Characterization of Fretting Damage in a Press-fitted Shaft below the Fretting Fatigue Limit

초록

Most of the railway axles are designed to endure 108 or more load cycles without failure. In addition, regular non-destructive tests are performed to detect the fatigue crack which may occur at the fitting part. In most cases, the dynamic stress applied to axles during operation is below the fatigue limit. However, because the fatigue crack can be takes place under the fatigue limit in railway axles, it is required to understand the characteristics of the fretting fatigue and fretting wear below the fatigue limit to secure the safety of railway axles. In this paper, the characterization of contact surface damage due to fretting in a press-fitted shaft below the fretting fatigue limit is proposed by experimental methods. A series of fatigue tests and interrupted fatigue tests on the small scale press-fitted specimen were carried out by using rotating bending fatigue test machine. Macroscopic and microscopic characteristics were examined using scanning electron microscope (SEM), optical microscope or profilometer. It is found that fretting fatigue cracks were initiated even under the fatigue limit on press-fitted shafts by the fretting. The fatigue cracks of press-fitted shafts were initiated from the edge of contact surface and propagated inward in semi-elliptical shape. Furthermore, the fretting wear rates at the contact edge are increased rapidly at the initial stage of total fatigue life. After steep increasing, the increase of wear rate is nearly constant under the load condition below the fretting fatigue limit. It is thus suggested that the fretting wear must be considered on the fatigue life evaluation because the fatigue crack nucleation and propagation process is strongly related to the evolution of surface profile by fretting wear in the press-fitted structures.