The structural problems of metal aircraft design largely centre round the difficulty of making efficient compression members. This difficulty is accentuated when loads are small in relation to the size of the structure. For example, the diameter of an aeroplane fuselage cannot usually be less than the height of a man, which results in such small forces at the surface of the shell that the lightest practicable beam is quite disproportionate to its strength.As a measure of load in relation to size, it is convenient to use a quantity that we suggest may be called the “ structure loading.” This quantity, due to H. Wagner (8), is simply the square root of the applied load divided by a characteristic dimension (such as the length) of the member.
WITHIN the last year or so we have learned to glue metals together with a strength which brings this method of joining materials into competition with riveting, at least in the thin gauges used in the aircraft and motor industries. Apart from this new extension of gluing to the metal working trades synthetic adhesives have already revolutionized the woodworking industries. This revolution is due to the superior quality of the resulting products and the increased rate of output made possible by the intrinsic high speed of setting of synthetic adhesives aided by such novel methods as high frequency heating, infra‐red heating and the like. In aircraft in particular the “weather‐resistance” of synthetic adhesives has largely removed the disadvantages of wood construction, due to the use of casein glues, so much in evidence in the first winter of this war. It may be said therefore that gluing has been raised from the status of a useful but humble convenience of daily life to a process of engineering significance. But before engineers can use gluing in the fabrication of structures they must be provided with data sufficient to enable thorn to compute the strength of the joints, and we at Aero Research Ltd. have therefore endeavoured to find a simple relation between the strength of a lap joint and its geometry. Such a simple relation is found in “the joint factor” which is defined3 as the square root of the thickness of the sheet divided by the length of the overlap.
Epoxy resins derived from bisphenol and resorcinol have been examined as to their adhesive properties in the presence of phthalic anhydride as hardener. From studies of their dielectric behaviour, the effect of hydroxyl‐group content and molecular weight in their monomolecular properties and their adsorption on to aluminium foil, it is concluded that the hydroxyl groups play a decisive part in anchoring the molecules to a (polar) surface. There was very good correlation (significant at the 1% level) between hydroxyl content and nominal breaking strength of joints on an aluminium alloy, and it was shown that the shrinkage of resin mixtures on cooling decreases with hydroxyl content.
An account that is to be of any value of a subject in such an early stage of development as the use of synthetic resin materials in aircraft construction, must necessarily contain a great many first thoughts and rough guesses, and anyone writing such an account must be prepared to take the risk of having to eat his words at a later date. But I believe it is better to have blundered than never to have thought at all, and it is my hope that what I have to say will at least arouse interest enough among those with special knowledge to contribute to the discussion at the end of the lecture.
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