Energy-dependent impact damage and post-impact recovery in self-healing honeycomb sandwich panels

This study investigates the energy-dependent impact damage and post-impact recovery of self-healing honeycomb sandwich panels comprising flax/epoxy composite face sheets, an epoxy honeycomb core, and an in-core dual-component healing system. The fabricated panels were evaluated under undamaged, damaged, and healed conditions. Controlled impact damage was introduced using a modified Izod configuration at four predefined energy levels (5.5, 11, 16.5, and 22 J), followed by three-point bending to evaluate residual flexural performance. The undamaged panels exhibited a peak load of 2.13 ± 0.07 kN and an initial stiffness of 0.89 ± 0.04 kN/mm. Following impact, the damaged specimens exhibited peak loads of 0.80–1.18 kN and initial stiffness values of 0.41–0.53 kN/mm, reflecting energy-dependent core crushing, face–core delamination, and mixed-mode damage. After thermal healing at 60°C for 24 h, peak loads increased to 0.94–1.97 kN, while stiffness increased to 0.53–0.90 kN/mm. Peak-load recovery relative to the undamaged reference ranged from 44.1% to 92.4%, whereas stiffness recovery ranged from 59.6% to 101% across the investigated impact-energy levels. The recovery response was non-monotonic with impact energy, indicating competition among localized and distributed damage mechanisms and differences in healing-agent accessibility. At higher impact severity, complete restoration became increasingly limited by irreversible damage mechanisms. The results demonstrate the feasibility of integrating localized self-healing functionality into natural-fiber-reinforced honeycomb sandwich structures and highlight the importance of impact severity in governing post-impact structural recovery.