Abstract
The saturation behaviors of damage in fretting fatigue at frequencies of 1 and 20 Hz in seawater under freely corroding condition were studied using high strength steels having tensile strengths of 490, 690 and 880MPa. Fretting damage saturated beyond a certain number of fretting cycles, and this number varied drastically with the steel and the frequency. The saturation behaviors of damage could be divided into two groups, that is, the smallest numbers of fretting cycles to cause the saturation were less than 0.1% of the fretting fatigue life (group 1) and more than 40-60% (group 2). They had a strong relationship to crack initiation and growth behaviors. In group 1, cracks propagated normal to the alternating stress axis from the early stage of crack propagation. However, in group 2 the cracks propagated at angles less than 90° to the alternating stress axis, but changed direction to become normal as they propagated beyond a certain length. This relationship could be explained from the mechanism that the decrease in the fretting fatigue lives of group 1 resulted from the decrease in crack initiation life caused by the acceleration of corrosion pit formation, and that of group 2 from crack initiation and growth caused by fretting.
