Assessment of the real contact area of a multi-contact interface from electrical measurements

The electrical supply of moving trains is provided by a sliding contact between the train's pantographs and the catenary. This electromechanical interface is composed of the pantograph's strips ? made mainly of carbon ? and the catenary contact wire. The temperature rising induces the rising of the carbon strip wear. Moreover, the heating produced at the interface depends on the surface quality. Indeed, the smaller is the surface contact, the greater is the electrical resistance which implies more heat production because of the Joule effect. The problem is multi-physical, there is a coupling between mechanical, electrical and thermal states. The objective is to define the real contact area with a simple electrical measurement. In many practical or fundamental situations involving contacting solids, the relevant notion of the real contact area is a very delicate one and especially its experimental assessment. Based on the Drude's classical transport model and within the linear elasticity approximation, a phenomenological model of a metal/metal contact is built up, offering a simplified interpretation framework of experimental data. The model accounts for the influence of the mechanical state of the contacting zone upon its electrical properties, such as its impedance. Interpreting available data within this framework leads to the assessment of the spots' number. The total contact force works on the spots and on the average contact length. In this model, the interface is treated as a new medium with its own conductivity and mean free time between ionic collisions. There are two types of measurement: - Electrical measurements carried out on two copper sheets with dimensions 50X50X1 mm3 acted upon with an external compression force, allowed to check and validate the model. In agreement with the conditions of the model and to avoid the complexification of the model due to the intricacy between thermal and electrical processes at the contact interface, the measurements were operated in alternative current at low voltages. - Additional measurements of surface states have been realized to join the electrical measure with roughness and the nature of the metal. In the case of contact between train's pantograph and catenary, this method allows predicting electric transfer's quality to control the heating of the interface. Being sensitive to the spots' mechanical solicitation conditions, properly interpreted, electrical impedance measurements should lead to a better understanding of the complex mechanical responses of these interfaces and their ageing process or even to detect a fatigue and prevent a potential failure