13th International Conference on Fracture June 16–21, 2013, Beijing, China -6- different crack growth mechanism emerges, developing a rather smooth fracture surface which together with the FGA forms the characteristic fisheye morphology. The cycle dependence of FGA formation explains the dual step S-N-curve (figure 1a). In the VHCF regime a minimum number of cycles is necessary before continuous crack growth in the form of the smooth fisheye fracture surface results in overall failure. a) b) Fig. 6: Correlation between area of the FGA and number of loading cycles (a) and correlation between fisheye formation and resonance frequency (b). The smooth fracture surface of the fisheye develops as long as the crack growth is still within the interior of the specimen. Hence, the size of the fisheye fracture is directly correlated to the distance of the crack initiating inclusion to the specimen’s surface as is depicted in figure 7 for several fracture surfaces. It is well known that crack growth under vacuum is retarded compared to crack growth in air for the material analyzed [20]. As a consequence, cracks will propagate very slowly since vacuum like conditions are still given while the crack front has not yet reached the surface, but will gain momentum the very moment the crack front touches the surface. A correlation between fisheye diameter (150 m up to 1000 m) and a predicted as well as measured change in resonance frequency during fatigue testing (figure 6b) revealed, that the formation of fisheye is limited to 2000-3000 cycles of the overall fatigue life. Hence, the major part of fatigue life in the VHCF regime is dominated by the formation of the FGA. As the development of a FGA is the fatigue life dominating feature, its formation mechanism should be further discussed. Different assumptions as to the origin of the FGA were presented in the introducing chapter. According to Shiozawa et al. [10] the FGA is formed by a decohesion of finely dispersed carbides in the microstructure. This assumption is not plausible for AISI304 with its negligible carbon content (0.02 mass-%). Neither carbides nor microcracks at carbides could be detected in the FGA environment of the specimens (depicted in figure 7) by means of scanning electron microscopy and EDS-analysis. The suggestion made by Sakai [9], that FGA is formed due to a polygonization of very fine subgrains is yet to be discussed as no pronounced fine grained structure could be detected in the crack initiation region. Murakami et al. [21] assumed that hydrogen trapped at the inclusion leads to the formation of FGAs due to the higher dislocation mobility in the presence of hydrogen. Hydrogen can lead to accelerated crack propagation in the martensitic phase of a metastable austenitic connected and moreover, the hydrogen diffusion coefficient in martensite is four orders of magnitude higher than that in the austenitic phase. Therefore, the model of hydrogen-assisted crack initiation at an areaFGA [m1/2] cycles cycles frequency [Hz] NF NF NF crack initiation at the surface, 1.17 x 106 Fish eye, 9.09 x 106 and at 1.49 x 108 end of test
RkJQdWJsaXNoZXIy MjM0NDE=