Some Observations of fatigue cracks of steels in various atmospheres

Abstract

It was clarified by the previous reports that crack propagation rate was predominantly affected by oxygen and/or humidity in atmospheres.
Then, plastically deformed zone at the tip of fatigue crack was observed by an optical and electron microscopies, in order to know why does the crack propagation rate change with atmospheres.
Crack propagation angle of fatigue tested specimen was also measured. A larger crack propagation angle will mean more plastically deformed cracking.
Generally speaking, the angle of fatigue tested specimen in the atmosphere without oxygen and/or humidity was larger than that in air. Significant difference of fracture surfaces were observed by electron microfractographies depending on the fatigue tested atmospheres. Some differences of hardness and slip line density at the tip of fatigue crack in different atmospheres were also observed. It can be concluded that crack in atmosphere without oxygen and/or humidity propagate with more plastic deformation than that in air.