1989 年 38 巻 426 号 p. 312-318
In order to observe the crack initiation and propagation behaviors, an original fatigue testing machine was designed and made, and it was combined to SEM in commercial type. By using this installation, fatigue crack initiation and propagation behaviors were directly observed on aluminum sheet specimens in attention to the long life region.
The main results obtained are summarized as follows; Some pores are formed at the intersections of slip bands in certain grains on the specimen surface, and these pores are connected to each other passing through the slip bands. Microscopic fatigue cracks formed in this mechanism are found in many surface grains, but the propagation rates of these microcracks have a large scatter. By adjoining some of these microcracks, a dominant crack appears somewhere on the specimen surface, and this crack grows gradually in company with the coalescense to further microcracks along the crack propagation path. The number of such predominant cracks in a specimen is only a few in the long life region of Nf>2×106. Another finding is that small fatigue cracks having the length such as L<0.1mm clearly open in full time of each cycle in pulsating tension.