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

Abstract

The crack initiation mechanisms and the fatigue life of α-iron polycrystals have been characterized in plastic-strain-controlled tests with small to intermediate plastic strain ranges Δ ϵ pl in the temperature range from 220 to 400 K. The fatigue life of pure decarburized α-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 valid, when the effective stress component σ∗ dominates (i.e. at low temperatures and small Δ ϵ pl). At low temperatures and up to room temperature, intergranular crack initiation is observed. At higher temperatures, mostly transgranular cracks develop in decarburized α-iron in the valleys of the surface rumpling, while in α-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.