The influence of prestrain and ageing on near-threshold fatigue-crack propagation (FCP) in as-rolled and heat-treated dualphase steels (DPS) was investigated over a wide range of 10(-10) to 10(-7) mm/cycle in laboratory air under a load ratio of R = 0. It was found that the fatigue-crack propagation threshold value increases with increasing grain size and decreasing yield stress, and that a combination of 10% prestraining with 175-degrees-C/30 min ageing showed almost no effect on the threshold level of as-rolled dual-phase steel, but decreased that of heat-treated dual-phase steels more than 37%. This different behaviour was suggested to result from the remarkable differences in grain size and volume fraction of martensite (Vm) in these two kinds of dual-phase steels. An estimation of the plastic zone size of fatigue-crack tip supported this suggestion.
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