Mechanics of balsa (Ochroma pyramidale) wood

• Mechanical properties of balsa are related to its cellular structure and modeled based on deformation and failure mechanisms.
• In axial compression, bending, and torsion, the elastic modulus and strength increase linearly with density.
• In radial compression, the modulus and strength increase nonlinearly with density, due to the combined contribution of fibers and rays.
• The cell wall elastic modulus determined by nanoindentation is about half of that extrapolated from mechanical models.

Balsa wood is one of the preferred core materials in structural sandwich panels, in applications ranging from wind turbine blades to boats and aircraft. Here, we investigate the mechanical behavior of balsa as a function of density, which varies from roughly 60 to 380 kg/m3. In axial compression, bending, and torsion, the elastic modulus and strength increase linearly with density while in radial compression, the modulus and strength vary nonlinearly. Models relating the mechanical properties to the cellular structure and to the density, based on deformation and failure mechanisms, are described. Finally, wood cell-wall properties are determined by extrapolating the mechanical data for balsa, and are compared with the reduced modulus and hardness of the cell wall measured by nanoindentation.