材料期刊网

Fatigue threshold under mixed-mode I and II loading and elastic plane-strain conditions has been studied in dual-phase steels (DPS) of two types of volume fraction of martensite (V(m)) in laboratory air at room temperature. Near-threshold mixed-mode (I and II) fatigue crack growth occurs mainly by two mechanisms: shear mode, and tensile mode. Particular emphasis was placed on the influence of the mode II component. The mixed-mode threshold is controlled not only by mode I displacement but also by the mode II component. Apparent- and effective-bound curves (corrected closure) are obtained for the threshold condition and discussed in terms of the shape and size of the plastic region of crack tip; crack surface rubbing; and especially, roughness-induced closure and shear resistance of crack surface that resulted in an extremely high extrinsic-toughening contribution to the mixed-mode fatigue threshold values. The ratio of the threshold value of pure mode II to that of pure mode I (DELTAK(th,II)/DELTAK(th,I) attained highly to 1.9 times; the maximum hoop direction stress-intensity factor range of pure mode II branch crack tip is 2.2 times that of pure mode I. Obviously, the resistance of pure mode II crack growth here is far larger than that of pure mode I; the former is just to introduce the shear resistance of crack surface, the latter, to reduce the driving force of crack tip for crack closure. It is proposed that the apparent- and effective-bound curves are nonconservative risky and too conservative for design purposes, respectively. So, the threshold data should be obtained under the specific conditions found by concrete mechanical, microstructural, and environmental factors.