Self-healing performance of flax/epoxy–epoxy core sandwich panels under interfacial delamination assessed by digital image correlation (DIC)

This study investigates the development and performance evaluation of self-healing honeycomb sandwich panels incorporating an intentionally introduced pre-delamination at the face sheet–core interface. Sandwich panels were fabricated with flax/epoxy composite face sheets bonded to an epoxy honeycomb core using an adhesive film, and three configurations were examined: intact, pre-delaminated, and healed specimens. Healing was achieved by embedding an epoxy–hardener healing agent within selected honeycomb cells adjacent to the delaminated region, enabling autonomous crack-bridging after thermal activation. Flexural testing under three-point bending revealed that pre-delamination caused a pronounced reduction in stiffness, peak load capacity, and deformation capability, accompanied by a distinct shift in failure mode. In contrast, the healed panels showed substantial recovery, restoring approximately 80% of flexural strength, stiffness, and mid-span deflection capacity. Digital Image Correlation (DIC) confirmed reduced strain localization, improved load-transfer characteristics, and a relocation of the final rupture site away from the original defect after healing. These findings demonstrate that incorporating healing agents into naturally occurring honeycomb cavities is an effective and structurally efficient method for mitigating interfacial delamination without adding vascular networks or embedded capsules. The results highlight a simple, scalable route to enhancing damage tolerance, reducing maintenance requirements, and extending the service life of lightweight composite sandwich structures.