CHARACTERIZING AND COMPARING FRETTING FATIGUE AND PITTING IN LOW-AMPLITUDE RECIPROCATING CONTACT

Fretting fatigue has consistently been described as an "insidious" fracture process due to the difficulty in modeling and predicting fretting-induced crack initiation. Components can develop fretting fatigue cracks even when they are designed for minimal relative motion, as in the case of a turbine blade root. Vibrations that are typically small enough to be considered negligible in engineering analysis can cause cracks that will lead to component failure. The sliding distance for fretting to occur is loosely defined as tens to hundreds of micrometers. To date, there has not been a good delineation between the fretting motion and gross-sliding regimes. Likewise, it is not well understood when a given component will experience fretting fatigue or pitting (which is associated with gross sliding and is often seen in gear components). Preliminary data suggest that pitting-like cracks can initiate in a hemisphere-on-flat linear reciprocating configuration at a low number of cycles (104) and fretting-sized displacements (200-300 mu m). Because of the differences between the mechanisms for failure in fretting and pitting, new insight must be developed to determine parameters under which to expect either failure mode. This work seeks to characterize these two forms of failure and to determine the conditions under which fretting or pitting becomes dominant to develop a new tool for the prediction and prevention of moving components.