The metallography of fatigue

Abstract

The object of this review is to consider the contribution that metallography has made in the last decade to an understanding of the fatigue process in solids, particularly metals and alloys. During this period purely observational experiments have become rare and metallography has therefore been broadly interpreted as the interrelation of structure and properties. A useful starting point is the comprehensive ‘state of the art’ review of Thompson and Wadsworth published in 1958. It was then well known that fatigue-crack initiation, in smooth specimens of ductile crystalline materials, often occurred in slip bands. Forsyth had observed the slip-band extrusion effect, and Cottrell and Hu1l and Forsyth had discovered the converse process, slip-band intrusion. Thompson et al. had reported the persistent slip-band phenomenon and demonstrated that, if a specimen had been fatigued for a substantial proportion of its nominal life, surface removal could restore the specimen to the ‘as-new’ condition. Moreover, this process could be repeated to extend the life of the specimen indefinitely. Cyclic deformation had also been observed to produce, under particular experimental conditions, polygonisation, recrystallisation, and grain-boundary migration.