Adhesive bonded structure

Of obvious importance to aircraft is the smoothness of exterior surfaces. Smooth aerodynamic surfaces reduce aerodynamic drag, resulting in higher airspeeds and increased efficiency. Mechanical fasteners, even countersunk flush fasteners, introduce disruptions in the airflow over the exterior surface. Even the slight deformation of thin sheets around fasteners produces drag. Adhesively bonded structure has no fasteners to disrupt airflow and is more capable of producing the smooth continuous contours that are so common on aircraft. 

It is clear that European civil aircraft manufacturers adopted adhesive bonding for major structural elements much more rapidly than their American counterparts, but it is difficult to determine exactly why. Certainly a number of contributing factors are obvious. One was a history of success in incorporating adhesively bonded structure in military aircraft such as the Mosquito. Although the Mosquito was the most unusual and extreme example of adhesively bonded structure, other European wartime aircraft contained bonded structure as well. American military craft of the time were almost exclusively riveted aluminum structure. 

Fig. 3. Adhesively bonded structure (shaded) on the Fokker F27. Reproduced by permission of Stork Fokker.

Designers must keep in mind that modem aircraft will likely be in service for thirty years and that repair of adhesive bonded structure will be inevitable. They must allow for reasonable disassembly of bonded stmcture for repair access as well as anticipate repair procedures that will not be unduly difficult for airline repair stations. 

Thrall, E.W. Jr., Failures in adhesively bonded structures. AGARD Lecture Series No. 12. Bonded Joints and Preparation for Bonding. North Atlantic Treaty Organization Advisory Group for Aerospace Research and Development, London, 1979. 

Potter, D.L. et al.. Primary Adhesive Bonded Structure Technology (PABST) Design Handbook for Adhesive Bonding. Report AFFDL-TR-79-3129, Douglas Aircraft Co., Air Force Flight Development Laboratory (FBA), Air Force Systems Command, WPAFB (November, 1979). 

Metals such as aluminium, steel, and titanium are the primary adherends used for adhesively bonded structure. They are never bonded directly to a polymeric adhesive, however. A protective oxide, either naturally occurring or created on the metal surface either through a chemical etching or anodization technique is provided for corrosion protection. The resultant oxide has a morphology distinct from the bulk and a surface chemistry dependent on the conditions used to form the oxide 39). Studies on various aluminum alloy compositions show that while the oxide composition is invariant with bulk composition, the oxide surface contains chemical species that are characteristic of the base alloy and the anodization bath40 42). 

Pearce, M. B., How to Use Anaerobics Successfully, Applied Polymer Symposia No. 19, Symposium on Processing for Adhesive Bonded Structures, M. J. Bodnar, ed., Interscience, New York, 1972, pp. 207-230. 

Shannon, R. W., et al, Primary Adhesively Bonded Structure Technology (PABST) General Materials Property Data, Douglas Aircraft Co., McDonnell Douglas Corp., Air Force Flight Dynamics Laboratory, Technical Report AFFDL-TR-77-107, September 1977. 

Crabtree, D. J. Ion graphing as an in-process cure monitoring procedure for composite and adhesively bonded structures, p. 636, Proc. 22nd SAMPE Symp., 1977... 

Adhesives have been used successfully in a variety of applications for centuries. Today, adhesives are more important than ever in our daily lives, and their usefulness is increasing rapidly. In the past few decades there have been significant advances in materials and in bonding technology. People now routinely trust their fortunes and their lives to adhesively bonded structures and rarely think about it. 

For bulk specimens, Kic and Gic convey the same information. However, for adhesively bonded structures, such as the specimens described in this report, Kic and Gic differ. E in Equation 3 is the modulus of the adherend (the PMMA) rather than the adhesive, and so Kic is not characteristic solely of the PS layer but of the composite as a whole. The other expression for fracture behavior is the effective fracture surface energy, y. It is related to the fracture energy by... 

It has often been observed that the application of adhesives to metal fabrication, in common with many other technological innovations, was pioneered by the aircraft industry. It is ironic that this industry, in which safety and reliability command paramount attention, should lead the departure from traditional methods of joining. Today adhesives are used to bond critical parts in commercial and military aircraft and helicopters, spacecraft, rockets, missiles and the US Space Shuttle. The American Primary Adhesively Bonded Structure Technology (PABST) Programme, which ran from 1976-81, was an imaginative attempt to advance significantly the use of bonded... 

Bascom, W.D., Cottington, R.L. and Timmons, C.O. Durability of Adhesive Bonded Structures (Ed. Bodnar, M.J.), Journal of Applied Polymer Science, Applied Polymer Symposia 32, J. Wiley and Sons, New York, 1977, p. 165. 

Hashim, S.A., Cowling, M.J, and Winkle, I.E. Design and assessment methodologies for adhesively bonded structural connections. Structural Adhesives in Engineering II 89, University of Bristol, September 1989. 

Today with the advance of structural and high temperature adhesives, aircraft, spacecraft, and missies (109) are built with secondary-bonded and some primary-bonded structures. The PABST (Primary Adhesive Bonding Structures Technology) program (110) was carried out at the Air Force Materials Laboratory between 1976 and 1981. That project led to the construction of a totally adhesive-bonded 42-foot-long circumferential fuselage section of a... 

At the end of the PABST program, there were several questions unanswered about (1) the ease and cost of maintaining the adhesive-bonded structures, (2) the durability in actual service environments where temperature cycles and moisture attack could reduce the life of the bonded joints, and (3) the feasibility and cost of curing an immense primary-bonded structure in an autoclave. 

The sustained load that is applied to the bond in relation to the bonds control strength is certainly a critical parameter. Whether or not the load has any effect on the adhesive itself (analogous to stress corrosion cracking of metals) cannot be ascertained from our data. The literature, especially that of Bascom (19) would indicate that it does. This work does indicate that designers of adhesively bonded structure should take durability testing of this sort into account in their design exper imentation. 

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