It is investigation in detail and study systematically o fatigue crack propagation threshold and crack tip shielding behavior in dual-phase steels (DPS) that will possess important, opening significance of theoretical direction and practical value applied to design of fatigue and materials of various new composite materials which are risen in near years. Hence, it is studied systematically in this paper to the fatigue crack growth threshold of mixed-mode (I/II); the shielding behaviors of long, short crack tip; the fatigue crack growth thresholds and the crack closure behaviors affected by a wide range of positive, negative stress ratios, specially, compression mean load; as well as the change of crack tip dislocation morphologies in different stages of crack growth, laying stress on the relation between the dislocation structures of long, short crack tip in fatigue threshold and effective fatigue crack growth threshold values, etc. DPS of two kinds volume fractions of martensite (V_m) were mainly chosen in this paper, in order to compare that different roughness on fracture surface would bring a series of change of fatigue crack growth threshold values, crack closure etc, result, it can revel common law of effective fatigue crack growth threshold value and controlling mechanism of crack closure behavior.
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