Effects of microstructure and strain ageing on fatigue-crack initiation in steel

Abstract Evidence of the effects of grain size, precipitation hardening, and dynamic strain ageing on the long-life fatigue endurance of smooth mild-steel testpieces is reviewed. The fatigue limit behaviour of annealed mild steel involves a grain-size transition. When dynamic ageing is suppressed, fine-grained conditions still retain a well-defined fatigue limit but coarse-grained samples do not. In strongly locked fine-grained conditions the limit stress corresponds closely with the cyclic stress required to spread plasticity across grain-boundary barriers, but its grain-size dependence is much less than that of discontinuous yielding in monotonic straining. Tests using prestrained and coarse-grained conditions show that dynamic strain ageing can be a sufficient cause of the fatigue limit, but the usefulness of room-temperature ageing as a general fatigue-strengthening mechanism is limited. Atmosphere-locked substructures developed at low cyclic stresses are unstable at higher stress amplitudes. Elevated...

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