Step-Wise S-N Curve Assisted by Humidity in a High Strength Steel
Masaki NAKAJIMA, Keiro TOKAJI, Toshihiro SHIMIZU, Hisatake ITOGA and Haeng-Nam KO
Abstract:The present paper describes the effect of humidity on the fracture mechanisms and associated morphology of the S-N curve in a high strength steel, JIS SNCM439. Fatigue tests were conducted on cantilever-type rotating bending fatigue testing machines operating at a frequency of 3150rpm in laboratory air and in dry air up to long life region around 109 cycles. The dew point of dry air was - 60. In laboratory air, fatigue failure occurred at stress levels below the conventional fatigue limit, leading to a step-wise S-N curve. Also in dry air, similar step-wise S-N curve was obtained, but the horizontal part of the S-N curve shifted to a higher stress level than in laboratory air. Fatigue lives were the same in both environments at stress levels above the horizontal part in dry air and below the horizontal part in laboratory air, because the same fracture mode operated in those regions, surface-related initiation in the former and subsurface crack initiation with a fish-eye in the latter. At stress levels between the horizontal parts in both environments, surface-related initiation and subsurface crack initiation were seen in laboratory air and in dry air, respectively. Fatigue lives were significantly shorter in laboratory air due to humidity-assisted crack initiation and small crack growth. Based on the experimental results, the environment dependence of fracture mode and associated morphology of S-N curve is discussed. Key Words:Fatigue, Step-wise S-N curve, Fracture mode, Fish-eye, Humidity, Dry air, High strength steel