Joint strength is controlled through precise bond line control.
The enormous strength of unidirectional carbon fiber composite rods is difficult to take advantage of at their ends because of inadequate joining technology. Bolting does not work with unidirectional composites, and bonding is difficult due to stiffness mismatches between the metallic and composite connections. Ideally, a thick bond is desired so that the relatively softer adhesive can shear and distribute shear stresses instead of peaking at the ends of the bond. Thick bonds are difficult to obtain and repeatedly control with conventional methods of beads, bonding wire, shim, or tooling. Most of these methods control the minimum thickness of the bond, but do not control the maximum thickness. In addition, traditional joint types such as lap, strap, and scarf are not ideal for this application.
The shape of this lug provides relatively high joint strength and allows a simple manufacturing process. The shape also provides a level of robustness as it is designed so that the fitting yields before the adhesive fails. Joint strength is controlled through precise bond line control, which is controlled by a tight-fitting bore in the root. This eliminates the need for traditional bond line control methods that impose imperfect constraints on the bond thickness and complicate the manufacturing process. The assembly process is streamlined in that the rod can be inserted and the bore will hold the rod in proper alignment. Adhesive is injected after fit-up, allowing flexibility in a larger fabrication process. An inspection hole provides confidence that the joint has been properly filled with adhesive, and prevents adhesive from detrimentally flowing into the alignment hole.
The fitting has three specific regions. First is a trivial clevis bolted with two fasteners that is not part of the innovation, but allows for traditional mechanical fastening. Second, on the other end is the bond section where the composite rod enters the fitting and transfers all of its load into the metallic connection. This region is specifically tailored to minimize the shear stress in the adhesive to maximize joint strength. To accomplish this, the metallic starts with a thin as-manufactured tube to minimize shear developed at its tip. Midway down the length of the bond, the fitting section increases so that its stiffness increases and drives more load across the middle of the bond that is traditionally not loaded. Near the end, the fitting section is oversized for its own strength, but needed to mitigate the shear stress peaking at the end of the bond. In this lap/taper joint combination, the adhesive shear peaking is reduced at the ends and increased in the center of the bond. An injection hole in the center of the bond area allows for adhesive application.
Third, in the middle of the fitting is a close tolerance bore hole that the composite rod engages. This location hole aligns the rod to the fitting, eliminating the need for traditional bond line control techniques. A key benefit in removing this constraint is that the bond thickness can easily be increased or tailored to an optimal size.
This work was done by Ian Fernandez of Ames Research Center.