In this research, we have investigated the ballistic resistance of sandwich structures with aluminum face-sheet and graded polyurethane foam core with different densities. The effects of graded changes of core foam density and the effect of the sequence of foam layers with different densities on energy absorption and ballistic limit of sandwich structures under the impact of hemispherical nose projectiles at high speeds (160 to 300 m/s) are studied. The results of this study showed that increasing the density and thickness of the foam core leads to increase in the ballistic limit and energy absorption; also, the ballistic limit of sandwich structures with the same mass with graded foam core for three and five layers panels is respectively, 10.37% and 5.57% more than single layer foam core with the average density in case the foam layer with less density is placed in the impact side. By using the graded foam core (laminate), the core resistance is increased and the damaged zone shape is changed, and the energy absorption of back face-sheet is increased
