Evaluation of fatigue damage and fatigue crack initiation process by means of atomic-force microscopy

Abstract

Researches on fatigue crack-initiation processes in metals by using atomic-force microscopy (AFM) were reviewed. For fatigue of a low-carbon steel and α-brass in air, morphologies and sizes of slip-bands and crack initiation processes were discussed, and it was found that the depth of an intrusion drastically increased with its outgrowth to a crack. With coalescence of cracks, the width of cracks increased rapidly. Transgranular cracks were initiated when their accumulated sliding distances, those were evaluated from measured intrusion depth as a function of intrusion angle relative to the stress-axis, reached a critical value. The critical value was independent of stress amplitude, mean stress, and grain-size. Change of surface roughness during fatigue process was also presented. In corrosion fatigue crack initiation study of a stainless steel and a high-strength aluminum alloy, even for a crack whose optical micrographs showed that it was initiated from corrosion pit, a micro-crack was sometimes found from AFM images before pit formation. On the other hand, for a crack whose optical micrographs showed that it was initiated from grain-boundary without corrosion pit, AFM image sometimes indicated that there was a small pit at the crack initiation site. These results showed that AFM is a very useful and powerful tool for elucidating micro-mechanisms of fatigue crack initiation process.