A comprehensive model to predict the contact resistance during the nth fretting cycle and the ultimate usable lifetime of the contact has been developed. This model incorporates contact wipe, fretting vibration amplitude and frequency, contaminant chemistry, material properties, plating thickness, asperity deformations, normal load, electrical load, and surface topography. It is assumed that fretting vibrations separate contacting asperities and expose virgin metallic a-spots to environmental contaminant attack. The model calculates the amount of corrosive product produced on the exposed surfaces during the separation phase of a cycle of fretting. As fretting motions pull the exposed corroded asperities back together, a mismatch in size occurs and some of the corrosive product is scraped off and deposited in the valleys. Eventually, the valleys fill and separate the a-spots, resulting in "ultimate" failure. A material balance between amounts produced and scraped off estimates the amount of corrosive product dragged into the contact. Shifting of molecules via plastic deformation mix particles of corrosive product into the asperity metal. Assumptions that correlate mixing to plastic flow and use of modern composite theory leads to an estimate of the conductivity within the contaminated asperity. Integration over the asperity volume gives the asperity resistance, and application of Greenwood's theory estimates the total contact resistance. Results show a monotonic increase in contact resistance over time. Initial increases are slow, followed by rapid increases. Predicted failure times are consistent with field measurements.
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