Distributions of Fatigue Life and Corrosion Pit of Low Alloy Steels, SCM435 and SNCM439, in Corrosive Environment.

Abstract

Statistical fatigue tests were conducted on low alloy steels, SCM 435 and SNCM 439, in room air and in 3%NaCl solution. Fatigue life distributions were examined at three stress levels, and were analyzed by the three-parameter Weibull distribution. In both environments, fatigue life distributions of both steels followed the three-parameter Weibull distributions. The scatter of fatigue life in 3% NaCl solution was smaller than that in room air. From the observation of the fracture surfaces in 3% NaCl solution, it was found that fracture of SCM 435 at all stress levels resulted from the growth of a single crack, while that of SNCM 439 occured by the growth of multiple cracks emanating from corrosion pits. The distributions of the size and aspect ratio of corrosion pits followed the log-normal distributions. The size increased and the aspect ratio decreased, with decreasing stress level. The stress intensity factors at which cracks emanated from corrosion pits increased with increasing stress level.