Experimental and theoretical study of creep effects in electrical contacts

Creep effects in electrical contacts have been mentioned by a few authors but never studied in detail. The aim of this work is to investigate this phenomenon in low level silver-nickel electrical contacts at room temperature. Experiments are described which carefully measure the evolution of the electrical resistance with time of hemispherical contacts under static normal loads. The results show a slow decrease of the resistance and a quasi-stabilization after a few hours. The final value of the resistance can be significantly low when compared with the initial one. It is also shown that the decrease of the resistance is more pronounced for rough surfaces than for smooth ones. We explain this behavior by the creep of the metal. Indeed the more the metal is stressed the larger is the creep phenomenon. In order to model this effect, a computer simulation based on the finite element method is used to calculate the visco-plastic deformation of a sphere-on-plane system, the sphere corresponding either to the whole contact or to a single asperity. The experimental variations of the contact resistance and those obtained by coupling the mechanical (numerical simulations) and electrical (contact resistance formulas) models are in good qualitative agreement.