Fretting

Fretting (or fretting corrosion) refers to combined wear and corrosion damage at the asperities of contact surfaces. This damage is induced under load and in the presence of repeated relative surface motion, as induced for example by vibration. The ASM Handbook on Fatigue and Fracture defines fretting as: "A special wear process that occurs at the contact area between two materials under load and subject to minute relative motion by vibration or some other force." The amplitude of the relative sliding motion is often in the order from micrometers to millimeters, but can be as low as 3 to 4 nanometers [ASM Handbook, Vol.13 "Corrosion", ASM International, 1987.] . The contact movement causes mechanical wear and material transfer at the surface, often followed by oxidation of the debris and the freshly-exposed surface. The oxidized debris can further act as an abrasive. However, fretting can also occur in inert gases or in vacuum (e.g., in space).

Fretting damage can often be identified by the presence of rouge (iron oxide powder) and the rough, torn appearance of the pitted surface.

Examples and significance

Some examples include the pitting of bicycle headset bearings due to fork flexing, wear of bicycle cranks caused by precession of the harder steel pedal spindle, and car wheel bearings becoming pitted due to the rocking motion they experience when new cars are transported by railway.

There is a current focus on fretting research in the aerospace industry. The dovetail blade-root connection and the spline coupling of gas turbine aerogines experience fretting during their service life.

Fretting fatigue

Fretting decreases fatigue strength of materials operating under cycling stress. This can result in so-called fretting fatigue whereby fatigue cracks can initiate in the fretting zone. Afterwards, the crack propagates into the material.

Mitigation

The fundamental way to prevent fretting is to design for no relative motion of the surfaces at the contact. Surface finish plays an important role as fretting normally occurs by the contact at the asperities of the mating surfaces. Lubricants are often effective in mitigation of fretting by reducing friction and inhibiting oxidation (e.g., reducing the exposure of the surface to air). Soft materials often exhibit higher susceptibility to fretting than hard materials of a similar type. The hardness ratio of the two sliding materials also has an effect on fretting wear. [A. Neyman, O. Olszewski, "Research on fretting wear dependence of hardness ratio and friction coefficient of fretted couple", Wear of materials, International conference No. 9, San Francisco CA, USA (13/04/1993). Wear, vol. 162-64, Part B, pp. 939-943, 1993.]

ee also

*Tribology
*MTCI
*Wear

External links

* [http://www.epi-eng.com/BAS-Fretting.htm EPI inc article]

References