Search results

THE development of synthetic resin adhesives has proceeded very fast since about 1937 when the first urea formaldehyde wood glues were being produced on a pilot scale. The excellent resistance of these materials to weathering conditions has enabled very considerable advances in structural design both in wooden structures and rather more recently in metal aircraft. These synthetic resin adhesives are basically ‘thermosetting’, that is to say, they harden by the addition of catalysts or by the application of heat, the chemical process which takes place being irreversible and not dependent upon the evaporation of solvents. This concept leads not only to good weather resistance, but also to the practicality of sticking together non‐porous materials such as metal. The first use of bonding of mstal in aircraft structures was in the de Havilland Hornet in 1943,1 and since then bonding has proved its advantages over other methods of joining for both primary and secondary structures. Other new methods of fabrication have appeared since that time and the requirements for structures in terms of loading and environment have changed considerably; but both mctal‐to‐metal bonding and its newer partner honeycomb sandwich show signs of much more extensive use in the future. These two most important applications of synthetic resin adhesives in modern aircraft will now be considered.

MANY engineers look upon plastics and allied materials as entirely new to the aircraft industry, but such is not the case. Phenol fibre sheet and resin bonded waterproof plywood have been used for years, and acrylic resin sheet has been in use for transparent enclosures for some time past; yet all come under the above category. The primary difference between the past and present uses of these materials is that they are now used in applications where structural loads are involved, while they were previously used only in non‐stressed parts where special characteristics, such as transparency or insulating qualities, were of paramount importance. If these materials are classified according to their major characteristics they fall into three categories; those made with thermosetting resins, those made with thermoplastic resins, and those made with wood veneer. This classification also in a general way divides them according to their principal uses; thermosetting materials being used mostly in the production of relatively small structural parts, the thermoplastics being used mostly for their transparent properties, and the wood veneer materials being used mostly in relatively large structural parts and assemblies.

The history of SAAB's development of adhesive bonded aircraft structures is briefly commented upon. The lecture deals mainly with Redux bonding of aluminium alloys, and the many design problems associated with bonded structures are treated mainly with the structural designer's interests in mind. He has to deal with a great number of different types of bonded joints, loaded and stressed in more or less complex ways, and often with high service temperatures, for which more design information is needed.

TWENTY years ago, there was no commercial production of metal‐to‐metal adhesive‐bonded structures, and much of the recent progress has resulted from a greater understanding of the molecular physics of the adhesive mechanism. Theoretical and experimental knowledge of the fundamentals of adhesion was reviewed as was experimental work in Germany on metal‐to‐metal adhesives. The European aircraft industry has put heat‐curing resins to general use. German research, however, has also covered room‐temperature‐curing adhesives which are specially valuable for repairs in the field. Since work in the United States has been confined largely to heat‐curing adhesives, the German work on room‐tempcrature‐curing adhesives was of considerable interest. Most of the metal‐to‐metal bonding adhesives used today are phenolic or epoxy‐modificd phenolic types. These, however, will withstand only short‐time exposure to temperatures above 500 to 1,000 deg. F. Prolonged high temperatures break down the polymer structure and drastically lower strength. Research was described in which an inorganic component (most satisfactory to date: arsenic pentoxide) was added to the organic adhesive to provide thermal resistance. Results definitely indicated that organic‐inorganic adhesive systems provide one solution to the thermal resistance problem. The high temperature resistance characteristics of five ceramic adhesives were presented. These inorganic adhesives were prepared as ceramic frits and applied to stainless steel, the material being considered for wings and control surfaces of missiles. Data from tests at 800 and 1,200 deg. F. showed great promise for this type of adhesive.

Fasteners, adhesives and sealants are as important to aerospace production as the most sophisticated advances in engine design. Without the correct nuts and bolts, no aircraft can function, or certainly not for long, efficiently and safely. Even the man in the street — as well as his wife and children — was made dramatically aware of this after the Amsterdam air crash last year when numerous daily newspapers clearly explained the function of sheer pins and showed, in large, simplified drawings, how they are used to attach the engines to the wings of the Boeing 747. But even though the proverbial “Man on the Clapham Omnibus” and his schoolboy son know the importance of fasteners in aircraft manufacture and maintenance, the industry itself does not appear to hold them in the same high esteem. Fasteners, it seems, are like discreet waiters in a decent restaurant: you don't notice them until they're missing. In order to compile this feature, we wrote to all known manufacturers and suppliers of fasteners, adhesives and sealants seeking information on products and their applications. Despite the opportunity for publicity, the response was negligible. The subsequent telephoning of non‐respondents (which was most of them) was only marginally more productive. The interesting thing was the air of indifference shown. Many of the companies bluntly said they were not interested — not even in free publicity. One firm even said that it did not need publicity, it never had and it never would. Such self‐sufficiency, though admirable, is hardly indicative of a dynamic, developing company and typified an attitude of many companies in this sector which seemed to spring from a general lack of interest — or pride, perhaps — in what they are doing. At the suggestion of our editor, I contacted the Society of British Aerospace Companies (SBAC) to obtain figures on the quantities and the value of fasteners produced and sold by their member companies. Here, at least, I thought, would be the basis of an article. But here I was wrong. The SBAC does not compile such figures. It is clear from all this that suppliers to the aerospace industry do not take fasteners as seriously as “The Man on the Clapham Omnibus” does. It is equally clear that the companies mentioned on this and the following pages consider fasteners, adhesives and sealants to be very important.

The author has prepared this book with the object of aiding the engineer or technician confronted with the problem of joining two metals, by helping him to choose the correct type of adhesive, use it and design an adhesive joint for optimum performance. The author succeeds admirably in this aim and in addition has included valuable information concerning new types of American adhesives, some of which have been developed specially for missiles and which are not widely known. He is a research engineer at North American Aviation Inc., Downey, California and among other things is responsible for the new NAA Hi‐Temp phenolic‐resin‐base adhesive which is suitable for structural applications at temperatures up to 600 deg. F. and which is, however, cured at a temperature of only 275 deg. F. for a 1 to 1½ hour period.

A Review of the Properties and Applications of High‐Strength Metal‐to‐Metal Joints and Sandwich Constructions Bonded with High‐Strength Adhesives. TODAY'S adhesives are the complex products of continually expanding technologies. Their applications appear to be limited only by the skill and ingenuity of the designer and his fabricators. Even in a single area such as metal‐to‐metal bonds in aerospace and electronic uses, evaluation and selection of an adhesive requires familiarity with a large number of adhesive systems.

THE launch of the new British Airways corporate identity has given the promoters of self‐adhesive aircraft livery and marking systems a major boost.

A theoretical analysis is given of the peeling test for Redux‐bonded joints, as devised by Aero Research Ltd. and generally accepted as a standard quality control test for metal‐bonding processes. Numerical values derived from computations appear to be in reasonable agreement with experiments, but more test‐data and better knowledge of the clastic and plastic behaviour of both adhesive and adherent are necessary to make the method more reliable in this respect. The practical value of the method, however, is the indication it gives about the sensitivity of the test for the variables involved. This may make it feasible to introduce corrections for variations of some parameters, thereby improving the reproducibility of the test.