EFFECT OF HUMIDIY ON LONG FATIGUE LIFE MORE THAN 107 CYCLES IN HIGH STRENGTH STEEL M. Nakajima 1, K. Tokaji 2 and T. Shimizu 1 1 Department of Mechanical Engineering, Toyota National College of Technology, 2-1 Eisei-cho, Toyota 471-8525, Japan 2 Department of Mechanical and Systems Engineering, Faculty of Engineering, Gifu University, 1-1 Yanagido, Gifu 501-1193, Japan ABSTRACT Long-term fatigue tests have been conducted in order to clarify the effect of humidity on the fracture mechanism in long life region more than 107 cycles. The material used was a low alloy steel, JIS SNCM439 (AISI4340). Tests were performed using cantilever-type rotating bending fatigue testing machines operating at a frequency of 3150rpm in laboratory air and in dry air. The dew point of dry air was – 60°C 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, the similar step-wise S-N curve was obtained, but the position of the horizontal line in the S-N curve shifted to high stress level i.e. short life region. Fracture mode at stress levels below the horizontal line was fish-eye type in both environments, i.e. cracks were initiated from a nonmetallic inclusion at subsurface. Since two different fracture modes, i.e. surface slip type and fish-eye type, coexisted, fatigue life distribution was scattered at the stress level of the horizontal line. Therefore, it is concluded that the appearance of horizontal line was due to the scatter of fatigue life corresponding to the fracture mode. INTRODUCTION It has been indicated that fatigue failure often occurred at stress levels below the conventional fatigue limit in long life region [1-3], particularly seen in high strength steels and surface-treated steels [4]. Provided design is conducted in accordance with the fatigue limit, the reliability of machine components could be faced with a difficulty by the above aspect. Such a failure is known to be caused by fish-eye which is developed from a nonmetallic inclusion in the interior of materials [1-4]. Furthermore, high strength steel is sensitive to the aqueous environments and hydrogen embrittlement is caused by the humidity in air [5]. It is recently indicated that the formation of fish-eye can be related to the environmental effects such as hydrogen embrittlement [6]. However, the source of this hydrogen is unidentified whether it is picked up in the manufacturing process or during fatigue test. If hydrogen is picked up during fatigue test, it is expected that the experiments in dry air would bring the interesting results which are different from those obtained in laboratory air. Many studies have been performed on the characteristic fatigue behaviour, while the effect of humidity on the fracture mechanism in long life region has not been fully understood. In addition, although fatigue testing machines operating at a high frequency more than 104 Hz are used to save the time in high cycle fatigue [1-2,7], most of the machine components are not subjected to such a high frequency of stress fluctuation. Moreover, it becomes difficult to detect the environmental effects in short time fatigue tests. Accordingly, it seems that fatigue testing machine operating at a moderate frequency should be used, while much time would be spent.
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