Study of Fatigue in Metals Using Ultrasonic Technique

Abstract

An investigation of the basic mechanisms associated with cyclic and fatigue stressing was undertaken employing as a research tool a megacycle frequency internal friction method (ultrasonic wave attenuation). The materials used were plain high-carbon and alloy steels (SAE 1095 and SAE 4340). The behaviour of lattice imperfections, dislocations and interstitial atoms (carbon) in particular, was studied in more detail, since these are reported to have a strong influence on the fatigue characteristics of iron-base alloys.

Initially, static stressing conditions were studied. Using a dislocation loop pinning model, the conditions for the break-away of a dislocation loop from interstitial atoms in annealed and in cold-worked material were investigated. These findings were applied to the experimental results obtained on cyclic stressing and an interpretation is offered in terms of the formation of active dislocation loops and their immobilization by interstitial atoms.

The results are discussed with reference to improving fatigue characteristics of iron-base alloys and a similarity with the work of Mason (19) on non-ferrous metals is noted.