Effects of Pressure-Dependent Contact Resistivity on Contact Interfacial Conditions

Adequate contact pressure needs to be applied at electrical contacts to overcome contact separation force due to current constriction effects. In the railgun operation, the initial contact pressure is provided by the interference pressure between the armature and the rail. As the magnetic field is established in the bore, the electromagnetic force provides the contact pressure, as the initial contact pressure reduces due to wear or contact melting. Excessive initial contact pressure may delay the motion of the armature and cause overheating of the armature. On the other hand, insufficient initial pressure would result in contact separation of armature and rail. The contact interface is never perfectly smooth. It only contacts at discrete asperities at micro scale and at discrete points due to deformation of the contact interface at macro scale. The interface characteristics are different from the bulk behavior. A detailed model of contact resistivity is essential to compute the interfacial current density and temperature distributions accurately. A contact resistivity model was developed that is a function of contact pressure, the hardness of the softer contact member, average bulk resistivity, and contact constants. The model was implemented into the code EMAP3D. This paper presents the simulation results of the start-up of a sample railgun with and without contact resistivity.