Compression and indentation behavior of lightweight foam-filled kraft paper honeycomb structure

Abstract

Honeycomb sandwich structure has been used extensively in engineering industries as an energy absorber to resist external loads due to its lightweight and high energy absorbing capability. However, the honeycomb core is the weakest part of the sandwich structure and they may fail or collapse through cell fracture or cell wall buckling depending on the loading regime and the core configuration. A foam-filled honeycomb structure has been proposed to provide an enhancement in the properties of the honeycomb core. The filler existence within honeycomb cells improves the honeycomb structure systems by strengthening the honeycomb cell wall and changes the structure's behavior. Therefore, statistical, experimental, and simulation works were carried out in this research work to investigate the effects of filling Kraft paper honeycomb with polyurethane foam. For the simulation, a three-dimensional finite element model for foam-filled Kraft paper honeycomb was developed. Statistical analysis was performed at the initial stage of this study to determine the optimum configuration of the Kraft paper honeycomb. Then, the optimized unfilled kraft paper honeycomb, polyurethane foam, and foam-filled Kraft paper honeycomb were subjected to quasi-static compression loading. The maximum force and energy absorption of foam-filled Kraft paper honeycomb were computed to study the improvements compared to the summation of unfilled kraft paper honeycomb and foam alone. The three-dimensional finite element analysis was performed using Ls-Dyna software to investigate the interaction between polyurethane foam and cell walls. Force-displacement behaviors obtained from numerical simulations were validated by experimental findings, and the distribution of energy absorption between cell walls and polyurethane foam in the foam-filled honeycomb was analyzed. In order to study the localized effect of foam-filled kraft paper honeycomb, experimental analyses and finite element analyses subjected to indentation loading were performed. As a result, the Kraft paper honeycomb with density 175gsm, 3 ply thickness of paper, and 10 mm cell size of honeycomb exhibit the optimum configuration with 724.80 J/kg of specific energy absorption (SEA) and 9.35 MPa/kg of specific compression strength (SCS). Moreover, the experimental results show that the peak force and energy absorption of the foam-filled honeycomb were increased up to 30% compared to the individual component. Meanwhile, the indentation at the vertical edge shows the higher peak force and energy absorbed which proves that the vertical edge of the cell wall behaves as a strong point to endure the indentation force. In conclusion, polyurethane foam filler has strengthened the honeycomb cell wall and improved the energy absorption capability of the Kraft paper honeycomb structure. The FEA results confirmed that the cell walls strengthened by the foam filler and the confinement of foam by cell walls increased the energy capability of the foam-filled honeycomb structure.