VI. The phenomena of rupture and flow in solids

TLDR

Rupture is predicted when the maximum tensile stress or extension exceeds a critical value according to standard hypotheses for elastic solids. The study sought to determine how surface treatments such as filing, grinding, or polishing affect the fatigue strength of metallic machine parts under cyclic loading and to estimate safe load ranges for scratched surfaces. Fatigue tests on steel and other common metals measured the alternating stress ranges, and stress–strain calculations were performed using Hooke’s law and elasticity theory to assess safe loading. The tests showed that the fatigue limit for scratched steel and other metals is lower than the elastic range, indicating that surface scratches reduce the material’s ability to withstand repeated loads.

Abstract

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 the work, which was carried out at the Royal Aircraft Estab­lishment, was the discovery of the effect of surface treatment—such as, for instance, filing, 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 could 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. Hence it was inferred that the safe range of loading of a part, having a scratched or grooved surface of a given type, should be capable of estimation with the help of one of the two hypotheses of rupture commonly used for solids which are elastic to fracture. According to these hypotheses rupture may be expected if (a) the maximum tensile stress, ( b ) the maximum extension, exceeds a certain critical value. Moreover, as the behaviour of the materials under consideration, within the safe range of alternating stress, shows very little departure from Hooke’s law, it was thought that the necessary stress and strain calculations could be performed by means of the mathematical theory of elasticity.