Chapter 8-Fretting Tests
Pages: 7 Published: Jan 2007
FRETTING DAMAGE OCCURS IN COUNTLESS applications where it is not recognized or where it is ignored. It is also relatively ignored in testing practices, and that is why an entire chapter is dedicated to this form of “nonabrasive wear.” Hopefully, this attention will counteract the relative neglect that it receives from the technical community. One significant reason for fretting damage neglect is that the damage is frequently “visually negligible.” A classic example is fretting damage on wires in rope and other metal-strand cables. Repeated use involves elastic elongations of individual wires. Because of the braiding, the wires do not elongate the same under load, and some wires are stretching more than others, which means microscopic relative slip. Eventually, the wires that are subject to the most fretting motion start to fail because the fretting damage produced a deep pit that acted as a stress concentration. However, the wire rope user may never see any visual external damage until enough wires have failed to produce dangerous damage. Often, the damage is never detected until a load is dropped when the entire wire rope or cable breaks. Fretting corrosion is a limiting factor in the use of wire ropes and similar cables subjected to alternating loads.
Fretting corrosion similarly is a limiting factor in turbine vanes. Jet engines and many turbine devices work by forces on a wheel of “vanes” that transmit combustion or fluid forces to rotary motion. The vanes are held in close-fitting dovetails and varying elastic deflections result in microscopic oscillating slip (fretting motion) between the vane's root and its mount. Fretting damage in this instance can lead to fracture and vane failure, which usually causes complete failure of the engine or device.
Fretting corrosion is the limiting factor in the life of many plastic injection molding molds. Micrometer-deep vents are ground in the mold surfaces to allow air to escape from the mold as the plastic fills the mold. Each time that the mold is closed, the contacting surfaces “adjust” or slip to accommodate machinery errors of form, and this rubbing produces fretting corrosion or wear. The vents eventually disappear (in 1 million cycles or so), and regrinding is necessary. Eventually, the mold dimension ends up out of specification, and the mold must be scrapped.
Paper ID: MNL11348M
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