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In a previous paper were recorded the results of an investigation into the effects of repetitions of stress on the micro-structure of various metals in the form of crystalline aggregates, the main purpose of the investigation being a study of the causes of fracture under repeated stresses of relatively low magnitude. One important conclusion derived from the experiments was that the action of slipping was not, as had been previously stated, a weakening process in itself. Up to a point the effect of slip was actually to increase the resistance of the metal to further slip. Eventually, however, this strengthening action was exhausted, and failure commenced by the formation of a crack. It was suggested that failure occurred when the amount of strain-hardening by slip exceeded a certain limiting amount. No definite evidence could be obtained on this point, but it was considered that further information might be obtained if attention was directed to a material more simple in structure than a crystalline aggregate. In particular, it was desired to eliminate the effects of the crystal boundaries, whose nature is at present unknown. This could be accomplished if specimens cut entirely from one crystal were employed. Further, it should be possible to verify the assumption, commonly made, that slip bands represent the traces of actual “slip planes” on the surface of the specimen, and to relate these with the atomic structure of the material. Through the kindness of Prof. Carpenter and Miss Elam a number of large single crystals of aluminium were prepared and presented and have been used throughout this work. At that time the necessary experimental facilities for X-ray work were not available to the authors at the National Physical Laboratory. Prof. Carpenter offered to arrange for the X-ray analyses to be undertaken by his assistant, Miss C. F. Elam, at the Royal School of Mines. This offer was gratefully accepted and the authors are greatly indebted to Miss Elam for carrying out this section of the work.

I. The behaviour of single crystals of aluminium under static and repeated stresses