Abstract: Interface defeat refers to the radial flow of the material along the surface of the ceramic target without penetration in a long-rod projectile’s impact. In order to reveal the mechanisms of the transition from dwell to penetration during the interface defeat, a detailed FEM-SPH numerical simulation model is built and validated. Two defeat modes, which are defined as the crush defeat and the radial flow defeat, are observed from the simulation. Hydrostatic pressure strength is introduced in deriving the formulas for determining the upper and lower velocities of the interface defeat transition zone, based on the Hertz contact theory. A semi-analytical method for determining the ceramic strength decreasing exponent, which can consider the effect of the projectile material and the impact velocity, is proposed and then the A-T formula is utilized to predict the dwell time. Further on, dimensional analysis is carried out for proposing a method to predict the dwell time quickly, with the parameters calibrated from the parameter analysis results. The proposed method is validated through a comparison with the results from the numerical study and the published experiments.