MIXED-MODE-I AND MIXED-MODE-II FATIGUE THRESHOLD AND CRACK CLOSURE INDUAL-PHASE STEELS

Fatigue threshold under mixed-mode I and II loading and elastic plane- strain conditions has been studied in dual-phase steels (DPS) of two t ypes of volume fraction of martensite (V(m)) in laboratory air at room temperature. Near-threshold mixed-mode (I and II) fatigue crack growt h occurs mainly by two mechanisms: shear mode, and tensile mode. Parti cular emphasis was placed on the influence of the mode II component. T he mixed-mode threshold is controlled not only by mode I displacement but also by the mode II component. Apparent- and effective-bound curve s (corrected closure) are obtained for the threshold condition and dis cussed in terms of the shape and size of the plastic region of crack t ip; crack surface rubbing; and especially, roughness-induced closure a nd shear resistance of crack surface that resulted in an extremely hig h extrinsic-toughening contribution to the mixed-mode fatigue threshol d 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 r esistance 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 fo r crack closure. It is proposed that the apparent- and effective-bound curves are nonconservative risky and too conservative for design purp oses, respectively. So, the threshold data should be obtained under th e specific conditions found by concrete mechanical, microstructural, a nd environmental factors.