Microscopic and Mesoscopic Evaluations of Materials. Observation of Fatigue Crack Initiation Process in .ALPHA.-Brass by AFM.

Abstract

Since the surface morphology of materials can be observed with atomic-scale resolution, scanning atomic force microscopy (AFM) is a powerful technique to study mechanisms of fatigue and fracture of metallic materials. In the present study, slip band formation and fatigue crack initiation processes in α-brass were observed by means of AFM. Surface of a fatigued specimen was observed under maximum stress. unloading state, and minimum stress at fatigue cycles. In the initial stage of the fatigue process, slip bands which formed only under tension stress or compression stress were observed. These kinds of slip bands, however, disappeared shortly. Under tension stress, cracks could be detected easily just after initiation. For specimens at the unloading state, cracks just after initiation were hardly detected from the AFM images, By measuring the size of slip bands at the unloading state, however, crack initiation from slip bands could be detected. Before crack initiation heights and width of extrusions and the depth and width of intrusions gradually increased with the number of cycles. After crack initiation, one of these values changed drastically, depending on the slip band angle relative to the stress axis and the shape of slip bands.