THE EFFECT OF MICROSTRUCTURE AND PRESTRAIN ON FATIGUE CRACK PROPAGATION OF DUAL-PHASE STEELS

摘要

The effects of microstructure and prestrain onthe propagation behavior of small fatigue cracks in dual-phase(DP) steels were investigated by in-situ observation withfatigue testing in scanning electron microscope. The materialwith a continuous martensite phase around ferrite grainsexhibited a higher fatigue strength and longer fatigue life thanthose of the material with martensite phase dispersed in ferritematrix. Cold rolling prior to fatigue testing resulted in adecrease of fatigue life in both materials, especially an extremedecrease in the martensite-dispersed material. It was found thatfatigue crack propagation was sensitive to microstructure andprestrain only in the small crack within a range ofapproximately 250#mu#m and the propagation life of thisregion governed the total fatigue life. Furthermore the effectsof microstructure and prestrain on macroscopic large fatiguecrack propagation were studied. The microstructure and coldreduction exerted a great influence on crack path and thatcaused a large difference in crack closure. A quantitativecorrelation existed between crack closure level and surfaceroughness of fatigue fracture.