite should be true. Some explanations are presented to explain how this misconception arises, based mainly on the issue that the estimates used for comparison are traditionally based on the costs of making only the first articles, whereas what should have been compared was the cost of making possibly very differently designed lower-cost parts and structures for series production. Some common misunderstandings about variable-thickness adhesives layers are put to rest by explaining how intense load transfer occurs only in small areas, where the adhesive layer is thinnest, with the rest of the bond area inevitably lightly stressed. The problem created by variable thickness bonds is not that of weak bonds. The thicker, and often porous, bond lines usually shed most of the load they were intended to transmit to nearby thin and stiffer areas. Indeed, the loss of load caused by softening can exceed the loss of strength caused by the porosity. No, the problem caused by variable thickness adhesive layers is that it increases the local stresses in the skin whenever the glue layer is thinnest. Traditional problems with release-agent-coated peel plies that prevent adhesion of the glue to the substrate are recounted. This issue is now fairly well known. What is far less understood is the adverse influence of pre-bond moisture that is unable to escape during the bond cycle. This prevents adhesion perhaps even more effectively than a layer of silicone. The importance of bonding only dry parts is explained. This is far easier to do at the time of original manufacture and less easy for in-service repairs and repainting. The paper concludes with a discussion of the important factors to be considered during design and analysis of bonded composite joints.