The effect of pearlite interlamellar spacing（d）from 0.31 to 0.77 μm of T8 steel on fatiguecrack propagation near threshold has been investigated at different stress ratios（R）.Theclosure stress intensity factor(Kcl),the threshold value(ΔKth)and crack tip opening dis-placement range（ΔCOD）were measured.The results show that pearlite interlamellar spac-ing in this range has no effect on Kcl,but has effect on the threshold value in low stress ratio.The threshold value is higher for materials with larger interlamellar spacing.No matter whatthe stress ratio is,during the propagation near threshold,the propagation rate will be thesame,provieded the crack tip opening displacement range is the same.Metallurgical examinations were also carried out to show that the crack propagationpath is sensitive to mierostructure.The crack always propagates along either boundary ofpearlite colony or ferrite lamellar in pearlite.
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