In the course of an investigation of the effect of surface scratches on the mechanical strength of solids, some general conclusions were reached which appear to have a direct bearing on the problem of rupture, from an engineering standpoint, and also on the larger question of the nature of intermolecular cohesion. The original object of this work was the discovery of the effect of surface treatment, such as, grinding or polishing, on the strength of metallic machine parts subjected to alternating or repeated loads. In the case of steel, and some other metals in common use, the results of fatigue tests indicated that the range of alternating stress which should be permanently sustained by the material was smaller than the range within which it was sensibly elastic, after being subjected to a great number of reversals.
