Small Fatigue Crack Initiation and Growth Behavior in Notch Root for Low Carbon Steel

Abstract

To clarify the small fatigue crack initiation, growth and crack closure behavior at the stress concentration area of welding, microscopic successive observation with automatic microscope system was conducted for notch root of heat treated low carbon steel, which simulate coarse grain (CGHAZ) and fine grain (FGHAZ) heat affected zone (HAZ) microstructure. As a result, fatigue life of notched CGHAZ specimen was shorter than that of FGHAZ. In addition, to evaluate the small fatigue crack growth behavior at notch root, this study proposed to use crack opening displacement (COD) as a related value of the crack depth. The COD, which is related to small fatigue crack size, of CGHAZ grow faster than that of FGHAZ microstructure. Furthermore, small fatigue crack opening stress were directly measured with digital image correlation (DIC) analysis. Small fatigue crack closure at notch root were successfully measured with microscope system with DIC analysis. As a result, CGHAZ microstructure shows lower crack opening stress than FGHAZ. This is one of the reason why CGHAZ shows shorter fatigue life and faster COD growth rate than FGHAZ.