Thermal stress in EML rail-conductor overlays

In an electromagnetic (EM) railgun, a thin, unbonded overlay on the rail conductor develops residual strains as evident from its post-shot curvature. Heating due to the transient EM field and a large thermal gradient due to deposition of a molten layer of armature material contribute to the straining of the overlay. In this paper, the authors solved the one-dimensional coupled magnetic and thermal diffusion equations to estimate the thermal gradient in the overlay. Heat sources included both Joule heating and the enthalpy from a resolidified thin layer of molten aluminum deposited by the armature. The thermal gradient so calculated was used in a one-dimensional stress model to compute residual stresses from which residual curvature was calculated. The calculated curvature compares well with the experimental value. Finally, based on the estimated residual stress, they present an estimate for the remaining life of the overlay.