Abstract
The smooth specimen of JIS S35C and SNCM 439 steels, which were heat-treated in several conditions, were low-cycle fatigued. Then, the growth rate of surface small crack, da/dN, was analyzed in terms of the cyclic J-integral range, ΔJ, and the strain intensity factor range, ΔKe. The crack growth rates plotted against ΔJ and ΔKe are approximately expressed by linear lines on a log-log diagram. The considerable scatter inherent in problems of a small crack, however, almost cover the small difference in the crack growth property among materials. As a result, the universal equations can be established for da/dN vs. ΔJ and da/dN vs. ΔKe relations, independently of kinds of materials used. The equations derived from combining the above two relations with the cyclic stress-strain curve describe well the fatigue life curve which bends downwards in the regime of the lower plastic strain range in materials of a high strength and low ductility as well as usual Coffin-Manson plots.
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