抄録
The fatigue test has been carried out on 0.18%C carbon steel in order to investigate the effect of atmosphere on its fatigue strength. The experiments were conducted under reversed plane bending at the vacuum levels ranging from ambient pressure to 8×10-5 Torr. The difference of fatigue failure modes at these environmental conditions was observed by means of electron- and scanning electron microscopies, and furthermore, the surface roughness of specimens subjected to stress cycling was traced using a surface profilometer. The influence of atmospheric-corrosion on the fatigue failure of low carbon steel was discussed from mechanochemical viewpoints. The main results obtained are as follows:
(1) The fatigue life of carbon steel has prolonged with evacuating the air surrounding of a specimen. The endurance limit was observed in the specimens in vacuum in which the value of applied stress at the endurance limit was higher than that in air, and the number of cycles to the endurance limit observed in vacuum was not so much different from the value in air.
(2) The prolongation of fatigue life of carbon steel, which was found to be dependent on the degree of test pressure, was analyzed theoretically using a capillary model.
(3) The surface roughness of specimens subjected to fatigue stressing has increased in three stages. The surface roughness of specimens tested under a constant stress level has increased with reducing the pressure of test environment.
(4) Many cracks of trans-granular type were observed in the specimens tested in vacuum, while those of inter-granular type existed in the specimens tested in air.
