Abstract
Near threshold fatigue crack growth and closure of a low carbon steel (0.18%C) with the duplex microstructure of ferrite-martensite were investigated. The highest value of 12MPa√m in the fatigue threshold stress intensity range, ΔKth, was obtained on a sample whose microstructure contained globular martensite in ferrite matrix with 47% in the volume fraction of martensite, among all the samples tested in the study. The threshold value, however, was less than those reported on AISI1008 and Fe/2Si/0.1C steels by Ritchie and his coworkers. No significant increase in ΔKth was observed in the microstructure where a continuous martensite phase encapsulated islands of ferrite. It was concluded that the excellent fatigue crack growth resistance at threshold in duplex steels was attributed to the optimum combination of the true crack growth resistance, ΔKeff, th, and the crack opening level, Kop.
