Recent Progress of Experimental and Measuring Technology. Quantitative Evaluation of Slip-Band Growth and Crack Initiation in Fatigue of 70-30 Brass by Means of Atomic-Force Microscopy.

Since the surface morphology of materials can be observed with atomic-scale resolution, scanning atomic force microscopy (AFM) in a powerful technique to study mechanisms of fatigue and fracture of solid materials. In the present study, slip-band formation and fatigue crack-initiation processes in 70-30 brass were observed by means of AFM. The slip-band angle relative to the stress-axis at the specimen surface varied from 15 to 90°, and it appeared most frequently around 60°. The depth of the intrusion drastically increased with its outgrowth to a crack, and with coalescence of cracks, the width of cracks increased rapidly. The critical value of the intrusion depth for crack initiation was given as a function of the slip-band angle through a geometrical model for the slip-direction and the slip-step, and the critical value was independent of stress amplitude. From presice observations of slip-band tips, slip-bands had steep slope when they were blocked by grain boundaries, and the slip-bands descended by gradual slopes to plain surfaces when they terminated within grains.