Fatigue crack growth retardation by control of microstructure in heavy steel plate

It has been generally recognized that the fatigue crack growth rate in the Stage II (b) regime is little influenced by the microstructure and strength of steels. In order to retard fatigue crack growth by control of microstructure in heavy steel plate, this study investigates the effects of morphology and distribution of the martensite phase in a ferrite matrix on fatigue crack growth behavior. Steels with various martensitic morphology and distribution are prepared in the laboratory, and their fatigue crack growth rates are measured. As a result, steel with an elongated and banded martensitic structure shows the highest resistance to fatigue crack growth in the thickness direction. The fatigue crack growth rate of the improved steel is less than one tenth of the rate of conventional steel. Statistical analysis indicates that the fatigue growth rate conforms to the Weibull distribution and that there is a significant difference between both steels. Fatigue cracks of the improved steel propagate with frequent deflection and branching at ferrite/martensite interfaces. These crack paths lead to a decrease in the driving force of crack growth and enhancement of roughness-induced crack closure. This can be attributed to the remarkable improvement of the fatigue crack growth property.