Slip-band formation and fatigue crack-initiation processes in α-brass were observed by means of atomic force microscopy (AFM), and the effects of the grain size and the stress amplitude were discussed. In a fine grain material, fatigue cracks were initiated only from slip-bands. In a coarse grain material, however, they were initiated either from slip-bands or grain boundaries. The depth of an intrusion drastically increased with its outgrowth to a crack, and with coalescence of cracks, the width of cracks increased rapidly.
The depth of an intrusion increased with the number of loading cycles, and when the depth reaches a critical value, a transgranular crack was initiated from the intrusion. The critical value was given as a function of the slip-band angle relative to the stress axis. From the AFM observations, it was found that the critical value of the slip distance was independent of the slip-band angle relative to the stress axis, the stress amplitude, and the grain-size. For crack-initiations from grain boundaries, however, the value of the grain boundary depth at the crack initiation was not a unique function of the grain boundary angle relative to the stress axis.