Bonded doublers

Skins with bonded doublers have been used successfully on a large number of civil aircraft and are still used on new designs. The only widespread in-service problem with bonded doubler assemblies has been delamination caused by unstable surface preparation. Early fuselage skins with bonded doublers and inadequate surface preparation experienced severe delamination and subsequent corrosion. A majority of these delaminated doublers were the fail-safe tear straps (Fig. 27). Although the tear strap bond does not cany structural load, the bond... 

The portion of this work to be discussed here is the longitudinal splices in the skin. Every second splice was designed as an entirely bonded double-strap splice, with no fail-safe rivets. (A small number of rivets located judiciously in low-stress areas could have served as both valuable tooling aids to position components and as a means for electrical grounding, but the decision was made to demonstrate that they were structurally unnecessary.) The intervening riveted splices at the manufacturing breaks employed selective bonded doublers at the most critical... 

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

This ability to control crack initiation and crack growth is utilised in fuselage construction to resist catastrophic decompression, which could result from a crack initiated by the cyclic pressurisation loads, which are applied during flight. The bonded doublers have the ability to turn the tip of any crack which might have been formed hence preventing catastrophic crack growth which can and, in the case of the de Havilland Comet 1, has led to uncontrolled decompression. 

If dismantleable components are to be attached to sheet metal, rivetted or screwed connectors have to be used. To avoid local distortions and to help to diffuse the point loads, bonded doublers are used as is also shown in Fig. 78. 

The grit-blast/silane (GBS) treatment was developed in Australia for use with bonded repair patches. One example is the repair of outer wing fatigue cracks, in which over 1,000 bonded doublers have been applied to primary structure with no crack growth or patch failures for 20 years. The US Air Force has extended the technology and optimized a grit-blast/silane surface treatment for patching other aircraft. 

Bonded sheet metal structures are inherently stiffer than their spot-welded equivalents. In the case of box section beams the flexural stiffness and ultimate strength can be doubled by changing from a welded to a bonded form of construction. Bonded doublers are also very effective. By implication, the stiffness of a vehicle shell will reflect the increased stiffness of its bonded sub-components. Therefore attention must be paid to the relative performance of the various major body sections in order that the vehicle will crumple correctly when it is impacted in an accident. 

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