CORROSION FATIGUE BEHAVIOR OF A HEAT-TREATED CARBON-STEEL AND ITS STATISTICAL CHARACTERISTICS

The investigation of fatigue damage in corrosive environments is an important problem, because such environments reduce fatigue strength far below the typical fatigue strength determined in air. In this study, rotating bending fatigue tests of plain specimens in NaCl solution were carried out using a heat-treated 0.45% carbon steel, in order to clarify the physical background of corrosion fatigue damage. The emphasis is to perform the successive observations by the plastic replica method. The results show that corrosion pits are generated at the early stages of cycling, then most of them grow with further cycling until a crack is initiated from each corrosion pit. The initiation of corrosion pits from slip bands is only observed in the case when the stress range is relatively large, in the range of stress under which slip bands are produced in air. After initiation of a crack, a crack propogates by accompanying frequent interaction and coalescence with other cracks. The growth rate of an especially small crack in NaCl solution is larger than that in air. However, the growth rate of a comparatively large crack is smaller in NaCl solution than in air. Moreover, the statistical characteristics of corrosion fatigue behavior were investigated by exhibiting the distributions of crack initiation life and crack length.