The Strength of Materials as Affected by Discontinuities and Surface Conditions J

JSGT_1932_V16_T182_T209

The paper describes experiments to determine the effect of discontinuities and surface conditions of metals on the behaviour of the materials when subjected to cycles of repeated stress. When test specimens are prepared from metals by turning and finishing the surface in the lathe or grinding machine, the safe range of stress that can be indefinitely applied to the metal at a specified mean stress can generally be expressed as a fraction of the ultimate tensile stress. This safe range, for a mild steel at zero mean stress is about equal to the tensile strength of the material. If however, during the test the surface is subjected to corrosive conditions (for example, it may be kept wetted by distilled water or by an acid or salt solution), the safe range of stress for say 10 million repetitions may be lowered to much less than one-half of the breaking strength and the tests described showed that the fatigue range may become very small. In other words, under such conditions, failure in time will occur at very small stress ranges. Photomicrographs are given showing interpenetration of corrosion lines into specimens that had not broken after more than 100 million repetitions of stress. Cooled and flat plate springs are heated during manufacture and are finally heat-treated and left in the black condition. The paper describes experiments which show that the safe range of repetition stress for such springs may be less than 30% of the safe range of the same material when the surface layers have been removed by machining and grinding. It is also shown that cold-drawn wires, although apparently quite smooth and exhibiting no surface discontinuities, have a fatigue range of from 16 to 60% of that which might be expected from a machined specimen of the same composition and same tensile stress. It is suggested, and evidence given that the decarbonised surface is not sufficient to account for the difference. The effects of scratches, screw threads, grooves, key-ways, drilled holes and other discontinuities in considerably reducing the safe fatigue range, both under cycles of stress and blows, are discussed and illustrated. Reference is made to experiments on specimens subjected to stress cycles as cathodes, nascent hydrogen being liberated at their surfaces and penetrating the metal and it is shown that discontinuity produced by this gas does not lower the fatigue range, but it changes the character of the fracture. The effect of nickel deposits on the surfaces of metals subjected to repeated stresses is discussed. It is suggested that the phenomena are explained by assuming stress concentrations at discontinuities and that in the case of heat-treated and cold-drawn materials imperceptible discontinuities at grain boundaries lead-to the failure at low ranges of stress.

F. C. Lea