The effect of microstructure and prestrain on fatigue crack propagation of dual-phase steels

The effects of microstructure and prestrain on the propagation behavior of small fatigue cracks in dual-phase (DP) steels were investigated by in-situ observation with fatigue testing in scanning electron microscope. The material with a continuous martensite phase around ferrite grains exhibited a higher fatigue strength and longer fatigue life than those of the material with martensite phase dispersed in ferrite matrix. Cold rolling prior to fatigue testing resulted in a decrease of fatigue life in both materials, especially an extreme decrease in the martensite-dispersed material. It was found that fatigue crack propagation was sensitive to microstructure and prestrain only in the small crack within a range of approximately 250 mu m and the propagation life of this region governed the total fatigue life. Furthermore the effects of microstructure and prestrain on macroscopic large fatigue crack propagation were studied. The microstructure and cold reduction exerted a great influence on crack path and that caused a large difference in crack closure. A quantitative correlation existed between crack closure level and surface roughness of fatigue fracture.