Experimental Behavior and Design-Oriented Analysis of Sandwich Beams with Bio-Based Composite Facings and Foam Cores

Sandwich beams made of bio-based fiber-reinforced polymer (FRP) composite facings and foam cores were studied. The FRP facings consisted of a plant-based bidirectional flax fiber fabric ($400\mathrm{g}/{\mathrm{m}}^2$) and a bio-based epoxy resin (30% bio content), and the foam cores were made of 75-mm-thick closed cell polyisocyanurate. A total of nine sandwich beam specimens (1,200 mm long and 150 mm wide) were prepared and tested under three-point bending. The parameters of the study were core density (32, 64, and $96\mathrm{kg}/{\mathrm{m}}^3$) and facing thickness (one, two, and three layers of flax fabric). Three failure mechanisms were observed during testing, including top face wrinkling/crushing, core shear, and tensile rupture of the bottom face. It was shown that the foam with the density of $96\mathrm{kg}/{\mathrm{m}}^3$ was stiff and strong enough to achieve the tensile rupture of the flax FRP (FFRP) facing. Also, a nonlinear behavior was observed for the sandwich beams. A bilinear stress-strain model for FFRP facing was proposed and, based on that, closed-form moment-curvature and load-deflection equations of the sandwich beams were derived for design applications. The proposed design-oriented model was verified against the test data of this study and an independent study capturing the stiffness, strength, and nonlinearity of the test specimens.