In this dissertation, near-threshold fatigue crack propagation behavior of the as-rolled dual-phase steel, and the heat- treated dual-phase steels, with different morphologies and volume fractions of martensite, has been investigated in the laboratory air and 3.5% NaCl solution under different stress ratios from both theoretical and experimental aspects. In the theoretical respect, the dislocation-free zone theory of the crack tip was firstly used to calculate fatigue crack propagation threshold. According to this, the threshold behavior of physically short fatigue crack and the variation of fatigue threshold with stress ratio can be successfully explained. Based on the effect of the volume fraction of martensite on the yield strength and crack growth resistance, a quantitative relationship between the fatigue crack growth threshold and volume fraction can be obtained, which is in agreement with the experimental results. In addition, the two empirical formulas between the fatigue threshold and stress ratio ad well as grain size can be deduced from the consideration above. In the experimental aspect, it was found that the morphology of martrensite in dual-phase steels can significantly influence near-threshold fatigue crack growth behavior. Compared with the microstructure with islanded martensite, the microstructures with acicular and netted martensite possess the higher fatigue thresholds and the lower fatigue crack growth rates in both laboratory air and 3.5% NaCl solution. As the volume fraction of martensite increases, fatigue threshold is found to increase firstly, and then decrease, i.e., a peak value of fatigue threshold appears. In the 3.5% NaCl solution, corrosion fatigue crack growth mechanism is found to be hydrogen embrittlement, resulting in the accelerative propagation of the fatigue crack. However, corrosion fatigue threshold is much higher than that obtained in the laboratory air. This is due to the strong crack closures caused by the higher roughness and the serious corrosion products. It was also shown that the fatigue crack propagation threshold of dual-phase steels decreases linearly with increasing stress ratio. Moreover, the fractal geometry was used to describe the fatigue fracture characteristic in the present research. It was found that fatigue threshold is closely associated with the fractal dimension of the fatigue fracture, and the larger the fractal dimension, the higher the fatigue threshold.
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