INFLUENCE OF TEMPERATURE AND CARBON CONTENT ON THE CYCLIC DEFORMATIONAND FATIGUE BEHAVIOR OF ALPHA-IRON - PART-II - CRACK INITIATION AND FATIGUE LIFE

The crack initiation mechanisms and the fatigue life of alpha-iron pol ycrystals have been characterized in plastic-strain-controlled rests w ith small to intermediate plastic strain ranges Delta epsilon(pl) in t he temperature range from 220 to 400 K. The fatigue life of pure decar burized alpha-iron cannot be described generally by means of a Manson- Coffin law, except under conditions under which distinct cyclic harden ing occurs. A Basquin law is found to be valid, when the effective str ess component sigma dominates (i.e. at low temperatures and small Del ta epsilon(pl)). At low temperatures and up to room temperature, inter granular crack initiation is observed. At higher temperatures, mostly transgranular cracks develop in decarburized alpha-iron in the valleys of the surface rumpling, while in alpha-iron with small carbon conten ts persistent slip bands form which then act as sites of transgranular crack initiation. Crack initiation rates and crack growth rates are e valuated ina semi-quantitative manner from crack depth distributions. (C) 1998 Acta Metallurgica Inc.