A Detailed Description Of Fretting Phenomena

A detailed description of fretting phenomena

Although fretting fatigue, fretting wear, and fretting corrosion phenomena are potential fail- are modes in a wide variety of mechanical systems, and much research effort has been devoted to the understanding of the fretting process, there are very few Gluantitative design data available, and no generally applicable design procedure has been established for predicting failure under fretting conditions. However, even though the fretting phenomenon is not fully understood and a good general model for prediction of fretting fatigue or fretting wear has not yet been developed, significant progress has been made in establishing an understanding of fretting and the variables of importance in the fretting process. It has been suggested that there may be more than 50 variables that play some role in the fretting process. Of these, however, there are probably only 8 that are of major importance:

1. The magnitude of relative motion between the fretting surfaces

2. The magnitude and distribution of pressure between the surfaces at the fretting interface

3. The state of stress including magnitude, direction, and variation with respect to time in the region of the fretting surfaces

4. The number of Fretting cycles accumulated

S. The material, and surface condition, from which each of the fretting members is fabricated

6. Cyclic freyue.ncy of relative motion between the two members being fretted

7. Temperature in the region of the two surfaces being fretted

8. Atmospheric environment surrounding the surfaces being fretted

These variables interact so that a quantitative prediction of the influence of any given variable is very dependent on all the other variables in any specific application or test. Also, the combination of variables that produce a very serious consequence in terms of fretting fatigue damage may be quite different from the combinations of variables that produce serious fretting wear damage. hlo general techniques yet exist for quantitatively predicting the influence of the important variables of fretting fatigue and fretting wear damage, although many special cases have been investigated. However, it has been observed that certain trends usually exist when the variables just listed are changed. For example, fretting damage tends to increase with increasing contact pressure until a nominal pressure of a few thousand pounds per square inch is reached, and further increases in pressure seem to have relatively little direct effect. The state of stress is important, especially in fretting fatigue. Fretting damage accumulates with increasing numbers of cycles at widely different rates, depending on specific operating conditions. Fretting damage is strongly influenced by the material properties of the fretting pair-surface hardness, roughness, and finish. Nor clear trends have been established regarding frequency effects on fretting damage, and although both temperature and atmospheric environment are important influencing factors, their influences have not been clearly established.

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