Influence of temperature and carbon content on the cyclic deformation and fatigue behaviour of alpha -iron. II. Crack initiation and fatigue life

The crack initiation mechanisms and the fatigue life of alpha -iron polycrystals have been characterized in plastic-strain-controlled tests with small to intermediate plastic strain ranges Delta epsilon sub pl in the temperature range from 220-400K. The fatigue life of pure decarburized alpha -iron cannot be described generally by means of a Manson-Coffin law, except under conditions under which distinct cyclic hardening occurs. A Basquin law is found to be void, when the effective stress component sigma exp * dominates (i.e. at low temperatures and small Delta epsilon sub pl ). At low temperatures and up to room temperature, intergranular crack initiation is observed. At higher temperatures, mostly trangranular cracks develop in decarburized alpha -iron in the valleys of the surface rumpling, while in alpha -iron with small carbon contents persistent slip bands form which then act as sites of transgranular crack initiation. Crack initiation rates and crack growth rates are evaluated in a semi-quantitative manner from crack depth distributions.