Tests have been carried out on round bar specimens of various strong aluminium alloys (unclad) in the fully heat-treated condition in both tensile and compressive loading. A careful study of the results establishes beyond reasonable doubt that there is a progressive change in the tangent elastic modulus with stress. The variation is appreciably greater than that predicted by the consideration of second-order effects in the conventional stress and strain representation. It is found that the modulus decreases with tension and increases with compression by about 5 per cent at 30 tons/in.2, the form of the variation being a continuous function. Tests on two alloys of aluminium other than those of the normal zinc-bearing and the copper-bearing types were also carried out. The first of these was a simple fully heat-treated binary alloy of 4 per cent copper which was very similar in behaviour to D.T.D.364; the second was a 5 per cent alloy (N.S.6-½H) which, after its initial loading, showed only very slight elastic modulus variation in the subsequent loading and unloading lines. An additional test to compare naturally and artificially aged D.T.D.364 did not reveal any difference in the degree of modulus variation.

In view of the present experimental findings, the use of the “second” modulus line in the proof stress determination of unclad strong alloys should be adopted generally, if practicable. A high level of internal stress can cause slight errors in this method because the elastic limit is then prematurely reached. As an occasional check on the method when applied to routine mechanical testing, the elastic component of strain can be found for the last point on the loading line by returning to the zeroing load.

In the Appendix the difference between true and nominal stress-strain relationships is calculated. The meaning and theory of modulus variation is also discussed and aluminium alloys are compared with a high-tensile steel.

A simple expression for the form of the variation is proposed.