Balsawood performs exceptionally well in static laboratory tests. The perpendicular end grains form a structure not unlike a miniature honeycomb, achieving a maximum compressive strength as high as any core material available. The high compression values contribute significantly to the stiffness of balsa-built sandwich panels. In a fire, balsa performs well, since it retains its structural load-carrying ability as it burns for a much longer period than foams do. While balsawood also exhibits exceptional shear values, the values typically presented are based on laboratory tests featuring a 0.25-inch thick panel, where balsa is at its peak in shear values. Shear values, however, are significantly reduced as panel thickness is increased.
Another weakness of balsa is its lack of impact tolerance. Specifically, its high compression stiffness causes impacts to be readily transmitted from the outside to the inside skin. One result is that the end grains are easily split, thus provoking delamination of the inside skin, which can often go undetected. Condensation can collect in the void created between the inner skin and the core and eventually result in severe water damage to the core itself. Even if the damage remains localized, repeated impact in the same area can eventually result in a catastrophic failure of the sandwich structure. Due to the low elongation of balsa, particular care must be given to the shear transfer bonding layers on each side of the core, ensuring that a low-modulus transition layer is used to “bed” the core. Timing is critical, since the core must be positioned and, preferably, vacuum bagged before this layer cures beyond gel point.
