EFFECT OF ENVIRONMENT ON THE MECHANISM OF FRETTING FATIGUE

A randomized complete block fretting fatigue test program was designed to determine statistically the relative roles of the mechanical factor and the chemical factor in the mechanism of fretting fatigue. Fretting fatigue tests were performed in both laboratory air and vacuum environments with the normal pressure and horizontal maximum fatigue stress kept constant at 6000 lbf in-2 and 50 klbf in-2 (within experimental error), respectively. It was found that the number of cycles to failure of the specimens tested in vacuum is between 10 and 20 times longer than that tested in laboratory air. It was determined that the three parameter Weibull distribution best fits the fretting fatigue data for both test environments. The difference in the mean lives of the specimens tested in vacuum and in laboratory air was determined to be significant with 95% confidence. Thus, the alternative hypothesis which states that the chemical factor plays the dominant role in reducing specimen life when fretting occurs simultaneously with fatigue is accepted. Fractographic analysis on the wear surface of the specimen tested in air revealed that pitting was very common while that performed on the wear surface of the specimen tested in vacuum indicated that rewelding of metal was very common. The overall fretting fatigue damage in vacuum was found to be less than that in laboratory air. The results of the fractographic analysis support the alternative hypothesis. © 1979.