Finite element model of the elbow interiorly impacted by flat-nosed missile was established using ANSYS/LS-DYNA. The Cowper-Symonds model was adopted. The rupture strain failure criterion was used to define the failure process. Numerical values were compared with experimental values obtained from the literature and the reliability of model was validated. The penetration failure mode of the elbow was analyzed. Factors of the critical rupture kinetic energy Er were acquired. It can be seen that the penetration failure mode is plugging induced by the extrusion and scraping dominated of axis stress. The effect of Do on Er can be neglected. Er increases with the increase of t/Do, Dm/t and R/Do when the missile mass m is invariable. The effect of m on Er should consider the factors of m and critical rupture velocity Vr.