Balsa sandwich composite fracture study: Comparison of laminated to solid balsa core materials and debonding from thick balsa core materials

We examined various fracture properties relevant to the use of balsa as a thick core material in sandwich composites with unidirectional glass fiber/vinyl ester composite skins designed for bridge decks. The first experiments compared the fracture toughness of a laminated veneer lumber (LVL) made from balsa to the toughness of conventional butcher-block balsa material. When using a good lamination adhesive, an LVL balsa core material has enhanced toughness compared to solid balsa and the LVL balsa also has enhanced fiber bridging effects. When balsa core is adhered to fiber glass composite skins, the vinyl ester resin used for bonding may infuse into the low density balsa. The second experiments looked at effects of infusing that resin on both LVL and solid balsa properties. Although the infusing caused a detrimental 15-20% increase in core material density, the infused resin had little or no affect on toughness compared to non-infused balsa. Finally, we looked at debonding toughness between fiber glass skins and a thick balsa core. The debonding toughness was higher than balsa toughness and increased with crack growth due to glass fibers bridging across the debonding fracture surface. Finite element analysis revealed that debonding a thin skin from thick balsa core in an asymmetric specimen is essentially a pure mode I process.