Effect of core type and thickness on the low-velocity impact behavior of hemp fiber-reinforced bio-based sandwich composites

Sandwich composite structures may be subjected to impact loads, and their impact behavior is strongly influenced by the core material. This study experimentally investigated the low-velocity impact behavior of bio-based sandwich composites with hemp fiber-reinforced epoxy face sheets and balsa wood or agglomerated cork cores, at thicknesses of 5, 10, and 15 mm. Specimens were manufactured using vacuum-assisted resin transfer molding (VARTM) and tested at impact energies of 5, 15, 30, and 40 J, in reference to ASTM D7136. Core type and thickness effects on contact force, displacement, absorbed energy, and damage mode were evaluated using force–displacement curves, energy–time curves, energy profile diagrams, and damage images. Agglomerated cork cores generally produced higher maximum contact force values, whereas balsa cores exhibited higher deformation and energy absorption at high impact energies, with the most significant difference observed at 15 mm. Approximate perforation threshold energies were also estimated, ranging from 5–15 J for the 5 mm specimens to 30–40 J for the 15 mm balsa-core specimens. By directly comparing these two core types under identical conditions, this study addresses a gap in the current understanding of core-type effects on impact behavior.