純鉛の疲労現象について

抄録

Lead is known to show a fatigue resistance in air much less than in vacuum, since it recrystallizes at room temperature. The another has made an optical-microscopic study on how the fatigue progresses in pure lead in air and how the structure of the metal changes.
As test samples, 99.998% pure lead was melted, cast, and extruded into 3.5 mm-thick plates. The samples were tested on a cantilever up- and -down-swing fatigue tester with the strain amplitude of 0.075% and at the speed of 600 c.p.m. Before the test, they vere polished chemically and etched for examination of their microstructures. The samples were subjected to 5,000 to 170,000 cycles of swing, and, on the way of the fatigue test, they were frequently inspected microscopically.
The observational results may be summalized as follows: Grain reveals slip band and a grain boundary migration occures. The latter is not a simple parallel migration of the grain boundary before fatigue, but shows a more complicated aspect, appearing as a black belt. This may be considered to be caused from the fact that the fatigue progresses while its grain boundary changes as influenced by oxygen air. This idea has been ascertained from the manner in which a grain boundary is corroded by chemical polishing and etching after fatigue. The depth of corroded furrows increased with the swing cycle. Thus, the fatigue is continued in this way, the strength of the grain boundary is decreased and cracks are developed, finally reaching the failure.