Adhesively bonded doublers

As mentioned above, aircraft structure is typically quite thin with numerous small fasteners to achieve efficient load transfer through joints. Mechanical fasteners are laborious to install. Adhesive bonding of large area doublers and joints can be accomplished at significant labor hour savings over equivalent mechanically fastened designs. 

Repair. Repairs for damaged bonded structure can be either mechanical or adhesively bonded. Mechanical repairs are metallic doublers on one or both sides of a damaged component, held on by fasteners. The fasteners transfer the load through the doubler around the damaged site and restore part functionality. Although common for metal-to-metal bonded structure, mechanical repair of sandwich structure is rare because of the risk of further delamination. Unless the doubler and fasteners are perfectly sealed, water can travel into the honeycomb core eventually causing freeze-thaw damage and delamination. 

A typical laminated joint is a butt joint on to which doublers are laminated, the result being a double-strap joint. The following features distinguish this laminated joint from the corresponding adhesively bonded double-strap joint ... 

Metal skins with bonded metal doublers If the doubler has disbonded and no corrosion is present, repair may be effected by riveting. If skins are too thin for countersunk rivets, then repair by adhesive bonding may be necessary. 

Fig. 21. Identity between adhesive shear stresses in bonded joints and bonded doublers.

Hart-Smith, L.J., Simple approximate analyses of adhesive bonds in joints, doublers, and bonded patches, Boeing Paper, to be published under the auspices of the CRAS program. 

The specimen, or model, which is selected is shown in Figure 1. It is named the skin-doubler specimen, because it comprises an infinitely long skin with a very long doubler bonded to it. When tension stress (strain) is introduced in the skin, the adhesive forces the doubler to strain until its strain and skin strain are equal. At this point, there is no longer any need for loading the doubler and so the adhesive shear force becomes zero. It will also be seen that the adhesive shear is a maximum at the doubler tip. 

The foregoing analysis of the skin-doubler specimen shows that it is essential to know the stiffness characteristics of the adhesive. Since good design practice places bond lines in shear, it was decided that the shear modulus is the primary stiffness parameter. Furthermore, it is recognized that more than the initial portion of the shear stress-strain curve was required. It was clear that the total curve was not linear. It was anticipated that the nonlinearity portion would bear heavily on creep and fatigue performance. Accordingly, the primary requirements for the strain measuring device were set as follows ... 

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