再現HAZ熱処理を施した低炭素鋼における微小疲労き裂の発生、進展挙動

抄録

Naturally initiated small fatigue crack and its growth behavior of several simulated HAZ microstructures were successively observed using automatic microscope system. Simulated HAZ microstructure specimen were prepared by heat treatment with controlling maximum temperature and cooling rate. In addition, crack initiation and early stage crack opening-closing behaviors were evaluated by Digital Image Correlation (DIC) technique. As a results, fatigue life of coarse grained microstructure was shorter than that of finer grained one due to the difference from not crack initiation life but crack growth life. Since initiated crack size, depending on microstructural unit size, of the coarse grained specimen was larger than the other, remained fatigue crack growth life became shorter. In addition, early stage crack opening stress of naturally initiated small fatigue crack were successfully measured by combination of automatic microscope system and DIC technique. This result showed that small fatigue crack just after initiation opens near the minimum load, under the compressive stress, and it was increased depending on crack length. However, crack opening stress of coarse grained specimen increased more slowly than that of finer grained specimen. This behavior also affected to fatigue crack growth life. Furthermore, early stage small crack growth life became comparable to long crack data by using effective J-integral range evaluated from measured crack opening point. Therefore, it was indicated that initial crack size and early stage crack closure model depending on microstructure are needed to calculate fatigue life.