CHARACTERIZATION OF FATIGUE CRACK INITIATION CONDITION OF STEELS WITH AFM

Slip-band formation and fatigue crack-initiation processes in a high strength steel and a stainless steel were observed by means of atomic force microscopy (AFM). In the high strength steel, fatigue cracks were initi-ated in the slip band just adjacent to the inclusions. In the stainless steel, they were initiated in the slip bands near the grain boundaries. The slip distance could be evaluated from intrusion depth and slip band angle relative to the stress axis. In both materials, the distance increased linearly with the logarithm of the number of cycles. When the slip distance reached a critical value, a transgranular crack was initiated from the slip band, and the slope of the relationship was changed with crack initiation. For every crack initiation site, the slip distance at the crack initiation was almost identical, and this critical value was about 80 nm for the two steels. In α-brass, this critical value was 380 nm. Then this critical value may be controlled by atomic binding energy or atomic spacing independent of microstructure of the material. 1