Observations of Fatigue Slip-Band Growth and Crack Initiation in alpha-Brass under Cyclic Shear Stresses by Means of Atomic-Force Microscopy

摘要

Slip-band formation and crack-initiation processes in a-brass under cyclic shear stress were examined by means of atomic force microscopy (AFM), and the slip-direction was identified with orientation imaging microscope (OIM). From AFM observations, it was found that slip-bands were not always formed along the maximum resolved shear stress directions, and slip-systems could be activated in the direction whose angles from the surface were larger than 22 deg. The depth of an intrusion increased linearly with the logarithm of the number of cycles, and the increasing rate of the intrusion depth drastically increased with its outgrowth to a crack. By combining the intrusion depth and the slip direction, those were measured with AFM and OIM, respectively, the value of slip distance could be evaluated, and the critical values of the slip distance for the initiation of transgranular crack was found to be constant for all crack initiation sites, while the intrusion depth was not constant. The critical value of the slip distance for cyclic shear stress (torsion) was identical for cyclic normal stress (bending). A unique relationship between shear stress amplitude in the slip direction and number of cycles to failure was obtained for cyclic torsion and bending loadings.