Low-velocity impact response of composite sandwich structures with hierarchical gradient foam/carbon fiber

Foam sandwich structures are widely used in aerospace, transportation, defense industries, and architecture due to their exceptional mechanical properties and physical characteristics. However, the impact resistance of these structures is significantly affected by their sandwich core. In this study, three types of foam sandwich panels were fabricated using uniform multi-layer foam, positive-density-gradient foam, and negative-density-gradient foam as core materials. The effects of core density distribution on the failure morphology, energy absorption characteristics, impact resistance, and damage evolution process of sandwich structures were systematically investigated through drop-weight low-velocity impact tests at energy levels of 20 J, 40 J, and 80 J, combined with finite element numerical simulations. The results indicate that the three specimen types exhibit fundamentally similar failure modes. Under equivalent impact energy conditions, the negative-density-gradient foam sandwich structure demonstrates optimal impact resistance. The positive-density-gradient and uniform-density specimens display progressive failure characteristics during impact damage propagation. In contrast, the negative density-gradient structure manifests face sheet and upper foam layer load-bearing failure, while the underlying low-density foam undergoes plastic deformation, with both synergistically resisting the impact load. The findings of this study provide theoretical foundations for the design and optimization of impact-resistant sandwich structures.