Original paper(Vol.45 No.8 pp.933)

Influencing factors on fatigue crack propagation of high strength steels in air

Sugiyama Yoshihiro; Fukuda Naohisa

Abstract:High strength steel is so sensitive to environment that it is liable to corrode in air. Therefore, it is necessary to consider the influence on fatigue crack propagation of high strength steel in air. In fact, the experiments in the past showed that the influence of moisture in air appeared as acceleration effect of da/dN. However, there are a large number of unknown factors about the details of the subject. In order to investigate the influencing factors of high strength steels on fatigue crack propagation characteristics at the stage 2, the fatigue crack propagation tests and SEM observations of fracture surface were carried out under the stress ratio R of 0.1 in air and dry air.
The results obtained are: (1) The acceleration of da/dN by moisture in air is dependent upon the static mechanical properties and the length of stage 2b area. And the acceleration is due to the appearance of intergranular fracture due to hydrogen induced cracking. (2) The acceleration effect differs with the difference of frequency as a result of quasi-cleavage of fracture due to hydrogen induced cracking. (3) The acceleration of da/dN decreases with an increase in pre-austenite grain size.

Key Words:high strength steel, fatigue crack propagation, acceleration effect, hydrogen induced cracking