Synopsis
It is not certain, from theory whether a preliminary period of dry fatigue should lengthen the normal corrosion-fatigue life by strain hardening, or shorten it by producing disarrayed metal which may be particularly susceptible to corrosion. An answer to this question was obtained by carrying out two-stage tests; a period of dry fatigue was followed by corrosion fatigue up to failure; for comparison, a second set was submitted to corrosion fatigue without previous dry fatigue. With both cold-drawn and annealed wires, little or no change in corrosion-fatigue life was brought about by preliminary dry fatigue under the conditions tested.
A metallographic study was also made of changes during air-fatigue and corrosion-fatigue processes. For this purpose, the annealed wires were reduced by electropolishing to remove surface defects and to avoid deformation. In air fatigue, the repeated application of a stress, greater than the fatigue limit, results in the plastic deformation of many grains as a preliminary to the development of micro-cracks and failure. Simultaneously, heat is generated in. the specimen and a superficial oxidation of the surface follow’s.
In corrosion fatigue, at a stress below the air-fatigue limit, a localized attack leads to stress intensification and hence to corrosion along slip bands. Attack spreads around the circumference of the wire, with subsequent weakening.
The major damage, estimated by loss in tensile strength, occurs towards the end of the process, both in air fatigue and corrosion fatigue. Cracking is mainly transcrystalline, but may occasionally be intercrystalline for distances of a few grain diameters.
