Axial Strength of Sandwich Panels of Different Lengths with Natural Flax-Fiber Composite Skins and Different Foam-Core Densities

This study examines the axial compressive strength of sandwich panels of different lengths, with flax fiber–reinforced polymer (FFRP) skins of different thicknesses, as an alternative to glass FRP (GFRP) skins. The panels incorporate polyisocyanurate (PIR) foam cores of different densities: 32, 64, and $96\mathrm{kg}/{\mathrm{m}}^3$. A total of 78 column specimens with a $100\times 50{\mathrm{mm}}^2$ core cross section were fabricated from the three PIR foam cores. FFRP skin layers ranged from one to five, giving a core-to-skin thickness ($c:t$) ratio of $15:64$. The length of the specimens ranged from 500 to 1,500 mm. Specimens were tested under concentric axial compression using pinned-end conditions. Panels with three FFRP layers showed equivalent axial strength to those with a single GFRP layer of 85% the total FFRP skin thickness. This was the case across all lengths studied and across all core densities. Doubling core density resulted in peak load increases, across all lengths, of 76, 57, and 79% for skins with one, three, and five FFRP layers, respectively; tripling the density resulted in respective increases of 114, 131, and 176%. As panel length increased from 500 to 1,500 mm, axial strength reduced by 40% for the $96\mathrm{kg}/{\mathrm{m}}^3$ panels but only by 20% for the 32 and $64\mathrm{kg}/{\mathrm{m}}^3$ panels. Panels longer than 1,250 mm experienced global buckling at peak loads, whereas those shorter than 750 mm experienced localized failures. The intermediate range of lengths saw mixed failure modes.