Flexural and axial behaviour of sandwich panels with bio-based flax fibre-reinforced polymer skins and various foam core densities

This study investigates the effect of foam core density on the behaviour of sandwich panels with novel bio-composite unidirectional flax fibre-reinforced polymer skins, along with a comparison to panels of conventional glass-FRP skins. Eighteen 1000 mm long flexural specimens and 18 500 mm long stub column specimens were fabricated and tested. All specimens had a foam core of 100 × 50 mm² cross-section with symmetrical 100 mm wide skins. The study compares the effect of three separate polyisocyanurate foam cores when used in conjunction with either three layers of flax fibre-reinforced polymer or a single glass-FRP layer for each skin. Flexural specimens were tested in four-point bending and stub columns were tested under axial compression with pin–pin end conditions. Doubling the core density from 32 to 64 kg/m³ and tripling the density to 96 kg/m³ led to flexural strength increases of 82 and 213%, respectively, for flax fibre-reinforced polymer skinned panels, and comparable increases in glass-FRP skinned panels. Similarly, flax fibre-reinforced polymer-skinned columns showed similar increases in ultimate axial capacity of 85% and 196%, while glass-FRP- skinned columns experienced lower increases when core density was varied. The three-layered flax fibre-reinforced polymer skin, only 17% thicker than the single layer glass-FRP skin, was shown to provide equivalent flexural and axial strengths at all three core densities, within −5 to +13%.